High Voltage Forum

General electronics => Electronic circuits => Topic started by: kamelryttarn on March 15, 2017, 08:22:34 PM

Title: Royer induction heater and high current power supply
Post by: kamelryttarn on March 15, 2017, 08:22:34 PM
This thread will be used primarily to share my build of a Royer induction heater and also a power supply capable of driving it with ease. I hope my findings can help others.

A while back I decided to start experimenting with induction heating. I had a very specific application in mind but I won't go into specifics. My goal was to heat up a large stainless pot, but I quickly decided to transfer the energy to a pipe instead. To start experimenting without spending a fortune I ordered a "1kW induction heater" from ebay and went to work.

One thing that was clear from the beginning was to make sure I had a power supply that could deliver quite a lot of current at 12V so I took a large UPS battery I had laying around. It quickly became clear that 20A or even 30A current draw was no problem and I had to find another, more permanent solution than a lead acid battery. Buying a large 2kVA or higher rated transformer cost a lot of money and if you want a three phase transformer to keep the ripple down it's even more expensive, but I managed to find a large welding power supply that I took apart. In it there was a 6kVA rated three phase transformer weighing 80kg that I will use to build my power supply. To build a three phase rectifier bridge I need 6 pcs of high current diodes but there was only 4 in the welding PSU so I bought 6 new ones, 3 "regular" and 3 reverse polarity. The original ones were rated 300A but my new ones are only rated for 150A but I think 150A should be enough.

The diodes in picture are a regular 4007, a fairly large MUR420 and my 45L10

I use the 500V primary which gives me a rectified secondary DC of about 37V
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on March 16, 2017, 09:25:04 AM
Most problems people have with this induction circuit is because of the PSU not being powerful enough so DON'T skimp on the power supply and make sure it can deliver at least 12V under HEAVY load. These induction heaters really need to be kick started so make sure your PSU can deliver a lot of current promptly, in other words make sure you have a large capacitor bank.

Here is some really good information about DC bus capacitors: http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/dc-bus-capacitor/

The reasons I looked for a three phase transformer were lower ripple than a regular one phase and that huge three phase transformer are easier to find than a single phase and also easier to power. Most single phase mains outlets are fused at 10A which gives a theoretical limit of ~2300W and I wasn't sure that was enough for me.

I would recommend anyone looking to build a beefy power supply to look for welding power supplies. They can be found really cheap sometimes the old ones are built like cold war era tanks.

I salvaged all parts from my welding psu, and among the many interesting parts there is a HUGE inductor/choke and 4 high current capacitors that I would like to re-use. Should I connect the rectifier first to the choke and then to the capacitors or vice versa? My thinking was that the choke that I measured to 1,8mH would act like some kind of soft start and make the power supply easier to power up.

For those of you who are interested I have attached the service manual/spare parts list for the welding psu and my diodes. (The spare parts list file was too large so here is the link instead)

http://www.esab.com/gb/en/support/upload/DTA-300-AC-DC.pdf
Title: Re: Royer induction heater and high current power supply
Post by: Mads Barnkob on March 16, 2017, 01:29:49 PM
You have really set sails for a large induction heater here!

How many bridges have you planned to use in parallel? I know that RogerInOhio on 4hv made one with 4 or 8 bridges in parallel, that would draw around 4 kW.

To experiment with the power supply design, where to place choke, capacitors etc, if you want a CLC or LC filtering, check out the Duncan amplifiers PSU designer: http://www.duncanamps.com/psud2/

From the part list for the welding machine, I can see that there is a shunt resistors and a digital Ampere meter in the front, you could use those for measuring the DC current drawn from the power supply.

I look forward to see the rest of this monster build and how it performs melting stuff :)

Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on March 16, 2017, 03:23:36 PM
First of all I will see how far I can take a single pair of mosfets. I aim for 1500W and if I reach that I don't think I really need to go much higher. Since I am going to use it for heating water and not melting metal it's hard to find relevant data. Everything is focused in crucibles and heating knives and bolts.

I remember psud2 from my vacuum tube days and it's a really nice software.

When it comes to current metering I first thought about using the regular shunt, but since I would like to use an arduino to both measure and control the output (just on/off) I think I will use a ACS712 hall effect current sensor instead. The shunt from the welder has a voltage output of 60mV at 300A and since I will have a maximum continuous output of 150A that translates to 30mV. Even if I use an external voltage reference of 1.023V for my arduino the A/D resolution will be too low, and also I will have to be careful about the ground and positive power supply since the arduino will be tied to to the circuit. The ACS712 will let the arduino float in regards to either the  positive or negative side of the psu and also give a much higher A/D resolution.

I hope to have crude psu ready for testing within a month or so. This entire project will take some time, but I will try and document all useful steps as good as I can.
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on April 08, 2017, 04:29:24 PM
Finally got around to wind the chokes. They are INSANE! Took quite a bit of work but the result looks pretty good. I think I managed to get 38 turns of AWG 9 wire around the T-300-cores. Will measure them with my LCR-meter later and post the values.

I put a glove there for size comparison.

Edit: measurements

Here are my measurements:

L1
@100kHz L=183uH Q=112
@10kHz L=184uH Q=142

L2
@100kHz L=180uH Q=114
@10kHz L=182uH Q=154

Pretty similar values. Can't wait to try them out :)
Title: Re: Royer induction heater and high current power supply
Post by: Mads Barnkob on April 08, 2017, 07:51:06 PM
Looks great, I can imagine that it must have been interesting winding those 38 turns of 6 mm^2 wire, that has to done carefully to get a good looking result like you have there.

What do you have in mind for filtering the DC supply? A huge stack of electrolytic capacitors?

Is the bridge/bridges the next part of the project?
Title: Re: Royer induction heater and high current power supply
Post by: Dylan Burger on April 08, 2017, 08:19:51 PM
This looks like its going to be an insane machine :)

What kind of switches will you be using to handle all that power ? I used to push 30A constantly through a set of irfp250's which didnt last very long. After pushing the voltage higher than 40v I had to move up to IRFP460's and as a last resort I switched to IGBT's.

Also , what resonant frequency are you expecting ?
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on April 08, 2017, 09:14:28 PM
Yes Mads, rectifier is probably the next step. I asked a friend to help me cut, drill and tap the heat sinks I salvaged from the welding power supply for my new power diodes.

I'm not sure about the psu caps. I would like a bunch of large electrolytic or high power film capacitors, but I can't spend an infinite amount of money on the just the caps alone. Also, since I am using a three phase transformer I don't believe I need as large bank of caps as I would have with a single phase setup.

For turning it on and off I will probaly use a pair of Infineon IRFP4468 driven by a high power mosfet driver, controlled by an Arduino. Initially the IRFP4468 was planned for the induction circuit but the voltage of my upcoming psu forced me to change IH mosfets to IRFP4568 with a 150V voltage rating instead. Expected resonant frequency is around 55kHz.
Title: Re: Royer induction heater and high current power supply
Post by: Dylan Burger on April 08, 2017, 09:43:07 PM

For turning it on and off I will probaly use a pair of Infineon IRFP4468 driven by a high power mosfet driver, controlled by an Arduino. Initially the IRFP4468 was planned for the induction circuit but the voltage of my upcoming psu forced me to change IH mosfets to IRFP4568 with a 150V voltage rating instead. Expected resonant frequency is around 55kHz.

Nice :) 

Your IH mosfets should also handle the resonant rise within the work coil. usually 3x input voltage. So , even if you somehow get inductive spikes from switching you will be covered, considering that 3 x input (36V) is ~108v.  55Khz is also a nice resonant point. I find that most objects heat fairly well at this frequency. I have gone to 200Khz and it doesnt seem to work well on thicker objects.
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on April 09, 2017, 08:03:02 AM
I wound the chokes myself by hand. They are for the induction heater circuit. I have a BIG choke that came with the welder power supply that I will use for my power supply. I measured it to 1,8mH and it's most likely rated for 300A continous use since that was the specs for the original welding psu.

I can take a picture of it and post here. I would like to get my hands on a milli or even micro-ohm-meter to check the DC resistance of it but I doubt it's much.

I realized almost immediately when fiddling around with the small ebay-IH that these suckers can draw A LOT of current so a beefy power supply was one of my first priorities. I guess this PSU will weigh in at close to 200kg and will be able to deliver 150A continuously. Should be enough ;)

[edit]

Here are some pics of the choke as promised.

Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on April 13, 2017, 01:31:07 PM
I received a PM about why I wanted to use a three phase transformer, so I thought I should take some time to explain in as good as I can in case someone else would like to know.

The reason is that with a three phase setup you have three voltages that are out of phase by 120 degrees.

(https://upload.wikimedia.org/wikipedia/commons/thumb/c/cc/3_phase_AC_waveform.svg/220px-3_phase_AC_waveform.svg.png)

When you rectify this with a full wave bridge rectifier (that requires only 2 more diodes than a single phase full wave rectifier) you end up with a DC voltage with lower ripple voltage and higher frequency which in the end requires less capacitors for smoothing out the ripple.

(https://i.stack.imgur.com/zVKAn.png)
Title: Re: Royer induction heater and high current power supply
Post by: Mads Barnkob on April 14, 2017, 11:01:08 AM
I also tried to summarize the differences between 1 and 3 phase, half and full wave rectification with some pictures and charts here: http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/rectifiers/
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on May 19, 2017, 04:49:58 PM
I have hit a bit of a wall with my power supply :(

I have found the capacitors I WANT but they are stupidly expensive. Dream scenario is 6000uF DC link film capacitors but that would cost about 1000 USD. Would it be possible to use a large bank of electrolytics bypassed with a few smaller film caps and achieve acceptable performance?

I found these but they are still quite expensive: http://www.ebay.com/itm/Ducati-Energia-DC-Filter-Link-Capacitor-1500-Micro-Farad-700-VDC-165-A-/142169549692
Title: Re: Royer induction heater and high current power supply
Post by: Mads Barnkob on May 19, 2017, 08:52:23 PM
I have hit a bit of a wall with my power supply :(

I have found the capacitors I WANT but they are stupidly expensive. Dream scenario is 6000uF DC link film capacitors but that would cost about 1000 USD. Would it be possible to use a large bank of electrolytics bypassed with a few smaller film caps and achieve acceptable performance?

I found these but they are still quite expensive: http://www.ebay.com/itm/Ducati-Energia-DC-Filter-Link-Capacitor-1500-Micro-Farad-700-VDC-165-A-/142169549692

I have been eye-balling those capacitors for a while too, but that should be used in a large DRSSTC that has to pull enormous amount of energy and dump it into a resonant LC circuit, they are way out of the scope for a high current low voltage power supply as yours.

You should build a high capacity bank of electrolytic capacitors with a voltage rating of no more than 200 VDC, it will be waste to go any higher for your low voltage application, and would only result in a lower capacitance bank. Parallel as many as needed for the capacitance needed for the desired voltage ripple. I guess you can find some inspiration in my DC link electrolytic capacitor part of the DRSSTC design guide (http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/dc-bus-capacitor/) to calculate what you need.
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on June 10, 2017, 05:13:15 PM
Finally got the rectifier assembled. I received the diodes a while ago but I needed help to drill and tap the salvaged heat sinks. Here is the result
Title: Re: Royer induction heater and high current power supply
Post by: Mads Barnkob on June 11, 2017, 10:18:08 AM
Really nice and beefy stud diodes :)

But are they mounted correctly in regard to be a 3 phase rectifier bridge? They would only be correct right now if you have the DC+ at one heat sink and DC- at the other.

You can see here how its normally done with stud diodes, the 3 inputs at the bottom and DC+/- is the rails at top.

(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fwww.gdrectifiers.co.uk%2Fuploads%2F3_phase_diode_bridge_4.jpg&hash=42f7c8c068f14a80898bbcd7e93014fa)
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on June 12, 2017, 08:28:41 AM
I think I have mounted them correctly. My intention was to have the DC output at the heat sinks and because of this I was forced to buy new diodes and half of them are reversed polarity. I do however need to double check the color coding of the sleeves before I connect everything.

I still haven't found any suitable capacitors. According to one site I found, a rule of thumb regarding ripple current is that ripple current over the capacitors is double that of the current draw to whatever load is connected. The limiting factor for this power supply is the diodes that are "only" rated at 150A meaning I would like a capacitor bank capable of handling 300A so I keep getting back to film capacitors that cost a small fortune. Just for fun I made some calculations for heat losses on a fairly high rated electrolytic capacitor with ESR of 8mOhm and maximum ripple current of 26A. This translates to roughly 5 watts. I would require 12 pcs of this, quite pricey electrolytic. The cost would be almost the same as using DC link film capacitors.
Title: Re: Royer induction heater and high current power supply
Post by: Mads Barnkob on June 12, 2017, 12:19:39 PM
What about if you made something yourself? A MMC of larger film capacitors, paralleling a lot of these should give you the current rating needed, but how much capacitance are you after?

http://www.ebay.co.uk/itm/132219914390

http://www.ebay.co.uk/itm/132219915352

http://www.ebay.co.uk/itm/132221070952

this seller has multiply auctions for more of the same capacitors, its possible to get a good deal with combined shipping.
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on June 12, 2017, 01:38:38 PM
I would like to reach somewhere around 4000-6000uF. Together with my DC choke that should give me somewhere near -30dB if I understand things correctly.

I found this one: http://www.ebay.com/itm/371440106874

It is 65 millifarad (65.000 uF). I double checked with the seller. I don't think I have ever wanted anything as badly as that behemoth of a capacitor ;)
Title: Re: Royer induction heater and high current power supply
Post by: Mads Barnkob on June 12, 2017, 02:06:27 PM
I would like to reach somewhere around 4000-6000uF. Together with my DC choke that should give me somewhere near -30dB if I understand things correctly.

I found this one: http://www.ebay.com/itm/371440106874

It is 65 millifarad (65.000 uF). I double checked with the seller. I don't think I have ever wanted anything as badly as that behemoth of a capacitor ;)

It will take a LOT of soldering, but it would be possible to reach with around 4-6 boxes of those capacitors :)

I also want that capacitor now, if only we knew what it looked like on the inside, maybe its just a array of smaller capacitors so that it could easily be taken apart and split between buyers, as no one really needs 65000 uF in hobby use. If you can get a really good offer from the seller, I am in on a group buy!
Title: Re: Royer induction heater and high current power supply
Post by: Mads Barnkob on June 13, 2017, 12:30:19 PM
I asked the seller for a picture of the insides and that revealed that it is only a huge stack electrolytic capacitors, so it is completely out of interest now, that is simply too expensive for something that is maybe worth 150€ in total.
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on June 13, 2017, 08:34:25 PM
Too bad but really nice of the seller to provide the picture. My search continues...
Title: Re: Royer induction heater and high current power supply
Post by: rikkitikkitavi on July 06, 2017, 10:39:55 PM

Kamelryttaren, have you considered larger server power supplies for server racks. There are two types in princip , 12V and 48V types. I was thinking about the later.

The good thing about these is that they are

1) high PF- this is important , utilising your AC outlet fuse to the max
2) high efficiency (90+%)
3) for the larger, 48V types floating chassis, ie it is NOT connected to either of the 48V terminals. It is of course connected to PE.
4) compact
5) whiny (as in fan whines) - maybe not so good after all :)
6) easily found at ebay etc for a reasonable price mostly


They can be paralleled at convenience.  They are designed for parallel operation by current sharing but there is a data bus protocl which  is propietary but for most of them you can find which pins to tie for startup as a standalone unit and run them as standalone supplies via current share resistors as output voltage is trimable a few volts.

They could also be series connected up until some critical insulation voltage but as most of them are compact and hold 400VDC bus with a high-pot aproval I am not worried about that.
I expect to be able to use 3 in series for 150V DC bus at 40A.

I was planning to use some for a induction heater project in the future.

They probably needs a soft start on the induction heater and proper filtration to prevent back emf interfering with the DC supply feedback loop, ie some secondary DC bus cap that can supply the current peaks.
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on July 18, 2017, 07:36:09 PM
Thank for the suggestion rikki. Haven't really thought about it but since I have most things apart from the caps I think I will stick with my +200kg three phase psu. I don't like SMPS and I think my 37VDC is already a bit on the high side. I would have preferred something closer to 24-30VDC but this will have to do.

I got another ebay package today with a Hantek 1008 8-channel USB oscilloscope and a 65A current clamp probe so I can keep track of what's going on with the induction heater circuit. I need a few 20:1 attenuators as well.

Btw, you wrote something about soft starting the induction heater. That's a great recipe for smoke and other undesirable special effects.They need to be jump started as brutally as possible. I have managed to get my hands on a pair of Infineon IPT007N06N that I will try and use as a power switch but I haven't really figured out how to water cool it and connect it. I think I will try and solder the thermal pad itself to a copper pipe and use the pipe as conductor as well as heat sink and then just solder the fattest wire I can to the source pins.
Title: Re: Royer induction heater and high current power supply
Post by: Mads Barnkob on July 18, 2017, 08:20:50 PM
The server power supplies are handy, cheap and small, if they can withstand the abuse and injected noise from a Royer oscillator, I got no idea, but I do have some of these supplies somewhere, maybe I should smack together a Royer just to test them :)

There is no doubt that your iron core power supply will be one of the few things left after 3rd world war along with the scorpions that will be sitting on top of it.

Good that you got yourself a oscilloscope, that is one of the best tools in electronics, if you need a cheap attenuator you can just use a ferrite ring core of suitable permability / AL above 4000, wind 20 turns on it and put the wire through you want to measure, this gives you a 1:20 step down.

The IPT007N06N MOSFET looks good on paper in a high current switching power supply, but take a look at figure 4, if you use this as a switch to conduct DC at high current, it will melt really fast, even at 50% duty cycle it will have a temperature rise of 2 Kelvin per Watt dissipated, how much power do you expect to dissipate across its R(DS)on? If you end up with more than 30 Kelvin rise, I think you will be on thin ice in regard to it living long.

Would it not be a better idea to get a large SCR to switch it on with?
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on July 22, 2017, 07:57:05 PM
If I assume worst RDS(on) of 0.75mOhm at about 50A current draw that would mean about 0.5W per mosfet and a voltage drop of 0.02V. Water cooling it should be sufficient I think unless I have severely misunderstood something in the datasheet.

I am not familiar with SCR. I tried searching but if you have a good source for info I would appreciate it. Since I have a pretty good grasp on mosfets I just thought that would be the easiest way to go, especially since I intent to use an arduino to control my power supply when it is finished.

I made some quick tests today with the scope and the current clamp. It works really well but I need to get my big power supply up and running. I took the old UPS battery just to get the heater to oscillate and it works but the gate voltage is just under 7V so they don't even fully "open". The gate waveform looks really bad as well. I was hoping for  something close to a square wave but it more resembles a saw tooth wave.
Title: Re: Royer induction heater and high current power supply
Post by: Mads Barnkob on July 23, 2017, 10:30:24 AM
You are right, I was being pessimistic, even at 100A through them you do not have more than 7 Watt dissipated.

If duty cycle D=0.5 has a 2K/W rating, I think we have to be pessimistic again and say that DC is around 10K/W, so you need to cool a 70 degree temperature rise, so water cooling is maybe a good idea, but maybe a large passive heat sink / forced air cooling is enough, you better test this before building a large water cooling setup :)

I did not have any good SCR resources in mind, just a fast googling it was something like this I had in mind: http://www.bristolwatch.com/ele/triacs.htm

Be careful with such low voltages, below 10V and you are in the MOSFET kill-zone  :'(
Title: Re: Royer induction heater and high current power supply
Post by: afk on July 23, 2017, 12:06:14 PM
On the topic, I'm having some questions regarding water cooling. Some sites warned me about tap water having minerals (mainly sodium, calcium and potassium ones) that make water more conductible, and this can cause electrolysis of water and these minerals, which then can corrode the copper tube. Is it true? I'm asking because distilling water is a time-consuming process and waste a lot of gas/electricity for heating. Furthermore the tank current is alternative at a few tens of kHz so the electrolytic effect shouldn't exist (or to be a problem). I'm thinking of taking advantage of the rice cooker in my house, though. If tap water can be used for around thirty minutes then that should be okay for my application.
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on July 24, 2017, 11:41:27 AM
You are right, I was being pessimistic, even at 100A through them you do not have more than 7 Watt dissipated.

If duty cycle D=0.5 has a 2K/W rating, I think we have to be pessimistic again and say that DC is around 10K/W, so you need to cool a 70 degree temperature rise, so water cooling is maybe a good idea, but maybe a large passive heat sink / forced air cooling is enough, you better test this before building a large water cooling setup :)

I did not have any good SCR resources in mind, just a fast googling it was something like this I had in mind: http://www.bristolwatch.com/ele/triacs.htm

Be careful with such low voltages, below 10V and you are in the MOSFET kill-zone  :'(

My water cooling setup for these mosfets will be very simple and nothing fancy at all. Will post pics when I have it soldered together, but it will probably be a while since I can not find any affordable caps :(
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on July 25, 2017, 11:13:05 AM
Regarding the dc bus capacitors, is there any advantage of the first configuration over the second one in this image?

I have purchased caps now. Proper film capacitors but I couldn't afford to reach more than 1900uF. I'm hoping it will be enough to start experimenting at least. I have tried to figure out the resulting ripple voltage but I don't know how to calculate this so I will just measure it when everything is up and running.

I got 5 of these for a good price: http://www.kemet.com/Lists/ProductCatalog/Attachments/123/F3303_C4DE.pdf

They are rated 100 Amps (10kHz 25C), will they tolerate more or less at 300Hz?

Now I have something fun to do on my upcoming vacation :)

[edit] Receied the caps and copper rails yesterday so today i drilled holes and test mounted the capacitors. I still haven't drilled the holes for connections but I will wait until I have decided how to mount everything. Hopefully I can do some test with my power supply soon since the caps where the last items missing.
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on August 18, 2017, 09:32:26 PM
It's alive!

I finally got the power supply together (but not completely finished) and fired it up. Great success and no smoke ;)

Tomorrow I will bring the IH and connect it to make some early measurements
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on August 20, 2017, 09:07:33 AM
Yesterday I connected everything and it is working great!

I made some triggered measurements with my Hantek 1008 oscilloscope and it looks ok but I am a bit puzzled about the gate waveform. Despite around 40V power supply voltage it does not look very "square-y" but I'm beginning to think that my oscilloscope knowledge or the actual oscilloscope itself may be at least part of the reason. Could it be that the bandwidth is a bit too low for what I am trying to do? Despite that it was very interesting an educational.

At first I did a few runs with nothing inside the coil. There was a large current spike at the beginning but it dropped quickly. Startup current at around 40V was roughly 20A.

Then I decided to see what happens when you put something inside the coil and try to fire it up. I found a piece if solid iron about 3cm in diameter and placed inside of the coil. Now things were starting to get exciting! Startup current was over 50A and it didn't drop as much as before. The iron piece got quite hot in the few seconds the induction heater worked before one of the mosfets blew up (as expected). Since the induction heater started to oscillate and worked well for a little while before blowing up I am assuming the heat losses is the most likely reason for failure. I will try my IRFP4568 next since they have MUCH lower RDSon the the IRFP250N. So far I am using un-modified ebay induction heaters to learn a bit more about them.

In the scope pictures below I have the current clamp hooked up to channel 1, mosfet gates are connected to channel 2 and 3, and finally channel 4 is the supply voltage connected through a 20:1 attenuator.
Title: Re: Royer induction heater and high current power supply
Post by: Mads Barnkob on August 20, 2017, 07:50:59 PM
I got 5 of these for a good price: http://www.kemet.com/Lists/ProductCatalog/Attachments/123/F3303_C4DE.pdf

They are rated 100 Amps (10kHz 25C), will they tolerate more or less at 300Hz?

In high frequency applications where these typically are used, it is the power dissipation that is the limiting factor, since losses are higher at higher frequencies. At lower frequencies it is the allowed ripple voltage that is the dominating factor, but since you are using MKP capacitors, this is not a problem, but watch their temperature rise as there is a large current de-rating factor for temperature.

Yesterday I connected everything and it is working great!

I made some triggered measurements with my Hantek 1008 oscilloscope and it looks ok but I am a bit puzzled about the gate waveform. Despite around 40V power supply voltage it does not look very "square-y" but I'm beginning to think that my oscilloscope knowledge or the actual oscilloscope itself may be at least part of the reason. Could it be that the bandwidth is a bit too low for what I am trying to do? Despite that it was very interesting an educational.

At first I did a few runs with nothing inside the coil. There was a large current spike at the beginning but it dropped quickly. Startup current at around 40V was roughly 20A.

Then I decided to see what happens when you put something inside the coil and try to fire it up. I found a piece if solid iron about 3cm in diameter and placed inside of the coil. Now things were starting to get exciting! Startup current was over 50A and it didn't drop as much as before. The iron piece got quite hot in the few seconds the induction heater worked before one of the mosfets blew up (as expected). Since the induction heater started to oscillate and worked well for a little while before blowing up I am assuming the heat losses is the most likely reason for failure. I will try my IRFP4568 next since they have MUCH lower RDSon the the IRFP250N. So far I am using un-modified ebay induction heaters to learn a bit more about them.

Congratulations on having it all working, at least for how long/short it lasted :)

You are right about your Hantek 1008 oscilloscope having a too low band width and sample rate. 2.4MSa/s is very low for a oscilloscope, which is why this is advertised for diagnostics on engines, not high frequency inverters. 2.4MSa/s is most likely also only if you used a single channel, so using all four it is even smaller, like my Rigol DS1054Z is a 1GSa/s if using one channel, but using all 4 its down to 250MSa/s.

Was it a loud boom when the MOSFET went? Did you get it on video?

Certainly looking forward to the repaired inverter updates! 
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on August 20, 2017, 08:06:42 PM
Yes the mosfet blowing up was louder than I expected. It's very nice to have a power supply that won't sag under heavy loads. I realized afterwards that I should have set my oscilloscope on a negative slope trigger to catch all values when it blew up but perhaps I will do that later on when I get it running again which shouldn't take too long.

I haven't inspected the pcb thoroughly yet but I noticed a very thin connection that blew up and it makes me wonder why they would make such a thin via if it was supposed to handle large currents. I will see if I can figure out what it was supposed to connect. The other, similar board I have does have the exact same configuration so I can not use that as a reference.
Title: Re: Royer induction heater and high current power supply
Post by: petespaco on August 22, 2017, 05:44:31 AM
Which "unmodified Ebay induction heater" are you using?  If you are using one of the "1000 watt" models, why would you allow 50 amperes to flow?
50 Amps X 40 Volts = 2000 watts.
  When testing the limits of one of these devices, I would not place  any work in the work coil before turning the power on unless I was already confident that the current flow would be acceptable .  I would turn the power on, confirm that the circuit was oscillating, and THEN slowly insert the work, watching the current, making sure not to exceed a practical limit.

In your pictures of the burned traces:
Those components appear to be gate components, not power handling components.  I think those traces are okay to handle several watts.  I think the components overheated when when the Mosfet failed.  I don't think my gate zeners have ever survived a failed Mosfet.

Pete Stanaitis
---------------
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on August 22, 2017, 08:44:02 AM
The application I intend to use the induction heater for requires my to have the work piece already in the coil when I start it up so I must make sure it can start to oscillate, which, as it turns out, it really can. Even at startup current of 50A! The heater I am using now is this one: http://www.ebay.com/itm/322519608639

And yes, I am fully aware that I am running it at least 100% over it's rated specs (that are probably optimistic to start with). At this stage I am not interested in seeing if it runs and oscillates. I know it does. I am trying to figure out it's weak points and how to address them. My goal is to get my own induction heater up to about 2kW, preferably using only one pair of mosfets and to have enough cooling for it to run continuously.

Another potential problem I might run into is that the work piece won't couple enough energy (not sure how to explain it). I now know that a solid piece of iron will mean a fairly large transfer of energy, but I will use a thin walled pipe and I have absolutely no idea if that will work or be enough. I will try and find a regular steel pipe to try with and see what happens, but for my finished application I will have to get hold of some martensitic stainless steel pipe which has turned out to be almost impossible.

[edit] the burned pcb vias were actually the source connection for both mosfets. I will bypass them with some serious and also hardwire the source leg of both mosfets to the ground connection.
Title: Re: Royer induction heater and high current power supply
Post by: Anders Mikkelsen on August 22, 2017, 11:09:50 PM
Nice and solid construction, it will take more than a line spike to kill that supply!

The rule of thumb of using capacitors with a ripple current rating of twice the load current doesn't sound right, so I did some simulations in LTSpice. With 1600 uF on the DC bus, 153 A RMS load current and an average output voltage of 38 V (with 5.7 Vpp of ripple), the capacitor sees about 7.5 A RMS of ripple. Using the Rifa PEH200 catalog as a reference, a 400 V 1500 uF electrolytic capacitor would have a sufficient ripple current rating.
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on August 23, 2017, 02:23:35 PM
Nice and solid construction, it will take more than a line spike to kill that supply!

The rule of thumb of using capacitors with a ripple current rating of twice the load current doesn't sound right, so I did some simulations in LTSpice. With 1600 uF on the DC bus, 153 A RMS load current and an average output voltage of 38 V (with 5.7 Vpp of ripple), the capacitor sees about 7.5 A RMS of ripple. Using the Rifa PEH200 catalog as a reference, a 400 V 1500 uF electrolytic capacitor would have a sufficient ripple current rating.

Thank you for the information about ripple currents and caps. Since I had no way of measuring beforehand and not enough experience with large ripple currents I had to rely on the internet as a source of information. The only actual downside of my current capacitor is the capacitance. Current handling and ESR are great!
Title: Re: Royer induction heater and high current power supply
Post by: Mads Barnkob on August 23, 2017, 07:38:18 PM
The only actual downside of my current capacitor is the capacitance. Current handling and ESR are great!

From Kizmo's experiences with using a 6000 uF DC link capacitor bank in his BiggerDR DRSSTC (https://www.youtube.com/watch?v=PPlVSYFNU0M), he saw huge improvements in the peak current delivery time, compared to electrolytics, that actually indicated that he needed less bus capacitance when using film capacitors, to get the same performance in the power electronics.
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on September 03, 2017, 05:46:18 PM
Did a new test run today. Replaced the broken IRFP250 with IRFP4568, zeners replaced with Russian surplus zeners rated for 8W and delivering about 13V gate voltage. I had done some testing with the zeners and the new 100 Ohm thick film power resistors and realized they got quite hot so I placed the resistors on the opposite side of each water cooling block for the mosfets. Hard to tell if it was the new mosfets, the water cooling or something else but I could run it at almost 40A for quite a while before one of the tank caps left the PCB with a loud bang and the everything just broke down. It's quite interesting to witness the violent fail of this circuit. I have broken stuff before but this is something else. Some pcb vias and components just seize to exist.

On closer post breakdown inspection I could see that the pcb trace connecting all the caps to the coil had burned off, but that was not all. The drain leg of one of the transistors had evaporated! It's not in focus but it's somewhat visible on a few of the pictures.

I am not sure of what actually failed this time, but water cooling is a must for the circuit when abusing it like I do. I did not cool the work coil which might have contributed to the circuit failing. The hose I used for the water block was a bit too large for the copper tubing in the coil so I would have had water everywhere. I think I will have to make the coil of a slightly larger diameter copper tubing so I can properly transfer the heat. I will make some measurements on the zeners do see I they survived but I think they are actually ok, and if they are not I still have a handful left.
Title: Re: Royer induction heater and high current power supply
Post by: petespaco on September 04, 2017, 04:10:09 PM
What do you gain with the 100 ohm resistors?
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on September 04, 2017, 04:16:36 PM
My assumption was faster rise time for the gate signal. I treated the gate part more or less as a voltage divider with 100 Ohm fixed resistor and the mosfet gate as a dynamic value with a large resistance when not charged. I haven't been able to do any measurements to confirm this as my oscilloscope does not have the required bandwidth but it seem logical.

At the current levels I am running it is crucial to keep the time the mosfets operate in the linear region to an absolute minimum.
Title: Re: Royer induction heater and high current power supply
Post by: petespaco on September 05, 2017, 05:11:36 AM
I agree that you don't want the Mosfet to be in the linear region any longer than they have to be.
  But, in my limited experience, as long as you are using 48 volts or so, the original design is pretty good.
that gate ckt appears to be more than  a simple  voltage divider to me.  The gate itself draws almost no current at all.  The apparent "voltage divider" would be the 470 ohms and the 10K ohms (in the original ckt).  So the difference there is 20:1 vs 100:1.  But that's not the end of the story.  when the zener gets forward biased, you now have  36 volts across it.  Now you get some heat.
But there's more, isn't there?
When the commutating diode forward biases, doesn't it essentially ground the gate to turn that Mosfet off?
If so, now you have 48 volts across that 470 ohm (or, in your case, 100 ohm resistor.
Here's how I see the power at that resistor.  Please correct me, someone, if I am wrong:
      Commutating mode (gating off)         
Voltage applied to R      48   48      
            R                  470   100                     
                 I=               0.102    0.48      
            P=                4.90   23.0   Watts   
               
      Gating Charging mode         
Voltage applied to R      36   36      
                         R   470   100      
                        I=     0.076   0.36      
                        P=    2.76   12.96  Watts   
               
Note: divide by two if you want, because each Mosfet is Off               
Half the time.
               

Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on September 05, 2017, 05:33:22 AM
I think you may be partially right. To my knowledge, once the mosgfet gate is charged it consumes almost no power at all, but to turn it on from an lmost non-charged state it takes quite a bit of power to do that quickly. The capacitive load of the mosfet gates basically presents a dead short initially.

You are right about the power dissipation from both the zener and my power resistors, hence the much beefier components. My resistor is a 20W TO-220 resistor that I actually used water cooling for and my zener diodes are quite big ones rated at 8W.

I'm not sure how you calculated 36V across the zener. Won't the resistor make sure the voltage never exceeds the zener voltage?
Title: Re: Royer induction heater and high current power supply
Post by: Mads Barnkob on September 05, 2017, 09:42:33 AM
I think you should really get some measurements done instead of just assuming that switching faster is better. It is a general rule of power electronics that you shall try to switch the current as fast as slowly possible. Meaning that too slow you have higher losses, but too fast and you have excessive ringing.

I do recall that when going up in voltage, someone else used 680 Ohm resistors instead, exactly to lessen the heat dissipation.
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on September 05, 2017, 09:50:46 AM
Excellent point. I never thought about the possibility of ringing. To measure that I do however need a better oscilloscope.

I think I will focus on making a better assembly of the whole thing. On closer inspection I realized I melted the solder on the pcb via connecting all the caps, the mosfets and the standoffs for the working coil. Since I didn't have the board flat, the solder flowed to one side of the board leaving the via much thinner in one section causing the via to break and the last connected capacitor leaving the pcb with violent force since the solder was melted.

What bandwidth would you recommend for measuring the gate signals? I don't think I will ever use it at frequencies above 1MHz.
Title: Re: Royer induction heater and high current power supply
Post by: petespaco on September 06, 2017, 01:16:21 AM
A quote:  "I'm not sure how you calculated 36V across the zener. Won't the resistor make sure the voltage never exceeds the zener voltage?"
Answer: No.

It's not 36 volts across the zener.  The zener clamps the gate to 12 volts, right?  If so, the resistor sees the difference between the supply and 12 volts.
eg:  48 (if that's what you are using) - 12 = 36.

By the way, I don't see any ringing to speak of using the "standard" values.
Regarding frequency:  I doubt that you will be running the tank at any higher than about 150 Khz unless you start removing capacitance and/or making coils shorter than about 6 turns or smaller in diameter than about 1" ID.
My little DSO-112 pocket "scope can see what's going on pretty well, but I use my ancient Techtronix 422 15 Mhz scope most of the time.  It was very conservatively rated, though and still has good response at 30 Mhz. (From back in my 27 Mhz radio control days.)
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on September 06, 2017, 05:18:41 AM
when the zener gets forward biased, you now have  36 volts across it.  Now you get some heat.

I don't understand what you mean by this. I thought that the resistor in combination with the zener diode made sure that the resistor never sees more than supply voltage (about 40V in my case) and that the zener never sees more than the zener voltage no matter what.

When I wrote 1MHz I meant frequencies for other possible future projects, not induction related. This was just to help decide what scope to maybe buy in the future, but I think I will borrow a 2 channel Hantek from my brother instead just to check the gate drive signal.
Title: Re: Royer induction heater and high current power supply
Post by: petespaco on September 06, 2017, 08:16:56 PM
Sorry at about this:
"But that's not the end of the story.  when the zener gets forward biased, you now have  36 volts across it.  Now you get some heat."
should have read: "But that's not the end of the story.  when the zener gets forward biased, you now have  36 volts across the 470 ohm resistor.  Now you get some heat.".
  I guess I had my mind stuck on your change to 100 ohms.
Title: Re: Royer induction heater and high current power supply
Post by: JardCrocker on November 23, 2017, 07:03:21 PM
Hi...i am a new user here. I want to knowwhat kind of switches will you be using to handle all that power ? I used to push 30A constantly through a set of irfp250's which didnt last very long. After pushing the voltage higher than 40v I had to move up to IRFP460's and as a last resort I switched to IGBT's.

pcb assembly (https://www.youtube.com/watch?v=-7AI18ugw1I)
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on November 23, 2017, 07:16:54 PM
My plan is to use a pair of Infineon IPT007N06N rated at 300A each. The current just when the circuit is closed doesn't seem too bad, but when it starts to oscillate the current draw is insane at first and it slowly stabilizes at a more reasonable level.
Title: Re: Royer induction heater and high current power supply
Post by: Mads Barnkob on November 23, 2017, 10:18:02 PM
Hi Jard and welcome to the forum :) If you have the time, please make a new thread about your induction heater and show us what you have built.

Kamelryttarn, it has been a while since your last update, is life keeping you too busy for making progress? :)
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on November 24, 2017, 06:09:09 AM
Kamelryttarn, it has been a while since your last update, is life keeping you too busy for making progress? :)

Yes, I'm afraid so. I know what I would like to try next, but I can't seem to find the time and money for it. Probably my next attempt at this project will be in a few months.

I saw a nice Celem cap on ebay the other day but it got sold pretty quickly. It as a 5uF 350B: http://www.celem.com/Item.aspx?ItemId=33&cid=11&hmenu=29

Title: Re: Royer induction heater and high current power supply
Post by: Mads Barnkob on November 24, 2017, 08:33:32 AM
If you are interested, we can find a reasonable price for one of these https://www.ebay.co.uk/itm/232565478339 . I have not sold a single one yet, so apparently noone needs these at 60£ a piece.

It would be better that they found new homes and got used instead of just being on the shelf in my lab :)
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on November 25, 2017, 10:46:19 AM
If you are interested, we can find a reasonable price for one of these https://www.ebay.co.uk/itm/232565478339 . I have not sold a single one yet, so apparently noone needs these at 60£ a piece.

It would be better that they found new homes and got used instead of just being on the shelf in my lab :)

Thanks Mads! Do you think they would survive a frequency closer to 40-50kHz?
Title: Re: Royer induction heater and high current power supply
Post by: Mads Barnkob on November 25, 2017, 12:34:15 PM
If you are interested, we can find a reasonable price for one of these https://www.ebay.co.uk/itm/232565478339 . I have not sold a single one yet, so apparently noone needs these at 60£ a piece.

It would be better that they found new homes and got used instead of just being on the shelf in my lab :)

Thanks Mads! Do you think they would survive a frequency closer to 40-50kHz?

Yes, you would just have to derate the current, I am not sure by how much, but looking at a similar capacitor type from Vishay http://www.vishay.com/docs/13057/phawo-ks.pdf

Where current ratings are given for a 1500V capacitor at 3000, 5000, 8000 and 10000 Hz, I extrapolated the derating further for the yellow marked numbers, these are just a guess.

 [ Invalid Attachment ]

So that would make my NWL rated at 680 A, should be derated to something like 400 Arms at 40 kHz.
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on May 06, 2018, 08:29:01 PM
I have thought a lot about how to connect the mosfets, providing both cooling and electric connection at the same time. Since I blow the mosfets up all the time they must be easy and quick to replace and I came up with this. The idea is co create 4 of the blocks made of brass and use threaded rods, insulating nylon washers/bushings and springs to clamp everything together. (The mock up TO-247 components are one mosfet and one power resistor)

In the middle I will put somewhere around 144 pcs of 47nF ceramic SMD caps giving me a total of 6,7uF of water cooled ceramic capacitor bank.

To provide good electrical contact between the surface and the components I plan to use small pieces of 0,3mm thick tin foil.

What do you think about it?
Title: Re: Royer induction heater and high current power supply
Post by: flyrod on May 09, 2018, 02:54:56 AM
What do you think about it?

I think it will work.  I did something similar:

(https://s14.postimg.cc/w59113iqp/annpost.jpg)

FETs are screwed directly to water cooled copper blocks with ceramic caps sandwiched in between.
Title: Re: Royer induction heater and high current power supply
Post by: kamelryttarn on October 16, 2018, 02:35:32 PM
There hasn't been much progress this summer as I have had other things to to but I have thought a lot about the project and I still want to get back to it. I bought some thick, soft tin toil which I hope will give each and every SMD cap ok contact. In order for the small SMD caps to stand upright and to make it easier to mount them I made a small frame that I intend 3d print in some plastic. Don't know if I need to use some special kind of filament but I will start with regular PLA and see what happens. The frame holds 144 pcs of 1210 SMD caps.
Title: Re: Royer induction heater and high current power supply
Post by: Max on October 16, 2018, 10:30:07 PM
I'm a little late to the party, but hopefully not too late  ;)


Yes, you would just have to derate the current, I am not sure by how much, but looking at a similar capacitor type from Vishay http://www.vishay.com/docs/13057/phawo-ks.pdf

Where current ratings are given for a 1500V capacitor at 3000, 5000, 8000 and 10000 Hz, I extrapolated the derating further for the yellow marked numbers, these are just a guess.



So that would make my NWL rated at 680 A, should be derated to something like 400 Arms at 40 kHz.

Actually I don't think that these values are that useful; they come from different capacitors with different capacities. Vishay doesn't indicate it, but a quick calculation based on Urms, Irms and the frequency gives the following values (sorted by voltage):
   Urms [V]    Irms [A]      f [Hz]     C [uF]
25016806001783
250264010001681
25026403000560
25025605000326
25024008000191
250228010000145
60012671502241
60017003001503
15002640300093
15002560500054
15002333800031
150022671000024
245020001000130
3000667150236
30001333300236
30002000600177

As you can see, your diagram is based on capacitors with 24uF to 93uF.

I'd like to point to another reference. Celem (manufactor of induction heating capacitors) indicates voltage and current deratings vs frequency for their capacitors. Take the example of a 6.3uF 600V 600A capacitor (http://www.celem.com/item.aspx?ItemId=36&cid=13&hmenu=31). Here's their diagram:
(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fwww.celem.com%2Fimage%2Fitems%2Flarge%2F36-219.jpg&hash=03f2190e952125b992c5bea3350e1171)
(their other capacitors look quite similar)
As far as I understand it, you simply have a maximum voltage and a maximum current which must not be exceeded. To do so, you have to derate the other value respectively. This would mean, that as long as you keep the voltage low enough so that the current does not exceed the maximum rating, you can go as high in frequency as you want (and vice versa for lower frequencies).
Now this seems quite simple and there're two situations where I think that this rule is too simple:
  • First, as visible on the diagram from Celem, around the point of maximum power both current and voltage are derated and I'm not quite sure why. Also this does not concern all of their capacitors.
  • Secondly with frequencies much higher than the nominal frequency I could imagine that the dielectric losses rise, too. Which means you'd have to decrease the current. However, I couldn't find such an example at Celem, where the maximum current always remains constant over the whole upper frequency range.

Kind regards,
Max
Title: Re: Royer induction heater and high current power supply
Post by: Mads Barnkob on October 21, 2018, 08:07:28 PM
There hasn't been much progress this summer as I have had other things to to but I have thought a lot about the project and I still want to get back to it. I bought some thick, soft tin toil which I hope will give each and every SMD cap ok contact. In order for the small SMD caps to stand upright and to make it easier to mount them I made a small frame that I intend 3d print in some plastic. Don't know if I need to use some special kind of filament but I will start with regular PLA and see what happens. The frame holds 144 pcs of 1210 SMD caps.

Wow that is a cunning plan, I am excited to see how it works out with a water cooled MMC for a induction heater, how about temperature swing micro-cracking problems with SMD ceramic capacitors?

Max: it must be that no matter what you do, what voltage or current, you have to uphold and stay below the KVAr rating.