Author Topic: ZVS Output Voltage Question  (Read 423 times)

Offline Andrew321

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ZVS Output Voltage Question
« on: December 21, 2020, 06:08:30 AM »
Its been a while since I broke my last HV winding on my ZVS driver and didn't have the time to build it all again until just recently. I completed my new winding with a longer transformer (same as the last one but I added 2 I sections to extend it so I could add more coils per layer) I got it all set up and going and it was a bit underwhelming. Aside from a longer core and more secondary turns, everything was more or less the same, yet I was only getting about 14kv peak out of the thing. So I started sciencing and taking notes to improve voltage output. I was hoping for 30-60kv. I don't know much about all the math involved so I'm hoping some one can shed some light on this.

I notice that, all else the same, decreasing primary turns causes output voltage to drop and current and frequency to rise. Increasing primary turns actually reduced primary current and increased secondary voltage considerably. In fact, I went from 11 turns on the primary  to 17 turns and the out put went form about 20kv to 37kv, oppicite form what i feel likei should have happened. However it saturated when I added one more cap and blew the IGBT's.

So I increased the core gap and added more capacitors. This is when I noticed something interesting. The turns ratio hasn't changed yet the more capacitors I add, the lower the frequency, the higher the primary current and the higher the output voltage. I was always under the assumption that the output voltage was the direct result of the turns ratio, yet the secondary voltage seems to be rising more with primary current than it is anything else. Why is this?

Also, I did hit 50KV output however it soon began to drop and I noticed bubbles coming form the secondary (its in mineral oil and i pulled much of the air out). It still kind of works, I knocked the voltage back for 30kv and there are still bubbles, just less of them. So I need to wind a new one but i would like ot understand the relationship between the current and secondary voltage a bit more before winding it.

Offline davekni

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Re: ZVS Output Voltage Question
« Reply #1 on: December 21, 2020, 09:09:18 PM »
The combination of high voltage and high frequency makes transformers more complex.  Inter-winding capacitance of the high-voltage secondary makes a resonant frequency with transformer inductance, a frequency that can easily be below your desired operating frequency.  For ZVS drive, the primary capacitance makes another resonant circuit.  So there are two coupled resonant circuits with two frequencies, the lower and upper poles.

In my experience, ZVS oscillators tend to lock to the upper pole unless secondary winding resonant frequency is well above primary frequency.  The upper pole produces relatively-little output voltage.  In one case where I couldn't avoid that (purchased transformer), I made the primary frequency much higher, then added a series L+C+R across the primary, tuned to damp the upper pole.

It will be difficult to know just what's happening in your case without scope measurements.  The one thing that almost always helps is to design the secondary winding for minimum capacitance.
« Last Edit: December 21, 2020, 09:18:14 PM by davekni »
David Knierim

Offline Andrew321

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Re: ZVS Output Voltage Question
« Reply #2 on: December 21, 2020, 10:20:28 PM »
Hey Dave, thanks for getting back!

I'm still reading up on things, but not sure what you mean by upper and lower pole. Could you explain this a bit more? Is it that the ZVS will resonate at secondary frequency if it has a lower frequency that the primary resonance? What if I matched resonance?

I'm not sure how close the primary and secondary need to be for the interwinding capacitance to be problematic, but mine are about an inch apart I  would say. I haven't actually measured it. From what I read on line a grounded metal plate between the two should fix the problem. Do I actually have to connect it to ground? Because I have a ground wire on the cord that I'm using and adding a another terminal to the box for the purpose would be easy enough.

I'm thinking my next coil is going to be a "pancake" coil. In my current design I put a few layers of paper between layers of coils and I still get those bubbles. I want a secondary that will last and I see people have good success with "pancake" coils.


Offline davekni

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Re: ZVS Output Voltage Question
« Reply #3 on: December 21, 2020, 10:30:34 PM »
Read about DRSSTCs, as they always have two poles.  It means there are two resonant frequencies.  If you have an easier time with mechanical analogies, search for coupled resonators, such as two swings with a springy connection between them.

Sorry, I should have said "intra-winding" capacitance.  The issue is capacitance between layers of the secondary.  Capacitance between primary and secondary is rarely of concern unless spacing is close enough to arc across.  Search for "segmented bobbin" - there are some threads here on that.  It's typical of high-voltage windings, a set of pancake coils.
David Knierim

Offline Andrew321

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Re: ZVS Output Voltage Question
« Reply #4 on: December 24, 2020, 01:38:58 AM »
DRSSTC's is a deep subject which I will tackle in due time as they are next on my project bucket list. However I don't think my problem is the frequency locking. I was reviewing my notes and all of my frequencies are in 10's of kHz range and they lower with increased primary capacitance. I don't have a scope and cant afford one at the moment (bucket list) so I take my measurements with a multimeter on either side of the capacitors between them and the primary coil. I started with one cap at 50kHz and ended about 18kHz as I added more, which is when I hit 50kV and damaged my secondary. So unless my multimeter is way off (which would not surprise me), I don't think I'm having a problem with the ZVS locking to the higher frequency. My question was more about why is it that as I add more capacitors to the primary the voltage rises on the secondary?

I noticed a large jump in voltage when I added more coils so I'm thinking that more coils = more magnetic flux and more caps = lower frequency = more current = more magnetic flux and maybe its just that the increased magnetic flux is generating the rise in voltage on the secondary? Because less caps = less current and a lower secondary voltage and less current means less magnetic flux.

From what I understand ZVS drivers are just flybacks and those store energy in the airgaps of the core (so I am told). So if I want a lot of power transfer and a high voltage I can increase the  core gap and the primary capacitance so that way the core doesn't saturate so quickly but I can still draw a lot of power and transfer it to the secondary.

Is any of that making sense or am I way off? Any ways, I hope you have a merry Christmas!

Offline davekni

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Re: ZVS Output Voltage Question
« Reply #5 on: December 24, 2020, 05:44:58 AM »
Have you ever used an analog simulator such as LTSpice?  That is one of the best ways to understand circuits and try variations without changing physical parts.  There are free online simulators and free downloadable ones such as LTSpice.  That's what I use for both home and work.

Most standard two-transistor ZVS oscillators generate a fixed primary voltage, with peak of PI * DC input voltage (less a bit of loss).  Changing capacitors changes frequency.   In a low-voltage transformer where secondary capacitance is not significant, output voltage remains roughly constant across frequency (if frequency is adjusted by changing capacitors).  Of course, at too low a frequency the core saturates.  A too high a frequency transistor switching time may limit operation.  Your output voltage changes must be due to output capacitance.

How are you measuring output voltage?  Do you have a HV diode and resistor divider to a meter?

If oscillation is stable, the meter is likely reporting the correct frequency.  If unstable, it may not report anything useful.  If you want to post ferrite core dimensions including length of gap(s) and primary turn count and capacitance for a couple frequencies, I can help you estimate what frequency would be without secondary capacitance.  That will show if it is close.  If you also post secondary data - number of layers, turns per layer, thickness of paper between layers, winding length, inside and outside diameters, then I can also help with a guess to secondary capacitance.  With that a good simulation model is possible.  (Knowing your ZVS circuit would be good too, but not as critical.)
David Knierim

Offline Andrew321

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Re: ZVS Output Voltage Question
« Reply #6 on: January 08, 2021, 12:30:53 AM »
I just started using LTSpice when over the past week for my class E tesla coil project and boy is it useful! It saves so much work, time and parts! It also shows all the different wave forms so not having an oscilloscope isn't such a huge limitation. Still plan on getting one in the future but for now LTSpice is very useful. I did try to simulate my zvs circuit but I couldn't get it to start up right away so I moved on to the work with the tesla coils. Ill try again later. I have soooo much to learn about how to use that program.

As for high voltage measurements I have a full bridge rectifier and capacitor bank rated for 100kv max and a resistive divider hooked up to my multimeter.

Dont worry about all the measurements on the secondary because I'm going with the segmented coil. I did a trial with 3 segments and with my lower voltage turn on I was getting about 20kv, which is insane! I do tests where I check the low voltage (30VDC) turn on and then high voltage turn on (160VDC). My highest voltage with the previous secondary was ~7kv low and 50~50kv high. So at 20kv low the high might have been above 100kv! I didnt want to break my bridge rectifier so I didnt measure it but the spark started up at about 6" and i could pull it more than 12" before the whole thing broke on me. So I'm inspired to get max voltage out of this thing now. Many segments on a longer core leg. I wanna get above 100KV :D

Offline davekni

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Re: ZVS Output Voltage Question
« Reply #7 on: January 08, 2021, 03:43:26 AM »
For oscillator startup in simulation, initial condition statements are useful.  For example, ".ic I(L1) = 1" will start simulation with 1amp in L1.  Current in the resonant inductor and 0V for the two FETs works well for the simple ZVS oscillators.  And some initial current in the supply inductors too, or the 0V FET voltages will draw infinite current.
David Knierim

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Re: ZVS Output Voltage Question
« Reply #7 on: January 08, 2021, 03:43:26 AM »

 


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