Author Topic: Size Comparison of Buck vs Phase Shift QCW  (Read 761 times)

Offline alan sailer

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Size Comparison of Buck vs Phase Shift QCW
« on: October 06, 2018, 10:43:53 PM »
Just a post to show a comparison between a Buck QCW and Phase Shift as far a size.

The Buck QCW was built as a clone of Loneoceans QCW1.5 (Thanks again Gao for the great documentation and help!).

The Phase Shift QCW was built using the SimpleDriver/Simple Interrupter from SimpleTesla.

Both have close to the same performance (within my limited tuning skills) with four to five foot sparks at around 350 volts on the buss.

As far as difficulty there is no comparison. The Buck was painful to troubleshoot, the feedback circuit that generates the ramp is very
sensitive and I was unable to find any way to probe it without affecting the regulation.

The SimpleDriver was true to it's name and was very easy to set-up and run. A series of jumpers presets the FPGA to run in many
different modes including DRSSTC and SSTC. In the QCW mode you can tell the driver to switch hard/soft drive between the bridge
transistors from one pulse to the next. this keeps the hard switch heating balanced between the two bridge half's.

The only downside to the phase shift method that I could see is that the transients on the bridge are larger than the Buck due to the
hard switching. Attention must be paid to avoid exceeding the transistors maximum CE voltage. At 350 V buss I am getting
~500 volt transients.

Cheers.

* For some reason (usually because I'm a dope) I could not get any photo to upload. At Mads suggestion I posted it on igmur
and pasted the link.

https://imgur.com/a/77RS0Js

Offline Hydron

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Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #1 on: October 07, 2018, 08:34:54 PM »
Thanks for the info regarding the two approaches - I am (very slowly) building a phase shift QCW, having rejected the Buck option as being too clunky (though I will have some added magnetics - a PFC stage).

I'm also a little worried about the spikes I'm seeing in testing of my prototype - similar to you I'm seeing stuff getting close to the Vce(max) rating of the IGBTs, and I had hoped to run a bit higher than 350V too.

I believe that most of it comes down to the inductance of the loop between the two transistors in each half bridge - my prototype PCBs had space for 2 sets of IGBTs to be fitted (for parallel operation), and when IGBTs are fitted in the pair of positions furthest from each other the spikes are significantly worse than when fitted in the pair closest together.

My final PCB design will have the smallest loop area I can get away with, some better decoupling (HV SMD MLCCs plus a variety of film caps) and an option to fit an over-voltage turn-off speed limiter (~550V Zener/TVS from Collector to Gate), so hopefully I can tame it enough to work at 400-425V or so.

I have also had a look at your JPG, nothing unusual about it that I can see!

Offline alan sailer

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Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #2 on: October 08, 2018, 03:14:05 AM »
Hydron,

I'd be curious what you find as far as the spikes. I am using a well designed PC board from Alex Yuan for the paralleled full bridge.
I am also using a bi-directional TVS rated at 440V on each device. I can't find any agreement on TVS, some hate them others don't.

I have done very little tuning so far but at one point I raised the primary up towards the secondary to increase coupling. It did cause
the spikes to go down, the current to go down and the spark longer. A good result. I'd like to get five foot sparks with some margin
on the CE spikes.

Another puzzle is that the transients are nearly as bad when soft switched as hard. Don't get that one at all.

The temperature issue (for me) is also not a problem. I have seen the IGBJT surface go to about 31 during my tuning. However, I'm
not running very long and don't intend to do any crazy pulse rates.

Cheers.

Offline Hydron

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Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #3 on: October 08, 2018, 09:50:14 AM »
I'm not convinced that TVS diodes will make a lot of difference just put straight across the IGBT - if you look at voltage vs current, at reasonable currents the voltage rises quite far above the rated breakdown spec. I may still put in provision to fit some on my design though if space permits - worth a go given how easy it is.

My plan for a TVS was not to clamp directly across the IGBT, but to build an "active clamp" circuit - by placing a TVS of appropriate voltage (i.e. somewhere above bus voltage but below Vce(max) ) between collector and gate; if a turn-off spike goes above the TVS voltage it will bypass enough current to partly charge the gate back up to flow appropriate current and clamp the voltage at approximately the TVS value. This only works because the spike is caused by the turn-off speed being otherwise too fast and uncontrolled during part of the IGBT turn-off and commutation process (something that is very hard to fix with a bigger resistor without increasing losses significantly). There are a few ways to do this active-turnoff (e.g. using a charged capacitor instead of a TVS) - I will need to have a look at the options.

I am a bit surprised that you are seeing spikes on both hard and soft switching transitions - I am seeing nice clean waveforms with soft and nasty spikes with hard. What IGBTs are you using? And do you have a link to the PC boards?

Offline alan sailer

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Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #4 on: October 09, 2018, 12:10:02 AM »
Hydron,

The devices are IRGP50B60PD1. I have no PC board layout. I bough the boards from Alex Yuan.

I'll look into the clamping circuit you describe. Sounds like feeedback! Do you have any references I could read in more detail?

Cheers.

Offline alan sailer

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Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #5 on: October 09, 2018, 02:02:15 AM »
I also did a quick look at the waveforms and there are no turn-off transients on my waveforms. Turn-on only.

I don't know enough to know if this makes sense but it is what I see.

Cheers.

Offline Hydron

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Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #6 on: October 09, 2018, 09:17:45 AM »
Turn-on vs turn-off transients depends on what type of modulation you're using for your phase shift.
- "Leading-edge" modulation, where you modulate the time between the zero-crossing and turning on the IGBTs, will result in reverse-recovery losses when the current is commutated from the free-wheeling diode to the IGBT, and this may cause turn-on spikes, but turn-off (at zero crossing) would be cleaner.
- "Trailing-edge" modulation, where you switch the IGBT on at the zero-crossing but then off early (modulating the time spent on, rather than off), will not have any losses from reverse-recovery and a clean turn-on, but Vce spikes at turn-off, as current flowing in the loop between the two IGBTs and the bridge output has to commutate from the turning-off IGBT to the free-wheeling diode of the opposing IGBT _very_ quickly.
- Many tried-and-tested IGBTs, in particular the FGH60N60SMD and family, have significantly lower turn-off losses than turn-on (taking into account the diode reverse-recovery), so trailing-edge has been the method normally used.
- This all assumes non-modulated switching occurs at zero-current or close enough for minimal losses and Vce spikes.

(decided to make this a generic summary - maybe of use to others. BTW your IGBTs seem to be pretty similar in turn-on/turn-off losses assuming the datasheet is accurate)

I have no idea what type of modulation your driver is using - would you be able to share some scope captures of the primary current and the output of the half bridges?

See my attachment for an example of trailing-edge modulation and some _really_ bad spikes on turn-off (I think this may have been from some of the faster IGBTs i'd tried). The cyan maths trace is the voltage seen by the bridge - see that it starts each cycle adding energy then switches off after a period.

Edit: Try googling "IGBT active clamping" to find out what I was describing. There are some fancy proprietary schemes, but can be done with just a TVS and a diode. If I find a good reference I'll link it here. With a DRSSTC you'd need to also be careful that the active clamp doesn't lead to shoot-through - it delays turn-off.
« Last Edit: October 10, 2018, 12:12:33 AM by Hydron »

Offline alan sailer

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Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #7 on: October 10, 2018, 01:44:05 AM »
Hydron,

After thinking about your earlier post I am somewhat embarrassed to realize that I have been unconsciously confusing what "on" is for a bridge transistor. My past experience has been only with low power analog and digital circuit and on means going from zero to high.

(Stupid red faced grin)

Yes, all my major transients are, like  yours, off transients.

I can show you scope shots. Are you interested in overall pictures or detailed ie seeing the individual ~500kHz switching?

Attached are the overall pictures.

Cheers.

Hard Switch 200V



Soft Switch 200V


« Last Edit: October 10, 2018, 02:33:33 AM by alan sailer »

Offline alan sailer

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Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #8 on: October 10, 2018, 01:48:45 AM »
Hydron,

Also here is a link to the English translation of the SimpleDriver manual;

http://tqfp.org/simple-tesla/simpledriver-v23-in-english.html

There is a section at the end talking about the phase shift technique.

Cheers.

Offline Hydron

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Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #9 on: October 11, 2018, 01:08:15 AM »
Thanks for the scope caps and link - will have a read tomorrow. Would also be interesting to see the detailed waveforms too (when convenient), possibly at different parts of the total on-period? I haven't seen too many real waveforms from QCWs posted, so would be of general interest as well as for this thread.

Offline alan sailer

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Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #10 on: October 11, 2018, 02:11:28 AM »
Hydron,

I can take some data for sure. It would be helpful to know what would be most informative for you. I can currently do things like CE (with an isolated scope) and primary current.
It would be nice to have a little data on-line showing some actual phase shift QCW waveforms.

By detailed would you means images like the one you posted with a few cycles of Vce and primary current? And doing this at points towards the front, middle and end of the "ramp".

I have no idea how to make simultaneous Vce measurements of two transistors. Since each is at a different potential. references each to ground seems like it could get pretty confusing quickly.

My set-up is in a minor state of flux right now. I originally tuned ZVS on the bridge using a low power DRSSTC mod but it has become apparent that this is not good enough.

The default value for the phase compensation section of the SimpleDriver was out-of-range so I added 100pF to the RC part of the network (the SD has two phase compensation sections, the first LR and the second RC). I have been trying to adjust the ZVS (for the soft switching) towards the end of the ramp since that is the point with the highest stress.

It would be nice to adjust the phase compensation while the unit was running but I don't want to die :-)

Cheers.

Offline Mads Barnkob

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Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #11 on: October 11, 2018, 07:47:31 PM »
Great discussion, I am following it closely as I have been pondering on a QCW ( https://highvoltageforum.net/index.php?topic=482.msg2976;topicseen#new maybe with a bridge like this, but from this discussion I can see that this bridge is maybe far from optimal in terms of inductance) to be a project within too long as I soon have been through and cleaned out most of the junk piling up everywhere.

http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline alan sailer

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Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #12 on: October 12, 2018, 01:53:36 AM »
Mads,

Glad it seems worth while to someone.  My main purpose was to illustrate how much smaller a phase shift QCW can be.

But I am will consider to making measurements on the units if it will help understanding. if there are any measurements
you are after, please let me know and I will try to help.

I am sure there are other people who have two types of QCW to compare side by side. But not a lot.

Cheers.

Offline Hydron

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Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #13 on: October 12, 2018, 02:06:34 AM »
I can take some data for sure. It would be helpful to know what would be most informative for you. I can currently do things like CE (with an isolated scope) and primary current.

By detailed would you means images like the one you posted with a few cycles of Vce and primary current? And doing this at points towards the front, middle and end of the "ramp".

I have no idea how to make simultaneous Vce measurements of two transistors. Since each is at a different potential. references each to ground seems like it could get pretty confusing quickly.
My Vce measurements were done on the lower transistor in each half-bridge, so they shared the same reference potential (-ve DC bus voltage). I was running off a bench supply so no issues grounding the bus - to do the same on mains one would have to isolate the bus or the scope (preferably the former unless the scope is designed for it).

To get the actual bridge output waveform (as seen by the LC primary circuit) I just turned on the maths trace with a subtraction operator.

And yeah, similar sorts of captures was what I was thinking, as you suggest in front/middle/end of ramp. I was asking as it'd be interesting to see how similar the spikes you're seeing are to what I get. Don't inconvenience yourself making measurements - was mainly asking in case you already had some scope shots, or already had the scope/coil setup anyway.

I would not stress too much about getting perfect ZCS - as long as the current isn't too high when you switch the losses will be minor, especially if switching is a little early rather than late.

Edit: I agree that not many people have both side by side - it's valuable to have the perspective of someone who has built both, especially when it agrees with the plan for my coil :P
« Last Edit: October 12, 2018, 02:09:19 AM by Hydron »

Offline alan sailer

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Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #14 on: October 13, 2018, 02:25:50 AM »
Hydron,

So here are some wave-forms. The data was taken (per your suggestion) on the low side of the bridge. I used an isolated scope and put
the "ground" clips on the Negative Buss. I wrapped the ground lead around the body of the probe to lower the loop inductance.*

The QCW was running at ~200 volts/14mS ramp. I took data at the beginning of the ramp, the end and two middle points. The true middle
point and a middle point selected for the worst overshoots.

The data seems to agree with what I understand about this circuit.  Less overlap of the two phases at the beginning, more at the end.
The drop in frequency due to spark loading. And the buss voltage declining as the capacitor gets loaded down.

Let me know if you see anything bogus or interesting.

* although I did an experiment making an ultra low ground/probe length (~1/2" and saw no first order changes).

Beginning of ramp.



Middle of Ramp.



End of ramp.



Worst transients.



« Last Edit: October 13, 2018, 02:56:07 AM by alan sailer »

Offline Hydron

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Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #15 on: October 13, 2018, 04:09:53 PM »
Interesting scope caps, thanks for sharing. Looks like you're seeing ringing up in the 10s of MHz on switch-off (~20Mhz), similar to me, though you are also seeing nastiness on turn-on due to poor zero-current switching.

I've attached another capture I dug up from previous testing with a very big spike well over what the IGBT is rated for. I'm not 100% sure how real some of the worst ringing (at about 70MHz, right after the big spike) is though, as I know that the differential probe I was using has it's own issues (input leads are a bit long and ring with the input capacitance at around this frequency).

It looks to me like the green trace is nominally zero-current switching for the whole burst, with the blue trace being phase shifted to perform the ramp. I'd expect the ZCS trace to be fairly clean, but it looks like it's switching a little late and you're getting spikes on that one as well due to diode reverse recovery at turn-on. I'm not familiar with the driver you're using, but the idea of a fixed inductor phase-lead and an adjustable RC delay following it seems sensible, and probably just needs some tweaking (delay seems too long to me - maybe take that extra capacitance out and reduce the resistance). I have actually adjusted phase lead with a coil running a few times - normally I'd use a dummy-load (cast iron pan or something with high losses) in place of the secondary or simply nothing in place (current can ring up _fast_ with no load though), and be very careful what I'm touching. The UD3 driver I'm playing around with lets you do it remotely though, which is very handy!

The other thing I'd look at is using the alternative phase-shift mode:
Quote
2. Mode in which shifted half-bridge change places every feedback cycle.
This is a very advanced mode that provides even better heat dissipation balancing, and without the necessity for interrupt to occur. It requires good attention to gate drive quality(isolated drivers are recommended, but was successfully tested with GDTs too). For activating it, set jumpers to (AltShift = 1, PSkip = 1).
This is what is seen in my earlier screenshot - it spreads the losses from hard-switching between the two bridges. This won't solve the spike problem, but is worth using if it works.
« Last Edit: October 13, 2018, 04:16:48 PM by Hydron »

Offline alan sailer

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Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #16 on: October 13, 2018, 08:03:43 PM »
Hydron,

I agree about the ZVS. For some reason it is far off from what I set before. It's frustrating no being able to tune
and run but that's just the way it is.

I spent some more time with it this morning and it appears that I can set the optimum ZVS point at points along
the ramp. When I do this the transients are best in a band around this tune point and growing larger outside.
Which seems plausible.

Screen shot shows two Vce (Blue and Green) Red is primary current. Notice the "notch" in the blue trace. This is where the ZVS was set.



By the way, the SimpleDriver is set in a mode which shifts the hard/soft side of the bridge from pulse to pulse.
This would tend to balance out the thermal stress. It also has a mode which shift from cycle to cycle although the
company says that this would require special gate drive techniques.

From the manual,

2. Mode in which shifted half-bridge change places every feedback cycle.
This is a very advanced mode that provides even better heat dissipation balancing, and without the necessity for interrupt to occur. It requires good attention to gate drive quality(isolated drivers are recommended, but was successfully tested with GDTs too). For activating it, set jumpers to (AltShift = 1, PSkip = 1).
In theory, this mode will allow users to control QCW with external audio signal, although operation in such a mode wasn't tested yet.


Offline Hydron

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Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #17 on: October 13, 2018, 08:34:11 PM »
Hmm, I'm surprised that the optimum ZVS point changes so much over the ramp - in a normal DRSSTC it doesn't seem to shift much. Did your other coil (the buck one) do the same thing? Would probably need to do some simulation to get to the bottom of it, though at some point if it works then it's not worth chasing too much more.

As for the alternative phase shift mode, the "special gate drive techniques" thing is a bit overblown - as long as your gate drive transformers have enough turns on them to handle half the QCW frequency (i.e. about 150kHz and higher) then they should work fine. You can always test it at low bus voltage, if there are no issues then it'll work at higher power too.

Offline alan sailer

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Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #18 on: October 13, 2018, 10:10:25 PM »
Hydron,

I'd have to examine the buck more carefully (my understanding of these beasts changes over time) but I remember at one
point tuning the ZVS in DRSSTC mode and then checking it during QCW mode and being surprised that it still looked in tune.
I mentioned it to loneoceans and he was not surprised at this.

If I get a chance I'll check out the waveforms of the buck.

Cheers.

Offline Hydron

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Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #19 on: October 13, 2018, 10:22:15 PM »
I'd have to examine the buck more carefully (my understanding of these beasts changes over time) but I remember at one
point tuning the ZVS in DRSSTC mode and then checking it during QCW mode and being surprised that it still looked in tune.
I mentioned it to loneoceans and he was not surprised at this.
Yeah I don't see any reason why the ZCS (not ZVS) tuning should change between QCW/standard interrupter modes - hence why I said I was surprised. I'd need to look at the maths regarding the phase lead technique in use - maybe it ends up being peak current dependant (though this would also apply in normal non-QCW mode), or possibly the switching noise throws it off.

High Voltage Forum

Re: Size Comparison of Buck vs Phase Shift QCW
« Reply #19 on: October 13, 2018, 10:22:15 PM »

 


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[Dual Resonant Solid State Tesla coils]
Michael_DA
October 14, 2018, 03:45:34 PM
post Re: Size Comparison of Buck vs Phase Shift QCW
[Dual Resonant Solid State Tesla coils]
T3sl4co1l
October 14, 2018, 05:14:29 AM
post Re: Size Comparison of Buck vs Phase Shift QCW
[Dual Resonant Solid State Tesla coils]
Hydron
October 13, 2018, 10:22:15 PM
post Re: Size Comparison of Buck vs Phase Shift QCW
[Dual Resonant Solid State Tesla coils]
alan sailer
October 13, 2018, 10:10:25 PM
post Re: Size Comparison of Buck vs Phase Shift QCW
[Dual Resonant Solid State Tesla coils]
Hydron
October 13, 2018, 08:34:11 PM
post Re: Size Comparison of Buck vs Phase Shift QCW
[Dual Resonant Solid State Tesla coils]
alan sailer
October 13, 2018, 08:03:43 PM
post Re: Size Comparison of Buck vs Phase Shift QCW
[Dual Resonant Solid State Tesla coils]
Hydron
October 13, 2018, 04:09:53 PM
post Re: Size Comparison of Buck vs Phase Shift QCW
[Dual Resonant Solid State Tesla coils]
alan sailer
October 13, 2018, 02:25:50 AM
post Re: Size Comparison of Buck vs Phase Shift QCW
[Dual Resonant Solid State Tesla coils]
Hydron
October 12, 2018, 02:06:34 AM
post Re: Size Comparison of Buck vs Phase Shift QCW
[Dual Resonant Solid State Tesla coils]
alan sailer
October 12, 2018, 01:53:36 AM

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