100kHz CM300 gate resistor choice

[Benjamin Lockhart]

I'm building a small half bridge coil. How do these gate waveforms look? Is the rise time too slow? and what about the ringing on turn off?
I've got 5.1 ohms gate resistors and 0.82 ohm turn off gate resistors. these values worked well for lower frequency coils but I might need to tweak them.

I'm scoping the low side gate because if I scope the high side, I get the primary current distorting the gate waveform somewhat.

I've also included shots of the bridge output with phase lead tuned as well as possible at 200A primary current. I'm planning to run it up to at least 600A But that was as high as I could get it with my isolation step down transformer at 40V AC in.

Also, do you know why I sometimes get the noise on the primary current waveform at the switching transitions but sometimes not? In this case it might be relevant to fix the noise because it lines up with some ringing/noise on gate turn off.

Thanks, Benjamin

[davekni]

How do these gate waveforms look? Is the rise time too slow?

I have no idea what rise time is because scope time/division is not listed.  Oh, after noticing title saying 100kHz, I can then deduce scope is at 2us/div for first capture.  Listing both volts/div and time/div for all traces would be helpful.  Or amps/div for current.
Rise time is probably fine.  Some rise time is needed to provide dead-time between turn-off and turn-on.

Also, do you know why I sometimes get the noise on the primary current waveform at the switching transitions but sometimes not? In this case it might be relevant to fix the noise because it lines up with some ringing/noise on gate turn off.

Presuming UD2.7 or similar driver, phase lead does not function well at low current.  Once current builds, phase lead works better.  Phase lead makes switching at proper time slightly before zero current which reduces spikes.

[unrealcrafter2]

On my drsstc I had a fair bit of noise. I found improvement on the RF ground helped.  Also if you have two MOTs you can put them together for an 120v isolation transformer

[Mads Barnkob]

I've also included shots of the bridge output with phase lead tuned as well as possible at 200A primary current. I'm planning to run it up to at least 600A But that was as high as I could get it with my isolation step down transformer at 40V AC in.

Low voltage testing is deceiving! To get a more accurate picture of switching transients/noise, you need at least 100 VDC on the bridge.

IGBT output capacitance (Coes) is a highly voltage dependant and decreases with increasing Vce voltage. This is a problem with zero current switching inverters, as low voltage testing them, introduces high capacitances to resonant with the inductances in the bus/IGBT/etc.

Switching spikes at several 100's Volt, at 50 VAC in, might be no bigger than several 10's Volt at full input voltage.

[Benjamin Lockhart]

I made an isolation transformer from MOT parts and added 46 turns to the primary so the core isn't saturating so bad. Now I can test up to 300V DC on the bus.

I just took some more measurements at about 250V bus, still with the same 5.1 ohm gate resistors.

Primary current is ringing up to about 450A here! Even at this current, adjusting the bus voltage messes with the phase lead somewhat.
I have a delayed timebase scope so these captures are zoomed in on the switching at highest current just before end of interrupt pulse.

Here's the low side switch, 250V ish DC bus voltage. (200ns/div), (Vce 50V/div), (Vge 10v/div)

turn on


Turn off


Here's the switching transition glitch where the interrupter pulse stops


I also still have significant spikes up to like 500V on the first few cycles! I know this is normal but should they be this high with 250V bus? I'm planning to run 680V bus voltage (1200V IGBTs)
(50V/div), (5us/div)


Different vertical position, same shot.


If I zoom in on the bridge output ringing on the first cycle, it's about 25MHz. Is this just from bus inductance, or could it be an artifact of poor measuring?

Should I try smaller gate resistors to speed up switching and reduce switching losses, or is it fine? the IGBT brick is staying pretty cold even after a few minutes at 450A.

[davekni]

Here's the low side switch, 250V ish DC bus voltage. (200ns/div), (Vce 50V/div), (Vge 10v/div)

Thank you for scope settings.  These first three zoomed-in traces look excellent!  Very clean switching with Vce transition occurring at turn-off and turn-on just before zero current.

I also still have significant spikes up to like 500V on the first few cycles! I know this is normal but should they be this high with 250V bus? I'm planning to run 680V bus voltage (1200V IGBTs)

Those spikes do look rather high, though may not be enough to damage anything.  Spike voltage likely scales with current more than with voltage.  Hope you get other responses and opinions too.

I see a snubber cap across top brick Vbus.  Is there another on the bottom brick?  Presuming so, and if other opinions say those spikes are high, a few options come to mind:
1) What CT ratio do you have?  If ratio can be reduced without exceeding UD2.7 feedback burden resistor (51-ohm) power, that will help.  Makes UD2.7 phase lead track sooner (at lower current).
2) Adding a laminated bus bar structure to H-bridge to further reduce stray inductance and allow connection of additional snubber caps.
3) Add R+C snubbers on brick outputs.
4) Make the self-oscillating mod's to UD2.7 driver that I've described in other treads and a couple people have implemented.  When self-oscillation is set to roughly primary frequency, phase lead starts almost immediately.  If you are interested in this option I can find those threads and add links.

If I zoom in on the bridge output ringing on the first cycle, it's about 25MHz. Is this just from bus inductance, or could it be an artifact of poor measuring?

The higher frequency ring is less important and is likely very sensitive to measurement details.  The more important frequency is width of each spike, which looks to be just under 1us, say 900ns.  That half-cycle spike is ~550kHz and not so likely to be a measurement artifact.  It is likely due to inductance, internal to bricks and snubber caps and external connections between bricks and caps.

[Benjamin Lockhart]

Thanks Dave,
This is just a single half bridge brick.
The build is really tight and the connections are quite short. A laminated bus would be very difficult here I think.




The snubber cap is 2uF

The CT ratio is 676:1 It was 728:1 before the last test but I reduced it thinking that would help with phase lead and it did help with the first cycle spikes somewhat. That's only about a 7% reduction in the ratio though. I'm planning to set the OCD around 700A or so. I think it may not actually hit that limit though.

Maybe the spikes are acceptable if they just scale with primary current. It it's linear then I might see 800 or 900V spikes at 750A peak.
The self oscillating mod sounds like the best solution if the spikes are problematic.

One more thing to note, this coil doesn't start oscillating until about 60V bus, and if feedback phase is reversed it actually does oscillate at roughly the same bus voltage, just the current doesn't ring up and looks similar to a triangle wave.
My full bridge coil starts oscillating at much less voltage.

This seems to me like not strong enough feedback soon enough. And reducing the CT ratio further might make it better, but I don't want to have too much past 1A of feedback current right?

[flyingperson23]

If switching spikes are caused by bus inductance dumping energy into parasitic capacitance, wouldn't the amount of overshoot decrease as bus voltage increases?

[davekni]

If switching spikes are caused by bus inductance dumping energy into parasitic capacitance, wouldn't the amount of overshoot decrease as bus voltage increases?

There are multiple possible causes for spikes.  I'd guess these are caused by diode turn-off (reverse recovery time ending) during post-zero-current switching.  Diode turn-off causes sudden drop in bus current causing sudden rise in voltage due to bus inductance.

This seems to me like not strong enough feedback soon enough. And reducing the CT ratio further might make it better, but I don't want to have too much past 1A of feedback current right?

Depends on power rating of 51 ohm CT burden resistor in your UD2.7 build and intended duty cycle of your coil (max setting of interrupter).  Yes, 1A is a reasonable number.

My full bridge coil starts oscillating at much less voltage.

Half bridges need twice the bus voltage to ring up at the same rate (same output voltage).

Edit:  One more thought:  What type of diodes are you using for feedback CT input clamping (input to comparitor, D1 and D2)?  If using 1N4148 or other non-schottky diodes with higher Vf, that will increase startup voltage and current required to get good phase lead.  Also, what value is your comparitor hysteresis feedback capacitor (C33 in many UD2.7 schematics)?  If using 2.2nF (or anything above ~1nF) that would be another contribution to increased startup voltage and phase lead current.

[Benjamin Lockhart]

MBR0530 schottky diodes, and C33 is 1nF.

[Benjamin Lockhart]

I tested the coil at 340V bus. I still don't want to test at 680V DC until I do something about the spikes or decide if it's fine. I don't really have a good way to measure the spikes at full bus voltage but I can still guess at how big they will be.
I'm still not sure what the best thing to do is.

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[davekni]

I don't really have a good way to measure the spikes at full bus voltage but I can still guess at how big they will be.

Can you run your interrupter at 60Hz?  Or even better, sync it to line voltage?  Then you can measure spikes.  Connect scope probe ground lead through a small cap such as 0.1uF to bus voltage.  Cap will provide an AC ground for spike frequencies while passing reasonably-low current at line frequency.

[Benjamin Lockhart]

I can do that but unfortunately I only have a 600V rated 10:1 probe, so I need to buy a 100:1 probe first.