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Half bridge inverter issues

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bob800

Sun Jun 29 2014, 03:14AM

Registered Member #18429 Joined: Sat May 18 2013, 02:05AM
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Posts: 19

I just finished building uzzors2k's "multipurpose inverter" (schematic here), which uses a TL494/UC3710T combo to drive a half bridge via GDTs. I did not modify anything in my build, except for the fact that I'm doing initial testing with an isolated variac (<100V).

It outputs a fairly nice square wave, and has no issues driving resisitve loads. However, I'm having issues with the built-in overcurrent protection being tripped when driving heavier inductive loads. I've connected it to both a flyback transformer and induction heater (with matching capacitor/inductor for the heater), and whenenver I drive the voltage past ~60V, the output waveform suddenly begins shaking up and down, with the OC protection tripping shortly afterwards. The unstable waveform also occurs when the output wires are not fed thru the OC CT.

Another thing I observed is that one of the IRFP450's becomes warm fairly quickly, while the other stays completely cool. I don't know how to account for this, because the output voltage swings all the way from the negative rail to 0 to the positive, so both transistors must be switching.

There is a little ringing on the output voltage waveform when under load, and I can't seem to completely eliminate it by tuning the frequency. I attached a scope shot of the ringing in case it is helpful (the ringing does not change significantly as voltage is increased). I tried adding some fast diodes (MUR240's) across the source/drain of the MOSFETs, but to no avail.

I can post more waveforms, but I've checked the gate and drain waveforms of each transistor, and nothing jumped out at me. If anyone could explain the one transistor hot/one transistor cool phenomenon I would really appreciate it!

--Also, aside from the slight ringing, everything seems normal before the unstable waveform/OC trip occurs. When connected to the induction heater, the current waveform is a nice triangle shape until I reach the resonant frequency, where it becomes more of a sine wave (as expected). It's just that after a certain point, it suddenly starts shaking.

--PPS, I had a DMM in series with the variac output, and the AC current reading stayed below 1.5A for the entirety of my tests.

Patrick

Sun Jun 29 2014, 04:15AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I always get suspicious of some transistors decieding to get hotter than others... given supposedly equal turn on and loading.

can i see more of your waveforms? right at the IGBT pins. (gate drive and load)

Sigurthr

Sun Jun 29 2014, 04:55AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

I had an inverter where one FET would get hotter than the other, it turned out to be increased switching time on the body diode. After that I started using full isolation of the body diodes (requires a series ultrafast diode AND an antiparallel ultrafast diode that is in parallel with both the fet and the series diode). That completely stopped the issue, but it certainly adds expense.

bob800

Sun Jun 29 2014, 05:24AM

Registered Member #18429 Joined: Sat May 18 2013, 02:05AM
Location:
Posts: 19

Here are the GS and DS waveforms (these are MOSFETs not an IGBTs btw)

No Load, Transistor #1, 50% duty cycle

No Load, Transistor #1, 100% duty cycle

No Load, Transistor #2, 100% duty cycle

No Load, Transistor #2, 50% duty cycle

Resistive Load, Transistor #1, 100% duty cycle

Resistive Load, Transistor #1, 50% duty cycle

Resistive Load, Transistor #2, 100% duty cycle

Resistive Load, Transistor #2, 50% duty cycle

Inductive Load, Transistor #1, 100% duty cycle

Inductive Load, Transistor #1, 50% duty cycle

Inductive Load, Transistor #2, 100% duty cycle

Inductive Load, Transistor #2, 50% duty cycle

Approaching OC shutdown (I can hear some quiet buzzing sounds... current draw is less than an amp, however)

RIght before shutdown

My setup

Both MOSFETs stayed cool during this run but the voltage was kept very low (5V?). The waveforms did not change in shape much as I raised the voltage (except for approaching the weird sporadic buzzing state, which always occured at the same voltage, maybe 50V)).

Patrick

Mon Jun 30 2014, 06:04AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I haven't forgotten about you, just been busy, I'm still pondering a solution....

Sigurthr

Mon Jun 30 2014, 06:23AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

It looks like the scope is unable to trigger accurately, which usually occurs when the drive frequency of the inverter is unstable or intermittent. I would assume from this that there isn't enough hysteresis on the overcurrent interrupter which is causing the gate drive to be rapidly enabled and disabled. This probably sends high frequency signals to the gates of the FETs, resulting in their heating.

I would use a similar OCD circuit to what we use in DRSSTCs; add a variable series resistance after the diode that's in series with the OCD CT and increase the capacitance of the 1nF to form a better peak detector with higher hysteresis. You could scope the junction of the 1nF cap and 39k resistor to investigate this node/s behavior before making modifications. I'm terrible with opamp circuits but I'm not seeing any way to vary the OCD limit in the circuit either, it may just be that the threshold is set too low.

Steve Conner

Mon Jun 30 2014, 10:34AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Edit: Forget it, I just noticed you are using a half bridge :-/ Your problem is probably just noise being picked up by the OCD circuit and causing spurious tripping.

Are you using a DC blocking capacitor in series with your load? This is a must-have when driving a transformer or inductive coil. High frequency coils tend to have low DC resistance, so without the DC block capacitor, even a tiny error in duty cycle will cause a large DC current to flow.

bob800

Tue Jul 01 2014, 03:50AM

Registered Member #18429 Joined: Sat May 18 2013, 02:05AM
Location:
Posts: 19

Thanks for the suggestions! Sigurthr/Steve I think you are right about the OC protection being way too sensitive... I desoldered the CT completely, leaving only the 6.8ohm resistor, and the OCD STILL tripped at higher power (albeit allowing higher power than before, but still less than an amp).

So I decided to remove the LM324 out of its socket entirely..

This finally allowed me to crank up the output power without anything tripping or messing up the waveforms.

The one transistor/hot one transistor/cold thing is still happening though... I'll order some more ultrafast diodes and try Sigurthr's series/antiparallel configuration, but should this even be necessary if I'm switching at ZVS? Couldn't I achieve this by tuning my series inductor/frequency until the DS voltage slightly lags the GS voltage, which should then eliminate the ringing thru the body diodes?

@Steve Connor: Yes I am using a 1uf film capacitor for DC block. (arbitrary value... should I calculate this somehow?)

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