Fullbridge LOAD TRANSIENT RESPONSE

[MacIntoshCZ]

So i should use lower voltage for GDTs or use only one GDT with 4 secondary outputs  (two noninverting drivers on same board and two inputs, outA and outB from tl494) as in the begining... What seems to be a better option? I think having two different GDT is better

Just for ilustration. Blue and Yellow are outputs from TL494
Purple is output of GDT with 4 secondaries and two noninverting drivers.

[MacIntoshCZ]

So this is overshoot causing mosfets to blow up. IRF460 is rated for 20V driving voltage and my overshoot is equal to 32V aprox. Seems its in rated specs  ...
QP35 - measured on output
QP36 measured on output, with added two 12V zener diodes in series (with common anode) across primary
btw VCC was 12,7V

[T3sl4co1l]

Do you mean IRFP460, by the way? The non-'P' version is TO-3...

Tim

[MacIntoshCZ]

Yes, i mean irfp460.
With to 247 package

[moffy]

So this is overshoot causing mosfets to blow up. IRF460 is rated for 20V driving voltage and my overshoot is equal to 32V aprox. Seems its in rated specs  ...
QP35 - measured on output
QP36 measured on output, with added two 12V zener diodes in series (with common anode) across primary
btw VCC was 12,7V

If you are having overshoot problems on your drive transformer, you could try loading the secondaries with resistors which will tend to dampen any overshoot.
The waveforms you see might be a result of scope probe capacitance, if you can measure the waveform with say a 100R resistor in series with the probe, that can reduce the capacitive influence of the probe somewhat.

[MacIntoshCZ]

Thanks very much =)
1) I tried first to measure overshoots with 100R with probe in series, Overshoot get down to 28V. So i have tried 50R, it gets down to 25V. 33R did nothing at all. So yes, "real" value of overshoot was way lower but still enough to blow up mosfets.
2) Now i have added 100R across secondary, overshoot was gone. But resistor was too hot. So i have added another one 100R in series-> 200R. This is the waveform. I think it will work now =)

[T3sl4co1l]

Oh, I think I see what's going on now.

It's a constant guessing game since no setups are specified for each waveform here, but I think the last waveforms must've been scope probe on secondary, unloaded (no gate connected), primary drive as shown (TL494 + UCC drivers).  So, you get the response of an unterminated transmission line, and apparently the winding is about that long (10-20ns electrical length, so, about 2m of wire?), so that's that.

This waveform is nothing of concern: the gate resistance swamps transmission line behavior, and leakage inductance dominates.

2m of wire at ballpark 100 ohms Zo is 0.7uH.  The gate average capacitance is 20nF per transistor, so the LC series resonant circuit has Zo (that's a different Zo now) 5.9 ohms, so the 6.8 ohm gate resistor will do well enough, and it'll have a time constant around 180ns.  We can expect about 1/3 that or 60ns commutation speed at the drain.  (Actual values may differ, but will likely be within 2x or so of these.)

To run faster, smaller gate resistors are not feasible: that'll just result in ringing.  A lower transmission line Zo is required to enable that.  That is, use more twisted pairs in parallel, or star quad(s).

The slanted waveforms shown earlier are suggestive of excessive magnetizing inductance; rewind with more turns or a higher A_L core.

Also consider an R+C in parallel with the GDT coupling capacitor, to dampen startup transients.  Probably there's some low frequency ringing in there.  Same calculation, R = sqrt(L/C), where L is magnetizing inductance and C is the coupling cap; use a series capacitor >=2.5 times the coupling cap.

Tim

[moffy]

Thanks very much =)
1) I tried first to measure overshoots with 100R with probe in series, Overshoot get down to 28V. So i have tried 50R, it gets down to 25V. 33R did nothing at all. So yes, "real" value of overshoot was way lower but still enough to blow up mosfets.
2) Now i have added 100R across secondary, overshoot was gone. But resistor was too hot. So i have added another one 100R in series-> 200R. This is the waveform. I think it will work now =)

That is a good result. Any time there is inductance(transformer leakage) and some capacitance(possibly the gates) you will have ringing, that is where a suitably placed resistor will absorb the energy and dampen the oscillation.

[MacIntoshCZ]

There is one thing i dont understand. I rewinded transformer for driving gates of mosfets.
 I am using this toroid https://semic.cz/!old/files/pdf_www/Lj_195BT3615C_CF.pdf
It has 40 turns (BTW before it has 10 turns only), which should be equal to 9,28mH and energy which was stored in coil after 50k cycles should be 39mW.
But when i byppas capacitor, which is in series with transformer, or i place paralel diode to transformer, it actually go to short circuit. I though those 39mW would "dissapear by heat" in chip itself, well actually i was nice to gate driver and placed 1R5 resistor in series with transformer, but still AMPS have a high ground =D.

[moffy]

There is one thing i dont understand. I rewinded transformer for driving gates of mosfets.
 I am using this toroid https://semic.cz/!old/files/pdf_www/Lj_195BT3615C_CF.pdf
It has 40 turns (BTW before it has 10 turns only), which should be equal to 9,28mH and energy which was stored in coil after 50k cycles should be 39mW.
But when i byppas capacitor, which is in series with transformer, or i place paralel diode to transformer, it actually go to short circuit. I though those 39mW would "dissapear by heat" in chip itself, well actually i was nice to gate driver and placed 1R5 resistor in series with transformer, but still AMPS have a high ground =D.

What is possibly happening when it "goes to short" is that the current is not being reset after each cycle, so a DC current and magnetic field builds until the core saturates and you get your short circuit. It is important to always balance the current through the core so that you don't saturate it. Any DC voltage across a coil will only be limited by the series resistance.

[MacIntoshCZ]

Just curious what is allowed negative spike which could possibly turn on next pair of mosfets? I cant get rid of it. At the moment its 1V aprox, i dont think 1V will turn mosfets on.
QP 38, measured on secondary of GDT, with mosfet connected
QP40, measured on secondary oif GDT, 20R and 25nF cap connected in series directly at output of secondary side, mosfet connected

Blue - Primary winding of transformer,
Yellow - secondary winding of transofmer,

[T3sl4co1l]

Why is the first one asymmetrical (different response around 0V vs. peak)?

Tim

[MacIntoshCZ]

Probably i dont know what you mean, but if i undestand correctly, first picture is without snubber and second one is with snubber.
I though you see a filename of pictures. One is named quickpicture38.png and second one quickpicture40.png.

[uer166]

But when i byppas capacitor, which is in series with transformer, or i place paralel diode to transformer, it actually go to short circuit.

Well, yes, that is expected since you're applying a DC voltage to transformer, which saturates the core and it becomes a short. In that circuit you need the series cap to maintain volt-second balance.

[T3sl4co1l]

I mean compare the two rising edges (or fallings), overlaid here for example.

Is that direct connection to G-S, nothing on D-S?  Or with 6.8R gate resistor?  Did you try other resistor values?

There's no need for an RC in parallel with the gate, that needlessly slows things down and consumes more drive power.

Tim

[MacIntoshCZ]

This is circuitry i measured on. With this setup capacitor before primary can be omitted.
WHy its assymetrical i honestly dont know  .
I did not try other values of gates resistor. SHall i play with it?

I added RC parralel to secondary becouse i am afraid of negative spikes when turning off, could lead to turn on others pair of mosfets which could lead to short circuit..

[MacIntoshCZ]

Looks good to me, except horrible ringing at gate waveform... I did not use 100R in series with probe so, who knows if its real...
I was scared that this ringing turns mosfet on but then i realized that it could be just body diode of mosfet that starts conduct when no mosfet its turned on.

BTW there is no RC parelel on secondary output of GDT transformer, should i think to add some?

[T3sl4co1l]

Wait you probed across the primary while both sides are driven? Do you have a floating scope/probe??

BTW there is no RC parelel on secondary output of GDT transformer, should i think to add some?

So C33+R21 are there, or aren't?

Tim

[MacIntoshCZ]

Sorry i forgot to explain what which waveform represents.
100V scale is measured Vds of Q2. 10v scale is Vgs of Q2.
I was checking for excesice high dv/dt.

[MacIntoshCZ]

C33 R21 are not connected.

[MacIntoshCZ]

First sorry for duplicite topic, i started another one, but then realised it would be better to keep just this one alive.

I tried to do load transient response test.  Turn ON time is probably too low i should make him larger.
In the video i was playing with RV3.
https://youtu.be/q_M8uPSxmOo?t=25
It seems "bumps" are alive =D.

Blue trace is just gate of mosfet that turns load on and off. BTW load is 23R, so at 188V its about 1,5kW aprox...
yellow trace is voltage at R9. I think i should not use this resistor to sense voltage. Becouse its also used by error op amp of tl494..

I already changed RV3 from 10k to 100K. I should play with C27  until there would be no "bumps" am i right?