TL494 DCDC converter wide range input issues

[Zac1]

With a help of few people I managed to design fairly wide range DCDC converter using TL494

I confirmed it works in range of 30-190V DC
Has fairly stable 12V output with ~40mV peak to peak noise
Can deliver up to 3A.

However it has one fatal flaw that haunts me in my dreams. It kills mosfet Q5 and diode D2 if you suddenly apply 160V+ (high dV/dt)
It does work properly if slowly raising voltage across 40-190V. Also works up until 140V (sudden turn on).

I have managed to capture the problematic moment on thermal camera

https://imgur.com/CHKR4Ew

Basically there is a full short on Q5 and D2 for a moment, then fuse pops and both components have dead short. I have tried to remedy this situation with help of PNP clamp (Q1) but it doesn't do much. I am out of ideas how to fix this... perhaps the driver is at fault? Or maybe diode is too slow?

 

[Easy peasy]

Please don't take this the wrong way - but looking at the heat signature in the zener - is about as opposite as you can get to designing a power supply properly

you know nothing about the peak currents in the inductor - so you can't tell if it saturates or not - this is kind of a big deal. It can be easily measured and seen on a scope.

The engineering approach is to observe the Vout & error amp output at start up, and adjust the control loop accordingly such that it is fast enough ( and with just enough gain ) to keep the overshoot to a couple of volts max at startup & max Vin - also with the speed-up circuit on the Vsense

The trouble with not having a good knowledge or circuit behaviour - is that some thing small can change later on and break the expected behaviour in a bad way.

For example - if some one changes the inductor later on - and the new one saturates at a slightly lower current - you will be back to blow ups at switch on.

--- Updated Apr 11, 2024 ---

" I may be wrong on this one but i don't think this is the real issue. Or to be precise, why that overshoot was caused. "

you are correct in the first part

--- Updated Apr 11, 2024 ---

p.s. the gate drive chip only starts to operate when it gets 10V on its supply - this is why you don't see much happening until this 10V level is reached,

if you had read the data sheet you would have been aware of this.

 

[Easy peasy]

OK - you have a solution that kinda works @ 190V in. If you ever re-spin the board, give some thought to using the other opamp in the TL494 to stop the pwm if the Vout goes higher than 12.5V

All the best going forward.

--- Updated Apr 11, 2024 ---

Oh, just out of interest could you set the blue trace Vin, to 100nS / div and apply start up ? it will be instructive to see what the Vin goes up to on the pcb.

trigger at 150V say on the blue trace

 

[Zac1]

OK - you have a solution that kinda works @ 190V in. If you ever re-spin the board, give some thought to using the other opamp in the TL494 to stop the pwm if the Vout goes higher than 12.5V

I have been extensively testing the board recently and as you say, under the load whole thing collapses, although it can output over 3A with ease I think you are right and so the revised schematic was born:

Long story short, new elements marked with arrows:
R28 R9 that forms a divider that should output 1.016V (current limit)
R27, 10mOhm shunt
U2 INA213 current amp with 50V/V amplification (2A * 0.01ohm * 50 = ~1V)
CFB and CFB- are connections to internal TL494 opamp.

As for the schematic i did put INA close to TL494 routed with (pseudo) differential pairs from resistor.

I am not sure however with placement of shunt, it makes most sense to me to measure current after the coil before voltage feedback. Or should I move it?
Are there perhaps any obvious issues?