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Flyback duty cycle locked

I built this 10W flyback. See schematic.

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It was working perfectly for several days of testing then I powered it up and it stopped regulating the duty cycle remains locked at a little over 4% the outputs are at about ten percent of there regulated values.

I noticed my once nice and clean DCM waveform over my current sense resistor (R4_D) is now oscillating at about 1.5MHz.

Scope shot current sense resistor (R4_D) 10mV/div.

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I suspected the overcurrent protection may be kicking in but there is no indication of this. If the OCP is active the SS pin should be ramping up and down trying a restart but this isn't happening.

The controller is the TPS40210.

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I'm down to switching out the controller, unless someone can offer an explanation.

I'd prefer not to pull the controller out because it would be a bitch without damaging the traces it's a fine pitch 10 pin MSOP-PowerPAD

Reply to
Hammy
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correction its oscillating at about 33MHz.

Reply to
Hammy

D9/Q4 could be causing the oscillation. Short D9 and see if the symptoms become clearer.

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Reply to
Kevin McMurtrie

Try taking the M1 MOSFET out and seeing what happens.

Put a scope on U1 pins 7 and 8. If the gate of the MOSFET is making a huge current spike or a swing below ground it can mess up the chip. The turn on current spike can be really huge if something has failed.

Reply to
MooseFET

I tried as you suggested. No luck now the controller gate drive is erratic almost like burst mode. I know some controllers have this feature but to my knowledge the TPS40210 doesn't.

I was going to try as MooseFet suggest but I think at this point its safe to say the controller somehow got damaged.

The gate waveform is almost triangular versus the nice sharp clean transtion when I first put this together. I must have inadvertently shorted the gate drive or something.

I'll redo the board . I'll place a shottky clamp on the gate drive just in case. I did check all the pins for negative transients and they were all fine. I always put a clamp on the CS pin regardless; D20 in the schematic.

Reply to
Hammy

The initial ramp-up is suspiciously steep. Sure nothing at or behind the transformer has shorted out? Shorted winding? Crack in the toroid core? D4/D4 shorted out?

Probably doesn't matter if you didn't get an unconnected pin warning upon netlist generation: The emitter of Q4 is sort of naked.

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Regards, Joerg

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Reply to
Joerg

I'm pretty sure the initial spike on the current sense isn't from drain source conduction but from the gate driver. The current sense signal is RC filtered then fed to the controller which also has leading edge blanking.

I pulled the transformer out so I could test for shorts and there is none. I can't see any cracks in the core but I measure the correct inductance with my LC meter. If there was a hairline crack in the core would you still get a correct inductance measurement?

The behaviour of the controller is just screwy.

Its just the way the model is drawn,its connected on the board. I didnt even noitice that good eye. ;-)

Reply to
Hammy

Then your Isense current would shoot straight up like a rocket.

Yeah, looks like the chip is kaputt :-(

Only because I wear 1.5x glasses when at the computer :-)

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Regards, Joerg

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Reply to
Joerg

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