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I am trying to make a step-down 350V to 220V voltage converter. I decided to go with rather simple circuit of full bridge MOSFET inverter since my loads are only resistive or switched.

I had built it and tested today, however, without any success. It runs well with intended loads then, randomly, it blows the fuse. Further investigation shows that two fets (upper & lower) on one side dead shorted. I install another pair, after some time (mostly from minutes to no more than one hour) it burn. Then again, but opposite side. In all cases, both high and low fets burn. When I lucky to run it for a long time without blows, I remove power then check fets temperature and they are OK.

My fets are IRFP460.

I use two IR2110 unified into full bridge, the load is purely resistive (a set of series lamps plus one single bulb) and draws 450mA @ 220VAC. This load is not the intended load, I plan to power more resistive loads with this circuit. IR2110 are driven by TL494 as signal generator which has a onboard trimmer to adjust duty cycle. My circuit diagram, without flyback SMPS shown.

This picture is reference only showing how my bridge is being built

TL494 and IR2110 are powered by small onboard flyback converter which is NOT isolated from network common. Whole circuit gets power directly, no diode bridges. The power is 350VDC line which is another SMPS driven by lead acid battery pack.

For two years I use exactly same circuit, but low voltage version: fets are IRF1404 (40V), and it gets power from separate 12V 1A low voltage flyback. Confirmed operation up to 30V on input. I probably miss something obvious, but can't figure out exactly what. Snubbers?

If needed, I can post my development PCB picture.

UPDATE:

This is my old scope showing signal between TL494 and IR2110 (IR2110's input). Those IR2110 are working and there are no shorts on board after them.

I can vary duty cycle from 0 to 97%.

duty cycle 67%

This is exactly signal waveform I want to see at device output.

full duty cycle

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    \$\begingroup\$ Time to get out the oscilloscope and measure things \$\endgroup\$
    – PlasmaHH
    Nov 26, 2015 at 15:25
  • \$\begingroup\$ Well, that's exactly a problem: I don't have a proper oscilloscope now. Only an old tube which will not tell me how fets burned. \$\endgroup\$
    – user92809
    Nov 26, 2015 at 15:29
  • \$\begingroup\$ You have STP8NM60 MOSFETs in the circuit - what other errors might there be lurking that make this question difficult to answer? \$\endgroup\$
    – Andy aka
    Nov 26, 2015 at 15:30
  • \$\begingroup\$ The picture is reference only... Just to show how my bridge is built. \$\endgroup\$
    – user92809
    Nov 26, 2015 at 15:31
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    \$\begingroup\$ Why are you using a full bridge to drive lamps? Do you need to reverse them? \$\endgroup\$
    – user_1818839
    Nov 26, 2015 at 17:45

4 Answers 4

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You would need something more sophisticated than TL494, a complete H bridge wit four outputs and dead time setting, to prevent conduction at once. Also with bootstrapping there is a possibility that you can't have 100% duty cycle (not sure on that). Furher there is no need both to switch both HI an LO side with PWM frequency, one can be ON/OFF and the other half is doing hard switching, you will get less loses.

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  • \$\begingroup\$ I simulated this circuit in LTSpice but with wrong parts and I see at boot it turns on only one side for a very short time and there is a chance of overcurrent. !However! I do not observe this situation and my actual circuit boots fine and works for a while, until sudden blow. And this is not an SMPS circuit - I need low frequency (about 400Hz) modified square wave output with duty cycle somewhere about 2/3 (63% or 350/220 = 0.63). I appreciate your answer and I will think about how to get lesser losses if I will face this problem. \$\endgroup\$
    – user92809
    Nov 27, 2015 at 5:27
  • \$\begingroup\$ This is also not a something sophisticated, just an inverter circuit which switches DC to square wave AC and dead time is large enough. So I probably hit something related to improper control, but can't easily solve this now. \$\endgroup\$
    – user92809
    Nov 27, 2015 at 5:28
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I cant see any fast cycle by cycle current limiting . Imagine if you put the square wave invertor into a load containing some capacitance .Many mains loads have low ESR metal film capacitance between P & N for EMC or PFC reasons.What now limits the prospective peak currents ? Wiring ? RDs on ? ESR of HVDC buss capacitance? So on your circuit you could get a prospective fault current of hundreds of amps .Chinese invertors that operate this way complete with TL494 are popular in my country and are marketed as "modified sine wave "Placing inductive reactance in series with the AC output has been shown to stop the fets blowing up on a case by case basis.Remember that George Ohm stated that resistance was proportional to Absolute temperature .Not always true for everything but filament lightbulbs can have one tenth of thier resistance when cold ,meaning that the surge current at cold turn on could be 10 times the load current so unprotected fet goes Bang again.This type of invertor is OK for powering a hotwater cylinder element as part of an off grid power system .Your simple invertor can be really efficient but its only good for resistive loads .I advise people to wire them in permanently to the known load ie hotwater element .This stops people plugging in stuff that will blow the FETs .In terms of $ per watt your invertor is one of the best if not the best .

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  • \$\begingroup\$ Yes! I try to implement that chinese things, only (in perspective) it should be more powerful. I understand that cold lamps have much lower resistance and surge current for a 10A fet can go up to 100A and more with them for a short time, but this is simply not the case: I played with circuit well, turned on the lamps when they were cold and circuit did not fault. But it once failed without any load at all. In other case it failed with hot lamp, i.e. it worked well for 5 minutes with load, no spikes anywhere, at input I see nice 350V reading and it blows, tripping fuse. \$\endgroup\$
    – user92809
    Nov 27, 2015 at 5:34
  • \$\begingroup\$ I also do not have any other load than resistive, I am trying to power a series of lamps with lower voltage. I have a low voltage version of this circuit and it works well and I really wonder why high voltage one blows for no reason with static calmed load. \$\endgroup\$
    – user92809
    Nov 27, 2015 at 5:35
  • \$\begingroup\$ I inadvertently posted comments here under Autistic's answer. I have moved my comments to the OP original question. Sorry. \$\endgroup\$
    – Marla
    Nov 29, 2015 at 16:13
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Two things i can recommend: refuce the 47R to even 0, try catching both high and low side conducting. It's almost certainly bad dead time management.

Also make sue the circuit is disabled on powerup.

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  • \$\begingroup\$ This (gate resistors) can be an option, I will try with spare parts from dismounted devices in next days. I hope my 2110's will not be damaged by this change if I will install jumpers instead. My dead time is large - or I do not understand something simple? And how do I check that my circuit is disabled on powerup? The powerup sequence is: cap charges from 350V to this voltage, then flyback turns on and powers up the control and from there it starts working. Please also note that I get boom not at powerup, but when it already has established operation. \$\endgroup\$
    – user92809
    Nov 29, 2015 at 8:30
  • \$\begingroup\$ And, because tube scope shows that before IR2110 there is a clean signal, then I have something after IR2110 which effectively makes two fets become short, and possibly large input hv capacitor charge at this time makes things even worse. Do my guess is correct? If so I am going to disconnect input powerline from capacitor and make it go through a small 15W lamp to see when it triggers and I hope I could catch this with scope too. \$\endgroup\$
    – user92809
    Nov 29, 2015 at 8:34
  • \$\begingroup\$ I don't completely understand what you are saying. My guess is that because of the high gate resistance the MOSFET is still conducting when the other one is being switched on. \$\endgroup\$
    – user76844
    Nov 29, 2015 at 8:44
  • \$\begingroup\$ Hm. Did you see shots of my scope in my question? The first one does display a 2/3 duty cycle.Second one however shows full duty cycle possible with TL494. That's what I mean. \$\endgroup\$
    – user92809
    Nov 29, 2015 at 8:55
  • \$\begingroup\$ It looks like waveform measured on the load, you should measure two gates, with isolated probe. \$\endgroup\$
    – user76844
    Nov 29, 2015 at 8:56
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Since my load does not require AC, I switched to DC "dimmer" circuit which requires only one MOSFET and it's robust and simple. Just flyback supply + NE555 with common ground.

The left thing unknown is only that I connect any type of load to it and mosfet does not blow at all, even if I short it when it is being supplied from limited power source. I still think that my full bridge attempt has control issues, but without proper scope I cannot test it.

ne555hvdimm

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