Hi all,

1500W 12Vdc to 110Vac modified sine inverter.

Two output MOSFETs shorted and took some of the driver components with them. I removed all the MOSFETs, and fed in some power carefully - once the voltage rose to 12V, I could see the large output caps were being charged to over 200V, and the input oscillator circuit was working. I concluded the input side was OK.

However, now I've put in some new MOSFETs, replaced their gate driver transistors and two ICs - TC4093BP. These are quad NANDs with Schmitt triggers, I guess for wave shaping. Sadly, the unit only powers on for a brief time then beeps and lights the "overload" LED. The output caps are charged for the brief time before overload kicks in.

There is no short on the output, so what is happening? I have a blurry schematic, and my plan is to follow the overload sense circuit, but it's hard going as the schematic refers to an older model and not everything is the same.

How would you approach this?
Many thanks

 

Hi,

I would look at the old driver vs the new one and see what changed. This of course requires that you either have a schematic or can reverse engineer and make your own.

You'll also have to try to figure out what causes the trip, as to what is being sensed. I does not have to be current it may also be voltage sensing so if the voltage is too high or too low it trips. It could even be using the output voltage sensing for too low as an indication that there is an overload. If you can figure out what is being sensed you can figure out what is causing the trip, and that could point to the problem right away.

You should also check any output caps for problems.

So that's two important things offhand.plus the cap functioning check.
A schematic of the original unit would be great here as that would tell us the whole story.

 

Excessive ESR in the output caps could cause excessive current. I'd test them or just replace them. That's cheaper and easier than testing them if you don't already have an ESR meter.

 

Thanks for you replies. Sorry, the schematics are hand-drawn and very hard to read. I removed the output caps and their ESRs (according to my ESR-V5 tester) were all under 0.2 ohm. Capacitances were all about 300 uF, marked capacity being 330.

The problem is still there even with the caps removed.

Looking at the second schematic, do you think that IC 14 or IC 3 is the oscillator?

Also, look at the big caps (the ones I've removed) at the top, next to L1. This is the high voltage DC rail +v1. There's a trace running down to IC3 pin 3 - could this be the voltage sense? Isolating it doesn't change the fault, though.

The overload circuit is on the second page, at the bottom. There is no IC5 on this board, but there is an IC6 in the same position. It's a TC4013BP, a flip-flop. This appears to drive the OL LED, but presumably other things happen to shut down the board at the same time. I'm not sure how to trace this back to the sense circuit.

 

Thanks for you replies. Sorry, the schematics are hand-drawn and very hard to read. I removed the output caps and their ESRs (according to my ESR-V5 tester) were all under 0.2 ohm. Capacitances were all about 300 uF, marked capacity being 330.

The problem is still there even with the caps removed.

Looking at the second schematic, do you think that IC 14 or IC 3 is the oscillator?

Also, look at the big caps (the ones I've removed) at the top, next to L1. This is the high voltage DC rail +v1. There's a trace running down to IC3 pin 3 - could this be the voltage sense? Isolating it doesn't change the fault, though.

The overload circuit is on the second page, at the bottom. There is no IC5 on this board, but there is an IC6 in the same position. It's a TC4013BP, a flip-flop. This appears to drive the OL LED, but presumably other things happen to shut down the board at the same time. I'm not sure how to trace this back to the sense circuit.

IC1 is the smpsu chip TL598 with complimentary pushpull outputs Q22,23, its feedback sensing is via IC 5 opto-coupler, so where are the output mosfets you're on about?

Datasheet
http://www.ti.com/product/TL598/technicaldocuments

 

Excessive ESR in the output caps could cause excessive current. I'd test them or just replace them. That's cheaper and easier than testing them if you don't already have an ESR meter.

How does effective series resistance increase the current draw of a capacitor?

Degraded filtering might confuse the regulation circuit though.

 

IC1 is the smpsu chip TL598 with complimentary pushpull outputs Q22,23, its feedback sensing is via IC 5 opto-coupler, so where are the output mosfets you're on about?

Datasheet
http://www.ti.com/product/TL598/technicaldocuments

There is no IC5 on my board - it's a newer version than the schematic. However, there is a Sharp PC817 opto (IC8) which, when given 1.4V 40mA at the diode end, has 46 ohm resistance at the other. I think this means it's OK. Pin 1 of this opto goes straight to pins 1 and 7 of an LM358, IC9. Should pins 1 and 7 be connected at all? Should I remove IC9 and check for shorts?

The output MOSFETs are Qs 34-41 at the right-hand end of page 2.

IC1 is certainly the oscillator for the input MOSFETs Qs 1-16, so do you think it's responsible for the output as well?

There's another opto, an LTV4N25. The emitter of this goes to pin 10 of the flip-flop ("IC5") TC4013BP. I'm guessing this senses the Vs rail.

 

To clarify, my IC9 is listed as IC3 in the schematic - it looks like this chip senses the output.

 

How does effective series resistance increase the current draw of a capacitor?

Degraded filtering might confuse the regulation circuit though.

Yeah, I didn't really mean a direct cause and effect link. Just pointing out that bad ESR caps are a common problem in power supplies.

 

There is no IC5 on my board - it's a newer version than the schematic. However, there is a Sharp PC817 opto (IC8) which, when given 1.4V 40mA at the diode end, has 46 ohm resistance at the other. I think this means it's OK. Pin 1 of this opto goes straight to pins 1 and 7 of an LM358, IC9. Should pins 1 and 7 be connected at all? Should I remove IC9 and check for shorts?

The output MOSFETs are Qs 34-41 at the right-hand end of page 2.

IC1 is certainly the oscillator for the input MOSFETs Qs 1-16, so do you think it's responsible for the output as well?

There's another opto, an LTV4N25. The emitter of this goes to pin 10 of the flip-flop ("IC5") TC4013BP. I'm guessing this senses the Vs rail.

Best solution is to take pictures of your inverter pcb and we can see what's happening...

 

Thanks for you replies. Sorry, the schematics are hand-drawn and very hard to read. I removed the output caps and their ESRs (according to my ESR-V5 tester) were all under 0.2 ohm. Capacitances were all about 300 uF, marked capacity being 330.

The problem is still there even with the caps removed.

Looking at the second schematic, do you think that IC 14 or IC 3 is the oscillator?

Also, look at the big caps (the ones I've removed) at the top, next to L1. This is the high voltage DC rail +v1. There's a trace running down to IC3 pin 3 - could this be the voltage sense? Isolating it doesn't change the fault, though.

The overload circuit is on the second page, at the bottom. There is no IC5 on this board, but there is an IC6 in the same position. It's a TC4013BP, a flip-flop. This appears to drive the OL LED, but presumably other things happen to shut down the board at the same time. I'm not sure how to trace this back to the sense circuit.

Hello,

You should never run the unit with missing capacitors, that's not good at all.

High ESR could cause the unit to trip on over voltage. The circuit may show over load for any bad condition.
High ESR on the input caps could cause the unit to trip on under voltage (on the input) or with reduced input it could make it look like a low output voltage and that could trip it also.

If you had a scope that would help too maybe.

 

You should never run the unit with missing capacitors, that's not good at all

Oops. Just getting a little desperate, I suppose.

Best solution is to take pictures of your inverter pcb and we can see what's happening...

Is this any help? IC9 LM358N (IC3 on the schematic) has been removed here, and pins 1 and 7 are still shorted. Another thing to note is that before I replaced ICs 11 and 12 (TC4093BP quad NAND Schmitt) and the blown driver transistors, the output caps were being charged to 240V. They are only rated to 200V, so I suspect the monitoring circuitry was always faulty. I just didn't click at the time.

 

Right, I'm getting more of a sense of what's going on now. Please forgive my slowness, I'm not trained formally in electronics.

I do have a scope - Hameg 203-5 - but I can't get a sensible signal anywhere as the unit shuts down too quickly.

Looking at schematic 0101, and considering the damage I found on the board, it appears Q36 and Q37 and two others shorted in service. I didn't note where every MOSFET came from - lesson learnt. The 4 failed components have damaged or even burnt away source legs. The gates are shorted to drain.

Would it be easiest to track back from the output MOSFETs and replace the ICs up to the optos, which should have stopped any faults from going further?

 

The unit is now working - I replaced IC3, 4 and 14 (all the ICs near the output MOSFETs) and also the KA3525A switching controller not shown on the schematic.

The unit would then power up, but only produce 40Vac.

I found that someone before me had put in 2222As at Q29 and 31 (as shown on the schematic, but wrongly), not realising that this transistor's max Vce is no more than 40V, whereas due to the signal diodes D29 and 31, the Vce would exceed that. The 2222As were breaking down and clamping the output.

Once they were replaced with KSP42s (Vce 300V) the unit returned to working order.

 

Nice job!