Inverter repair

Fabian · January 2020

I have a 24v 2000watt @ 120vac pure sinewave inverter that is in need of repairs. A output short circuit damage it. I analyse it and realize that the mosfets needed to be replace. It uses (16) pcs of irf3710 and 4 (pcs) of 85GT33SW Transistor mosfets.

Upon Changing them I proceeded to try and power it up. So I connect it to a bench power supply that I have 24v at 15amps. The power supply has 4 connections. Which is divided into 2. One set has the positive and the other set has the negative. Upon connection I accidentally connect both negatives to the inverter. Where one negative was connected to the positive of the inverter and the other negative was connected to the negative of the inverter. The positive from the power supply was not used thou.

When i try to turn it on which I did several times before I saw the mistake. The blue and red light on the front came on together which suggest that something was wrong but he lights were very dull. And the screen was blank.

When I realize the connection error and connect it correctly this time the lights came on brightly and the inverter screen comes on but it is showing low voltage input of around 8v with no output.

Each time u turn it on the same thing comes up and you see where the output voltage jumps up.to. around 45 or 75v or close to.that region. And then it just dial back down to 0 volt output in about 1 sec.

You can see the red light blinks every 2 or 3 secs too and you can hear a slight ramp up sound from the power supply And the screen displaying the input voltage of around 8 volts slowly decline until it now showing 0 volts input. Now when you turn it on it is showing just 0 volts input with 0volts output.

I test the input at the back of the inverter and it is showing 26volts dc coming in so I am kinda confused as to why the screen is showing 0volts input. Several sections on the inverter u test and it is showing 26volts dc.

I am wondering if is the wrong connection that I made at first cause the issue now or is the short circuit problem which it had originally did damage not only the mosfets but some others components too.

I am wondering if the mosfets got damage again even thou at first the screen would not come up and now it is showing even thou there is no output/input displayed.

What do you suggest that i do?

Would connecting negative from a power supply to the inverter positive and negative cause a issue like reverse polarity even thou the positive from the power supply was not used.

Please help me out someone I really want to fix this as my system is currently down now.

Your help would be greatly appreciated.

BB. · January 2020

In general, a large problem with integrated circuits, transistors, and other active/silicon components are dramatically shortened lives from stresses (electrostatic discharge, over voltage, reverse voltage, thermal and mechanical stresses, etc.).

Even if you get it running again, it is possible that you may have other failures in the near future because of the reverse power connections.

You don't even know which parts where exposed to the voltage issues, without a schematic.

I just don't see this as worth the time and money to fix.

Bill

clockmanfran · January 2020

I agree with Bill.

Here below ..... is a copy of my OzInverter build manual from the tech section. Its a straight to the point contribution by a well respected, good and clever engineer in Australia.

"‘Warpspeeds’ comments on limitations of HF (high Frequency) Inverters.

“If you build say a 2Kw high frequency switching power supply to first generate a high dc voltage, and then use PWM to generate a sine wave from that high dc voltage, you are always going to be limited by that 2Kw dc to dc converter.
You cannot suddenly draw 3Kw from it to supply some momentary power surge. There will only ever be 2Kw of dc available.
Now a great steel lump of a transformer driven with PWM straight from a battery is only limited in power by heating of the wire in the transformer, and by the maximum safe current capacity of the mosfets driving that transformer.
Although constant continuous long term power output may still be limited to 2Kw (in this example) you could draw short term power surges of multiples of 2Kw for very short periods without any problem at all.
If you want an inverter that has some real peak surge load grunt, a big lump of iron for the transformer core has all the advantages for a practical inverter.”

‘Warpspeed’ on a Chinese G…..l brand of High Frequency Inverters.

“A High frequency Inverter …………….. It’s the crappy high frequency type that uses five small dc to dc converters in parallel to generate several hundred volts of dc, and then turns that into a modified sine wave. It’s all very highly stressed with light duty parts, and just looking at it the advertised power rating is just a complete total joke.

High frequency inverters are rather fragile and very prone to blowing up. Not necessarily G…..l, but high frequency inverters in general are definitely best avoided.

Much better are the larger transformer inverters, even the modified sinewave types. Better still are the PWM pure sine wave transformer inverters, but all that "goodness" comes at very great extra cost, size, and weight.

A high frequency inverter takes some low voltage dc (12v for G…..l) and generates high voltage dc, usually about 340 volts. It does that with a high frequency switching supply, the G…..l runs about 40Khz, all fairly typical.
Now that is the problem right there. If your switching power supply is designed to deliver 1Kw of power, that is its maximum. It cannot suddenly supply a peak power of 1.1Kw, or 3Kw or 5Kw for a second or two.

Many loads, in fact most loads have high inrush currents when initially switched on, and a high frequency inverter has absolutely zero short term overload capacity, despite what it claims on the front of the inverter.
So you take your 99,000 watt inverter that has a surge capacity of a gazillion watts and plug in one of your power tools pull the trigger, and the smoke escapes from the inverter. Happens all the time...

Now a transformer inverter has a vastly higher overload capacity. It has multiple huge mosfets to ensure low conduction losses, but that also provide massive short term overload capacity. The large transformer and heatsinks can absorb a lot of flash heat, so the inverter will not complain too much about short term overloads.

Now a tiny high frequency switching power supply will have much smaller parts, and just cannot absorb the same type of short term overloads without going bang. Much more stressed, far more fragile. Just a toy really and best avoided for serious usage.”

Fabian · January 2020

i check the diodes at the input stage and even desolder all the fets and there is still a short circuit existing on the input battery connection.
any ideas guys as to what else could be causing the shorts?

BB. · January 2020

Any large electrolytic capacitors on the DC input? Look for leaking, swollen, many have an "X" on the back side of the can that expands and bursts if over pressure.

Also, there are different voltages you can use to Ohm check... Typically, modern DMM will use a pretty low voltage to measure Ohms. And there is the "Diode Check" which is >1-2 volts source... With low voltage resistance checks, most diode junctions measure high resistance. At diode check typically somewhere around 0.3 to 0.7 volts or so (forward biased junction in diode/transistor/etc.).

To check for shorts... use a low voltage current source (0.1 to 1.0 volts or so) and get 0.1 amps or so flowing through the circuitry. With a DMM set to 200 mV (0.2 volts) full scale, measure the voltage drop (one meter lead on the source, a second meter lead probing points along the current path(s)). Typically, you will see a "steady voltage" (0.2 volts for example on the + side, or near 0.0 volts on the negative side--Depending on your choices), and you will move around the current paths until your find a big drop (or jump) in voltage... That is typically where the current path went to a dead short to return.

This technique almost always works... Finds shorted components, solder bridges, and even Printed Circuit Board internal shorts. And since you are using low voltage and low currents, you don't usually damage and components.

For diodes, the typically fail open, but diodes (and any silicon devices like transistors, FETs, etc.) can fail open or short.

Good luck,
-Bill

706jim · January 2020

clockmanfran said:

Much better are the larger transformer inverters, even the modified sinewave types. Better still are the PWM pure sine wave transformer inverters, but all that "goodness" comes at very great extra cost, size, and weight.

A high frequency inverter takes some low voltage dc (12v for G…..l) and generates high voltage dc, usually about 340 volts. It does that with a high frequency switching supply, the G…..l runs about 40Khz, all fairly typical.
Now that is the problem right there. If your switching power supply is designed to deliver 1Kw of power, that is its maximum. It cannot suddenly supply a peak power of 1.1Kw, or 3Kw or 5Kw for a second or two.

Now a transformer inverter has a vastly higher overload capacity. It has multiple huge mosfets to ensure low conduction losses, but that also provide massive short term overload capacity. The large transformer and heatsinks can absorb a lot of flash heat, so the inverter will not complain too much about short term overloads.

An interesting read. I nearly bought a 2000 watt Xantrex inverter charger to replace my aging Trace DR1524 MSW. Now I'm thinking perhaps I won't bother. The Trace has a heavy transformer, 4400 watt surge and has proven to be bullet proof. And seems to be just fine running my electric fridge. It was interesting to see that the Xantrex transformer 2800 watt inverter was three times the price of the similar capacity solid state unit they had displayed. I asked he salesman how they could still keep selling a $3000 inverter when they had a perfectly good solid state for less than $1000. I never got a very definitive answer.

Photowhit · January 2020

706jim said:

clockmanfran said:

Much better are the larger transformer inverters, even the modified sinewave types. Better still are the PWM pure sine wave transformer inverters, but all that "goodness" comes at very great extra cost, size, and weight.

An interesting read. I nearly bought a 2000 watt Xantrex inverter charger to replace my aging Trace DR1524 MSW. Now I'm thinking perhaps I won't bother. The Trace has a heavy transformer, 4400 watt surge and has proven to be bullet proof.

While I feely admit to having limited knowledge about the limits of High frequency inverters. I'm at the point of callinng my 20+ year old l800 watt Prosine pretty much boom proof. It has been pretty much abussed since I decided to forgo buying properly sized unit for my new home, regularly exceeding it's max output and popping it off maybe 50 times the first year and just resetting and continuing, tht was 7 years ago. It lives outside and works from -l0 - l00 degrees. I'm fine with this high frequency inverter. It's now wired in as backup, just replaced this year.

Inverter repair

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