Help on Inverter problem

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Ogu Reginald

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I have 2kVA inverter that I just built. During testing I just connected a 100watt bulb and it was working for about 10minutes but when i connected a 150watt fan to it , the bank of MOSFET got burnt.
Since then I have replaced the MOSFETs and connected no load to the Inverter output but at switch ON the MOSFETs still got burnt.
At no load should the inverter draw current from the battery to the extent that it can burn the MOSFETs?
Please can someone help me and tell me the possible cause of the problem.
 

At zero load, the inverter will draw no or negligible current. Atleast the mosfets will not burn...
Did you added proper heat sink with fan or other cooling system??? Can you upload any picture or schematic of your inverter???
 

Genovator said:
At zero load, the inverter will draw no or negligible current. Atleast the mosfets will not burn...
Did you added proper heat sink with fan or other cooling system??? Can you upload any picture or schematic of your inverter???
Click to expand...

I am very sorry because i didn't take a picture of the MOSFETs before i dismantled them. I think you have a point.
1. I didn't add any fan.
2. The part of MOSFETs that are burning are the ones with small heat sink.

But at no load should the MOSFETs dissipate much heat?

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pradeep.g.belchada said:
their is a problem in pwm driver section
Click to expand...

I didn't use any driver circuit as my inverter is operating on 5OHz. I just connected the two output of SG3524 to the parallel section of MOSFETs.
 


The problem you mentioned might be in oscillator circuit(drive circuits),if you have oscilloscope ,mesure the oscillator output and it should be approxmately 10v pp at both inputs of switching mosfet.If it's ok then the fault lies in ewitching circuit.
 


I have measured the frequency and voltage. The frequency is 50Hz while the voltage is 10V. Also I connected it directly to the MOSFETs without any driver circuit.
 

Since you have already dismantled it, there is no help of asking you to take measurements. Otherwise you could have taken measurement of current draw by the battery. You said mosfets were attached with smaller heatsink and you successfully used a 100watt bulb, then maybe there are some unnecessary current drawn by the battery. Or you may haven't connected enough mosfets for that much output???
Can you tell us what transistor you were using, and how many in series???
 

am a nigerian like you too,i also had the same problem then.first question;are you using a 12volts battery?if so,your output at d oscillator should not be 10volts.(2) are you driving your mosfets directly from your pwm ic i.e sg3524?(3) is it a square wave inverter
 

SG3524 can't drive mosfets directly as it has only opn collector or open emitter output. SG3525 has that totem pole output.

https://www.edaboard.com/threads/289492/

See this page,

https://tahmidmc.blogspot.com/2012/12/low-side-mosfet-drive-circuits-and_23.html
 

I am using IRFP150 MOSFET, and I am using four in parallel. So I am using a total of eight MOSFET.

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I am using the SG35524 ic and I connected the pins 12 and 13 to VCC through pull up resistors of 10 Ohm and I took my output from pins 11 and 14.
Between the gate of the MOSFETs and pins 11 and 14, I also connected 27 Ohms resistors and between the gate and the source I connected resistors of 2k2 Ohm.
 

Delay to prevent common mode conduction and soft start feature is very useful in high power apps. Show your oscillator and driver section diagram.
 

In my experience and from everything I've read, MOSFET gates must be driven with a totem pole that forces the gate on AND off. This is usually setup using 1 PNP and 1 NPN transistor. This image shows a very simplified totem pole used to drive MOSFETs or IGBTs. I don't usually build this circuit from discrete components and instead use gate-driver ICs such as the IR2110, ect.
 

The diagram is tiny and I cannot read the component values, but from what I see you are only relying on pull-down resistors to turn off your MOSFETs. This will not shut the MOSFETs off fast enough and sometimes will cause them to sporadically turn on when they are supposed to be off(when the other set of FETs turn on the pull-down resistors can't keep the gates pulled to ground). I've tried, mistakenly when I didn't know, to build a push-pull switching stage using only pull-down resistors to turn off the FETs. I've tried values as low as 10Ohms and they still did not turn off fast enough and the resistors didn't effectively hold the FETs in the Off-State properly either. To ensure they FETs switch properly and stay off when they're supposed to be off you must use either a driver IC or a transistor totem-pole to drive their gates. Once I learned this and inserted these totem-poles into my circuit, I no longer had any switching issues with my FETs.
 

You have not shown regulatd supply for SG524. Drive it with 12V regulator such as 7812. At low frequency such as 50Hz, resistors between gate and source can work as you do. If 24V is used for IC supply rather than using 12V, it will cause damage.
This is a similar circuit.




 

It's not so much the frequency, its the dead-time between the first set of FETs switching off and the second set switching on that is the problem. Using ONLY G-S resistors to turn your FETs off requires much more dead-time than driving them properly with a totem-pole, driver IC, or even a Gate-Drive-Transformer, and in my experience using only G-S resistors just won't hold the FETs off under high di/dt or high dv/dt through the FETs. I mostly work with SMPSs both low and high power and I have never been able to get one to work long-term when allowing the gates to be pulled-down with resistors.

This is how I drive a Push-Pull configuration. You might need larger transistors, that all depends on the MOSFETs you're using and the total gate-charge of all of them in parallel.

 
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I agree with you. it is not frequency problem because I have built an oscillator based on CMOS IC and the problem still continued. Your diagram is not clear< I don not know whether what I am seeing is a totem pole in the diagram. If it is what type of transistors should I use? Should I use power transistors like tip41 and tip42 or BC547 and BC558.

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I agree with you. it is not frequency problem because I have built an oscillator based on CMOS IC and the problem still continued. Your diagram is not clear< I don not know whether what I am seeing is a totem pole in the diagram. If it is what type of transistors should I use? Should I use power transistors like tip41 and tip42 or BC547 and BC558.
 

Totem ple is better as I earlier recomened. It becomes essntial when frequency rises. I manufacture UPSes, I used to use totem pole driver circuit. Then I decided to give this resistor network a try just to save a few transisors. Mostly I supplid to people nearby with 1 year warrenty. I know some instances wthere they are in use for more than six years. There was not a single instant where it had FETs damage problem. Thre are other reasons influencing more.For example, uneven current sharing due to improper wiing, shoot though ue to ineducate dead band, instabily due to vulnerable circuit layout. voltages and current not within optimum parameters leaving safe margin, etc.
 

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