Why would MOSFETs heat up in an inverter while charging with the same inverter

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sam1145

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Heating up mosfets in an inverter while charging with the same inverter
am trying to build an inverter of 3.5kva,so far it is working but not up satisfaction because my transformer first of all was heating up seriously when charging then i replaced it with a higher rating next the mosfets started heating also beyond normal, now i am stock i need help don't know what to do next,.am using irf150[2x6] pieces and sg3524 for the oscillation...
24vdc
50hz
modified sine wave
about 3.5kva load
220vac
about 15amps

so that 220vac is as both input for charging and output from inverter
output with a curent of about 20amps
2x12v battery of 200ah] to give 24v 200ah
 

Hi Sam
You're simply talking about inverters and a problem without showing any schematic or talking about topology of them . how you expect toy obtain a good answer ?
Best Wishes
Goldsmith
 

SG3524 should be in off position while charging. In charging mode body diodes are used for rectification. Gates shroud be in off state. A voltage drop on diodes(0.7V) in contrast to when FETs are hard on is a cause of a little heating of FETs while charging. Check your charging current for max 20A.
 

thanks this is realy helpful but please are you saying i have to add component to do the switching of the sg3524 and the gate of the mosfet?

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thanks this is realy helpful but please are you saying i have to add component to do the switching of the sg3524 and the gate of the mosfet?
 


Were the MOSFETs also heating up excessively before the transformer was replaced?
 
no are not heating up
 

    T

    technetic

    Points: 2
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So, the overheating of the MOSFETs during charging has occurred with the new transformer and not the old one?

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If the overheating of the MOSFETs during charging has occurred with the new transformer and not the old one, it indicates that the overheating issue has been introduced by the new transformer. So, one potential problem is transformer winding method that contributes to excessive spikes due to too high leakage inductance.
 

but the the former transformer fried out while charging suggesting the inadequacy the reason why i change it
 
Check the outputs of the driver stage (to MOSFET gate) to ensure that no gate drive signals are generated.
It is best to observe the signal on an oscilloscope. You can also use a voltmeter to see if the outputs show a steady 0V.
 

You can check by
1) Removing connections from ic to gates of FETs.
2) Shorten gate with source through a resistor.
3) Charge the battery and observe the difference.
If problem is cured the cause is naughty gate drive.If not check if charging current is within maximum 20A.
Check if all FEts are heating equally or some single is heating more. It could be a damaged FET.
If inverter is delivering full load without overheating then it may be wrong drive to FETs or over voltage transformer causing extra charge current and heating. you have to check charging current and open loop transformer secondary voltages'
If you can post a circuit diagram of your UPS, it maybe helpful to locate the problem.
While charging, pin # 15 of SG3524 must be disconnected from +ve power supply. PIn 10 is shutdown pin but its use is more tricky.
 

these are the schematics

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what amount of heat in Celsius is to be expected from the heat sink
 

The 3rd circuit diagram shows that the SG3524 is always powered on (pin 15 directly connected to battery through current-limiting resistor). But pin 10 (shutdown) is left unused. So, that means that the SG3524 is not shut down through this pin, even when electricity is present. This is possibly the reason the MOSFETs get too hot. During charging, check pins A and B and see if there's any signal or any voltage present, either stable or fluctuating.
 

sam1145 said:
Heating up mosfets in an inverter while charging with the same inverter
am trying to build an inverter of 3.5kva,
24vdc
50hz
modified sine wave
about 3.5kva load
Click to expand...
It is not advisable to make 3.5Kva inverter using 24vDC , you are dealing with almost 146 A, which is too high. Battery voltage should be at least 60Vdc.
sam1145 said:
my transformer first of all was heating up seriously when charging
Click to expand...
Because you are using a transformer of 146A capacity if the charging is not controlled using some circuit it will surely heat up. please post the charging circuit.
regards ani
 
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Dear sam1145,

I have also faced this kind of situation.In inverter mode it was working fine but in charging mode both channels start heating

(sometimes it burnt).I have checked sg3524 osc. was off but there was about 0.3v present at the gate of FET's. Changing

the driver transistor solved my problem (it was a leakage current ). In your case replacing Q13 & Q14 may solve the problem.

Thanks
 
Last edited:

One more thing is that the outputs of the SG3524 should be pulled down. Either connect pull-down resistors from pins 11 and 14 to ground or connect pull-down resistors from bases of Q13 and Q14 to ground.
 

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