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voltage drop in inverter dc bus

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amirtaha2541

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what whenever i attach a load to my inverter the dc bus voltage drops and in load=400watt input mosfets burns?
inverter is modified sinewave
 

Tahmid

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The high current that flows as a load is attached may cause the dc bus voltage to drop, depending on the battery, input capacitance, etc.
The MOSFET burning is another thing. The inrush may damage it, but MOSFETs usually have a high surge current capacity. Maybe the MOSFETs aren't capable of taking the amount of current going through, or heatsinking is insufficient and the MOSFETs overheat and burn.

Hope this helps.
Tahmid.

---------- Post added at 18:18 ---------- Previous post was at 18:16 ----------

There could also be other reasons. Is this the first time the MOSFETs burnt? Did you use this inverter successfully at a lower power level? Is it the first time you're using this inverter?
 

amirtaha2541

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The high current that flows as a load is attached may cause the dc bus voltage to drop, depending on the battery, input capacitance, etc.
The MOSFET burning is another thing. The inrush may damage it, but MOSFETs usually have a high surge current capacity. Maybe the MOSFETs aren't capable of taking the amount of current going through, or heatsinking is insufficient and the MOSFETs overheat and burn.

Hope this helps.
Tahmid.

---------- Post added at 18:18 ---------- Previous post was at 18:16 ----------

There could also be other reasons. Is this the first time the MOSFETs burnt? Did you use this inverter successfully at a lower power level? Is it the first time you're using this inverter?

Thanks but The main problem is dc bus voltage drop :
dc bus voltage( no load)=333 volt & input voltage=13 volt
dc bus voltage( load=40 watt)=295 volt
dc bus voltage( load=145 watt)=277 volt
dc bus voltage( load=245 watt)=254 volt
dc bus voltage( load=345 watt)=223 volt
dc bus voltage( load=400 watt)=205 volt. in power 400 watt after a few seconds input mosfets burns
 

Tahmid

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In this case, I guess, the problem is in the feedback part.
From the figures you posted, you can see that as load is increased, the DC bus voltage drops. This means that the PWM on the primary side is not adjusting pulse width properly for proper feedback.
Do you have an oscilloscope at hand?
Can you show the waveforms at the gate of the MOSFETs at 40W, 145W, etc? This would help identify if the problem is in this section.
Is it an inverter you made? Did you wind the transformer? What are the transformer turns? You may have insufficient secondary turns.

Is there any feedback there at all? Do you have a schematic? Can you identify the PWM controller? Is it SG3525, SG3524, TL494, LM5030 or some other controller?

---------- Post added at 01:55 ---------- Previous post was at 01:55 ----------

It is possible that some part in the feedback section is damaged and thus feedback isn't working properly.
 

amirtaha2541

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My problem Not resolved
MY PWM controller is SG3525
Feedback is working properly in no load
My problem is that How do I control sg3525 for "heavy load" and "light load"?

In this case, I guess, the problem is in the feedback part.
From the figures you posted, you can see that as load is increased, the DC bus voltage drops. This means that the PWM on the primary side is not adjusting pulse width properly for proper feedback.
Do you have an oscilloscope at hand?
Can you show the waveforms at the gate of the MOSFETs at 40W, 145W, etc? This would help identify if the problem is in this section.
Is it an inverter you made? Did you wind the transformer? What are the transformer turns? You may have insufficient secondary turns.

Is there any feedback there at all? Do you have a schematic? Can you identify the PWM controller? Is it SG3525, SG3524, TL494, LM5030 or some other controller?

---------- Post added at 01:55 ---------- Previous post was at 01:55 ----------

It is possible that some part in the feedback section is damaged and thus feedback isn't working properly.
 

Tahmid

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If feedback is working properly, then you don't need to individually compensate for low or high load (provided there is a minimum load present).

One problem I can think of is insufficient transformer secondary windings. It would be great if you can provide gate waveforms.
If you don't then use your voltmeter. At the output of SG3525 (pin 11 or 14), measure the voltage using a DC voltmeter. Measure the voltage at different loads starting from low load to a high load, making sure you don't burn the MOSFETs. This would be a good starting point.

Hope this helps.
Tahmid.
 

tpetar

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One question for amirtaha2541.

Inverter is Home DIY or from market ? If DIY provide circuit to see maybe is there design error.

Did you check MOSfet's are original somehow ? (I mean not fake)

Did you put MOSfet's on proper heatsink ? Maybe dont have cooling/transfer to heatsink, check cooling grease.

I supose that MOSFet's are proper power for that job.

Voltage drop can be bad battery, but that is not reason for malfunctioning of MOSfet's.
 
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dick_freebird

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The more the input voltage sags, the more the controller tries
to make it up with input current. At some point of low-line that
will overstress the high side FET against its current rating (or
thermally, against the finite heat-throw of your thermal path).

Input undervoltage lockout is meant to protect the components
but if your input is not up to the job, you'll just motorboat in
and out of UVLO.

If this is at turnon, maybe you have a slug-the-line problem
and want more attention to soft-start.
 

Tipu606

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without posting the full circuit diagram, we can only make intelligent guesses
as to what can be the problem and solution
 
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dick_freebird

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It's worth noting that the "400W" FET is only 400W up to
some case temperature, and is subject to the thermal
performance of your heatsink and fan.

You should do the math on input current at low line /
high load, with the FET max RDS(on) or better yet the
measured Vds(on), and see your FET dissipated power
@ duty cycle.

You should also be sure that at that corner, duty remains
low enough to reset the core (100% voltage duty on an
inductor being universally a bad idea).
 

joelyoung

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Hello Tahmid I have tried to catch up with you for weeks now but I guess u are busy . I have a problem of my 170 v dc falling offf with a load of about 9 watts in my inverter circuit. I have few questions. Without feedback applied to the dc to dc circuit about how much more should your dc volt go up to. Without feed back my volts go up to 185v is that enough or it should be more, What components in the feedback loop can i adjust to get better stability in my voltage. I am using Sg3525 as pwm Thank you
 

joelyoung

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hello Tamid thank you for your reply I got the problem sorted out I changed the output fets in the dc to dc stage I was using irfp260n but I changed this to 75n75 this worked much better . the other problem I am having is the 60hz frequency is not stable in the ac output it floats from like 56 58 59 61Hz and back around a gain not stable but the volts is stable. When I use the dc 170 from a working inverter the frequency is stable. i am using ka3525 as pwm in the dc to dc circuit and tl494 as 60Hz oscilator. Please give me suggestions thank you I await your reply
 

joelyoung

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thank you for your reply here is the circuit I only use the 60 hz oscilator section of this circuit that uses tl494 to drive the h bridge output dont know if anything is missing from the schematic. I await your reply. Thank you
 

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joelyoung

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hello Tahmid I tried another inverter design and this time my 60Hz was stable. It uses 556 as oscilator and 4013 to drive the H bridge. My question is can these two ics be used in higher wattage inverters like 2000watt because I always see they use ics like Ka3525 and tl494 in high wattage inverters and the 4013 and 556 in small wattage inverters . Is there a disadvantage. Thanks for your support
 

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