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Push Pull Converter MOSFETS Overheat

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azadfalah

azadfalah

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Hi friends,

In a simple DC to DC converter Push Pull -> No inductor at the output

Frequency = 60KHZ
Core = ETD39
Primary Turns = 2 ~ 10.9uh
Secondary Turns = 58
Power switches = IRF2807 75V 83A
12V to 348V

At a load of about 100 watts MOSFETS overheat

PP.PNG




I do not know why the current waveform in these MOSFETs drain is not a ramp
WhatsApp Image 2021-06-28 at 13.13.58.jpeg





Drain To Ground Voltage ( at load of about 100 watts )
WhatsApp Image 2021-06-28 at 17.17.37.jpeg




Something unusual is seen in the waveforms ?


Thanks,
 
Last edited:

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Hi,

a lot of information missing:
* how is the DC power supply bypassed?
* how is A and B driven
* how is the internal circuit of the transformer (we just see pin numbers, which are not needed in a schematic)
* what is the VBatt voltage?
* what is the current scale in the scope picture
* where and how did you measure the current?

Klaus
 
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KlausST said:
Hi,

a lot of information missing:
* how is the DC power supply bypassed?
* how is A and B driven
* how is the internal circuit of the transformer (we just see pin numbers, which are not needed in a schematic)
* what is the VBatt voltage?
* what is the current scale in the scope picture
* where and how did you measure the current?

Klaus
Click to expand...

Hi

Driver is SG3525
transformer Primary = 2 + 2Turn
transformer Secondary Turns = 58 (AC0 AC1)
transformer AUX = 2Turns
VBatt = 12V
I add 0.05R Resistor in MOSFET Source and measure the current
 

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Why no output inductor?

Tell us a story about heat sink and airflow. 100W output on
a generic push-pull probably means at least 10W of waste
heat.
 
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Your power supply is crapping out ( current limit ) because the transformer is saturating - most likely because there are not enough turns to limit Bpk to +/-200mT - or you have a massive gap in the core, or a very low Ur core ....
--- Updated ---

just by the way - 1V in your scope shot is 20A for 0.05 ohm ....
 
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dick_freebird said:
Why no output inductor?

Tell us a story about heat sink and airflow. 100W output on
a generic push-pull probably means at least 10W of waste
heat.
Click to expand...
In this project, there was no need to keep the output voltage constant at a certain value and the voltage change in the expected range is acceptable

Is the MOSFET Drain Source current waveform acceptable?
Is Drain Voltage Spike Acceptable?


Easy peasy said:
Your power supply is crapping out ( current limit ) because the transformer is saturating - most likely because there are not enough turns to limit Bpk to +/-200mT - or you have a massive gap in the core, or a very low Ur core ....
--- Updated ---

just by the way - 1V in your scope shot is 20A for 0.05 ohm ....
Click to expand...
The drain current value is normal , RMS Voltage of Drain Shunt Resistor is 0.484V so
RMS Current = 9.68A and the load is ~ 100Watt So -> the current is normal and there is no saturation do you agree ?



mtwieg said:
Why would you expect the current waveform to be a ramp if you're not using an output inductor?
Click to expand...
There is a diode bridge and a capacitor at the output
With this in mind, do you think it is wrong to expect a ramp waveform?
Most sources showed the waveform as a ramp, and this made me doubt it
Do you think the current waveform is normal?
 

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just by the way - 1V in your scope shot is 20A for 0.05 ohm ....
Click to expand...
The drain current value is normal , RMS Voltage of Drain Shunt Resistor is 0.484V so
RMS Current = 9.68A and the load is ~ 100Watt So -> the current is normal and there is no saturation do you agree ?

No - it is more likely your power source cannot supply enough current for what you are asking it to do - further discussion is pointless without knowledge of the load, also are you trying to deny the peak current is ~ 20A ... ? Show a simultaneous scope shot of the supply voltage ...

are your mosfets on a heatsink ?
--- Updated ---

Also - what capacitors are on the pri side ? from CT to gnd
 
Last edited:

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azadfalah said:
There is a diode bridge and a capacitor at the output
With this in mind, do you think it is wrong to expect a ramp waveform?
Click to expand...
Without an inductor, you are basically short circuiting the input and output sides when either FET is on. The current waveform will be defined by the source impedance, transformer leakage inductance, rectifier diode, and load impedance, but those are all usually very low impedances. It could be that some parasitic inductance is resonating with your input/output capacitors, giving the rounded shape you see.
azadfalah said:
Do you think the current waveform is normal?
Click to expand...
No, but a push-pull converter without a filter inductor is abnormal in itself, in my opinion.
 

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Also you have nothing to dissipate the leakage inductance power.
But yes, as above, further discussion pointless until Easy Peasys question above is answered
 
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