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12v 10a smps using forward topology

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emiljose88

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i try to do a 12v 10A smps using forward topology. The ic used is UC3842. instruments. the mosfet used is 8n80 (fairchild). The gate waveform is not correct. it has some spikes,the supply is steady. so the mosfet is getting hot.

Input voltage= 150vac-275Vac
Output voltage=12Vdc
Output current=10A
Frequency(switching)=115KHz
bobbin-etd34
Here i attached the circuit diagram

how can i solve it???????????????
View attachment 12v 10a.pdf
 
Last edited:

Is it a single MOSFET forward? In that case, the MOSFET voltage is going to be 2X the rectified DC voltage (provided the AUX winding has as many turns as the primary winding). So, I don't see how you can have 300V. Did the TI simulation use 110V AC? Are you sure it was 150VAC-275VAC?

To limit the MOSFET voltage, you could use a clamp. But if you have as many turns on the AUX winding as on the primary winding, I don't see how you could limit the voltage to less than 2X DC bus voltage.

Hope this helps.
Tahmid.
 

thanks for your reply. here I post the corrected details. Please verify the new post.
 
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What is the value of the gate resistor (from pin 6 to MOSFET gate)? Which diode (across the gate resistor) is used? What are the values of the clamp resistors and capacitors? You can decrease the resistance and capacitance in the RCD clamp (between drain and +ve DC bus) to reduce drain voltage. However, you need to take care that you don't dissipate too much power in the resistors and capacitors.

In my flyback circuits using UC3845 (similar to UC3842 with some differences) running at around 80kHz, I used 10k 2W resistor, 1nF 1kV capacitor and BYV26E diode for the RCD clamp. For the clamp/slowdown (from drain to ground), I used BYV26E diode, 10k 2W resistor and 10nF capacitor. The voltage is clamped to about DC bus + 50V.

Do you have the same number of turns on the winding discharge winding and the primary winding?

What is the voltage rating of the zener diode connected between VCC (pin 7) of UC3842 and ground. It should be rated at >18V. You can leave this out. The UC3842 has an internal zener for start-up if you limit the current.

Hope this helps.
Tahmid.
 

Thanks.
The gate resistor is 10R/.25W and diode is 4148 (pin 6 to MOSFET gate). The reset winding turns is 65 turns and the primary turns is 45. now i don't use RCD snubber, because i use reset winding. but still the mosfet is over heated (8N80-fairchild). the supply zener is 18V (VCC (pin 7)).
 

Can you post the waveforms at the drain and at the gate of the MOSFET?

How hot does the MOSFET get? And at what load are you testing?
 

My oscilloscope has no serial port to connect to computer,to show the wave forms. Now i use 2 mosfet in parallel. now the mosfet's heat reduced and,it works properly. Is it ok (suggetable)?
 

If the MOSFET doesn't burn, it's okay. Try measuring the temperature or at least getting some indication as to what the temperature could be. If it's normal heating due to conduction, then it's okay.

You can take a photo of the oscilloscope display with a camera.

Hope this helps.
Tahmid.
 
here me attached the gate wavforms (drwn by me)
 

Attachments

  • gate wave forms.bmp
    2 MB · Views: 162

It's not too bad. Initially voltage is lower. Then rises up. Shouldn't be too much of a problem, provided it's all above 8V or so. Are those squiggly lines at the top? Ringing?

How hot does the MOSFET get? Can you draw the drain waveforms?
 

Haere i attached the drain waveforms
 

Attachments

  • DRAIN WAVE FORMS.bmp
    2 MB · Views: 152

The duty cycle is small (about 1/3), so the transistor current is respectively high. Did you verify, that the observed transistor power dissipation actually involves excessive losses? It may be normal operation according to your design.
 

720V peak to peak (drain to source voltage).
duty cycle is 25%-30%
 
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So that's a peak amplitude of 360V. That's fine. So, the problem is natural heating as drain and gate waveforms are fine. The heating is normal and will occur due to the design of the circuit. The way to reduce this would be to use a MOSFET with lower RDS(on) or multiple MOSFETs in parallel. Both methods rely on lowering the RDS(on) to reduce conduction loss to reduce the heat.

Hope this helps.
Tahmid.
 
Can be hardly related to the shown Vds waveform (if it's drawn to scale), I think. What's the DC bus voltage for this measurement?
 

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