Please check your Transformer Polarity.Dear All,
I designed a flyback converter with folllowing specification: input voltage of 230V and output 20V, 3A.
For testing the circuit I am giving input through a variac. However, even at 20V Ac input the diode rectifier (using 1n4007) is getting damaged. There is also heating in the current sense resistor. But at this low voltage the circuit should not even have started right. The output load that I have connected is 50 ohms. The controller used is UC 3842. I have simulated it in PSIM and the system is working fine. I then removed, input CM choke, NTC and Capacitor however, there is still no effect. What could be the issue and how to resolve it. Thanks
Thanks for the reply. This means that i should quickly go to 230W incase of 60W load. However, my doubt is When input is 20Vac rms, the device should not turn on , then how is the diode getting damaged. What I am still not able to understand is that the even if I remove the IC and power on the circuit, the diode is still getting hot. Ideally when IC is not there, there should not be any path for the current flow.60W converter - the power factor on the input will be about 0.55 thus the input current, rms = 500mA at full power
for 230Vac input, at 115 Vac in I = 1A for same power
at 40Vac in ( assuming the converter can still work at full power here ) Iin = 2.9 amps rms ....
you see the issue here? also the peaks will be about 4-5 times the rms, so 500mA rms invites 2.5A pk ...
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also check your build carefully - and the polarity of the transformer winding ( phasing - dot notation )
The diode is stil getting damaged when the IC is removed from the circuit and when I give 20 Volts AC.50ohm load resistor is much less. IN4007 diode is 1 amp . you may have to use at least 2-3 amp rectifier diode.
2nd check the output filter capacitor, put a load resistor of 330 ohm/1 watt, check polarity of xformer
& also check power FET. if still the problem not solved then remove the ic from circuit then check the diode.
I have replaced 1n4007 with UF4007 for both aux diode and snubber diode. The turns ratio is 52(input): 8(output):5 (aux). At present I have only UC 3842 with me, so will have to resolve the issue with it. I will try with supplying vcc of ic separatelyThe startup hysteresis is very low on UC3842...you may need a much bigger C6......and c10 reduce to 100nf.
UC3842 takes loads of bias current, so will drain down c6 before your bias coil takes over...try a UCC38C42 instead...its pin-for-pin, and takes much less bias current.
Change D4 to a super fast diode , or at least put say 10R in series with it.
Your schem doesnt fully show extent of vout net.
Can you divulge your turns ratios...?..is your aux diode getting overvoltaged...its schottky and not snubbed...beware killing it with OV spikes.
Have you used fast sec diode? (<75ns)
Why not try getting it working with a isolated lab psu feeding vcc first.
C6 i have increased to 100uF. However, there is not much effect.The startup hysteresis is very low on UC3842...you may need a much bigger C6......and c10 reduce to 100nf.
UC3842 takes loads of bias current, so will drain down c6 before your bias coil takes over...try a UCC38C42 instead...its pin-for-pin, and takes much less bias current.
Change D4 to a super fast diode , or at least put say 10R in series with it.
Your schem doesnt fully show extent of vout net.
Can you divulge your turns ratios...?..is your aux diode getting overvoltaged...its schottky and not snubbed...beware killing it with OV spikes.
Have you used fast sec diode? (<75ns)
Why not try getting it working with a isolated lab psu feeding vcc first.
The voltage across Vcc is postive as seen in the Figure, therefore the polarity should be correct. However, the IC may be damaged when the gate resistor got burnt during earlier testing. Will have to check with updated transformer with turns ratio of 60:9:8(aux) and also perhaps replace the IC.put a zener on pin 7 - likely you have now killed the chip - the electro on pin 7 will be a bit suspect now too ...
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make sure the phasing (dot notation ) of the transformer is absolutely correct.
The core I have used is ETD 29.Yes, as Easy Peasy implies...when a mains flyback fet fails...the drain and gate can get shorted together with low Z, so the high voltage on the drain can rip through the controller circuitry, via the gate of the fet.
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by the way, can you post gerbers.
Attached is an smps pcb layout doc too.
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Here is ETD25 datasheet..
https://www.tdk-electronics.tdk.com/inf/80/db/fer/efd_25_13_9.pdf
…You say you have a gap of 0.25mm…..that is not a standard gap size….how did you gap these?
With 52 turns and a gap size of 0.22mm, and N87 ferrite, your i(sat) would be 1.04A.
What is your primary sense resistor…it should be more than 1/1.04 = 0.96R
Are you sure your CS pin cap is correct value…is it 100p?.....you have it as “0.1n”, and maybe mistook and put a 100n cap there instead?
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