Flyback converter problems (UC3845 + MOSFET) - diyAudio
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Old 4th July 2011, 05:12 PM   #1
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Default Flyback converter problems (UC3845 + MOSFET)

Hi all,
I'm trying to make an offline flyback converter. Since I can't get readily wound transformers, I need to wind my one. I got a few EE25 cores and bobbins and am aiming to make a 14V 2A flyback converter. I've been reading a lot on flyback and I think I got the idea. Last few times I tried (about a few months back), there was output for a few seconds and then BANG!!!

So I've been reading on flyback. I carried out calculations and stuff and arrived at this:
Primary: 147 turns
Secondary: 9 turns
Bias: 10 turns
I used thick paper and glue to get a gap of about 1mm (measured with slide caliper) between all three legs.

Before anything blew up again, I set up a circuit with UC3845 running at around 90kHz at 48% (max) duty cycle and pulsed an IRF3205 off a 12v line. The IRF3205 drove the secondary of the transformer and I set up a diode, cap and a load resistor on the bias just to check the drain waveform. The IRF3205 got EXTREMELY HOT and burnt. So, I placed an IRF840, which also gets EXTREMELY HOT, but didn't burn out.

This is the drain waveform with no snubber:
Click the image to open in full size.
Click the image to open in full size.

This is the drain waveform with an RCD snubber (diode - BYV26E, cap - 4.7u 100v, res - 4.7R 2W):
Click the image to open in full size.

I'm doubting one of two things here:
1) It is due to poor winding or any such thing relevant which caused excessive leakage inductance.
2) My snubber values are completely wrong.

Can someone help identify the problem? Are the transformer calculations and airgap right?
I think if I make my offline PSU, the MOSFET will go kaboom! So, what can I do?

By the way, I didn't measure the secondary and bias windings, but the primary winding (147 turns) was about 1.47mH. Without the air gap, it was about 16mH.

Thanks in advance.
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Old 5th July 2011, 08:13 AM   #2
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Help, anyone?
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Old 5th July 2011, 09:27 AM   #3
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Taswar View Post
Help, anyone?
How could someone help you if you don't provide even the most basic information: schematic, input voltage, etc..?
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Old 6th July 2011, 04:45 PM   #4
rajudp is offline rajudp  India
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Location: Kerala
what is the calculated inductance of the primary and how you got the gap as 1mm
which core materiel you are using ie n27, n87
what i your input specification , input volt min and max , frequency, dutycycle etc
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Old 6th July 2011, 09:55 PM   #5
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I have no idea of the core material.

As I already stated before:
Quote:
I set up a circuit with UC3845 running at around 90kHz at 48% (max) duty cycle and pulsed an IRF3205 off a 12v line.
Input voltage is 12v. Duty cycle is 48%, frequency is 90kHz. The primary (147 turns) has an inductance of 1.4mH. But I didn't use it, instead I used the secondary and bias for testing.
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Old 7th July 2011, 11:21 PM   #6
hesener is offline hesener  Germany
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Location: Munich, Germany
Basically, in your first experiment you were forcing a 55V mosfet into repetitive avalanche, no wonder it exploded at some point..... the 500V mosfet didnt go into avalanche, but since the secondary inductance is significantly smaller (0.36mH if I am not mistaken) the peak current you reached was much higher than it would have been with the primary inductance (that would have been a safer experiment by the way, you had very high voltage peaks on that primary....). I calculated a peak of 0.2A, quite a bit for such a small core but without the material we can only speculate what happened....
And, your snubber values look way off. With a primary of 1.5mH, the leakage should be around 40uH, in which case the snubber should be in the range of 2-3nF and 68K....

maybe you should try the transformer again with a proper snubber, and something like 40V applied to the primary winding, just to see if you get something useful at the output. The transistors should definitely not get hot at these levels...

just my two cents...
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Old 7th July 2011, 11:45 PM   #7
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What is your secondary load? A rectifier, filter cap+dummy load for 2A@14VDC?
I agree your snubber cap is way huge at 4.7uF try a few nF.
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