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Help with flyback smps

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Newbie level 6
Jun 15, 2009
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I am a newbie in smps, I making a smps with input 33VDC, output 12VDC 1A using UC3842. It work but there are some problem:
- The mosfet, and diode D1 overheat. I dont understand what happen. Please explain to me. This is schematic


The flyback transformer use EE25 with air-gap 0.25mm, primary winding is wouned first with 15 turns 2x wire 0.4mm,cover 1/2 length of bobbin, bias windings is wounded next with 8 turns wire 0.3mm, and last secondary windings with 6 turns 2x wire 0.4mm. Bias and secondary windings cover the rest of space bobbin. All windings are wounded as clockwise.

P/s I have a question that if air -gap of transformer is larger, the leakage inductance will be high?
Thanks in advance

The mosfet, and diode D1 overheat.


You are drawing 1A output. The diode D1 is carrying 1A average. This is sufficient to make it too hot to touch, even though it is rated for 2A (your schematic shows it is an FR207).

It would run cooler if its leads could dissipate the heat to a heatsink, or some substantial piece of metal, or wire, etc.


It is important that the mosfet turn fully on and fully off. Its resistance should alternate between very high and very low. That is the only way it can run without dissipating heat. If it spends much time at a midway resistance, then it is wasting power needlessly.

Can you verify that the mosfet is turning fully off and fully on?


For a given transformer there is a suitable range of operating frequencies, and duty cycles, in order to obtain the power you wish. That is, if it is even within possibility for that transformer.

The control IC must find the correct combination, in an effort to produce your target volt level at the load. If it has trouble settling at the right combination, then it may result in the mosfet overheating.

Mosfet can not turn on and off saturation because duty cycle max is about 45%, so Voltage at gate mosfet is 45%*16V (Vcc of UC3842) = 7.2V. Mosfet turn on saturation at 10V

Mosfet can not turn on and off saturation

The gate needs to be raised to a high enough volt level, to drive the mosfet into saturation.

This ought to be possible as long as the UC3842 is getting 18V supply as your schematic indicates.
(Based on the 18V zener diode D5, fed by the 1k safety resistor).

Can you confirm that the UC3842 has 18V supply at all times? And that it never drops?
I ask this because if its current draw ever goes above 15 mA, the supply loses regulation and drops.


Do you have an oscilloscope? If you can observe the waveforms, that will tell you a lot.
Such as whether your primary is saturating. Whether your mosfet is turning full on and full off. Etc.

Unfortunately, I don't have oscilloscope. So it is difficult to design a smps. Actually, I only want to design a smps 12V 1A from 110-220VAC inut to supply for IC 4046 and LM324 working. Beside, smps must stop working at the mains below 90VAC. So I choose UC3842.

Hi vantusaonho can u plz tel me how u found the number of turns for your flyback transformer i have a turns ratio of Np/Ns =17 and Lp = 18.6mH but how do i find number of turns. Input voltage 310V output 18V.

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