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Smps Mosfet 2N65 heating up

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My smps providing three voltages 12v, 4.2v and 5v. However transformer used here only providing two voltages 12&4, i m using 7805 to get 5v. My Mosfet 2N65 which I am using for switching heating even at no load. And on load it's getting too hot. What are the causes that MoSfet heating. Please help me troubleshoot this problem.
 

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ronsimpson

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Where did you get this design?
What data do you have on the transformer you used "12&4", and the transformer " 12v, 4.2v and 5v"?
 
Bought it from a vendor. Transformer providing two voltages 12v and 4v. I am getting all voltages properly only problem is mosfet heating.
 

dknguyen

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What is the pinout of the transformer? Right now, it's a black box with 8 pins coming out of it which is not helpful.

Can you scope the Gate terminal of 2N65 relative to ground? I suspect it's not hard switching but the circuit is difficult to follow so I'm not sure.

Also, how how is hot? Did bother heatsinking the the 2N65?
 
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ronsimpson

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I was hoping that you copied this from some where so we can see it before you drew it.
I found an error. I think there are more.
1550173652070.png
 
so you saying error between R4 and c3. i bought this from a vendor we didn't provided me the circuit diagram. so i draw it with the help of multimeter and looking the track on pcb.
 
What is the pinout of the transformer? Right now, it's a black box with 8 pins coming out of it which is not helpful.

Can you scope the Gate terminal of 2N65 relative to ground? I suspect it's not hard switching but the circuit is difficult to follow so I'm not sure.

Also, how how is hot? Did bother heatsinking the the 2N65?
transformer pin 6: 12v, pin 7: 4v, pin 8&9 : grounded.
not measure the actual temp, but it so hot u cant touch it (40-50 c). heat sink added, but dont you think heating will reduce its life.
 

dknguyen

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transformer pin 6: 12v, pin 7: 4v, pin 8&9 : grounded.
not measure the actual temp, but it so hot u cant touch it (40-50 c). heat sink added, but dont you think heating will reduce its life.
That's not what I'm asking for. I'm asking you to tell us what pins go to what windings inside the transformer.
 

dr pepper

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Does the Ps have a minimum load rating.
If so and its underloaded the fet will receive very narrow pulses, it and the rectifiers might not like that.
I see R12 is a load of some sort.
 
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