Need help to understand flyback circuit Part 2

[Nora]

Hello-
I built a simple switching power supply based on a number of different circuits I found online. I do not know what parameters to look for in sizing the MOSFET and the capacitor on the HiV output.

This circuit partially works. I put 5VDC into the circuit and get as much as 240VAC out -almost a sawtooth wave- measuring at TP1 and disconnecting C2 from the circuit.

Leaving the circuit as shown and measuring across either C2 or R5 has less than 1 volt.

What am I doing wrong? I want a high DC voltage on the output.

Also- R2 gets reeeeeally hot. Q1 does not. I put an alligator clip on R2 for now. But is this much heat normal? Should I get a 1W resistor?
How do I know how much current if flowing through the MOSFET? BTW the control wave is nice clean square. The wave going into T1 not so much square as spiky.
Thanks!
N_N

 

[keith1200rs]

Do you mean R3 or R4 get hot, not R2? What is the duty cycle of the drive waveform?

Normally you wouldn't try to limit the current into the inductor with a resistor. It would be limited by the transistor switching off. The time taken for the current to reach some limiting value will depend on the inductance and the supply voltage.

Keith

 

[Nora]

Do you mean R3 or R4 get hot, not R2? What is the duty cycle of the drive waveform?

Sorry I mean R4 gets really hot. The switching frequency is designed (with R2 and R1) for 100kHz (my very old scope measures 88kHz).

Normally you wouldn't try to limit the current into the inductor with a resistor. It would be limited by the transistor switching off.

OK. Am going to try a different MOSFET IRF510, maybe TO220 packaging is better than TO92?

The time taken for the current to reach some limiting value will depend on the inductance and the supply voltage.

Inductance of T1 is 25uH and supply is 5VDC.

 

[keith1200rs]

You might want to look at a faster diode as well - the 1N4004 is very slow.

Simulation is useful - saves cost on components while you blow things up experimenting.

I think the main problem with your original idea was the diode - try changing it for a fast diode such as a Schottky and you should see some results. Removing the current limiting resistor will also make a big difference but the current may build up too quickly for the small transistor you are using and blow up the transistor.

Keith.

 

[karesz]

Hi Nora,
Not a wonder if your unneeded R4 well be too hot; you have ca 1.5W (OK, pulsed, but as middle value is ca at 0.5W_ or more) on a half Watt component :-(...
K.