building boost converter using flyback transformer

Can anyone help me building boost converter using flyback transformer. I have build one but output voltages are very high i.e. 780V and mosfet gets heated repeatedly. My requirments are

Vin 12v dc
Vo 220v dc

Circuit diagram?
Frank

Hello chuckey
Thanks for your nice reply..
Here is the circuit which i have designed according to some reference circuits in google etc.. I have made this circuit hardware without the use of RCD Snubber.

Input Frequency : 25kHz
VGS (IRF740) : 10V (Approx.)
Vin : 12V DC
Vout (Desired) : 220V DC
Flyback Transformer : (Used the one found in Homage Inverters 220-12V)

Results:

MOSFET (IRF3205) got burnt, I replaced with IRF740 but it gets very heated even running circuit for 2-3 seconds.
Output voltages are more than 780V.
After reading some websites, i came to know this can be an issue of RCD Snubber, i added this to my circuit.

Problem

First of all please do tell me if this is problem due to absence of Snubber or any other fault in circuit?
If the fault is due to snubber, please suggest me proposed values for
C3,C5,Cs,Rs,D4,D5?

Thanks in Advance!
Please help ASAP

At least this problems:
- RCD not working (wrong D4 polarity)
- very slow MOSFET turn-off due to bad gate driver circuit. Should use push-pull transistor buffer.

It's also unclear if the transformer has right winding polarity for flyback operation.

Thanks for correction!
Transformer has the polarity configuration like i found in this google image.

Also one thing i require to design values for these R,C,D? Can you please guide?
Also Would you please elaborate push-pull transistor buffer?

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Would it be a better idea using gate driver IC instead of this transistor configuration? Such as IR2110

Winding polarity is good as shown.
You can use a low-side driver IC like TC426.
RCD snubber dimensioning is described in some application notes and internet guides like this https://www.daycounter.com/Calculators/Snubbers/Snubber-Design-Calculator.phtml

Thanks for your reply FVM. I have visited above link. Four things are required for RCD calculation. I want to ask following questions related to my case.

What would be Ipk in my case, as still i'm only testing the voltage, means no-load condition, but my transformer winding (i.e. charging transformer extracted from smps inverter) is capable of providing 10Amp at 12vDC and vice versa at 220VDC. And I'm using it in reverse condition i.e. boost stage. So would my Ipk be 10Amp?? or Its something else??

What is the Rise time asked in above RCD Calculation? Of MOSFET or something else?

Vrail = 12Volts?

Frequency is 25kHz as I stated.

Can I practically use the values designed from above RCD calculator.