schootky diode vs ultrafast one

[Snub]

every suggest is appreciated..

 

[FvM]

Circuit as expected. The diode will be most likely never forward biased.

But there's a good chance that MOSFET and/or schottky diode are driven into avalanche breakdown during flyback period.

 

[Snub]

please be more precise explaining me this phrase: " But there's a good chance that MOSFET and/or schottky diode are driven into avalanche breakdown during flyback period ". thanks..

 

[FlapJack]

Snub, back in post 11 i told you to check your gate drive. Now that you have posted a schematic it all but confirms you have a gate drive problem.

First, most people start with 15vdc on controller chip so they have enough voltage to drive the FET gate. After the internal drops of the controller and the gate resistor a 12vdc input is considered low.

But even if you change it to 15vdc you still have a 100 ohm gate drive resistor limiting your turn on speed. If you would have put an oscilloscope on the gate you would have seen a slow turn on.

At the very least do this. The TL494 has 200 mA drive. 12vdc / .2A = 60 ohms, compared to your 100 ohms this should give you a noticeable improvement.

You can also try some bulk capacitance at the TL494 power and ground pins. Start with a .1uF and parallel in a .22 or .33 and look at your gate drive for improvements.

Many times people will not even use the TL494 output to drive a FET because of its low drive capability, they will add a totem pole driver to get a real power pulse of .5 to 1A or more.

 

[FvM]

please be more precise explaining me this phrase: " But there's a good chance that MOSFET and/or schottky diode are driven into avalanche breakdown during flyback period ".

1. Know that irf3205 has a minimum Vds breakdown voltage of 55V, typical Vds,br may be 70 or 80 V.
2. Understand working principle of a flyback converter.
3. Estimate expectable primary peak voltage in flyback freewheeling phase

TV line stages with low voltage supply have transistors with at least several 100 V voltage strength. In addition, they are clamping the freewheeling voltage on the primary side, which doesn't happen in your circuit (except for unintentional transistor and diode breakdown).

 

[FlapJack]

Yikes,

Snub, i did a search for avalanche and got lucky on the first result.

www.vishay.com/docs/90160/an1005.pdf

Good explanation and it uses a flyback for an example.