Gate driver Isolation

Question

I have designed a 50 W, 12V-5V buck converter operating at switching frequency 200kHz.This is just for understanding practical converter circuits, so I made a totem-pole bjt driver circuit for IRF640N.
Can someone help me out with the isolation required for the MOSFET? At first I had placed the switch in the return path, but this design is for closed loop control, so I think I might have trouble with feedback noise if I don't place it in the go-path.
I know there is pulse-transformer isolation & digital isolation, but I'm not sure how to incorporate that.
EDIT: Parallel to trying to do the isolation part, I was looking for a gate driver IC, but I'm stuck at how to choose one.Any inputs on that front is helpful too.

mr_js · Answer

One option that I have successfully used is a bootstrap MOSFET driver or a half bridge driver with the HIN input controlling the high side MOSFET. e.g. LM5109 from TI. This will allow you to use a high-side N-MOSFET. An example circuit is shown below using an half-bridge driver. To modify this for a buck converter, forget the LIN input and can omit the power MOSFET Q2 during normal operation. Use a power diode in place of Q2.
When the diode is conducting, i.e. the high side switch is off, then Cboot gets charged up to 15V (you could also use a lower voltage than 15V, just ensure that it is higher the the gate threshold voltage of the MOSFET).
When the high side switch is on, the supply for the MOSFET gate drive comes from the boot capacitor, which has a potential of 15V higher than the source.
The only caveat with this circuit is the maximum duty cycle is limited to approx 95-98%, since the boot capacitor must have time to charge up. You also need a small npn instead of Q2 that you briefly turn on at power up to precharge the boot capacitor.

Gate driver Isolation

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