How to drive these MOSFETs?

[lithium]

hi all,

i am designing a power supply which employs a 2 stage down converter. A buck followed by a push-pull converter.

the circuit has 3 MOSFETs(IRF840) whose PWM will be controlled by PIC16F877. i have the following queries:

1) while switching the Buck converter MOSFET, during the off state, the source will be floating & hence PIC may be damaged. i undertsand that i have to connect an optoisolator. i would like to know how the connection is done without using drivers like IR2125, etc. also how to increase the current drive of the PIC to drive the MOSFET.

2) do the MOSFETs used in the push-pull configuration need isolation? if NOT, then can i drive them using a transistor(2N2222) with collector pulled up to Vcc?

if there is a better way to do it.....i would like to hear from u. any application notes i could refer to will be appreciated.

thanx in advance...

Lithium.

 

[flatulent]

M2 M3

Transistors M2 and M3 are referenced to ground and so you can do a simple drive. One cheap way would be to use 74HC4049 buffers all six in parallel for each transistor drive. You can drive them from your PIC and they will source and sink about 300 mA each package.

The floating transistor will need some sort of isolation. Opto types are slow (microsecond or so respnse times) you might want to try pulse transformers.