I'm having a hard time finding practical info on buck converters.
I've draw what seems to me to be a basic synchronous buck converter but I am confused on a few points.
The high and low side FETs for conversion are on the right. Since I want this to be a charge regulator as well, I've put a FET on the left to short the panel when the batteries are charged.
![](http://www.otherpower.com/images/scimages/6527/buck_1.gif)
The micro has complementary PWM outputs with dead band timing generation to make sure that both FETs are synchronized; this seems to be a requirement but I haven't found anything that tells me how long the dead band should be? I think I can do multiples of about 20 nanoseconds. I'm guessing that I need a deadband that insures that the turn-off delay and turn-off time finishes before the turn-on delay finishes plus, perhaps, a little bit more for safety?
If I read correctly, I can use either a P-channel or N-channel FET on the high side. Right now I have it drawn as a N-FET but I've noticed that shorting the panel shorts the battery too. I can throw a diode up top but that seems like wasting power.
I've never used a P-FET in my life but it seems to be the opposite of an N-FET and I'd need to change the pull-down on the gate to a pull-up?
I'd like to handle an input of 4 amps and 100v; 100v so that it can handle the Voc of four 12 volt panels. I'm hard pressed to find a logic level P-FET that can meet those specs. But does it have to? ... Isn't the max voltage across the high side Voc - Vbattery? It might be easier looking for a 80 volt part. I can do the math of loss across a diode's Vf vs. a P-FETs Rdson and realize that the P-FET is only a marginal winner over an N-FET and diode. (That assumes that I really can use and N-channel FET on the high side?)
Hm... I think the issue goes away with a diversion load instead of shorting the panel but I'd like the option of not using a diversion load.
Occasionally, I've seen a schottky diode inserted across the low-side FET without explanation. Is that a snubber or is it part of the rectification? ... it just seems a little strange to me since I put the FET on the low side just to get rid of diode in exactly that position!
Comments, help and pointers are greatly appreciated!
- Ed.