I am making an hbridge PCB for a robotics project. I've been looking at some of the posts on the forum and have found out that most people prefer MOSFETs to BJTs in hbridges. This is probably because of the cooling.
However,
http://www.electro-tech-online.com/v...ic.php?t=21414
So does this mean I cant use FETs with PWM but I can use BJTs.
I am currently using a BJT circuit (without freewheeling diode). Are they essential for operation? I need to design for a current of possibly 10A. What kind of diode will I need? Obviously not an 1N4001

 

First things first Yes you can use BJT's or MOSFET's for a H-bridge
Equally you can PWM a FET-bridge or a BJT-bridge
either way that quote
isn't strictly true.
correct quote would be

Whether you are PWMing or Static, likewise using BJT or FET's, the switches need to be TURNED-on hard. Either cases, either switches


Next
Free-wheel diodes are ESSENTIAL!!!! to a H-bridge. The load of a H-bridge is usually inductive (even if it wasn't the diodes are still needed). When you turn-on the switches the current in the inductor will build up. When you attempt to turn-off the switches IF those diodes were NOT their, then hte iductor would produce as high of a voltage it needs to keep that current flowing, always results in a nice spark and DEAD switches (as well as other stuff) so PUT THEM IN!!!
the intrinsic diode of FET's is good for 12V application (above and their "zener" proporties, ie SLOW can be a problem), but since BJT's dont have this the anti-parallel diode NEEDS to be added


Next.
BJT's have alot of advantages over FET's, but FET's advantages (or faster switching and easy gate-drive) has relegated BJT's to all but the higher current converters

BUT for 10A BJT's are fine, is all a question of do you HAVE to use BJT's? the problem with them is 1) slow at switching (w.r.t FET, might not be an issue) but 2) base-drive

the GATE-DRIVE/BASE-DRIVE cct always seems to be overlooked (esp on the H-bridges flying around this board). Yes for non-PWM type H-bridges the gate-drive can be simple BUT when you start to PWM a decent driver is NEEDED!!!!

but back to BJT's: To switch a BJT you need to provide current to its base (then a potential ~0.6V). THIS can be quite demanding on some drivers.
As opose to FET's where you must provide a GATE-potential (and a blat of current at the transition).


If you are after a PWM'ed H-bridge I would suggest going for a FET-based as well as a decent gate-drive (some good chips out there)

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thanks for the quite comprehensive reply.
seems to me the bjt will draw a lot of current at the base drive. i am using 2n2955 transistors which have a current gain of about 10. i'll have to change over to fets
what i dont understand is that the circuit worked fine on bread board (without freewheels, no pwm), but flopped on a pcb. must have been some sort of mistake. But i cant pin it.
I was using optocouplers for logic circuit isolation at the base, but i guess i wont need that on a mosfet right.
what should be the package i should use for 10A. will a to220 work without heatsinks? i was using to3 for the bjts