That's NOT a "normal" h-bridge driver. In fact it's an example of how *not* to do it properly. Consider, the use of driver outputs to drive the out-of-phase drivers can be a disaster. That's because the driver (and I use that word advisedly), with its 10k pullup resistors against the high IRF9Z30 gate capacitance (900pF), slows the gate-drive signal (t = RC = 10us) enough to cause cross conduction. That's when the p-channel MOSFET conducts through an n-channel MOSFET from the supply rail to ground. Ouch! We're talking about 20A or more during the up-to-10us supply-short time. That's not my definition of normal. :-)
Thanks for all the feedback, Im now a even more confused novice.
Basically I have a 4 amp motor for a robot . Will have a pic giving it control logic and PWM. Was going to build this
but upgraded the driver to TO-220 (5pin) 4422 but now concerned the
2.8ohms is a waste of power and source of heat. I need to connect external mosfets to
4422 driver. I dont want this to self distruct so am asking if someone has a link or a circuit to connect external mosfets to this driver. Thanks for all advice so far.
you probably can find a suitable H-bridge IC for your motor.
if you DIY with FETs you need to ensure a reasonable amount of DEAD TIME between the drive pulses. the original circuit you posted a link to was really, really bad, in that it required dead times on the order of
20-50us to prevent both the upper and lower FETs from being on simultaneously, thereby shorting out your supply thru the FETs - if this doesnt outright fry the FETs (most likely) it will certainly make them get very hot. If you use a FET driver IC to drive the FETs, the dead time can be quite small (1us or less), but it still needs to be there.
(among others) make full-bridge FET driver chips that have dead time control pins, for exactly this purpose.
the answer is a definitive maybe. within the chip they will have some way of dealing with shoot-thru (eg with deadtime). if you try and parallel these, you would want individual chips to switch at exactly the same time, to avoid shoot-thru between adjacent devices. that may well be a nsaty trick.
the real problem is you are playing with a little bit of power, so the Rdson is fairly low, so shoot-thru is bad. if OTOH you want to parallel individual HC14 gates, go right ahead, it works real well. I havent tried paralleling different chips though, but ultimately they have much higher Rdson, so the problem is a lot less severe.
I suspect you are going to have to do it properly, if you want it to work reliably. OTBS its a great learning experience. just buy plenty of FETs, and use a current limited supply until you are convinced it works. no scope = no hope.
"John E." schrieb im Newsbeitrag news: firstname.lastname@example.org... | >
| | This circuit uses a single power supply (+9 to +18 volts). With such a setup | you can use one kind of MOSFET, probably N-channel to connect your supply to | the motor. | | If you use dual (+ and -) supplies, you have to use complementary pairs of | MOSFETS (N- and P-channel) to conduct each supply to your load (N for the +, | P for the -). | | Just something I think no one has pointed out yet...
Thanks, interesting circuit and very well documented. But I work about one million times higher in frequency. Obviousely it seems that you overlooked that.