Driving lots of H-bridges thru isolated SPI, design ideas? - Page 1
Forum section: electronics / projects

author: hlavac
author_rank: Frequent Contributor
timestamp: March 29, 2014, 12:22:10 am
content: Hi,I need to find a way of driving lots ( 8 ) of reasonably powerful H-bridges (~48V,~5Aish) from an isolated SPI bus. I'd like the H-bridges to be all discrete N-MOSFETs.I have looked at the various gate driving ICs but they seem too expensive to use for driving so many gates (4 per channel, *8 = 32 gates, 16 of them high side). This will get very expensive very fastSo, I'm thinking, I could create a shared gate driving power rails (Vmotor+10V, GND+10V) on the high power side, and switch the gates with some simple BJT stuff... can that fly? I'd also like some hardware shoot-through protection there to avoid the thing blowing up in my face when wrong data gets sentAny ideas, component suggestions on how to do this, cheaply? Similar projects I could look at? Maybe protection circuitry suggestions? Is there a magic IC that will make this easy?

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 29, 2014, 12:37:52 am
content: Do the high sides need to be able to do 100% duty cycle or is less OK? Is each output being either high or low OK or does it have to be able to go open? What maximum switching frequency do you want?

author: hlavac
author_rank: Frequent Contributor
timestamp: March 29, 2014, 12:49:03 am
content: These are for stepper motors, so I need 100% duty cycle I guess.Going open is probably not required although would be nice for current control (slow decay). Or maybe it is to be able to let go of the motors.I want quite fast PWM, above 25kHz, maybe up to 100kHz, and want it to be pretty high res as I want to play with sinusoidal driving of stepper motors to reduce the motor noise.

author: Psi
author_rank: Super Contributor
timestamp: March 29, 2014, 12:52:51 am
content: You can get optoisolators designed to drive mosfet gates. (2A peak output)Maybe a multichannel one of those would be cost effective.

author: han
author_rank: Frequent Contributor
timestamp: March 29, 2014, 01:30:49 am
content: I always use MC33151 for Mosfet Driver, you can get <$1 in large qty

author: djacobow
author_rank: Super Contributor
timestamp: March 29, 2014, 02:05:45 am
content: I just ordered some FOD3182 for a similar purpose, and you are right -- they are pretty damn expensive, $3-ish and 10 doesn't get you a much lower price.Without a driver and with something simple like a single-ended pulldown arrangement, you might get very slow rise times, along with a fair amount of power dissipation during the transition, but if transitions come very infrequently, eh, so what? 'It might be worthwhile spicing some simple circuits to do this and see what the actual power dissipation in the FET is given a less than awesome driver -- maybe you can live with it.The MC333151 looks cheaper but doesn't have isolation. However, driving so many gates, it probably makes sense to isolate at the SPI bus not each individual driver.

author: johansen
author_rank: Super Contributor
timestamp: March 29, 2014, 02:36:23 am
content: HIP4081A if you really need a charge pump.unless you are severely missmatching your steppers and power supply voltage, you will not need but maybe 10-20% pwm duty cycle to send 5 amps into the motor under stalled conditions, so you don't need a charge pump

author: eneuro
author_rank: Super Contributor
timestamp: March 29, 2014, 07:54:59 am
content: Quote from: hlavac on March 29, 2014, 12:22:10 amI was also looking for something like this and decided to use something like fast optoisolator 6N137 & 2 channel mosfet driver TC426 (2x1.5A) at each high & low side in 3 phase H bridge, however I do not know whether is it the chipest solution, but I need reliability in electric vehicle motor driver, so decided to make additional isolated power supply for those 6 isolated gate drivers per 3 phase H bridge.Of course fast diode in output power stage is only for other applications, where this driver switches inductive loads standalone.