Protecting logic level mosfets in a 24v system - Page 1
Forum section: electronics / beginners

author: senso
author_rank: Frequent Contributor
timestamp: June 25, 2014, 11:15:48 pm
content: Good evening, I'm going to use a couple(more like a dozen) N channel mosfet's, and I would like to know how to protect/limit the gate voltage so I don't end up with a lot of burned mosfets.I'm thinking about using this mosfets:PSMN041-80YLXhttp://www.mouser.com/ds/2/302/PSMN041-80YL-269396.pdfAnd drive them either directly from the output pins of an AtMega or using a totem pole made up of MMBT3909 and MMBT2222A's, the swithcing speed is pretty low, 100Hz or lower.I have designed an H bridge driver that used this arrangement, I saw it in some AppNote from IRF:In the same board I have to drive a biggish stepper(around 3Amps), and it just used as a sort of slow DC motor, so I only need full steps, can I just make a simple mosfet H bridge like this:All this will be powered by a 24v bus, ence my doubts, I have done this with lower voltages, in the range of 10 to 12 volts, so it was always inside the Vgs tresholds.I think I have everything nailed down, but would like to have a smarter, more experienced opinion.Best regards.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 26, 2014, 12:37:54 am
content: Worth asking... is the stepper 3A at 3V or thereabouts, or is it rated for full 24V continuous?Point being, you don't want to cook it at 24V. You can run it from 24V -- or more, and this is quite normal, because it provides higher top speed. But you need a current PWM circuit -- usually, this is integrated with the driver, or a companion (e.g. the L292 / L293 pairing, or a lot of far better modern chips with good CMOS switches).Which begs a follow-up question: if the motor is rated for 24V continuous, then would you like the overhead to run it from an even higher voltage (50-100V?) for the same speed advantage?Tim

author: senso
author_rank: Frequent Contributor
timestamp: June 26, 2014, 01:01:37 am
content: Yes, its rated at 3v, not 24v, I suggested the use of an L6472 dSPIN driver, this one employs current chopping so I would limit it to around 2.5A, the motor will not run continuously, but in bursts of a couple seconds, stop, then another couple seconds, and work like that for hours.He has a proof of concept using an all N chan mosfet and he says that it works, but he has been advised about burning out the stepper.For the gates protection I cannot find anything very specific, going with resistors to limit current and zeners to clamp the voltage is really the best option?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 26, 2014, 01:14:07 am
content: Well then why not use that chip? It looks like it does everything for you.I would strongly suggest against driving a stepper motor directly from a microcontroller. Sooner or later, the code will lock up, the motor gets stuck on, and BANG, out goes the magic smoke from both transistor and motor. A 3V 3A motor will draw some 24A when shorted across a 24V supply for more than a few miliseconds.Since chopping is required, the drive frequency will be more like 10kHz, not under 100Hz. This determines the minimum amount of drive required, if you insist on using discrete MOSFETs.If you don't want to go to the hassle of rolling your own circuit, or using an existing driver, then you should consider a gearhead DC motor or something like that instead.Tim

author: nuno
author_rank: Frequent Contributor
timestamp: June 26, 2014, 01:42:33 am
content: Quote from: senso on June 26, 2014, 01:01:37 amWho's "he"?...A few years ago I designed a similar h-bridge (N and P FETs, 24V) with discreet FET drivers, but these were designed to switch the FETs "fast" and have low latency and so they kind of have a lot more hw than I suppose you'd like to have. Nevertheless, what I did was to use zener diodes to eat half of the supply away; 1 in series with the pull low top driver transistor (thus provinding only down to -12V to P-FET) and 1 to limit the pull high on the bottom driver (thus providing only up to 12V).You should probably get an integrated solution, because you have lots of FETs (2 full bridges for a stepper). (also, I don't think you need the 1N4004 diodes).