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| H-bridge motor control circut burns MCU after some time |
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| Chris56000:
Hi! A lot of commercial and industrial motor drive and servo control amplifier PCBs I have seen at work use opto–couplers between the control MCUs and the motor drive amplifier, sometimes on both sides of the MCU with both input control voltages and the motor control voltages isolated, and I also think many of the purpose made motor bridge drivers are internally isolated between MCU drive pins and MOSFET driver gates as well, precisely to prevent the high peak voltages due to motor parasitics, self inductance etc., etc., getting back into the MCU and damaging it! As Member "Oxdeadbeef" said, a purpose–made H bridge driver is preferable as these devices contain built in dead–time to prevent "cross–conduction" between upper and lower halves of the drivers, and also precautions to prevent one half or the other "bottoming" or "latchup", and this is why he recommends their use! Have a look amongst T.I., ST and the other major Semiconductor Vendors for "H Bridge Motor driver" in their "Parametric Search" or "Search By Function" pages – you should find recommended devices and possibly even reference designs to use as a starting point! In fact, there are similarities in design between full bridge motor drivers and full bridge power supply designs, with the same MOSFET drive precautions being needed for each type of circuit! You can find examples of MOSFET gate–drive circuits in Keith Billing's "Switch Mode Power Supply Handbook" free from the Internet Archive or a few Uni repositories! If you get mega–stuck I'll see if there's any scrap at my work that uses a full bridge motor driver and if there is I'll draw a bit of it out for you to provide a commercial example of how it's done, but please have a look in the semiconductor maker's pages first! Chris Williams |
| rasul:
Hi Chris! That's a great advise I was looking for! I had to do it before I failed. |
| 0xdeadbeef:
I'd use something like an ST L9958 unless you need really high currents. These things are more or less foolproof, have SPI diagnostic/configuration, dynamic overcurrent limitation, freewheeling on low side and what not. Stuff like this is used in automotive environments for throttle blades and actuators like that. |
| TurboTom:
The major problem is capacitively turning on the P-MOSFETs while the N-FET is turned on. Provided the control scheme is sound, the P-Channel FETs are held non-conducting only by the 47k resistors. The reverse transfer capacitance is specified to be 110pF @ 25V, at 12V probably a little more. Gate-source capacitance is 680pF (calculated by the gate-source charge) which forms an inductive voltage divider of approx. 0.14 ratio. This means, a fast voltage change of 12V at the drain will momentarily induce a gate voltage of 1.68V which is well within the specified threshold range. So switching on the N switches, the P switches will always conduct for a certain time as well (until the input capacitance is discharged via the 47k resistors). This "punch-through" effect can only be eliminated by actively driving the corresponding gate below the threshold voltage. This effect gets the more severe, the higher the supply voltage is. Some high voltage circuits, especially IGBT stuff thus requires negatively biasing the gate (for an N-Channel switch) when the element is supposed to stay turned off. Better use proper gate drivers to eliminate the problem or at least an additional PNP transistor to reverse the drive scheme of the high-side switches. Good luck, Thomas |
| rasul:
TurboTom thank you very much. Actually those p-chanel MOSFETs were damaged during one of the tests iteration. I changed 47k resistors to 1k to reduse that switching time. Anyway I should read more about this "punch-through" effect and probably stay away from designing custom H bridges for powerful DC motors for a while. |
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