The setup is pretty typical: battery - MCPs - motors. With a host of additional electronics on the side, but which were not operational for the tests and are thus irrelevant.
- Each MCP236 drives a pair of PMDC servomotors (basically ordinary motors designed for servo applications, not actual servos).
- Each motor is rated for 48V and has a max continuous current of 10A, and a stall current (according to specs) of 48A. (L = 1.5 mH, armature resistance R_a = 0.8 Ω, terminals resistance R_t = 0.6 Ω)
- The battery is (currently) a Li-Ion battery with a nominal voltage of 48V (full 54.6V), and is fitted with a BMS. The BMS lists a max continuous discharge current of 30A, a peak discharge current of 90A, and a max continuous charge current of 5A.
- There are no encoders, so according to the manual, in open-loop PWM mode, with motor parameters provides, the controller should perform current control.
I opened up the controller, found out that two of the MOSFETs (one side of one of the bridges) had failed with the typical drain-source short. This first time I thought it was due to the voltage spike (from the motor being stopped) exceeding the 100-V rating of the FETs, and (through reverse-transfer-coupling) destroying the gates, so I went ahead and installed the protective parts recommended in the manual. I hadn't done so at first due to being rushed to demonstrate the prototype by the company.
- A 0.1 μF ceramic capacitor across the motor leads, and one across each lead and the casing.
- A bidirectional TVS diode across the motor leads (DigiKey part number: F7678CT-ND). Diode stands off up to 58V, begins breaking down at 64.4V, and clamps to a max of 93.6V, with a max current of 54A, so it should adequately protect the MOSFETs in the controller.
- A 1 KΩ precharge resistor with its own, dedicated switch parallel to the controller's fuse.
The only protective device I have not installed is a voltage clamp. Is it possible that the regenerative braking current is so high that the battery BMS cannot let it through ?? Granted, the battery is fully charged before every test...
I cannot think of any other reason the MCP burns, as while running, even for several seconds, at 8A motor current, it never even gets lukewarm. I have mounted each controller on a heatsing with a fan, seems to cool it enough.
Also, I replaced the two burned FETs on the older controller, since they had markings on them and I could read the part number (Infineon 070N10NS). However, on the new controller, the package has an exposed metal pad on top (presumably for better thermal conduction), so I can read nothing. What MOSFETs should I use to replace the two burned ones on the new MCP? Do they come in the metal-pad version, or is it custom-made for you?
On the old controller, after replacing the FETs, it works, but only the channel I repaired actually drives the motor. The other channel seems dead. Is it possible the FET driver is also burned out? I see no burn marks on the pcb around the part though. How could I diagnose the board? Use an oscilloscope to verify if the driver actually outputs PWM when switching? Are there othe test points I can check out?
In short: Help, plz...??