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I’m developing a motor controller based on a dsPIC33EP64MC504 from Microchip. The motor is a brushless permanent magnet synchronous motor of about 500W on 50V. The controller is based on an SVM algorithm from Microchip application notes. The bridge uses 6 MOSFETs SiR882ADP (100V) from Vishay. No speed nor current regulation is achieved meaning the motor is open loop controlled.

We‘ve first implemented a simple BLDC controller based on Hall effect sensors and it worked great. We’ve then decided to improve the control using a 1deg resolution encoder to achieve a sinusoidal control (for a later Vector Control and current control). Here we are facing some problems of exploding/burning/blowing MOSFETs at middle range power. I will not explain all the design choices and issues : we are working on it but I would like to investigate one soft/hard choice: the hardware current limitation strategy we've changed between the 2 designs.

The current limitation is an hardware dsPIC embedded function which overrides the PWM commands normally driven by the SVM based on an analog signal from current sensor.

Here are the main differences between the 2 approaches: Differences between the 2 current limitation approches

  1. The first strategy (version 1): the bus current value controls the 3 PWMs (all MOSFETs are open as soon as the current limit is activated and overrides the PWM signals out from the SVM).
  2. The second one (version 2): the low side current of each half bridge controls each PWM independently: the corresponding half bridge is open in case of a current limit.

I understand that in the first strategy, the body diodes of the MOSFETs turn the driving bridge into a classical 6 diode rectifier: current is recharging our battery is that right? Anyway, this strategy seems really safe!

But what happen in the second case? Could the opening of only one half-bridge be the responsible for voltage overshoot and MOSFET destructions due to current loop behaving in a different way than in the first version? For example what happen in the case where the current limitation on PWM2 (the blue one) opens the corresponding half-bridge in the following condition ? Configuration of current limitation

Thank you for your time and the answer you will give me (or not).

Regards, MG.

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  • \$\begingroup\$ Details of "A" are needed. \$\endgroup\$ – Andy aka Feb 17 '17 at 10:28
  • \$\begingroup\$ Do you mean the 'A' in circles ? It's the symbol of an ampermeter, isn't it ? It's just to show you where the current are measured (by shunt resistor or hall effect sensor). \$\endgroup\$ – mgarivet Feb 17 '17 at 10:47
  • \$\begingroup\$ As far as I can tell, the only difference between the two circuits is the position of "A" and if one circuit works and the other blows FETs then, the problem rests in how you have implemented "A" unless there is something I've missed. \$\endgroup\$ – Andy aka Feb 17 '17 at 10:58
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    \$\begingroup\$ My question is not about how 'A' is implemented (In first case it's a shunt resistor, in the second case it's three hall effect current sensors). My question is about the strategy of the current limitation algorithm (if you read by first post, I'm not asking about the circuit but about the current limitation algorithm). To be more clear : Is there any reason that the current limitation half bridge by half bridge based on the measurement of each low side current could be more harmful for the MOSFETs than a current limitation of all 3 half bridges at the same time ? \$\endgroup\$ – mgarivet Feb 17 '17 at 12:11
  • \$\begingroup\$ My two cents: in your last sketch energy back from the motor is not returned to the battery but has to be torched in between the involved MOS . This is may exceed SOA especially because given "low" voltage drops (just forward biased diodes and rds(on)) transient may take quite a long time to extinguish. \$\endgroup\$ – carloc Dec 16 '17 at 8:12
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This may not apply here, but, I often see some kind of current limiting resistors along the Source to Drain paths on PWM cicuits, but, not on this. Yes, they are frequently low resistance values, but, seem to be needed in that application. They might be needed here, as well.

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