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| 3-phase Motor Switching FET Power Consumption |
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| bdohler:
Hi all, I've been tasked with measuring the power consumed by the switching FET's on a board with a similar topology to the following. Problem is, I can not measure the current through the FET's, only the current through the motor phases. I can, of course, get the voltage across the FET's. All online solutions seen to be either analytical, or assume you can get the current through the FET's. Does anyone have a good solution? Thanks! |
| MagicSmoker:
You need to clarify if you just have to make a one-time measurement (ie - for prototype verification or troubleshooting or the like) or if you need to incorporate measuring the current through individual fets into the design itself. Different solutions pertain to each case (as well as what sort of equipment you have on hand and how much kludging of the board is permissible). |
| bdohler:
To clarify, I am doing this to determine the efficiency of the motor for design verification purposes. It only needs to happen once. The FETs are fixed to the board. I can not measure the current through the FETs because they are fixed to the board, only the voltage across them. There are sense resistors which will allow me to determine the current through the low side FETs, which will allow me to integrate current by voltage over time. But because it is a 3-phase BLDC motor, if I know the current flowing through a bottom FET, I still don't know the current flowing through the top FET. So I am still unable to determine this. Because a charge pump is used to generate the gate voltage for the high side FETs, I am concerned that the gate voltage may not be symmetrical to the low side gate voltage. The thermal approach is tempting, and I will do it to verify my voltage/current based approach. I can do it for the low side FETs at least. I was hoping for some clean mathematical solution, but I don't seem to be able to find one. Using V^2/Rds(on) is still an approximation, because I don't know my gate current, either. |
| Phoenix:
If your modulation technique is fair each switch should see the same current and number of switching cycles so you should only need to measure 1 of the 6 (you can validate this assumption in simulation). As you mentioned, using the current sense resistor should get you one of the lower switches thermals doing integral(V x I). You will see the contributions of both the transistor and diode switching and conducting, but I'm sure this will be what you want anyway. You'll need to make sure your measurement has enough time resolution to accurately measure the switching loss and enough samples to measure a full fundemental cycle to get it more accurate - that's a lot of samples. |
| MagicSmoker:
--- Quote from: bdohler on June 18, 2016, 12:16:53 am ---To clarify, I am doing this to determine the efficiency of the motor for design verification purposes. It only needs to happen once. The FETs are fixed to the board. I can not measure the current through the FETs because they are fixed to the board, only the voltage across them. --- End quote --- One simple approach is to connect the positive terminal of a low voltage power supply (or a 9V battery) in series with a 1k to 10k resistor and a fast diode to the MOSFET drain and the negative of said supply to the source (cathode of diode towards MOSFET drain). Measure or probe between the anode of the diode and the source. When the MOSFET is off you'll read the voltage of the external bias supply and when the MOSFET is on you'll read the voltage drop of the MOSFET and the blocking diode; subtract the latter and Bob's your uncle. It should go without saying that the bias supply voltage has to be lower than the drain voltage (and if the drain voltage is that low then just measure the drop directly!) and the PIV rating of the diode must be higher than the drain voltage. |
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