3-phase Motor Switching FET Power Consumption

[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]

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.

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.

[r0d3z1]

Interesting question, do you manage to do the measurement ? how did you do ?

[schmitt trigger]

As Phoenix has mentioned, a balanced converter in steady state, all IGBTs should be dissipating the same amount of power.

The poor man's way I would do it:

-Attach the motor to a steady state mechanical load. A dynamometer would be best, but driving a large fan or a water pump with a set head, could suffice.
-With a fairly common three phase power analyzer, measure the real power consumed by the motor.
-Measure the power consumed by the converter. The best would be to measure the power supplied by the high voltage DC bus.
-Subtract both. The residue is the bridge's loss. Divide by 6, and voila!

[Doctorandus_P]

If you're looking for "long" time averages instead of instantaneous switching losses then you could use a thermal mass (such as the existing heatsink) and measure temperature differentials over time.

You can calibrate the temperature differentials by using the FET's as a current sink and have them dissipate a DC current from a DC voltage.

Next step up in the calori meter approach is to use water cooling and measure temperature rise and flow rate of the water.

[r0d3z1]

I am more interested in measuring the instantaneous switching loss....so one leg is enought...but istantaneous. The difficult part is measuring the current, I have seen some paper where high bandwith low inductance shunt resistor are used
http://www.ib-billmann.de/koax_e.php
but probably they are not cheap and they are also difficult to solder on a SMD board.

[jbb]

Ahhh, so you want to measure the FET switching losses. They way to do that is a double pulse test. I think Semikron (and probably Infineon and IXYS) has an app not about that

There are some things to look out for:
1: you will need a high speed current measurement on the device under test. A SMD shunt could work, or a thin Rigowski coil could be slid between legs on the device. A very short wire loop through a high bandwidth current transformer can also be effective (Pearson make good ones). Try to chenge the circuit as little as possible.
2: the diode (in this case I think it’s the complimentary FET’s body diode) matters a lot. The double pulse test really tests a pair of devices.
3: be careful to check you haven’t combined the gate and drain currents in your measurement
4: make sure to check that your scope and probes are fast enough to measure the signals. This can be done by comparing the rise time of measured signals to the rise time of the scope and probe. If the rise time is the same, you’re measuring the scope and not the device.
5: FET switching losses increase with temperature, so consider using a heater pad to warm up the board to the maximum expected operating temperature
6: FET switching losses change with DC bus voltage and output current so you’ll need to do a few test cycles

[r0d3z1]

thank you for the useful tips, my major issue is measuring the current on LFpack mosfet. Due to this package, rogowski coil and current transformer are not feasable. A low side SMD shunt is already in place for current control, however it is a normal one, not a dedicated one for high bandwith measurement. Than probably the parasitic inductance of SMD shunt could affect the measurement. I expect a voltage spike across low side shunt at low side mosfet turn-on.