Quote:
Originally Posted by s1900ahon
I didn't see any response from Mindsensors on this here, so I'll speculate.
There are two contributors to an efficiency loss. The IR drop due to Rds(on) is one and is the contributor most are familiar with. The other is the refresh of the boot capacitor.
To turn on the high-side MOSFETs, the Vgs must be high enough for efficient conduction, but since the source voltage is also that connected to the motor, it means that the gate voltage relative to ground is greater than the battery voltage (by more than 5V, typically). So, the high-side gate drivers use a capacitor as a source of voltage/current and that cap (the boot cap) is periodically refreshed. This means that when driving the motor at 100%, the gate driver cannot actually drive a high-side MOSFET on for 100%, it is more like 99.9%. The 0.1% is enough to refresh the boot cap.
Since the test setup uses a resistive load and RMS multimeters, the refresh will appear like further IR drop. To figure out the refresh component, a scope can be put across the resistive load and the duty cycle measured.
Some newer gate drivers (like the one announced by TI earlier this year) use an integrated charge pump and can keep a boot cap charged without requiring this loss.
So, Mindsensors *could* have updated their firmware to reduce the refresh time/duty cycle and improve efficiency.
Scott
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Thanks for your input, Scott. It is interesting to get your perspective as a power electronics professional.
If changing the refresh time will make the MOSFETs saturate properly, they could update the firmware in the original and improve it's performance too. The stated change to make the "B" version was to remove the under-voltage lockout feature which should have no effect on how the MOSFET gates are controlled during "normal" operation.
It would be interesting to see if anyone who has an SD540 or SD540B can put a scope probe on the output. It is possible that the SD540B uses a switching frequency lower than 32.25 kHz to cut the switching losses in half. I would also be interested to see how clean the output waveforms are. I have had to fix inverters where poor internal layout led to excessive voltage surges which led the designers to greatly increase the gate resistance (increasing switching losses and allowing the Miller Capacitance to cause oscillations).