SD540 Motor Controller

[s1900ahon]

Originally Posted by philso

Mindsensors - How do the firmware changes for the SD540B (other than removing the undervoltage-lockout safety feature) change it's behaviour relative to the SD540 to account for the increase in efficiency? Has the switching frequency been changed? Have the switching edge rates been changed? ...

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