Active Precharge Circuit
Designed a buck-converter-style active precharge circuit to improve upon the car's inefficient RC-style precharge.
- Combined negative feedback of shunt current readings and a MOSFET to maintain a constant precharge current
- LTspice simulations to determine inductor value and minimize ripple current to 50mA pk-pk
- Negligible power loss compared to the RC-style precharge
- Up to 4 times faster than previous RC-style precharge
- Hardware adjustable target current using a simple voltage divider and comparator
- Rated for 1200V, providing a 2x safety factor for our 588V HV system
Design
An RC precharge dumps its energy into a resistor and slows down as the bus voltage rises. This circuit switches the current through an inductor instead, so the bus charges at a constant rate and almost nothing is lost as heat.
Simulation
The current is held around the target, the green sawtooth, and its constant average is what gives the linear bus voltage ramp in red. The full first principles derivation is in the design proposal.
