Q1 You are to design the gate drive circuitry for a MOSFET buck converter for a forklift motor drive.
The Forklift battery pack is 72 V nominal (12 by 6 V lead acid batteries), with a maximum voltage under charge of 90 V, and under load of 84 V.
The motor is a DC series wound motor, with a maximum ratings of 100 V and 300 A. The motor has high inductance, so no additional inductance or output capacitor is required.
a) Draw the circuit for a synchronous buck (half bridge) converter for this application. How would you reverse the direction of operation of the motor?
b) Choose suitable MOSFETs, with discussion justifying your choice.
c) Calculate maximum conduction losses when operating at maximum current and a duty cycle of 5% (maximum torque at start).
d) Design a gate drive circuit.
e) Calculate switching times and switching losses for 4 kHz switching frequency, neglecting MOSFET intrinsic diode reverse recovery.
Q2 Design a suitable snubber for the above MOSFETs.
a) Draw the circuit for an RCD snubber circuit.
b) Calculate the values which lead to approximately the minimum overall losses, and calculate the MOSFET and snubber resistor losses.
c) Choose suitable resistors, capacitors, and diodes.
Q3 Model the above circuits in LTSpice and compare your results to calculations.
a) Working LTSpice circuit, operating at 84 V supply, 300 A load current and 5% duty cycle, no snubber circuit.
b) LTSpice plots and calculations as necessary to calculate conduction losses (no snubber).
c) LTSpice plots and calculations as necessary to calculate switching losses (no snubber). Discuss the effect of MOSFET diode reverse recovery.
d) Working LTSpice circuit, operating at 84 V supply, 300 A load current and 5% duty cycle, including RCD snubber circuit as calculated above.
e) LTSpice plots and calculations as necessary to calculate MOSFET conduction losses and snubber losses for circuit including snubber values calculated above.
f) LTSpice plots and calculations as necessary to calculate switching losses for circuit including snubber values calculated above. Highlight and discuss the effect of MOSFET diode reverse recovery.