Two MOSFETs are employed as current-bidirectional two-quadrant switches in a bidirectional battery charger/discharger based on the dc-dc buck converter. This converter interfaces a 16V battery to a 28V main power bus. The maximum battery current is 40A. The MOSFETs have onresistances of 35mΩ. Their body diodes have forward voltage drops of 1.0V, and exhibit recovered charge Qr of 25µC and reverse recovery times tr of 200ns in the given circuit. You may assume that all diodes in this problem have “snappy” reverse recovery characteristics, and also assume that diode stored charge is the dominant cause of switching loss in this circuit. You may neglect all losses other than the semiconductor conduction losses and the switching loss induced by diode stored charge. The current-bidirectional two-quadrant switches are realized as in the figure below, utilizing the MOSFET body diodes.
Answer the following questions:
(a) Estimate the switching energy loss, conduction loss, and converter efficiency, when the battery is being charged at the maximum rate. The switching frequency is 100kHz. External diodes are now added as seen below. These diodes have forward voltage drops of 1.0V, and exhibit recovered charge Qr of 5µC and reverse recovery times tr of 40ns in the given circuit.
(b) Repeat the analysis of part (a), for this case.
(c) Over what range of switching frequencies does the addition of the external diodes improve the converter efficiency?
