A Simple Equation for the Energy Stored by Voltage-Dependent Capacitances

The parasitic capacitances of semiconductor power devices that contribute to the switching losses are voltage dependent, which can make calculations of their stored energy difficult. Typically, manufacturers will provide effective capacitance values to aid in circuit design and component selection. However, stored energy calculations using these effective capacitor values are erroneous. In this letter, we derive a new equation for the stored energy in the voltage-dependent capacitance associated with a semiconductor depletion region, such as in diodes and transistors. In particular, we show that the 1/2 term in 1/2 CV2 should he replaced by a new term gamma, which depends on the device structure. By applying our proposed method to several commercial diodes and transistors, we show that it matches the measured data much better than using the effective capacitances. The proposed equation will enable better power circuit design by improving the accuracy of stored energy calculations.