Switching Power Supplies Draw Energy Storage from Output Capacitors
On the voltage charge relationship diagram, the capacitor is represented by a diagonal line, and the energy stored in the capacitor is the area contained below this line. Although the output capacitance of power MOSFETs is nonlinear and varies according to changes in drain source voltage, the energy storage in the output capacitance is still the region contained offline by the nonlinear capacitance. Therefore, if we can find a straight line that gives the same area as the output capacitance curve shown in Figure 1, then the slope of the line is exactly the equivalent output capacitance that produces the same energy storage.
For some old-fashioned planar technology MOSFETs, designers may use curve fitting to find the equivalent output capacitance based on the output capacitance values in the data table at the typically specified 25V drain source voltage.
Figure 2 shows the measured values of the output capacitance and the fitted curve obtained from formula (3). Compared to the old-fashioned MOSFET technology shown in Figure 2 (a), its performance is good. However, for MOSFETs with more nonlinear output capacitance characteristics using new technologies such as super junction technology, simple exponential curve fitting is sometimes not good enough. Figure 2 (b) shows the measured output capacitance of the new technology MOSFET and the fitted curve obtained using formula (3). For the equivalent output capacitance value, the gap between the two in the high voltage region can cause a huge difference, because in the integration formula, voltage is multiplied by capacitance. The estimation in Figure 2 (b) will result in a much larger equivalent capacitance, which can mislead the initial design of the converter.
If the output capacitance value varies according to the drain source voltage, the energy storage in the output capacitance can be calculated using formula (4). Although the capacitance curve is displayed in the data table, it is not easy to read the capacitance value from the chart. Therefore, based on the drain source voltage, the energy storage in the output capacitor is given by the chart in the * new power MOSFET data table. By using the curve shown in Figure 3 and formula (5), the equivalent output capacitance at the desired direct current (DC) bus voltage can be obtained.
