Output Capacitor Design for Soft-Switching Converters

Figure 2(a) shows the measured output capacitance and the fitted curve obtained by equation (3). However, for new superjunction MOSFETs with more nonlinear characteristics, a simple exponential curve fit is sometimes not good enough. Figure 2(b) shows the measured output capacitance of a state-of-the-art MOSFET and the fitted curve obtained by equation (3). The difference between the two in the high voltage region will lead to a huge difference in the equivalent output capacitance because the voltage and capacitance are multiplied in the integral formula. The estimate in Figure 2(b) will result in a much larger equivalent capacitance, which will mislead the initial design of the converter.

Figure 2: Output capacitance estimation: (a) old MOSFET, (b) new MOSFET.

If the output capacitance value as a function of the drain-source voltage is known, the output capacitance energy storage can be calculated using formula (4). Although the capacitance curve is shown in the data sheet, it is not easy to read the capacitance value accurately from the graph. Therefore, the output capacitance energy storage as a function of the drain-source voltage will be given by the graph in the latest power MOSFET data sheet. Using the curve shown in Figure 3, the equivalent output capacitance at the desired DC bus voltage can be obtained using formula (5).

Figure 3: Energy storage in the output capacitor.

Common Problems with Output Capacitors

In many cases, switching power supply designers question the MOSFET capacitance temperature coefficient because power MOSFETs usually operate at high temperatures. In general, it can be assumed that the MOSFET capacitance is always constant with temperature. The MOSFET capacitance is determined by the depletion length, doping concentration, channel width, and silicon dielectric constant, but all of these factors do not change much with temperature. Moreover, MOSFET switching characteristics such as switching losses or on/off transition speed do not change much with temperature because MOSFETs are majority carrier devices and their switching characteristics are mainly determined by their capacitance. When the temperature rises, the equivalent series gate resistance increases slightly. This slightly reduces the switching speed of the MOSFET at high temperatures. Figure 4 shows the capacitance variation with temperature. When the temperature changes by more than 150 degrees, the capacitance value does not change more than 1%.

Figure 4: MOSFET capacitance vs. temperature.

Another area of ​​interest for designers is the test conditions for MOSFET capacitance. In most cases, the output capacitance is measured at 1MHz and Vgs = 0V. In reality, there is gate-drain capacitance, gate-source capacitance, and drain-source capacitance. But in practice, it is impossible to measure each capacitance separately. Therefore, the sum of gate-drain capacitance and drain-source capacitance is called output capacitance, which is measured by connecting the two capacitors in parallel. To connect them in parallel, the gate and source are shorted together, that is, Vgs = 0V. In switching applications, when the MOSFET is turned on by applying a bias voltage to the gate, the output capacitance is shorted through the MOSFET channel. Only when the MOSFET is turned off, the output capacitance value is worth considering. Regarding frequency, as shown in Figure 5, the output capacitance at low voltage increases slightly at low frequency. At low frequency, due to the limitations of the test equipment, it is sometimes impossible to measure the capacitance at low drain-source voltage. In Figure 5, when the drain-source voltage is less than 4V, the capacitance at 100kHz cannot be measured. Although there is a small change in output capacitance, the equivalent output capacitance is almost constant because a small change in output capacitance at low voltage does not have as big an effect on energy storage as shown in Figure 3.

Figure 5: MOSFET capacitance vs. frequency.

Conclusion

The output capacitor is an important part of the soft-switching converter design. Designers must carefully consider the equivalent capacitance value instead of fixing it to a single value at the drain-source voltage.