The key function of the bus capacitor is to smooth the bus voltages and provide a transition current in the switch to keep the bus voltage ripple small enough to change the FET time becoming shorter, the amount of command required by the capacitor becomes smaller, so using a higher PWM switching frequency can reduce the required bus capacitance value. Typically, bus capacitors are placed as electrolytic capacitors. The following test attempts to replace electrolytic capacitors with ceramic capacitors, increasing the frequency of PWM ripple electrolytic capacitors and ceramic capacitors at different frequencies.

The following tests were taken on a working 400μF ceramic capacitor, compared with 1000μF electrolytic capacitor data.

As the results show, as the frequency increases, the ripple on the bus decreases, so capacitors with smaller capacitance can be used. But ceramic capacitors have significantly larger voltage ripple at low frequencies. Because the actual capacitance of this 10μF ceramic capacitor is only 4μF at a voltage of 36V (from the GRM32EC72A106KE05 data sheet), the actual effective capacitances corresponding to the 400μF ceramic capacitors is 160μF. When the PWM frequency increases to 80kHz, the voltage ripple of a 400μF ceramic capacitor (160μF effective capacitances) and an 1000μF electrolytic capacitor is similar. Therefore, the ultra-low switching loss of GaN can be used to increase the PWM frequency to 80kHz. At the same time, the electrolytic capacitor can be replaced with a ceramic capacitor of the same capacity to achieve a smaller size.