The selection of the two capacitors, C2 and C7, is important to minimize the ripple on the generated power supply. The following is an example to calculate the capacitor value under extreme conditions. The good news is that the integrated low-dropout (LDO) regulator on the high-side of the AMC23C11 eases the pre-regulation requirements of this power supply. As an example, this design is capable of taking a ripple voltage of 3 V, and other parameters are as follows:

• Half-bridge circuit switching frequency f = 1 kHz
• Duty cycle D = 20%
• Current required for isolated comparator I = 3.3 mA (max)
• Low-side driver supply U_low side = 15 V

Switching frequencies in motor drive systems are typically 1 kHz to 20 kHz. Lower switching frequencies result in larger ripple because of longer discharge times. Increasing the capacitance reduces the ripple but increases the charging time. Assume the switching frequency is 1 kHz and 20% duty cycle in the extreme case. This case means that the isolated comparator is powered only by the capacitor C2 of the charge pump for 80% of a PWM cycle, the minimum capacitance required for C2 under this condition is:

In this design, C2 takes a capacitance of 1 μF and the bootstrap capacitor C7 is also 1 μF. R4 and R6 limit the high currents that may occur during initial power-up. Typical values for this resistor are 5 Ω to 10 Ω. A larger resistance increases the time constant of the RC circuit and prolong the time to reach the minimum supply. When R4 is taken as 10 Ω, the maximum current at initial power-up is 1.36 A:

The two diodes D1 and D2 should be able to withstand the voltage of DC bus. For a motor drive system with a 380 VAC input, the diodes need to have a withstand voltage value ≥ 1200 V, and must have fast reverse recovery characteristics to minimize the recovery charge and thereby ensure the stability of the supply circuit. The circuit can theoretically generate a supply voltage of 13.6 V, which is 15 V minus the voltage drop of the two diodes (0.7 V).