Since we’re using anelectrolytic capacitor, a ceramic capacitor should be used in parallel withthis capacitor. For high frequencieslike 30kHz to 50kHz, I use between 4.7 ♟ and 22 ♟. For low frequencies suchas 50Hz, I use between 47 ♟ and 68 ♟ capacitance.
So, I just estimate the required capacitance. However, there are many parametersinvolved, some of which we may not know – for example, the capacitor leakagecurrent. Yes, there are formulaeavailable for calculating the capacitance. The higher the dutycycle, the higher the required capacitance for C1.
Thus, the lowerthe frequency, the higher the required capacitance for C1. C1 must also not be too large that charging is tooslow and the voltage level does not rise sufficiently to keep the MOSFET on.The higher the on time, the higher the required capacitance. A large enoughcapacitance must be chosen for C1 so that it can supply the charge required tokeep Q1 on for all the time. When LIN = 0 and HIN = 1, this charge onthe C1 and C2 is used to add the extra voltage – VB in this case – above thesource level of Q1 to drive the Q1 in high-side configuration. When LIN = 1 and Q2 is on, C1 and C2 get charged to the level on VB,which is one diode drop below +VCC. D1, C1 and C2 along with the IR2110 form the bootstrapcircuitry.
