The below waveform shows peak current occurs at the instant when diode starts conduction. Why is the peak current not happening at the max input voltage point?
Usually the rectifier output caps are large in value, making the output voltage more or less constant.
Current = (Vin-Vc)/(R1) ( neglecting diode drop)
-> As Vin is increasing, shouldn't the current also keep increasing?
What am I missing here?
Best Answer
When the diode starts conducting and dumping current into the capacitor, the rate of change of the AC voltage is much bigger than when the AC waveform hits the top of its peak. Given that capacitor current is proportional to \$\frac{dv}{dt}\$ it follows that the biggest peak in the capacitor current occurs when the diodes start conducting. Consider this simpler scenario: -
Image from here showing how when an AC voltage is applied to a capacitor, the maximum current occurs when the voltage waveform is at its maximum \$\frac{dv}{dt}\$ AND zero current flows when \$\frac{dv}{dt} = 0\$.