# Battery Charger PWM determination

Discussion in 'General Electronics Chat' started by assassin___0, Mar 27, 2013.

1. ### assassin___0 Thread Starter New Member

May 10, 2011
19
0
Lets say I'm charging a 12V car battery with a constant 24V input from a power supply using a buck converter.

I always need the current flow to be towards the battery and not away from it, so this means that my duty cycle will never be under 0.5, right? Because that would cause lower voltage (<12) at buck output and the battery voltage will be higher so current flows away from battery. Is this correct thinking?

My other question is, the input power and output power will be the same, assuming no losses. So if I increase the output voltage by increasing the duty cycle, will that lower my output current (in order to maintain same power)? But I would assume that providing a higher voltage to the battery will give it more current. So how should I be looking at this situation?

2. ### crutschow Expert

Mar 14, 2008
20,486
5,800
No. Think about the input and output voltages. By definition, a buck converter has an input voltage greater than the output voltage, so current can never flow from the output to the input, independent of the duty cycle. If the duty cycle gets too small then the output current will simply stop.

Yes, ideally the input power and output power are the same, so the input VI equals the output VI. If you increase the output voltage that will increase the output current (power) and the input current will also increase to maintain this equality.