# Choosing appropriate capacitor value from ripple

I have a H-bridge driving a 12V motor that at 100% duty cycle draws around 15A. To start the motor I am applying a soft-start, the duty cycle of PWM to H-bridge goes from around 60% to 100% and PWM frequency is around 500Hz.

How can I calculate the needed capacitor value on 12V for 10% ripple? At such high currents and low PWM frequency would it be better to go for more than 10% ripple (this 12V line isnt driving anything else than motor)?

I am aware of: $$I=C\times\frac{dV}{dt}$$ But plugging the numbers in I get: $$C=\frac{I\times dt}{dV}=\frac{15*\frac{1}{500}}{1.2}=25mF$$

Which seems quite high to me, is this correct?

• The calculation you did does not include the current delivered by the power supply. You need to specify what the 12 V power supply is (current, ripple), and what drop in the 12 V supply do you see currently on starting? Dec 3, 2016 at 21:03
• @JackCreasey Its night here, so I will do this tommorow. But so you dont wait, what would be the calculation then? Dec 3, 2016 at 21:05
• Essentially the same, though this time you will actually know what the ripple value is. I doubt your power supply is dropping by 10% (1.2 V), and if it is, then you probably need to be working on it first. I'd expect that you might expect droop values of around only 1% maximum from your power supply. Dec 3, 2016 at 21:17
• May I ask why would you reduce voltage ripple out of your H-bridge? And exact schematic you are planning for? Dec 3, 2016 at 23:12
• Okay . I believe you over estimate capacitor. I'd rather try to analyze an RLC circuit including power supply and cabling stray L and R and capacitors with their ESR. A few runs on a simulator with different duty cycle are going to be the fastest way. Dec 4, 2016 at 23:46