Output Filter design David Baba

Output Filter – Why? Performance Size Reducing noise difficult to obtain single stage Buck Would need large value of capacitance and extremely low ESR to accomplish Effective reduction of noise spikes Using small values of L and C filter Reduces output ripple significantly

Addition of an output LC filter Our Value Advantage Design Rules #1 Filter Capacitor #2 Filter inductor #3 Resonance of LC filter #4 Quality Factor of LC Filter Make First Capacitor smaller than 2nd Capacitor Typically ceramic Make 2nd inductor smaller than 1st inductor Make sure Isat in greater than Iout max Make filter resonance 3 times larger than the cross over frequency Make sure the LC filter is damped to a Q of 1

Example Buck Convertor

Design Calculations

Design Calculations

Design Check Mathcad

Spice verification

Output Filter Noise spike reduction