[052] Designing a Two-Stage Output Filter for Low Output Impedance

How to arrange the components of your second-stage output filter for optimum performance. 

Introduction

Dr. Ridley demonstrates how the proper configuration of a second-stage output filter can significantly lower the output impedance of your power supply. The correct arrangement of components is essential for optimal performance, and is often overlooked.

Output Filter Configurations

A powerful design technique for reducing the noise on the output of a switching power supply is to utilize a two-stage output filter. Dramatic reductions in noise can be obtained with small filter components. If the design is done properly, the filter can be incorporated into the main feedback loop for optimal regulation.

Figure 1 shows the schematic of a 1 kW boost converter, operating at 75 kHz. Notice that there are two different types of output capacitor, one used for bulk energy storage, and one to provide low impedance for the high pulses of current that flow in the output capacitor.

Figure 1: Boost Converter with Two Different Output Capacitors.  

While the two-capacitor approach to an output filter works reasonably well, it is far more effective to incorporate a second-stage inductor between the two capacitors to attenuate the noise further. Figures 2 and 3 show two different ways to configure the output filter using a 300 nH inductor.

In Figure 2, the large capacitor is on the left, and the small capacitor is on the right. In Figure 3, the position of the capacitors is reversed. Both of these arrangements produce similar power-stage control and attenuation characteristics, as analyzed in [1] and [2], and at first glance appear to be equivalent.  

Figure 2: Two-Stage Output Filter with First Capacitor Larger than Second Capacitor.