Understanding output filters for Class-D amplifiers

Figure 6: A hybrid output filter. C1 is a metal film capacitor and the other capacitors are X7R multilayer ceramic capacitors.

A hybrid filter has split inductors and R-C networks between the speaker terminals and ground like a common-mode filter, plus a capacitor across the speaker terminals like a differential low-pass filter. At first this would seem to be counterproductive since it adds a fifth capacitor to the design. The total cost can be reduced by making the value of the differential capacitor significantly larger than the common-mode capacitors and only using a metal film capacitor for the differential cap.

The other four capacitors can then be less expensive X7R multilayer ceramic capacitors. This makes the cost of the hybrid filter only slightly more expensive than the cost of a differential L-C filter while still providing some common-mode attenuation and some damping under no-load conditions. The drawbacks of the hybrid filter are:

1.) The differential attenuation is the same as for a normal common-mode filter but the common-mode attenuation is not as good. Because the common-mode capacitors in a hybrid filter are smaller than they would be in a pure common-mode filter, the center frequency for common-mode filters is higher and therefore the attenuation at the switching frequency and its harmonics is lower.

2.) The differential-mode damping of a hybrid filter under no-load conditions is not as good as a pure common-mode filter because most of the high-frequency current flows through the larger capacitor across the speaker terminals. Normally this isn't a problem because the speaker provides the differential-mode damping, but if the amplifier is operated without the speaker connected then the damping will not be as good. Care needs to be taken to insure that the damping of a hybrid filter is good enough to protect the amplifier under no-load conditions.

3.) The harmonic distortion with a hybrid filter will be slightly higher than with a common-mode filter because ceramic capacitors provide some of the filter capacitance, while a pure common-mode or differential low-pass filter would normally only use metal film capacitors with much better characteristics.

Despite these drawbacks, hybrid filters can provide some of the benefits of a common-mode filter while keeping the cost close to that of a differential-mode filter.

As might be expected, calculating the component values for a hybrid filter are somewhat more complex because choosing the component values involves making performance trade-offs. In order to prevent harmonic distortion from being a problem, the value of the ceramic common-mode capacitors should be smaller than the value of the film differential capacitor.

However, making the ceramic capacitors too small will hurt the common-mode EMI attenuation and the no-load damping. Amplifier manufacturers will normally recommend hybrid filter component values for common speaker impedances that provide good filter performance.

Hybrid Filters for Single-Ended Outputs
Hybrid filters for amplifiers with single-ended outputs are slightly different than hybrid filters for amplifiers with BTL outputs. At high frequencies the impedance of C1 is much less than the series combination of C2 and C3 so capacitor C3 is not needed.

Snubbers
When the output of a Class-D amplifier switches there normally is a "dead" time between the time when one transistor is turned off and the other transistor is turned on. The dead time is necessary to insure that both transistors are never conducting at the same time, which would cause large currents to flow from the power supply to ground through the transistors. However, the dead time causes a problem because it interrupts the current flowing through the inductors. Snubbers are normally used on the amplifier outputs to provide another path for the inductor current during the dead time.

There are two types of snubbers. Amplifiers with BTL outputs can use a differential snubber, with a single resistor and capacitor in series in between the two outputs. Common-mode snubbers have a resistor and capacitor in series from the output to ground and can be used with either single-ended or BTL outputs.

Common-mode snubbers for amplifiers with BTL outputs use twice as many parts as a differential snubber but they may reduce harmonic distortion. The type of snubber to use will depend on the application. The amplifier manufacturer will normally recommend values for the snubber components.

Filterless Amplifiers
No discussion of Class-D output filters would be complete without talking about filterless Class-D amplifiers. The primary purpose of the output filters is to reduce EMI. However, it is possible to operate a Class-D amplifier without any filters on the outputs. Although there is a large amount of high-frequency switching noise on the amplifier outputs, this noise is far outside of the response range of most speakers so filters are not necessary for good audio quality. Filterless Class-D amplifiers are less efficient because the high-frequency energy that is normally absorbed by the filter is dissipated as heat and EMI.

Filterless Class-D amplifiers should have controlled rise and fall times to limit the high-frequency content in their output spectrum. Filterless Class-D amplifiers also require very careful attention to circuit board layout to prevent EMI problems. In particular, the distance between the speaker and the amplifier must be kept as short as possible, and loop area between the amplifier output and its return path (either another output or ground) must be as small as possible.

The wires from the circuit board to the speaker should be twisted together in order to keep the distance between them as small as possible. Of course, these measures are good practice for Class-D amplifiers with filters also.

The second article in this series offers some pc-board layout guidelines designed to help optimize the performance and reliability of Class-D amplifiers.

About the authors:
John Widder is a Market Development Manager at STMicroelectronics. During his eight years at ST, John has focused on design and development support for printers and audio products. Before joining ST, John spent 20 years working in printer design and development. John has a BSEE from the University of Portland and a Master's degree in Engineering Management from Washington State University.

Yun-tao Zhao is a Senior Application Engineer at STMicroelectronics where he focuses on design and support for audio products. Prior to joining ST, Yun-tao spent four years in consumer audio video electronics product design and development. Yun-tao has a BSEE from Xi'an Jiao tong University.

Related links:
Class D Audio Amplifiers: What, Why, and How
How Class D audio amplifiers work
Design and analysis of a basic class D amplifier
Filterless Class D simplifies audio amplifier design
Webcast: LC filter design for Class-D audio vs. switching supplies
Why Class D Amplifiers May Test Well But Often Sound Terrible