8.3.13 Efficiency: LC Filter Required with the Traditional Class-D Modulation Scheme

Many traditional Class-D amplifiers are based on the AD modulation. Due to the out-of-phase nature of a BTL or PBTL amplifier operating in the AD modulation, if no LC filter was present, the load sees the full PWM signal across its terminals. This causes a high-frequency ripple current to pass through the load, which leads to high power dissipation, poor efficiency, and potential speaker damage. The ripple current is large in the AD modulation scheme, because it is proportional to voltage multiplied by the time at that voltage. The differential voltage swing is 2 × VCC, and the time at each voltage is half the period for the AD modulation scheme. An ideal LC filter is required to store the ripple current from each half cycle for the next half cycle, while any resistance causes power dissipation. The speaker is both resistive and reactive, whereas an LC filter is almost purely reactive.

The modulation schemes implemented in the TPA3126D2 have little loss in the load even without a filter because the pulses are short and the change in voltage is VCC instead of 2 × VCC. As the output power increases and the pulses widen, the ripple current can go up. In this case, the ripple current can be filtered with an LC filter for increased efficiency. However, in most applications the filter is not required.

With an LC filter, specifically as the cut-off frequency of the LC filter is smaller than the PWM switching frequency of the amplifier, the ripple current is reduced such that only a small residual ripple voltage is present after the LC filter. The filter has less resistance but higher impedance at the switching frequency than the speaker, which results in less power dissipation, hence increasing efficiency.