A multiphase pulse width modulator for class d audio amplifiers

[0006]A first aspect of the invention relates to a multiphase pulse width modulator producing N mutually phase shifted pulse width modulated signals having a predetermined cycle time. The multiphase pulse width modulator is particularly well-suited for application in class D audio amplifiers, but may be useful in numerous other applications such as power inverters benefitting from accurate and well-matched phase shifted pulse width modulated signals. The multiphase pulse width modulator comprises N+1 analog triangular waveform generators configured to produce respective N+1 mutually phase shifted analog triangular waveforms each comprising a rising segment with substantially linearly increasing signal amplitude, a falling segment with substantially linearly decreasing signal amplitude and an idle signal segment with substantially constant signal amplitude such that a duration of the rising segment and falling segments corresponds to the predetermined cycle time and a duration of the idle segment corresponds to a phase shift between two adjacent phases of the N mutually phase shifted pulse width modulated signals. The multiphase pulse width modulator further comprises N+1 comparators each comprising a first input operatively coupled to respective ones of the N+1 mutually phase shifted analog triangular waveforms and a second input coupled to an audio signal to generate at least N+1 mutually phase shifted pulse width modulated phase signals. A crosspoint or matrix switch comprises N+1 input terminals, coupled to respective ones of the N+1 mutually phase shifted pulse width modulated phase signals, and N output terminals configured for supplying respective ones of the N mutually phase shifted pulse width modulated signals. A crosspoint switch controller of the cross-point switch is configured to selectively connect each of the N+1 input terminals to each output terminal of the N output terminals for a duration of the predetermined cycle time in a predetermined time sequence to simultaneously generate the N mutually phase shifted pulse width modulated signals such that each of the signals comprises interleaved time segments of the N+1 mutually phase shifted pulse width modulated phase signals;

Both of these modulation schemes generate rather large ripple current in the output inductor of the LC lowpass filter when the class D amplifier is idling which cause significant power losses.

However, such large inductors lead to significant increase of costs and size of the class D amplification solution or assembly.

However, generating such accurate multiphase PWM signals presents a significant challenge for various reasons, in particular generating sufficiently well-matched signal phases of multiphase analog triangular waveforms produced by a number of separate analog triangular waveform generators.

While it is possible to digitally control a phase shift and frequency or time period of each of these analog triangular waveforms by the application of clock frequency locked digital control signals, maintaining accurate control of the amplitude and offset voltage of the multiphase triangular waveforms produced by such separate triangular waveform generators presents a challenge.

While certain well-known integrated circuit design and layout techniques can be utilized to reduce these component variations between the separate analog triangular waveform generators these techniques are insufficient or impractical to make such separately generated multiphase triangular waveforms sufficiently accurate to reach optimal performance in multilevel Class D amplifiers.

This lacking accuracy or matching of the multiphase PWM signals degrades various important performance metrics of multilevel Class D amplifiers such as power efficiency, flying capacitor stability and general audio performance.