Adaptive low-pass filter for zero-crossing detection

A method for detecting a zero crossing in a power signal, comprising:

• receiving an input signal indicative of the power signal at a low-pass filter (LPF) which introduces a phase shift dependent on a frequency of the input signal;
• passing an output of the LPF through filter circuitry which applies a fixed phase shift thereto and which adjusts phase of the output of the LPF based upon received control signals to produce a filtered output signal;
• detecting zero crossings of the input signal and the filtered output signal and asserting a digital zero cross signal at each zero crossing, and determining a phase shift and DC offset correction between the input signal and filtered output signal;
• and
• generating the control signals based upon the determined phase shift and DC offset correction such that a total phase shift between the input signal and the filtered output signal remains constant and there is a same duty cycle between the input signal and the filtered output signal to thereby provide for accurate detection of the zero crossings of the input signal.

A circuit configured for detection of a zero crossing in a power signal, comprising:

• a low-pass filter (LPF) configured to receive an input signal indicative of the power signal and to introduce a phase shift dependent on the frequency of the input signal;
• filter circuitry configured to:
• receive output of the LPF and to apply a fixed phase shift thereto, and adjust phase of the output of the LPF based on control signals to produce a filtered output signal;
• and
• control circuitry comprising:
• a zero crossing detector configured to receive the input signal and the filtered output signal, detect zero crossings of the input signal and the filtered output signal, assert a digital zero cross signal at each zero crossing, and determine a phase shift between the input signal and filtered output signal;
• and
• a controller configured to generate the control signals, based upon the phase shift, such that a total phase shift between the input signal and the filtered output signal is constant and there is a same duty cycle between the input signal and the filtered output signal, thereby providing for accurate zero crossing detection by the zero crossing detector.