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PS18/28/35 Troubleshooting Guide

Theory of Operation

1.5 Feedback Control Circuit

The feedback circuit controls the above power converter in such a way that its output voltage

follows or tracks the power supply control (PSC) signal. This section describes the feedback circuit

shown in Figure 9.

Figure 9. Feedback circuit

The feedback control circuit senses the negative output of the power converter and compares it

with the PSC signal. The error is amplified by an integrator. The amplified error is compensated by

a nonlinear amplifier for loop stability and is then applied to the resonant controller IC via an opto-

coupler.

If the magnitude of the negative voltage is lower than what the PSC signal demands, the output

voltage of the opto-coupler is higher, lowering the oscillation frequency of the resonant controller

IC. The IC moves the switching frequency closer to the resonant frequency of the power converter,

increasing the magnitude of output voltage. Such a negative feedback control minimizes the error

between the rail voltage and the PSC signal. The voltage at the power supply output terminals

tracks the audio signal. Figure 10 shows waveforms of a PSC voltage, two output voltages, and an

amplifier output voltage for a tracking power supply used as power source for one audio amplifier.

The PSC is derived from the audio signal and an offset is added for voltage drop at the amplifier

(amp saturation voltage). The two rails track PSC very well.

Figure 10. Power supply waveform showing tracking voltage

1.6 Protection Circuits and Others

This section describes protection circuits and some other circuits that are added to the power

supply. Figure 11 shows an over-temperature protection circuit and an AMP_FAULT protection

circuit.