Bose Lifestyle PS 28 Troubleshooting Manual page 9

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.
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