Bose Lifestyle 28 Troubleshooting Manual - Page 5

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PS18/28/35 Troubleshooting Guide

Theory of Operation

AC power comes from line source to an EMI filter first, which prevents noise generated by high

frequency switching from flowing back to the line source. A bridge rectifier BR1 converts AC power

into DC power. There are three versions of the power supply; US, Euro, and Dual. For the US

version, the bridge rectifier BR1 is configured to a voltage doubler rectifier. For the Euro version,

the bridge rectifier BR1 is configured to a full wave rectifier. For the Dual version, an automatic

voltage selector IC turns on or turns off the triac Q401 depending on the line voltage, re-configur-

ing the bridge rectifier into a full wave rectifier for 220V/240V line or a voltage doubler rectifier for

100V/120V line. Electrolytic capacitors C110 and C111 filter out line frequency and its harmonics.

A DC voltage at about 340V is obtained across the two capacitors regardless of the line voltage

variation.

The DC voltage is then inverted into a high-frequency quasi-square voltage by two power

MOSFETs Q101 and Q102 which are turned on and off at a high frequency (100kHz ~ 200kHz) by

the controller IC U101. The high-frequency quasi-square voltage is applied to the primary winding

of the center-tap transformer T102 via a capacitor C115. Capacitor C115 and leakage inductance

of the transformer form a resonant circuit, which shapes the resonant current waveform into a

quasi-sinusoidal waveform. The power to the transformer is controlled by adjusting the switching

frequency relative to the resonant frequency. A rectifier connected to the secondary of the trans-

former converts the high-frequency power into DC power and a low pass filter removes high-

frequency contents. DC power is obtained at two output terminals. This circuit which converts DC

power to DC power is called a half-bridge high-frequency resonant DC-DC converter.

A negative feedback circuit made of an integrator, a nonlinear amplifier, an opto-coupler, and a

controller IC controls the above power converter in such a way that the output voltage of the power

converter follows or tracks the power supply control (PSC) signal, which is generated at the DSP

board to track the audio signal.

In addition to the above basic functions, overvoltage, amplifier fault, and over temperature protec-

tion circuits are designed to prevent the power supply and amplifiers from catastrophic failure. The

power-down circuit is also designed so that the power supply outputs decay slowly when the AC

input power is turned off. Two linear regulators are tapped off the two output rails to provide two

regulated low voltage rails.

The high-frequency transformer and the opto-coupler provide electrical isolation between the AC

line source and audio circuitry that is powered by the power supply.

Since the transformer is operated at a high frequency, it can be designed to be very small and yet

very efficient. The size of power supply is significantly reduced compared with a power supply

using a line-frequency transformer.

1.3 Resonant Controller IC

This section describes the high-voltage resonant controller IC that controls the resonant power

converter which is the brain of the power supply. Figure 2 shows the block diagram of the IC. Its

main function is to generate a high-frequency signal at a voltage-controlled-oscillator (VCO) and to

drive two power MOSFETs in a half-bridge circuit.

Figure 2. Block Diagram of high-voltage resonant controller IC