Bose Lifestyle PS 28 Troubleshooting Manual page 5

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
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