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PS18/28/35 Troubleshooting Guide

Theory of Operation

Figure 11. Over-temperature and AMP_FAULT protection circuit

A positive-temperature-coefficient (PTC) resistor RT100 is placed next to a heat sink for two power

MOSFETs. When the PTC RT100 is heated up to 125°C by the heat sink, the resistance of the

PTC increases dramatically, resulting in a logic-high at the PTC. Independently, when any audio

amplifier is in fault condition, the AMP-FAULT is pulled down, setting a logic-high at the output of a

second opto-coupler U103. Either over-temperature or amplifier fault sends a voltage pulse to pin-

8 of the resonant controller IC, shutting down the IC and hence the power supply.

Referring to the resonant controller IC circuit shown in Figure 3, the Vs-pin (pin-12) of the IC will

be slowly charged up beyond 12V in shutdown mode by a current through R156 since the IC

consumes very little power after it is shut down. Zener diode ZR101 starts conducting, charging

voltage at pin-9 up. Once the voltage at pin-9 reaches 1.2V, the IC is enabled. The power supply

resumes operation. The above shut down and recovery process repeats until both over-tempera-

ture and AMP-FAULT conditions are removed.

During normal operation, the resonant controller IC is powered by rectifying the dv/dt current

through capacitor C114 that is connected across MOSFET Q101 (Figure 5).

An overvoltage protection circuitry shown in Figure 12 is designed to protect the rail-to-rail voltage

from exceeding 52V. If the rail-to-rail voltage exceeds 52V, Zener diode ZR300 starts conduction,

activating transistor Q301 and hence transistor Q300. Current to opto-coupler U102 is pulled away.

Figure 12. Overvoltage protection circuit

The consequence of such is a lower rail-to-rail voltage.

A power-down circuit, shown in Figure 13, is designed for the output voltage to decay slowly after

the power switch on the AC line is turned off. Upon detection of low voltage at the AC line, transis-

tor Q104 turns on, turning on transistor Q103. With the turn-on of transistor Q103, the power

converter operates in an open-loop manner. The output voltage of the power supply follows the