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Design Considerations

5.2.3 Voltage Rail Switching

The feedback loop to regulate the rail voltage uses the optocoupler U8 and the shunt regulator U4. When

Q10 is OFF the voltage is regulated to approximately 26V. When the Clip-Detect signal goes high, the

gate of Q10 is charged and the transistor turns ON effectively paralleling R37 and R38. At the same time

Q17 is turned ON signaling, through the optocoupler U9, to turn ON the PFC converter. When the clip

detect signal goes low again, the gate of Q10 is slowly discharged. The rail voltage is kept at 50V until the

gate voltage drops into the active region where Q10 starts to behave as a resistor slowly increasing in

value. This ensures slow rail voltage ramp down. During this action the boost PFC will shut down as the

gate voltage of Q17 reduces to below the threshold.

5.2.4 OVP

The overvoltage protection kicks in at about 53V. The divider of R50 and R53 together with U6 senses the

rail voltage and signals through U9 to the primary side. If the OVP is triggered, the current through the

optocoupler is sufficiently large to discharge the soft start capacitor C32. The supply restarts using soft

start after the rail voltage is below the OVP limit.

5.2.5 Power Limit

The power limited looks at the secondary output current across sense resistors R104 and R105 connected

in parallel. The effective resistance of 2.5mΩ gives a low voltage drop maximizing efficiency. The low

sense voltage is amplified by U12, a current shunt monitor – INA210, using U7 as reference voltage, and

U13 as comparator. The ratio R56/R45 can be used to adjust the current limit.

When the power limit is reached its recommended to switch the output from 50V to 25V or turn down the

audio signal in the analog front end.

5.3 Auxiliary Converter

Follows standard design procedure when using the UCC28600.

SLOU293C–June 2010–Revised September 2012 A 600-W, Isolated PFC Power Supply for AVR Amplifiers Based on the

TAS5630 and TAS5631

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