SoundCraft CPS275 - Manual (Page 18)

Circuit Description
The CPS250 and CPS275 power supplies provide +/-17V at 4 Amps and 5.25 Amps
respectively. Both give +5V at 1 Amp, and +48V at 0.3 Amp. The PCB is SC3803.
THE +/- 17V RAILS.
This consists of two identical +17V supplies connected together to give +/-17V.
Each 17V supply is a linear regulator with conventional fullwave rectification and
20,000uF reservoir capacitors. This is combined with a second power supply that sits
on top to power the driver transistors and the control IC; this reduces the minimum
voltage drop across the series-pass device and so improves efficiency.
The main series-pass element is a 250W discrete bipolar transistor mounted on a
fan-cooled heatsink. The voltage reference and servo control amplifier are provided
by a 723 regulator IC.
The circuitry of the +17V regulator is described in detail below:
The +17V Supply.
The transformer secondary is fused by F1. Secondary fuses will normally only blow
in the event of serious failure, such as a major short in the AC wiring or a failed bridge
rectifier.
The fullwave bridge rectifier is mounted on the heatsink, at the hottest end as it is the
most heat-tolerant of the semiconductors. For safety reasons, the reservoir capacitor
is discharged by bleeder resistors R39,40 at switchoff; this takes about 10 seconds.
The unregulated DC voltage is approx +26 to +29V.
The 723 IC contains a 7.15V nominal Zener diode, defining the voltage that appears
at Pin 6. R5 is in series with the positive input of the servo opamp (Pin 5) and is made
roughly equal in resistance to the feedback divider R7,11 etc to minimise bias-current
offsets.
The feedback divider R7,8,11, & PR1 derives a fraction of the output voltage and
delivers it to the negative input of the servo opamp. (Pin 4) The negative feedback
keeps this point also at 17.15V, so the actual output voltage is determined by the
feedback divider ratio, which is trimmed over a narrow range by PR1.
The output of the servo opamp controls the output through TR1, an emitter follower
driving the series-pass device TR3. The power supply to TR1 is taken from the +12V
supply developed across C23. This has its 0V side connected to the positive
unregulated DC supply of 26 - 29V, (at nominal mains) giving a total voltage of approx
38 - 41V for the driver; this also powers the 723 IC via R1. This subrail markedly
reduces the minimum drop across the pass elements and therefore increases the
efficiency.
10 Amp Schottky diode D25 on the output allows supplies to be paralleled for
redundancy. This is only fitted for the CPS275 option.
Overcurrent Protection.
The power supply is protected against short-circuits and overload by a dual-mode
protection system; brief overloads are dealt with by constant-current protection, while
prolonged overload causes both 17V rails to shutdown. Reset is by turning the mains
power off for not less than 6 seconds.
14 Technical Description