Functional Description - 5V/4A Module (Pl320Qmt); Circuit Description - 5V/4A - TTI PL Series Service Manual

Functional Description - 5V/4A Module
This is very similar to the linear section of the 5V 7A module. Refer to this section for the
description.

Circuit Description - 5V/4A

Power Supply - Mains
This consists of a full wave bridge rectifier feeding the reservoir capacitor C1. The full load
secondary voltage is 11V rms giving approximately 15Volts off load and 12V on full load.
Power Supply - Auxiliary
This consists of twin full wave rectified supplies from a 21V centre tapped winding.
D10 and Q5 generate +7V5 and D9 the -5V.
0V of this supply is connected to the positive output at R32.
Voltage Control
IC1A buffers the voltage control pot and VR5 sets maximum output voltage of the unit. D8 is the
reference which is nominally 2.45V.
IC1B is a differential amplifier with a voltage gain of 13.3. For 6V output this means the voltage at
IC1A will be 450mV. VR4 trims differential gain and effects voltage regulation. This can be set
more accurately by generating a few hundred millivolts between positive output and positive
sense, see calibration section.
Current Control
IC1D is the error amplifier and compares the voltage on the wiper of VR3 with the voltage
generated across the current sense resistor R32. When this limit is reached IC1D takes control.
This turns on Q1 which lights the current limit LED.
To limit power dissipation in the series regulator the power supply has foldback current limiting.
When the output falls below 3.5V IC1C reduces the output current by reducing the voltage on
VR3.
Over Voltage Protection
When the voltage on the output terminals exceeds 7V, SCR1 is fired crowbaring the output. When
the output switch is turned off, Q4 is turned on, which turns the series regulator off, which allows
SCR1 to turn off.
18
(PL320QMT)