AMT Datasouth Accel-7350, Service Manual

Page: 91 / 119

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Power Supply PCB
Outline
This is a self-oscillating, RCC type switching power supply circuit. The output voltage is
controlled by the switching frequency. The rated input is 120V, 60Hz for U.S.A. and 100V for
JAPAN setting (adding to the Jumper at CN2) and 230V, 50Hz for European setting (Removing
the Jumper at CN2). The output voltages are +40V, and +5V. To stabilize the output voltages,
the switching frequency of the HIC (P1) in the primary circuitry is controlled by the signal feed
back from the +40V output voltage detection circuitry through the photo-coupler (PC2).
Operations
See the voltage waveform and circuit diagrams.
(1) Power switch is turned on.
(2) The DC voltage Vin is converted through the rectifying circuitry (DB1, L2, C7, C8) and is
supplied to the HIC (P1) through initial one-time flow resistor (R5). The P1 starts oscillating
when the 4 pin of P1 reaches 16V.
(3) Initiated by the oscillation of P1, the Power MOSFET (in P1) is turned ON, then the current
starts flowing through C7 (+) → TR1 (5 → 2) → P1 (4 → 2) → R2 → C8 (-).
(4) The drain current at P1 increases in proportion to the ON time interval of P1 due to the
transformer (TR1) inductance.
(5) When P1 is ON and the current flows in the primary coil (5-2) of the transformer (TR1), the
voltage is generated on the control coil (6-7) of the TR1. The voltage is rectified through D2
and C11 and is supplied to the Vcc of the P1 at approximately 20V to sustain the oscillation
in P1. At that time, the voltages for 40V is also generated in the secondary coil (12,13-15,16)
of the TR1.
(6) The voltage in the secondary coil (12,13) is rectified through the diode D7 and smoothed by
C16 and C17.
(7) When the voltage e40V in the output and reaches over the rated value of the Zener
diode (ZD5) at the voltage level divided down by the ratio of R19 and R20, the transistorQ1)
is turned on. Simultaneously, the photo-couplerPC2) is turned on. Consequently it increases
the voltage on OCP/FB1 pin) of P1 and the switching frequency of P1, and it decreases the
output voltages. When the output voltages decrease below 40V the PC2 is turned off. It in
turn, increases the output voltages.
(8) By the repetition of the process at step 7 above, +40V output voltage is stabilized. Because
the current flow in the PC2 responds linearly and inversely proportional to the load, the
switching frequency of the P1 is controlled to stabilize at the +40V output.
(9) +5V output voltage is also leveled to a constant value by the chopper operation in the
regulator IC (P2) and smoothing circuit. +40V output voltage flows into P2 and generates a
square waveform with a different duty ratio by the switching on and off operation (chopper
operation). These square waves are flattened and smoothed out to be in 5V voltage level in
the rectifying circuitry