General; Operations - Acroprint ATR120 Service Manual

9-1. General

This circuit is a forward converter type switching power supply. It controls the output voltage by pulse width
modulation. The ON duty of the power MOSFET is controlled by negative feedback from the secondary output
through a photo-coupler in order to produce a stable output voltage.

9-2. Operations

1 start up
Connect the AC cable to the socket. The secondary voltages are output in the following order:
The input AC power is rectified and smoothed by a diode bridge (PD1), capacitor PC1. The rectified and
smoothed voltage is supply through a starting resistor (PR6) to a switching regulator control IC (PU1). When the
voltage reaches the start-up voltage (16V), PU1 begins to oscillate.
As soon as PU1 begins to oscillate, the MOSFET (PQ1) goes active, and current flows in the order of PC2,
T1, PQ1, PR1.
When current flows the main coil of the transformer (T1), Voltage is produced in the auxiliary coil (5-4), The
voltage rectified and smoothed by PD3, PR7 and PC7 is supplied to PU1, which keeps oscillating.
At this time, voltage is produced in the secondly coil (6-10) of the transformer (T1), yielding output.
2 oscillation
The oscillation frequency of PU1 depends on PC9, PC10 and PR8, and is about 100Khz. The period when the
output is turned on depends on the product of the capacitance of PC10 and resistance PR8. The period when the
output is turned off without fail depends on the product of the PC9.
3 Voltage control
If the secondary main output voltage to be controlled (on the anode fide of diode PD3) exceeds the set value
(approx 11.5V), the reference voltage of PQ1 increases, the current flowing through the photo-coupler (PU3)
increases, the voltage at the F/B pin on PQ1 drops, the output ON duty lowers, and the output voltage is reduced. On
the contrary, of the output voltage is dropping, the output ON duty increases and accordingly the output voltage
increases.
4 Current limit circuit
The voltage at both ends of PR1 is detected at the CLM-pin of PU1to control the drain current flowing through
the MOSFET PQ1. When this voltage reach approx. 200mV, the output is cut off between that point and following
cycle.
5 Over-current protector circuit
When the current limit is turned on and the voltage at the VF pin of PU1 is approx. 3V or less, and intermittent
operation circuit functions to cause intermittent oscillation.