19.3 THEORY OF OPERATION
Input power is fed through terminals A23-25, C23-25 and A15-17, C15-17. The first circuits provide fusing and
EMI suppression.
Diode CR1 protects the power supply form reversal of input voltage. Under such conditions, a fuse on the power
supply clears. Inrush limiter TH1 allows charging of capacitor C5 from a stiff dc source without excessive input
currents.
The forward converter is designed around PWM integrated circuit U1. The device has an internal clock set by
resistor R12 and capacitor C11. Power for the device is initially provided by a series regulator consisting of
MOSFET Q3, resistors R2 and R8, and diode CR5. With power applied and internal clock set, pin 11 of U1
goes high causing MOSFET’s Q1, Q2, and Q5 to conduct. This causes the dc bus voltage to be applied across
windings 1 and 2 of transformer T1 and conduction of output diodes CR9, CR11, and CR14. Upon conduction
of these diodes, power flows from the input to the output of the power supply. The dc bus current, which flows
through MOSFET’s Q1 and Q2 is detected across resistors R5 and R6 and is monitored at pins 3 and 4 of U1.
When the peak increases to a level set at pin 7 of U1, pin 11 of U1 is set low causing transistor Q4 to conduct.
This drives MOSFET’s Q1 and Q2 to an off state. Transformer T1 voltage reverses and diodes CR21, CR9 (op-
posite device), CR11 (opposite device) conduct and circulate current stored in inductor L3. Inductor L3 and ca-
pacitors C24, C26, and C34 form a low pass filter producing a dc that is the half cycle average voltage produced
by transformer T1. Inductor L3 is a multi-winding inductor that is matched to transformer T1.
The auxiliary winding of transformer T1, terminals 5 and 4, provides a source of power for the supply’s control
circuit. After the first few cycles, the voltage at capacitor C17 increase rendering MOSFET Q3 non-conductive.
The secondary auxiliary winding of transformer T1, terminals 3 and 4, capacitor C6, and diode CR2, is a clamp
to limit voltage transients across MOSFET’s Q1 and Q2.
The 5 Vdc output is unique because its level is monitored across diode CR102, resistors R113 and R115, and
potentiometer R114.The wiper arm of potentiometer R114 is fed to pin 6 of dual error amplifier U3 and is com-
pared against an internal reference set at pin 7 of U3. If the 5Vdc output is greater than the reference, pin 14 of
U3 goes high causing optical isolator U2 to conduct. This reduces the voltage applied to pin 5 of integrated cir-
cuit U1, reduces the peak current through transformer T1, and lowers the output voltage. The negative feedback
path is compensated with resistor R110 and C104.
To protect the 5 Vdc output from excessive output currents, the voltage across shunt R122 is monitored be-
tween pins 2 and 3 of U3. Like the voltage feedback path, exceeding limits, established by resistors R102 and
R103, causes pin 14 to go high and reduces the output voltage.
Integrated circuits U102 and U103 are series regulators which provide post regulation for the secondary out-
puts. Post regulation is required to meet the voltage regulation requirements of the supply. Feedback for regula-
tor U102 is provided by resistors R117, R118, potentiometer R116, and CR103. Feedback paths for the other
regulators are similarly structured.
NOTE
RFL 9300 power supply modules are equipped with several protection circuits. These circuits will
cause the module to shut down if its voltage, current, or temperature limits are exceeded. If the power
supply has shut down, it can be reset by placing power switch S1 to the OFF position, and then plac-
ing it back to the ON position. If it shuts down again, troubleshoot the RFL 9300's chassis to make
sure that nothing external to the power supply is causing it to shut down. If nothing is found in the rest
of the chassis and the power supply is still shutting down, the power supply may be defective, and
should be serviced.
RFL 9300 RFL Electronics Inc.
May 5, 2002 19 - 3 (973) 334-3100
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