DER-56
UPS Battery Charger - LNK520
April 20, 2005
Constant current (CC) operation controls secondary-side output current by reducing the
primary-side current limit. The current limit reduction characteristic has been optimized to
maintain an approximate constant output current as the output voltage and bias voltage is
reduced.
If the load is increased further and the CONTROL pin current falls below approximately
0.8mA, the CONTROL pin capacitor C5 will discharge and LinkSwitch will enter auto-
restart operation.
Current limit control removes the need for any secondary-side current sensing
components. Removing the secondary sense circuit dramatically improves efficiency,
giving the associated benefit of reduced enclosure size.
Diode D5, C4, R1, and R2 form the primary clamp network. This limits the peak DRAIN
voltage due to leakage inductance. Resistor R2 allows the use of a slow, low cost rectifier
diode by limiting the reverse current through D5 when U1 turns on. The selection of a
slow diode improves radiated EMI and also improves CV regulation, especially at no
load. A glass passivated diode should be used with specified recovery time.
4.3 Output Rectification
Output rectification is provided by Schottky diode D7. The low forward voltage provides
high efficiency across the operating range. Low ESR capacitor C6 achieves minimum
output ripple and maximizes operating efficiency.
4.4 Output Feedback
Resistors R6 and R7 divide down the supply output voltage and apply it to the reference
pin of error amplifier U3. Shunt regulator U3 drives Optocoupler U2 through resistor R8 to
provide feedback information to the U1 CONTROL pin. Capacitor C7 rolls off the gain of
U3 and is sufficient to compensate the control loop of the power supply.
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