DER-75
2.4W Power Supply using LNK520P
September 12, 2005
4 Circuit Description
4.1 Input EMI Filtering
Resistor RF1 acts as a fuse for the entire power supply and also limits different surge.
Diodes D1, D3, D5, D7 and Capacitors C5, C6 rectify and filter the input waveform to
produce a high voltage DC-bus. Inductors L1 and L3 work in conjunction with C5 and C6
to filter and attenuate conducted EMI.
4.2 LinkSwitch Primary
Diode D10 and capacitor C12 rectify and filter the bias voltage. The diode is in a low-side
configuration to allow the bias-winding to act as a primary cancellation winding.
Components C3, R3, R4, and D4 form an RCD clamp to capture the leakage spike at
Drain turn-off. A slow diode (D4) is used to allow recovery of some of this leakage
inductance energy. The remainder is captured in C3 and dissipated in R3.
4.3 Output Rectification
Output diode D2 and capacitor C7 rectify and filter the output voltage.
4.4 Output Feedback
Resistor R9 is used to bias the Zener reference (VR1), adjusting this resistor will adjust
the output voltage. Resistor R10 is used to control the opto-coupler current, and
depending on the value can also change the output voltage set-point. The opto-coupler
U2 transfers the feedback signal across the isolation barrier to the primary side of the
supply. Resistor R6 set’s the maximum power point before the supply transitions into
constant current mode.
4.5 No Load Consumption
No-load consumption is affected by the choice of bias winding components. Use of a
slow diode (1N4005GP) makes no-load consumption worse (by about 50mW at low-line),
but gives the best CC regulation. Use of a fast diode D6 (such as 1N914) on the bias
winding dramatically improves no-load consumption. However use of a fast diode also
makes the constant-current (CC) regulation significantly non-linear. The absence of an R-
C snubber on the bias-winding, slightly improves the no-load consumption (10mW at low-
line). Also the choice of Zener current setting resistor R9 affects the no-load consumption
(again by approx 10mW at low-line). The value of resistor R7 has no effect on the no-load
consumption. The set-point of the output voltage has a significant effect on the no-load
consumption (high output voltage, higher no-load consumption) – e.g. going from Vout =
5.44 V to Vout = 6.5 V, the no-load consumption increased by 50mW.
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