DER-57
Industrial – LNK501P
April 20, 2005
4 Circuit Description
The schematic shown in Figure 2 provides two isolated outputs from a universal input
voltage range of 90 VAC to 265 VAC.
4.1 Input Stage
The incoming AC is rectified and filtered by D1, D2, D3, D5, D6, D7, D8, R4, C2, and C8
with a high power factor. Resistor R1 is a flameproof fusible type to protect against fault
conditions and is a requirement to meet safety agency fault testing. Please consult with a
safety engineer or local safety agency.
L1 and L2 provide differential mode filtering of switching frequency noise generated by
the LNK500 power stage. C6 provides local energy storage for the Flyback stage. C5 is a
safety X1 capacitor that helps filter switching noise of the bridge rectifier and maintains
compliance to the conducted EMI requirements of EN55022 Class B in conjunction with
safety Y1 capacitor C9.
4.2 LinkSwitch Operation
When power is applied to the supply, high voltage DC appears at the DRAIN pin of
LinkSwitch (U1). The CONTROL pin capacitor C1 is then charged through a switched
high voltage current source connected internally between the DRAIN and CONTROL
pins. When the CONTROL pin voltage reaches approximately 5.7 V relative to the
SOURCE pin, the internal current source is turned off. The internal control circuitry is
activated and the high voltage internal MOSFET starts to switch, using the energy in C1
to power the IC.
As the current ramps in the primary of Flyback transformer T1, energy is stored. This
energy is delivered to the output when the MOSFET turns off each cycle.
The secondary of the transformer is rectified and filtered by D4 and C7 to provide the DC
output to the load.
Control of the output characteristic is entirely sensed from the primary-side by monitoring
the primary-side VOR (reflected output voltage). While the output diode is conducting, the
voltage across the transformer primary is equal to the output voltage plus diode drop
multiplied by the turn’s ratio of the transformer. Since the LinkSwitch is connected on the
high side of the transformer, the VOR can be sensed directly.
Diode D9 and capacitor C3 form the primary clamp network. The voltage held across C3
is essentially the VOR with a small error due to the parasitic leakage inductance.
The LinkSwitch has three operating modes determined by the current flowing into the
CONTROL pin.
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