EPR-89 Datasheet, PDF (7/32 Pages) Power Integrations, Inc. – Engineering Prototype Report for 2.0 W CV Adapter using LNK362P

08-Nov-2005
EP-89 6.2 V, 322 mA Adapter
4 Circuit Description
This converter is configured as a flyback. The output voltage is sensed and compared to
a reference (VR1) on the secondary side of the supply, and the results are fed back to U1
(LNK362P) through optocoupler U2 (PC817A). This enables U1 to tightly regulate the
output voltage across the entire load range. Past the point of peak power delivery, U1
will go into auto-restart, and the average power delivered to the load will be limited to
about 5% of full load. This circuit takes advantage of Power Integrations Clampless™
transformer techniques, which use the primary winding capacitance of the transformer to
clamp the voltage spike that is induced on the drain-node, by the transformer leakage
inductance, each time the integrated MOSFET switch within U1 turns off. Therefore, this
converter has no primary clamp components connected to the drain-node.
4.1 Input Filter
Diodes D1 through D4 rectify the AC input. The resulting DC is filtered by bulk storage
capacitors C1 and C2. Inductor L1 and capacitors C1 and C2 form a pi (π) filter that
attenuates differential-mode conducted EMI noise. Resistor R1 dampens the ringing of
the EMI filter. L2 also attenuates conducted EMI noise in the primary return. This
configuration, combined with the LinkSwitch-XT‘s integrated switching frequency jitter
function and Power Integrations E-shield technology used in the construction of the
transformer enable this design to meet EN55022 Class-B conducted EMI requirements
with good margin. An optional 100 pF Y capacitor (C4) can be used to improve the unit-
to-unit repeatability of the EMI measurements. Even with C4 installed, the line frequency
leakage current is less than 10 µA.
4.2 LNK362 Primary
The LNK362P (U1) has the following functions integrated onto a monolithic IC: a 700 V
power MOSFET, a low-voltage CMOS controller, a high-voltage current source (provides
startup and steady-state operational current to the IC), hysteretic thermal shutdown and
auto-restart. The excellent switching characteristics of the integrated power MOSFET
allows efficient operation up to 132 kHz.
The rectified and filtered input voltage is applied to one side of the primary winding of T1.
The other side of the T1 primary winding is connected to the DRAIN pin of U1. As soon
as the voltage across the DRAIN and SOURCE pins of U1 exceeds 50 V, the internal
high voltage current source (connected to the DRAIN pin of the IC) begins charging the
capacitor (C3) connected to the Bypass (BP) pin. Once the voltage across C3 reaches
5.8 V, the controller enables MOSFET switching. MOSFET current is sensed (internally)
by the voltage developed across the DRAIN-to-SOURCE resistance (R_DS(ON)) while it is
turned on. When the current reaches the preset (internal) current-limit trip point (I_LIMIT),
the controller turns the MOSFET off. The controller also has a maximum duty cycle
(DC_MAX) signal that will turn the MOSFET off if I_LIMIT is not reached before the time
duration equal to maximum duty cycle has elapsed.
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