EPR-89 Datasheet, PDF (8/32 Pages) Power Integrations, Inc. – Engineering Prototype Report for 2.0 W CV Adapter using LNK362P

EP-89 6.2 V, 322 mA Adapter
08-Nov-2005
The controller regulates the output voltage by skipping switching cycles (ON/OFF control)
whenever the output voltage is above the reference level. During normal operation,
MOSFET switching is disabled whenever the current flowing into the FEEDBACK (FB)
pin is greater than 49 µA. If less than 49 µA is flowing into the FB pin when the
oscillator’s (internal) clock signal occurs, MOSFET switching is enabled for that switching
cycle and the MOSFET turns on. That switching cycle terminates when the current
through the MOSFET reaches I_LIMIT, or the DC_MAX signal occurs*. At full load, few
switching cycles will be skipped (disabled) resulting in a high effective switching
frequency. As the load reduces, more switching cycles are skipped, which reduces the
effective switching frequency. At no-load, most switching cycles are skipped, which is
what makes the no-load power consumption of supplies designed around the
LinkSwitch-XT family so low, since switching losses are the dominant loss mechanism at
light loading. Additionally, since the amount of energy per switching cycle is fixed by
I_LIMIT, the skipping of switching cycles gives the supply a fairly consistent efficiency over
most of the load range. [NOTE * Termination of a switching cycle by the maximum duty
cycle (DC_MAX) signal usually only occurs in an abnormal condition, such as when a high-
line-only design (220/240 VAC) is subject to a brown-out condition, where just slightly
over 50 V (the minimum drain voltage required for normal operation) is available to the
supply, and the current through the MOSFET is not reaching I_LIMIT each switching cycle
because of the low input voltage.]
4.3 Feedback
The output voltage of the supply is determined by the sum of the voltages developed
across VR1, R2 and the (forward bias voltage) LED in optocoupler U2A. As the supply
turns on and the output voltage comes into regulation, U2A will become forward biased,
which will turn on its photo-transistor (U2B) causing > 49 µA to flow into the FB pin, and
the next switching cycle to be skipped. Resistor R2 limits the bias current through VR1 to
about 1 mA. Resistor R3 can be used to fine-tune the output voltage, and also limits the
peak current through U2A during load transients. Since the controller responds to
feedback each switching cycle (the decision to enable or disable MOSFET switching is
made right before that switching cycle is to occur), the feedback loop requires no
frequency compensation components.
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