EPR-54 – LinkSwitch 2.75 W Low Cost Charger / Adapter
07-May-2004
When power is applied, high voltage DC appears at the DRAIN pin of LinkSwitch (U1).
The CONTROL pin capacitor C5 is then charged through a switched high voltage current
source connected internally between the DRAIN and CONTROL pins. When the
CONTROL pin reaches approximately 5.6 V relative to the SOURCE pin, the internal
current source is turned off. The internal control circuitry is activated and the high voltage
MOSFET starts to switch, using the energy in C5 to power the IC.
Once the output has reached regulation, PWM control maintains CV regulation by
indirectly sensing the output winding voltage. Ideally the DC output voltage is equal to
the bias voltage plus the forward drop of D6B multiplied by the transformer secondary
winding to bias winding turns ratio minus the forward drop of D7. However, leakage
inductance causes errors that vary with load, causing the output voltage to rise at no-
load. To give the best regulation, the bias and secondary windings should be physically
close to each other in the transformer.
Diode D6B rectifies the output of the bias winding, which is then smoothed by C3 to
provide a DC voltage to be fed to the CONTROL pin via R4. Resistor R3 is added to filter
noise due to leakage inductance. The value of R4 is set such that, at the peak power
point, where the output is still in CV regulation, the CONTROL pin current is
approximately 2.2 mA.
As the output load is increased, the peak power point (defined by 0.5 × L × I2 × f) is
exceeded. The output voltage and therefore primary side bias voltage reduce. The
reduction in the bias voltage results in a proportional reduction of CONTROL pin current,
which lowers the internal LinkSwitch current limit (current limit control).
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
0.8 mA, 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 (sense resistor, transistor, optocoupler and associated 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.
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