DER-57
Industrial – LNK501P
April 20, 2005
During start-up, as the output voltage, and therefore the reflected voltage and voltage
across C3 increases, the feedback current increases from 0 to approximately 2mA
through R3 into the CONTROL pin. The internal current limit is increased during this
period until reaching 100%, providing an approximately constant output current.
Once the output voltage reaches the regulated value, the output voltage is regulated
through control of the duty cycle. As the current into the CONTROL pin exceeds
approximately 2mA, the duty cycle begins to reduce, reaching 30% at a CONTROL pin
current of 2.3mA.
If the duty cycle reaches a 3% threshold, the switching frequency is reduced, which
reduces energy consumption under light or no load conditions.
As the output load increases beyond the peak power point (defined by ½·L·I²·f) and the
output voltage and VOR falls, the reduced CONTROL pin current will lower the internal
current providing an approximately constant current output characteristic. If the output
load is further increased and the output voltage falls further to below a CONTROL pin
current of 1mA, the CONTROL pin capacitor C1 will discharge and the supply will enter
auto-restart.
4.3 Transformer
The transformer is designed to always be discontinuous; all the energy is transferred to
the load during the MOSFET off time. The energy stored in the transformer during
discontinuous mode operation is ½·L·I²·f where L is the primary inductance, I² is the peak
primary current squared and f is the switching frequency.
Since the value of LinkSwitch current limit and frequency directly determines the peak
power, the parameter of current squared times frequency is defined in the datasheet.
This parameter, together with the output power, is used to specify the transformer
primary inductance.
As LinkSwitch is powered by the energy stored in the leakage inductance of the
transformer, only a low cost two winding transformer is required. Leakage inductance
should be kept low, ideally at less than 2% of the primary inductance. High leakage
inductance will cause the output current limit characteristic to walk out as the output
voltage decreases and increases the no-load consumption of the supply.
4.4 Clamp and Feedback Components
Diode D9 should either be a fast (trr <250 ns) or ultra-fast type to prevent the voltage
across LinkSwitch from reversing and ringing below ground. A fast diode is preferred,
being lower cost.
Capacitor C3 is typically fixed at 0.1 µF and should be rated above the VOR and be stable
with both temperature and applied voltage. Low-cost, Metalized plastic film capacitors
are ideal; high value, low-cost ceramic capacitors are not recommended. Dielectrics
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