EPR-16 - LinkSwitch 2.75 W Charger/Adapter
17-May-04
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 or CV/CC transition point in the output characteristic, 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. With a primary
inductance tolerance of ±10%, the EP-16 is designed to provide the output current
∗
characteristic shown in Figure 3 .
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 CC characteristic to walk out as the output voltage decreases
and increases the no-load consumption of the supply.
With a figure of 50 µH for leakage, this design is able to meet a voltage tolerance of
±10% at the peak power point, including the effects of output cable drop. For tighter
voltage tolerance across the whole load range, a secondary optocoupler can be added.
For most battery charging applications, only the voltage at the peak power point is critical,
thus ensuring sufficient voltage for charging.
4.4 Clamp and Feedback Components
Diode D5 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. Leakage inductance is filtered by R2, the optimum value providing the
straightest CC characteristic.
Capacitor C4 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
used for these capacitors such as Z5U and Y5U are not stable and can cause output
instability as their value changes with voltage and temperature. Stable dielectrics such
as COG/NPO are acceptable but are costly when compared to a metalized plastic film
capacitor.
∗
This includes LinkSwitch tolerance and line variation.
Power Integrations
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