EPR-16 datasheet(10/36 Pages) POWERINT

10 / 36 page

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.

Page 10 of 36

Power Integrations

Tel: +1 408 414 9660 Fax: +1 408 414 9760

www.powerint.com

∗ This includes LinkSwitch tolerance and line variation.

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