EPR-16 - LinkSwitch 2.75 W Charger/Adapter
17-May-04
4 Circuit Description
The schematic shown in Figure 5 provides a CV/CC (constant voltage/constant current)
type output characteristic from a universal input voltage range of 85 VAC to 265 VAC.
The nominal peak power point at the transition from CC to CV is 5.5 V at 500 mA.
The precise output envelope specification is shown in Figure 3.
4.1 Input Stage
The incoming AC is rectified and filtered by D1-4, C1 and C2. Resistor RF1 is a
flameproof fusible type to protect against fault conditions and is a requirement to meet
safety agency fault testing. This component should be a wire wound type to withstand
input current surges while the input capacitors charge on application of power or during
withstand line-transient testing. Metal film type resistors are not recommended, they do
not have the transient dissipation capabilities required and may fail prematurely in the
field.
Lower values increase the resistor dissipation (V²/R power term) during transients,
increasing resistor stress, while higher values increase steady state dissipation (I²R
power term) and reduce efficiency.
If a suitable flame proof resistor cannot be found (during failure flame proof resistors do
not emit flames, smoke or incandescent material that may damage transformer
insulation), then a standard fusible type may be used as long as a protective heat shrink
sleeve is placed over the resistor. Please consult with a safety engineer or local safety
agency.
The value of C1 and C2 were selected to provide the smallest standard values to meet
3 µF/W, in this case two 4.7 µF, 400 V capacitors. Smaller values are possible (either
2.2 µF or 3.3 µF) however, the lower DC rail voltage will increase LinkSwitch dissipation,
lowering efficiency and increasing line frequency output ripple. Differential mode EMI
(<500 kHz) also typically increases.
The input capacitance is split between C1 and C2 to allow an input π filter to be formed
by L1.
This filters noise associated with the supply to meet EN55022B /
CISPR 22 B and FCC B conducted EMC limits, even when no Y safety capacitor is used.
Ferrite bead L2 is optional, fitted to improve radiated EMI.
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