EPR-86 Datasheet, PDF (6/28 Pages) Power Integrations, Inc. – Engineering Prototype Report for EP 86 - 6.6 W Multi-Class Powered Device (PD) for Power over Ethernet (PoE) Using DPA-Switch-(DPA423G)

EP-86 – 6.6 W, 3.3 V, 2 A PoE Powered Device
13-Apr-2006
4 Circuit Operation
4.1 General
A flyback topology was used to minimize circuit board size, parts count and cost. This
topology also provides excellent operating efficiency across the input voltage range.
4.2 DPA-Switch Primary
The DPA423G IC implements PWM control of the internal power MOSFET and initiates a
soft start-up function when it first powers up. The IC also monitors die temperature as
part of its over-temperature protection function and also monitors the input voltage as
part of its under-voltage detection and overvoltage shutdown functions. The integrated
220 V MOSFET provides excellent switching characteristics at the selected 400 kHz
operating frequency. The MOSFET and controller consume very little power, giving good
efficiency across the entire input voltage operating range.
Diodes D3 through D9 ensure that the incoming DC input voltage is correctly polarized.
Capacitors C1 and C2 and inductor L1 form a low-cost pi (π) filter that attenuates
conducted EMI noise, to keep it from being passed to the incoming line.
Resistors R4 and R6 program the internal current limit of the DPA423G, so that it
reduces as the input voltage increases. This helps to keep the variance of the maximum
output overload current below 5%, across the entire input voltage range.
The IC’s integrated MOSFET is protected from overvoltage stresses that could damage it
(during a line surge) by a primary-side Zener diode clamp (VR3). Zener diode VR3 does
not conduct under normal operating conditions.
The primary bias winding provides CONTROL pin current after start-up. Diode D2
rectifies the bias winding voltage, while R8 and C11 attenuate high frequency switching
noise and reduce the peak charging of the bias voltage.
4.3 Output Rectification
The secondary winding voltage is rectified by a low-loss Schottky diode (D11). Low ESR,
tantalum output capacitors, C7 and C8, filter the output voltage. The LC output filter (L2,
C9 and C10) further attenuates switching noise and ripple from the output voltage.
4.4 Output Feedback
Resistor divider (R12 and R13) senses the output voltage and feeds it into the reference
pin of a 1.24 V reference IC (U4). The conduction of U4 pulls current through the LED of
optocoupler U5, which controls the conduction of its phototransistor (U5-B).
The phototransistor modulates the current that flows into the CONTROL pin of U1. Since
the DPA-Switch is a current-to-duty-cycle converter, it uses the varying CONTROL pin
current to pulse-width modulate the duty cycle of the MOSFET switch. Resistor R10 sets
the gain of U4, while R11 and C13 compensate for the variation in gain of U4 over the
frequency range of the feedback loop’s bandwidth (about 10 kHz). Feedback
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