DER-96 datasheet(7/33 Pages) POWERINT

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DER-96

10 W VoIP

September 12, 2005

Page 7 of 33

CONFIDENTIAL

Power Integrations

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Circuit Operation

4.1

General

The power supply uses a DPA423 device (U1), with integrated MOSFET and controller,

in an isolated flyback configuration. The circuit also uses the under-voltage shutdown

feature of the device along with current limit setting to minimize transformer size. The

device operates at a switching frequency of 300 kHz.

4.2

Description

The input fuse F1 protects the supply against catastrophic failure – although the built-in

protection features of the DPA-Switch should render this redundant.

The components C1, C2 and L7 form a pi-filter to limit both conducted and radiated EMI

emissions. These components work to limit EMI emissions in conjunction with y-capacitor

C16 and the shielding in the transformer. Resistor R1 programs the input under voltage

startup threshold (and over voltage shutdown voltage). Diode D1, R7, C4, R2, and R3

implement an RCD clamp circuit to limit the leakage inductance spike on the Drain pin.

Capacitor C3 and resistor R4 implement a snubber to limit high frequency ringing on the

due to drain switching. Diode D2, R5 and C6 implement a bias voltage supply to provide

operating power to the DPA-Switch (U4) with integrated PWM, controller and main

switching MOSFET. Resistor R6 and C5 provide diode snubber for D2. Resistor R20

programs the current limit of the DPA-Switch.

Capacitors C7 and C8 provide device

decoupling with C8 also program the startup and auto-restart period of the device.

Resistor R8 provides feedback compensation in conjunction with C8. The inductance of

transformer T1 provides the energy storage and conversion component of the circuit.

The 3.3 V output is rectified and filtered by diode D8 and capacitors C10, C11 with C12

provided output decoupling. The 5 V output is DC-stacked on the regulated DC output of

3.3 V and is rectified and filtered by diode D6 and capacitors C13 with capacitor C18

providing output decoupling. The 12 V output is AC-stacked on the 5 V transformer

winding and is rectified and filtered by diode D7 and capacitor C14 with capacitor C17

providing output decoupling. Transistor Q2 and components R15, R26, R27, R28, C24,

U4 implement a linear regulator to eliminate peak charging voltage from the 12 V output.

Resistor R26, D12, D13 and D14 all form pre-load networks between the outputs to

improve cross-regulation.

Components R21, C21, R22, C22, R23 and C23 provide

snubbing on output diodes. Diode pulls down the 5 V output when 12 V output is shorted

thus forcing DPA-switch auto-restart. Using this diode removes the need for short-circuit

protection in the 12 V linear regulator circuit.

Resistors R11 and R10 sense the voltages on 5 V and 3.3 V outputs respectively. In

conjunction with R12 they provide the input signal for the LM431 (U3) reference.

Components R13 and C15 provide compensation for U3, to make sure that it’s frequency

response is limited only to low-frequency signals.

Resistor R9 programs the high-

frequency gain of the control loop and with opto-diode U2A transmits the feedback signal.

Diode D3 and C9 provide a soft-finish circuit to limit output overshoot at startup. Diode D4

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