Exceeds Blue Angel efficiency requirements, achieving Line under-voltage is implemented using a single

Design Idea DI-10®

TOPSwitch-FX^®                                                               POWER

INTEGRATIONS_, INC_.

PC Standby

Application

Device

Power Output

Input Voltage

Output Voltage

Topology

PC Standby

TOP232Y

17 W

200 - 375 VDC

3.3 V / 5 V

Flyback

Design Highlights                                      current and voltage stresses on the internal power MOSFET,

clamp circuits and output rectifier.

•  Exceeds Blue Angel efficiency requirements, achieving Line under-voltage is implemented using a single 3.9 MΩ

78% at 3.9 W output (5 W input) resistor (R1) to eliminate glitches during turn-on/off.  This

•  Soft-start capability reduces start-up component stresses

value was chosen to set V          at 195 VDC.  This is just below

•  132 kHz operation allow small, low cost EE19 transformer      ^UV

the lowest operating DC input voltage for a doubler input to deliver 17 W and prevents audio interaction with main configuration. converter

•  Under-voltage detect eliminates turn-on/off glitches   The input bypass capacitor C1 is needed only if the main

•  Dual sensing ensures 5% accuracy on 3.3 V and         supply filter is located remotely.  Diode D1 and Zener VR1

5 V outputs                                                                                are used to clamp the leading edge voltage spikes caused by

•  15 V bias supply for primary side circuitry   transformer leakage inductance, to a safe value.  The bias winding is rectified and filtered by D2 and C6 to create a

Operation bias voltage for the TOP232. This 15 V primary referenced voltage may also be used to power the main converter The TOPSwitch-FX PC Standby supply provides 17 W of control circuitry.  Shunt regulator U3 along with R6, R7, output power.  Three outputs are generated, 3.3 V and 5 V for R9, R10 & R11 are used to sense and regulate the two output the processor and peripherals, and 15 V for the main power voltages. supply switcher.

The design utilizes the TOP232 and takes advantage of many Key Design Points of the device's built in features.  Soft-start is one of these,

•  The value of the under-voltage detection is set according requiring no external components.  During the first 10 ms of operation, the duty cycle is raised from 0 to 78%, lowering to the equation VUV = 50 µA x R1.

•                                                                   Suitable layout and filtering (L1, L2, C11, C13) results

* Optional                                                                                                                                                                                                      PI-2537-071200

Figure 1. TOPSwitch-FX 17 W PC Standby

in low output ripple on 3.3 V and 5 V channels.  At full load, peak-to-peak ripple is significantly below 50 mV.

•  Y1 capacitor (CY1) is tied between secondary return and primary positive DC rail to minimize potential noise coupling to SOURCE pin during AC common mode surge events.

•  Good cross regulation on the main channels is maximized with dual sense feedback to the 3.3 V and 5 V outputs (R9, R10).

•  Capacitor (C8) is added to eliminate output start-up overshoot on DC output channels.

•  Feedback compensation is accomplished with capacitor C7, across TL431 reference amplifier (U3), and correct choice of CONTROL pin capacitor and series resistor (C5, R5). Resistor R6 sets feedback gain while resistor R7 provides bias to the TL431 when the 5 V output is lightly loaded. Compensation is optimized for the transformer design and the power supply provides satisfactory phase margin and operating bandwidth across wide operating conditions.

Table 1.  Transformer construction information

1

0                                                  50                                                100

Time ( s)

Figure 2. DC output ripple seen on +3.3 V output, full load

Figure 3.  Transformer build diagram For the latest updates, visit our Web site: www.powerint.com

Power Integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability.  Power Integrations does not assume any liability arising from the use of any device or circuit described herein, nor does it convey any license under its patent rights or the rights of others.

The PI Logo, TOPSwitch, TinySwitch and EcoSmart are registered trademarks of Power Integrations, Inc.

©Copyright 2001, Power Integrations, Inc.

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