AN708

Vishay Siliconix

Document Number: 70581

www.vishay.com

S FaxBack 408-970-5600

1

Low-Power Universal-Input Power Supply

Achieves High Efficiency

Expanding global markets have created a demand for what

have become known as universal-input power supplies –– that

is, power supplies that allow devices to be plugged into wall

outlets anywhere in the world. These power supplies must be

able to operate directly from 100-, 110-, and 220-V ac power

lines without the use of selector switches or jumpers. A power

supply with the ability to operate under such conditions while

remaining cost-effective is now becoming a necessity.

In the under 30-W power range, meeting the above

requirements while maintaining high efficiency has been a

challenge. Add to this the need to meet various international

safety standards, and the circuit designer has his hands full.

The demands of low-power universal-input power supplies are

met by the Si9120 pulse width modulation (PWM) controller

from Vishay Siliconix. Using the Si9120, the flyback circuit

presented in this application note demonstrates that designing

universal-input supplies can be a simple task.

CIRCUIT TOPOLOGY

For the low power levels that are of interest here (under 30 W),

the discontinuous-mode (DCM) flyback converter is the

preferred topology. The biggest advantage of this topology is

simplicity. The parts count in the power path cannot get any

lower.

The peak-to-average primary current ratio in a DCM flyback is

high relative to other topologies; however, at low power levels,

this is not a serious drawback. On-state losses are minimal.

Magnetics are small. Also, the transformer reset voltage is set

by the minimum input voltage and remains fairly constant as

the line voltage changes. As a result, a 600-V MOSFET proves

adequate, even with ac inputs up to 300 V RMS.

The

DCM

flyback

converter,

when

operated

under

current-mode control, provides a natural input volt-second

limit, which helps keep the drain voltage from getting out of

control during line or load transient conditions. Also, today’s

power MOSFETs are able to withstand avalanche current

many times greater than a low power circuit can typically

deliver (see appendix A). As such, the MOSFET will serve as

a clamp for the occasional spike which may result from a short

circuit or extreme load transient.

Cross regulation is fairly good, especially if leakage

universal-input application, meeting VDE input-to-output

isolation requirements is essential. Depending on the end

product, this can be as high as 3750-V RMS, primary to

secondary –– a figure that is totally inconsistent with the desire

to achieve low leakage inductance. As a result, cross

regulation between primary and secondary- referenced

windings will be poor. This complicates the regulation of the

primary-side

bootstrap

winding

used

to

avoid

secondary-to-primary feedback across the isolation boundary.

The addition of a simple spike-blanking circuit solves the

problem (see AN707, “Designing Low-Power Off-Line Flyback

Converters Using the Si9120 Switchmode Controller IC”).

When using the Si9120 for universal-input applications, it is

recommended that a bootstrap winding be employed. While

not strictly necessary, the power dissipation and chip

temperature are higher if bootstrapping is not utilized. As an

dissipation on the chip without a bootstrap is 600 mW. If a 10 V

bootstrap supply is used, the dissipation is only 15 mW. This

becomes more of a concern as the gate charge requirements

controller is largely dependent on gate drive demands.

Another advantage of the DCM flyback converter is its

single-pole loop response. This makes compensating the

feedback loop comparatively simple. In addition, transient

response can be quite good in DCM flyback converters. It is

possible (though not practical in a closed-loop system) to slew

the power stage from no load to full load in only one switching

cycle.

DESIGN EXAMPLE

The circuit shown in Figure 1 is an 11.1-W, 3-output off-line

supply. The input voltage is specified from 90- to 260-V ac.

Outputs are +5 V at 1.5 A, +12 V at 150 mA, and –12 V at

150 mA. The design features full VDE isolation, primary side

regulation, and true foldback current limiting. Operating

frequency is 100 kHz.

DCM

flyback

operating

principles

are

generally

well

understood and will not be presented here. Refer to

Vishay Siliconix Application Note AN707 for a detailed design

example. References 2 and 3 are also recommended.

Sizing the input capacitor and rectifiers for universal input

requires more thought than for comparable single-input

converters. Keep in mind that while the maximum input voltage

occurs at high-input line, the maximum current stresses will

occur at low line. This implies that the input capacitor value

must be sized at low line while the voltage rating is dictated by

the high-line condition. The bridge rectifier should be rated at

600-V dc minimum. The RMS current rating is calculated

below.