71180| Datasheet

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AN730
Vishay Siliconix

Designing A Wide Input Range DCM Flyback Converter Using the Si9108

FEATURES
D D D D D Wide 10-V to 100-V Input Voltage Range Enables Designs With Efficiency Above 80% "12-V Outputs At 125 mA Total 3-W Continuous Power Discontinuous Conduction Mode Flyback dc-to-dc Converter D D D D 500-V Input/Output Isolation Minimum No Optocoupler Feedback Needed Low Component Count, Low-cost Design Complete Solution Occupies t 2 In2 Of Board Space In A Single-Sided FR4 Board D Maximum Component Height Is t 1/4 Inch.

INTRODUCTION
The Si9108 is a highly integrated, high-efficiency current-mode regulator designed for telecom dc-to-dc converters. Its wide operating duty cycle of up to 99.9% is suitable for many power conversion applications, especially those with a wide operating input voltage range. This application note is intended to guide the user to design a very wide input voltage range, discontinuous conduction mode (DCM) flyback converter. Why use a flyback converter? Because the flyback design combines simplicity, a low parts count, and affordability for low-power applications that require input/output isolation. Why use discontinuous conduction mode? Here are some of the advantages: D maximizes energy storage in the magnetic component (a smaller core is needed for a given output power) D eliminates the output rectifying diode reverse recovery problem, hence there is no current shoot through in the main input power switching device D higher efficiency, especially at high input voltage D no right half plane zero in the control l
oop, simplifying feedback compensation design, allowing a very stable and wide-bandwidth feedback loop. A DCM Flyback Design Has Some Limitations: D the peak primary current and output rectifying diode current are large, although this is not a major concern for low-power applications D duty cycle varies with both the input voltage and output load, thus wide duty cycle operation is usually required. The design of the Si9108 regulator helps to overcome this limitation. Figure 1 illustrates a simplified flyback converter designed with the Si9108 regulator.

D1 VIN ip CIN D Np iD1 Ns1 iD2 Ns2 Ris GND1 D2 iC1 C1

IO1 V1

GND2

iC2 C2
IO2 V2 (NEG)

S SI9108

FIGURE 1. Simplified Flyback Converter Using Si9108 Regulator

Document Number: 71180 08 Jul 02

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