71923| Datasheet

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AN738
Vishay Siliconix

Design Of A Small, Efficient, Isolated Flyback Converter For 24-V Input Systems with Si9121
Jess Brown, Jerry Bernacchi

INTRODUCTION
The Si9121DY is an excellent choice for a non-isolated buck-boost converter IC capable of operating from a wide input voltage range of 10 V to 60 V to produce a +3.3-V or +5.0-V output. The power IC contains both the controller and an internal switch to enable it to be used as a converter with the minimal amount of external components. This allows a small, cost-effective converter to be built. The design of this converter is described, in detail, in the Vishay Siliconix application note AN732, "Designing a High-Voltage Non-Isolated Buck-Boost Converter With the Si9121DY." In some circumstances there is a requirement for a cost-effective, low-power, isolated converter with a minimum amount of external components and design expertise required. Although the Si9121 is designed for a non-isolated buck-boost converter, the internal MOSFET lends itself to use in isolated converters. This application note describes the design and construction of such a converter with a nominal 24-V input and a 3.3-V output with a
continuous power rating of 1.3 W. The design uses the Siliconix Si9121DY power integrated circuit, described above as the controller, and implements its internal MOSFET as the primary switch. Using the internal MOSFET, coupled with a flyback transformer and a nominal amount of external parts, makes for a cost-effective power supply with a very small size envelope. The power IC includes an internal floating feedback-error amplifier, a fixed-frequency oscillator, an output-voltage-sensing resistor divider, and a depletion-mode MOSFET for start-up VCC regulation reducing the external component count to less than 15. The IC features current-mode control to achieve good line transient response, and also includes undervoltage lockout, programmable soft-start, pulse-by-pulse current limit, hiccup mode with negligible power delivery and dissipation during continuous short-circuit conditions, automatic recovery from hiccup after fault removal, and overtemperature shut-down. The Si9121DY is available in a narrow-body
SO-8 package and allows dissipation of up to 1.3 W. A detailed study of the design of the flyback transformer is presented. An Excel spreadsheet design tool is also provided to enable readers to tailor the design to their specific requirements.
+ VCC

D1 VOUT

VIN

Controller

Q1 MOSFET

GND

FIGURE 1. Block Diagram of Flyback Converter

CIRCUIT TOPOLOGY AND DESCRIPTION
For this power level the most suitable isolated topology is the flyback converter. These converters can provide either single or multiple outputs. Flyback converters are more suitable than forward converters for relatively low power levels because of their lower circuit complexity as shown in Figure 1. Their relative simplicity results from the elimination of the output inductor and freewheel diode that would be present in the secondary stage of a forward converter. The energy acquired by the transformer during the on-time of the primary MOSFET is delivered to the output in the non-conducting period of the primary switch. During the conduction period of the primary MOSFET, the current flows into the positive terminal (+) of the primary winding through the switch to GND. The secondary winding is connected with reverse polarity, so there can be no current flow to the output, due to the blocking diode (D1). When the primary MOSFET ceases to conduct, the induced voltage is reversed by the collapse of the magneti
c field, and the output capacitor is charged through the diode (D1). Since the circuit only requires one magnetic component--the transformer-- flyback converters are simpler and cheaper than forward converters to design and build.

Document Number: 71923 01-Oct-02

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