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AN732

更新时间： 2024-11-28 22:29:19

品牌	Logo	应用领域
威世 - VISHAY		转换器
页数	文件大小	规格书
7页	74K
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Designing A High-Voltage Non-Isolated Buck-Boost Converter with the Si9121DY

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AN732

Vishay Siliconix

Designing A High-Voltage Non-Isolated

Buck-Boost Converter with the Si9121DY

Nitin Kalje

INTRODUCTION

The Si9121DY is a non-isolated buck-boost converter IC,

operating from a wide input voltage range of –10 to –60 V with

minimal external components. This polarity inverter converts

–48 V to +5 V or +3.3 V, making it suitable for applications

including digital phones and ISDN power supplies. A

non-isolated buck-boost design with the Si9121 eliminates the

need for an expensive transformer, while its integrated low

on-resistance MOSFET driver, floating feedback error

amplifier, fixed-frequency oscillator, output voltage sensing

resistor divider, and depletion mode MOSFET for start up/V_CC

regulation reduce the external component count to less

than 10. Current-mode control technology achieves good line

transient response, making the Si9121DY suitable for the high

source impedance environment. Other features include the

under-voltage lock out, 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 over

temperature shutdown. The Si9121 is available in a narrow

body SO-8 package and allows power dissipation of up to

1.25 W.

while the output capacitor supports the load current. Since the

current in an inductor cannot change instantaneously, the

voltage across it reverses to maintain a constant current when

the main switch turns off. The reverse voltage is clamped to

(V_O+ V_D), and the inductor is discharged through the diode,

output capacitor, and load.

Theenergystoredanddischargedtotheloadisproportionalto

the square of the peak inductor current. At full load, the circuit

can be designed to operate in a continuous conduction mode

to reduce the peak and RMS currents in the MOSFET switch

and an inductor. Operation is in discontinuous mode at lower

loads. The constant volt-second product determines the

operating duty cycle in continuous conduction mode while the

amount of energy needed to be stored in the inductor

determines the duty cycle in discontinuous conduction mode.

The peak and average inductor currents for continuous mode

operation are:

I_OUT

I_L(AVG)

(1)

(2)

1–d

DEVICE OPERATION

I_OUT

DI_L

I_L(PK)

)

1–d

The Si9121DY uses a current-mode flyback topology to

convertthe–48-Vinputtoa+5-Vor+3.3-Voutput. Asshownin

Figure 1, the inductor stores energy during switch-on time,

COM

L(PK)

L(AVG)

∆I

Si9121

OUT

–V

FIGURE 1. Buck-Boost Converter

Document Number: 71156

04-May-00

www.vishay.com S FaxBack 408-970-5600

12 3 4 5 6 7

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