9014 查看數據表（PDF） - Fairchild Semiconductor

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9014

Fairchild QFET for Synchronous Rectification DC to DC Converters

Fairchild Semiconductor 

Switch MOSFET (Q1)

PWM

Control

IC

Synchronous

Rectifier (Q2)

(L2)

(D2)

(C2)

DC Input

(Vin)

RL

Figure 3-2. Synchronous Rectification Step-down DC-DC Converter

3-2. Buck Converter

The Buck converter is the most basic and simplest DC-DC converter topology suitable for

obtaining low capacity, low voltage output. The Buck converter uses the step-down technique,

in which the output voltage is always less than or equal to the input voltage. The output to

input voltage ratio is controlled with the duty ratio.

Figure 3-3 shows the basic circuit of the Buck converter, and Figure 3-4 shows the operating

waveforms for each portion.

When Q1 turns on at time t1, the reverse-biased D turns off, and current flows through L2,

inducing vL(t)=L×diL(t)/dt voltage at both L2 terminals according to Faraday’s law. Then the

power is delivered to the output load. When Q1 turns off at time t2, the current energy that was

stored in L2, during Q1’s on period, discharges through the forward-biased D, and power is

delivered to the output load.

D in this circuit is used to make the pass required to reduce the energy that was stored in L2

during Q1’s on period to the original value. L2 and C2 are used to eliminate the ripple of the

output voltage by forming a low pass filter.

5

Rev. B, July 2000

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