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9014

Fairchild QFET for Synchronous Rectification DC to DC Converters

Fairchild Semiconductor 

verter used to supply power to the microprocessor. Section 3-2 briefly explains the operation

of a Buck converter.

PWM

Control

IC

Switch MOSFET (Q1)

DC Input

(Vin)

(L2)

Freewheeling

Diode (D)

(C2) RL

Figure 3-1. Step-down (Buck) DC-DC Converter

3-1. Efficiency of Buck Converter

In order to increase the overall efficiency, it is important to reduce the loss generated in each

component, as much as possible, in low output voltage power supplies. Equation (3-11) in

Section 3-2 shows the reduction in system efficiency due to the loss associated with semicon-

ductor components such as the switch MOSFET (Q1) and freewheeling diode (D). If the free-

wheeling diode (D) is substituted with the MOSFET (Q2), the semiconductor component loss

can be reduced because the loss from the diode's forward voltage drop is greater than the loss

from the MOSFET’s RDS(on). This effect is represented by the following equation. First,

assume the following conditions: output current Iout=18[A], duty ratio D=0.49, freewheeling

(Schottky) diode's forward voltage drop VD=0.52[V] (the typical value when iF=18[A],

TJ=100[°C] in MBRD835L data sheet Figure 2), and MOSFET RDS(on)=14.70[mΩ] (the typical

value when VGS=10[V], TJ=100[°C] in Figure 2-3). Then, the freewheeling diode (D) loss can

be calculated by the following equation (3-1):

P = VDIout(1 – D)

= 0.52[V] × 18[A] × (1 – 0.49)………(3 – 1)

= 4.77[W]

If substituted with the MOSFET (Q2), the MOSFET (Q2) loss is calculated by equation (3-2) as

follows.

P = [Iout

(

1

–

D

2

)]

RDS(on)

= [18[A] ×

(

1

–

0.49

2

)]

×

14.70[

mΩ

]

…(

3

–

2

)

= 2.43[W]

The calculation results of equations (3-1) and (3-2) verify the increase in efficiency if the free-

wheeling diode (D) is substituted with a MOSFET(Q2). This method is called synchronous rec-

tification, and the MOSFET, a synchronous rectifier (Q2). Figure 3-2 is a remodeling of the

circuit shown in Figure 3-1 employing synchronous rectification. If the Schottky diode (D2) is

used in parallel with the synchronous rectifier (Q2) in this circuit, the inductor (L2) current that

previously flowed through the parasitic diode of the synchronous rectifier (Q2) starts flowing

through the faster and low loss Schottky diode (D2) during the switching dead time of the two

MOSFET’s (Q1, Q2). This flow pattern makes the circuit more efficient.

4

Rev. B, July 2000

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