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 

Equation (3-8) can be obtained as in Equation (3-6) by referring to Figure 3-5:

(Vin – vds(t) – Vout)DTs = (Vout + vD(t))(1 – D)Ts…………(3 – 8)

Equation (3-9) represents this equation with respect to Vout:

Vout = DVin – [Dvds(t) + vD(t)(1 – D)]…(3 – 9)

Rearranging the above equation (3-9) in terms of D,

D = V-----o---u---Vt----+-i--n--v--–-D---v-(---td--)-s--(-(-1-t---)-–-----D-----)…………………(3 – 10)

The efficiency (η) can be obtained from equation (3-11) using the input to output current ratio,

Iin/Iout=D, and equation (3-10):

η = V----V-o---u-i--nt---II--oi--n-u---t = V--V---o--i-u-n--t V-----o---u--V-t----+i-n----v---–D----(-v--t-d-)--s-(--1(---t--–)-----D-----) < 1………(3 – 11)

As shown by equations (3-9) and (3-11), the forward voltage drop of the switch MOSFET and

the freewheeling diode reduces the output voltage and the entire system efficiency. Besides

these factors, the inductor coil resistance and the capacitor ESR also drop the efficiency.

Hence, a low coil resistance inductor and low ESR capacitors must be used. In the case of the

capacitor, it is advantageous to use several capacitors in parallel.

A high quality output DC voltage with low ripple, if possible, is considered the best. The follow-

ing is a description of the Buck converter output voltage ripple.

iL(t)

∆iL/2

∆Q

∆iL

Ts/2

iL=Iout

t1

t2

t3

t

Figure 3-6. Inductor Current

Ripples appear in the output voltage as the L2 current ripple component, ∆iL charges and dis-

charges C2 as illustrated in Figure 3-6. C2 is charged during the period when iL(t) is greater

than Iout. The charge (∆Q) that flows into C2 at this time divided by the value of C2 (C) is the

output voltage ripple component. First, the following equation calculates the ∆iL in Figure 3-6:

∆iL = DTsV-----i--n----–--L---V----o----u---t= -D----(---1-----–-----D-L----)--V-----i-n---T-----s- ………(3 – 12)

8

Rev. B, July 2000

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