9014 查看數據表（PDF） - Fairchild Semiconductor
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9014 Datasheet PDF : 16 Pages
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A DC-DC converter is primarily used to obtain a specific DC voltage from any DC voltage. The
output to input voltage ratio of the Buck converter is described as follows. First, assume an
ideal circuit, where the losses associated with each portion of the circuit have been ignored.
Because L2 current iL(t1)and iL(t3) must be equal, as shown in Figure 3-4, equation 3-3 is
derived as follows:
∫ ∫ L-1--
t3
vL ( t ) dt
t1
=
t3
diL(t) = i(t3) – i(t1) = 0………(3 – 3)
t1
Equation (3-4) can be obtained from Equation (3-3):
t3
∫ vL(t)dt = 0……………(3 – 4)
t1
When this is divided into Q1’s on and off periods, the following can be derived:
t2
t3
∫ ∫ vL(t)dt = – vL(t)dt………(3 – 5)
t1
t2
The left hand side of Equation (3-5) represents the integral of L2 voltage with respect to time
during Q1’s on period and the right hand side represents the integral of L2 voltage with respect
to time during Q1’s off period. When this equation is applied to the Buck converter, the follow-
ing equation is derived.
(Vin – Vout)DTs = Vout(1 – D)Ts………(3 – 6)
Where Ts = period
The Vin - Vout of the left hand side of the equation represents the L2 voltage in the (DTs) period
when Q1 is on, and Vout of the right hand side represents the L2 voltage in the ((1 - D)Ts)
period when Q2 is off. Equation 3-6 can be reduced to the following to calculate the output to
input voltage ratio.
Vout = DVin………………(3 – 7)
Equation (3-7) shows the output to input voltage ratio for an ideal case. However, the forward
voltage drop component of the switch MOSFET Q1 and the freewheeling diode D exists in an
actual system operating environment.
+ vds -
Vin
ids(t)
iL(t)
Iout +
Vout
-
[Ton]
iL(t)
Iout +
-
Vin
vD
iD(t)
Vout
+
-
[Toff]
Figure 3-5. Equivalent circuit of an ON and OFF Step-down (Buck) Converter, which accounts for the
forward voltage drop of the switch MOSFET(Q1) and the freewheeling diode (D)
7
Rev. B, July 2000
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