Buck 电路
5, 电容的输入输出平均电流为 0,在整个周期内电感平均电流=负载平均电流,所以有:
I
L
=I
o
6, 二极管只在 sw 关断时流过电流,所以 I
D
=I
L
×(1-D)
7, 则平均开关电流 I
sw
=I
L
×D
8, 由基尔霍夫电压定律知:
Sw 导通时:V
IN
=V
ON
+V
O
+V
SW
→ V
ON
=V
IN
-V
O
-V
SW
≈V
IN
-V
O
假设 V
SW
相比足够小
V
O
=V
IN
-V
ON
-V
SW
≈V
IN
-V
ON
Sw 关断时:V
OFF
=V
O
+V
D
→ V
O
=V
OFF
-V
D
≈V
OFF
假设 V
D
相比足够小
9, 由 3、4 可得 D=t
ON
/(t
ON
+t
OFF
)
=V
OFF
/(V
OFF
+V
ON
)
由 8 可得:D=V
O
/{(V
IN
-V
O
)+V
O
}
D=V
O
/ V
IN
10,直流电流 I
DC
=电感平均电流 I
L
,即 I
DC
≡I
L
=I
o
见 5
11,纹波电流 I
AC
=ΔI/2=V
IN
(1-D)D/ 2Lf=V
O
(1-D)/2Lf
由 1,3、4、9 得,
ΔI=V
ON
×t
ON
/L
=(V
IN
-V
O
)×D/Lf=(V
IN
-DV
IN
)×D/Lf=V
IN
(1-D)D/ Lf
ΔI/ t
ON
=V
ON
/L=(V
IN
-V
O
)/L
ΔI=V
OFF
×t
OFF
/L
=V
O
T(1-D)/L
=V
O
(1-D)/Lf
ΔI/ t
OFF
=V
OFF
/L=V
O
/L
12,电流纹波率 r=ΔI/ I
L
=2I
AC
/I
DC
在临界导通模式下,I
AC
=I
DC
,此时 r=2 见 P51
r=ΔI/ I
L
=V
ON
×D/Lf I
L
=(V
IN
-V
O
)×D/Lf I
L
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