Toshiba MM20E45, Техническое учебное пособие

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9-9
(1) Fundamental theory of Chopper Regulator
Chopper regulator is a circuit to supply intermittently d.c.
voltage to smoothing circuit and to take out the average
voltage as a result. In this section, the operation theory and
basic formula of voltage and current are explained. The
voltage is supplied to the load via choke coil for period Ton
that FET is on, and when FET turns off, energy stored in
choke coil is supplied via flywheel diode D. Output voltage
is average value of which voltage is added to smoothing
circuit, and is shown by the next formula.
Therefore, by controlling the ratio of Ton and Toff, output
can be stabilized. The current iL in the time when FET is on
is :
And just before Tr is off, maximum value iL2 of iL becomes:
After FET turns off, the current flowing to flywheel diode is
expressed as below.
As output current Io is equal to average of iL,
• Ton ......... (5)
2L 2
Io =
i
L1
+
(i
L1
+ i
L2
)
When Io becomes small, i
L1
becomes small. To control
circuit stably, current i
L
of choke coil is required to flow
continuously, that is; the condition i
L1
>0 should be satisfied.
Vo =
Ton + Toff
Ton
• V
t
..................................(1)
i
L
= i
L1
+
(V
i
– Vo)
L
• t ................................. (2)
(V
i
– Vo)
• Ton .......................... (3)
L
i
L2
= i
L1
+
• t ............................................ (4) io = i
L2
–
L
Vo
(V
i
- Vo)
T
OFF
T
ON
Vi
0
0
i
L1
i
L2
i
L1
i
L2
i
L
0
0
Io
(b) Operation waveform
(I)
Voltage across
FLYWHEEL diode
(II)
Drain current
of FET
(III)
Current of
FLYWHEEL diode
(IV)
Current of
choke
FET
i
L
I
O
Vo
OUTPUT
i
D
-
+
PWM
Circuit
-
+
(a) Basic circuit
Vi
INPUT
Fig. 11