The buck converter and the boost converter can be combined to produce a single
buck-boost converter. In this case the load voltage can be either less than or
greater than the supply voltage by adjusting the duty cycle m of the chopper (a
unidirectional near-ideal switch). The circuit diagram of a buck-boost converter
is shown in
6.7. Also shown are the ideal waveforms associated with a very
high value of capacitance C for virtually constant load voltage. For such a situation
in the steady state the average voltage across the inductor is zero. Therefore, for
continuous current in the inductor the input-output relations are
t
(6.4.1)
\-m
and
(6.4.2)
Unidirectional switch
Sw
Frequency/"
ij)
Diode
Duty cycl es
DC
load
2
L
a
v
i<*l
-'max
*/>
1 -'mm
0
*ON
I
0
\T
*ON
]
0
(b)
Fig. 6.7 Buck-boost converter.
(a) Circuit diagram, (b) ideal waveforms.
For m< 0.5 the chopper is a buck converter, and for m> 0.5 the chopper is
a boost converter. The PSpice simulation of a buck-boost converter will take into
account the
value of the capacitor C. Accordingly, the simulation results will
be more like those that would be found in practice.
EXAMPLE 6.4.1
Consider the buck-boost converter illustrated in Fig. 6.7a. The parameter values
are
the switch is a MOSFET operating at 30kHz, L - 500//H, C = 250/JF
and
For a duty cycle m =
execute a PSpice simulation, (a) Plot a
trace of the inductor current iL over the interval t=0 to t=5ms. (b) From the trace
of iL determine the inductor current excursion, A/=/ max - / min . (c) Plot a trace of
the load voltage and determine approximately the average value.
(b) 0.6A, (c) 12V.)
Solution
Two steps in the solution, PSpice circuit configuration and circuit-file creation, are
described below.
A PSpice configuration of the circuit is shown in
6.8. A PSpice
switch models the MOSFET and a dependent voltage source models
the diode. See Chapter 4.
IS
r^
VD = 0
Gate
control
y ^MOSFET
\ R>Switch
V •f(Kvc^ 10 V
Control
voltage
L=500^H
[3 DIODE [3]
DIO GD
RL=10n,
Fig. 6.8 PSpice configuration of a buck-boost converter.
From the PSpice configuration in
6.8 we can write a circuit file.
In this example no comments are written in the circuit file. It is left
as an exercise for you to interpret the statements. We have chosen
subcircuits MGJDEAL and DIO_GD from POWER.LIB (and Section 4.7 and
1 88 Chap.6 DC-DC Converter Circuits
4.2.3) to represent the MOSFET switch and the diode respectively. The driver
circuit for the model MOSFET could be the subcircuit MG_GATE (in POWER.LIB
and Chapter
We have arbitrarily chosen VCHOPPER. The parameters needed
are PERIOD (T= 1//), PULSE_WIDTH ( m l ) and VCC (the gate voltage VG in
Fig. 6.8). The circuit file EX6_4_1 .CIR follows. Run the simulation. Plot the
inductor current and the load voltage.
»»»»»EX6_4 1 .CIR»»»»»»»»»»»»»»»»»»»»»»»»»»»»»»»»»»»
BUCK-BOOST CONVERTER
.LIB POWER.LIB
.LIB DRIVER.LIB
.OPTIONS RELTOL=0.01
VS
2
1
24V
VD
5
2
0
L
2
0
500uH
IC=0A
C
3
0
250uF
IC=OV
RL
3
0
10ohms
. PARAM
FREQ=30kHz
PERIOD={1/FREQ}
. PARAM
DUTY=0.38
PULSE_WIDTH={DUTY* PERIOD}
.PARAM
VCC=10.2V
XMOS
1
0
4
MGJDEAL
XDRIVER 4
0 VCHOPPER
XDIODE
3
5
DIO_GD
.TRAN
1us 5ms UIC
. PROBE
i(VD),
v(3)
.END
Drill Exercise D6.4.1
Consider the circuit diagram of the buck-boost converter in Fig. 6.7. Let the values I
of the elements and parameters be as follows. Vs = 12V, L =
C = 120pF and I
The chopper frequency is 100kHz with a duty cycle m = 0.7. Use any of
the appropriate devices and drivers described in Chapters 4 and 5. For example,
let Sw be the near-ideal MOSFET (subcircuit MGJDEAL in POWER.LIB), let D
be the near-ideal diode (subcircuit DIO_GD in POWER.LIB), and let the MOSFET
driver circuit be a pulse circuit (subcircuit VCHOPPER in DRIVER.LIB).
Determine from a PSpice simulation (a) the peak diode current during the initial
transient interval, (b) the time to reach steady state, (c) the steady-state inductor