M2-3 Buck Converter
Objective is to answer the following questions:
1. How does a buck converter operate?
Buck Converter
12 V
Control
5V
• The input voltage is always greater than
the output voltage
Buck Configuration
VIN
VIN
VOUT
ISW
20V
VGATE
15V
10V
7.5V
IL
VOUT
10V
VM
5V
0V
time
L
C
5V
2.5V
0V
time
• The input voltage is always greater than the
output voltage
Switching Regulator Components
Switching Power Supply
VIN
Network
Switch
Network
PWM
Controller
Error
Amplifier
Bandgap
Reference
Switching Power Supply Block Diagram
VOUT
External Network
•
An external network (consisting of an inductor, capacitor,
and diode) transforms the energy from the PWM
controlled power switch into a desired output voltage
Switch
VIN
VIN = 12 V
Network
VOUT
VOUT = 5 V
Recirculation Diode
VIN
VIN
SIN
VOUT
VM
VOUT
VM
SGND
How a Switching Regulator Works
VIN
Switching Regulator
50%
Duty Cycle
Controller
Voltage
OK
Output
Monitor
Filter Network
VOUT
5V
time
VOUT
How a Switching Regulator Works
VIN
Voltage Regulator
50%
Duty Cycle
Controller
Voltage
OK
Output
Monitor
Filter Network
VOUT
5V
time
VOUT
How a Switching Regulator Works
VIN
Voltage Regulator
50%
Duty Cycle
Controller
Voltage
OK
Output
Monitor
Filter Network
VOUT
5V
time
VOUT
How a Switching Regulator Works
VIN – 1V
Voltage Regulator
60%
Duty Cycle
Controller
Voltage
Low
Output
Monitor
Filter Network
VOUT
5V
time
VOUT
How a Switching Regulator Works
VIN – 1V
Voltage Regulator
60%
Duty Cycle
Controller
Voltage
Low
Output
Monitor
Filter Network
VOUT
5V
time
VOUT
How a Switching Regulator Works
VIN
Switching Regulator
50%
Duty Cycle
Controller
Voltage
Ok
Output
Monitor
Filter Network
VOUT
5V
time
VOUT
Step Down Switching Regulator
Steady State Operation
VIN
VGATE goes high
VM ~ VIN
ISW
VGATE
t
VM
VL = VM – VOUT
VGATE
IL
VOUT
VM
VF
+
+ VL -
RLOAD
t
-VF
ISW
t
IL
COUT
t
VOUT
t
Step Down Switching Regulator
Steady State Operation
VIN
ISW
VL  Constant
dIL VL
=
 Constant
dt
L
VGATE
t
VM
IL and ISW increase
VGATE
IL
VOUT
VM
VF
+
+ VL -
RLOAD
t
-VF
ISW
t
IL
COUT
COUT is charged by IL
and
VOUT
t
VOUT increases
t
Step Down Switching Regulator
Steady State Operation
VIN
VGATE = 0V
VGATE
The pass transistor
ISW
is turned off
t
VM
ISW = 0A
VGATE
IL
VOUT
VM
VF
+ VL COUT
RLOAD
-VF
ISW
VM goes negative
VL = VM – VOUT
dIL VL
=
< 0 A/s
dt
L
t
t
IL
+
VOUT
IL cannot go to
0A instantly:
t
dIL VL
=
dt
L
t
Step Down Switching Regulator
Steady State Operation
VIN
ISW
VGATE
VM = -VF
But, VM is clamped
to -VF
and IL decays
through the diode
IL
VOUT
+ VL -
VF
+
RLOAD
VGATE
t
VM
t
-VF
ISW
t
IL
COUT
VOUT
COUT stabilizes
the output voltage
so VOUT will only slowlyt
decay
t
Step Down Switching Regulator
Steady State Operation
VIN
ISW
VGATE
VM = -VF
The MOSFET is
turned on and off
to repeat
the sequence
IL
VOUT
+ VL -
VF
+
RLOAD
VGATE
t
VM
t
-VF
ISW
t
IL
COUT
t
VOUT
t
Volt-Second Balance
VIN
diL VL

 Constant
dt
L
ISW
VGATE
VGATE
IL
VOUT
VM
+ VL -
t
RLOAD
IL
COUT
t
Volt-Second Balance
•
•
VL
In steady state, the inductor current
ripples about an average, IL,AVG:
Therefore, the total area (or voltseconds) under the inductor voltage
waveform is zero.
VIN - VOUT
DT
T
(1-D)T
t
-VOUT
T
DT
1-DT
 VL (t)dt =  VL (t)dt + 
+ VL -
0
T
0
VL (t)dt
DT
 VL (t)dt = (VIN - VOUT )DT + (-VOUT )(1- D)T = 0
0
Voltage-Second Principle
and the DC Transfer Function
• From:
T
 VL (t)dt = (VIN - VOUT )DT + (-VOUT )(1- D)T = 0
0
we can calculate the transfer function of the step down switching voltage
regulator
VINDT +(-VOUTD+ VOUTD - VOUT )T = 0
VINDT +(-VOUT )T = 0
VOUT
=D
VIN
VIN vs. VOUT
and Duty Cycle, D
VIN
• During steady state:
VL,AVG = 0V
ISW
VL
SIN
VIN - VOUT
IL
DT
VOUT
L
T
time
(1-D)T
+ VL SGND
COUT
RLOAD
-VOUT
 VIN - VOUT  D = VOUT (1- D)
VOUT = DVIN
VOUT Increases with D
VOUT = DVIN
VGATE
VIN
ISW
VGATE
SIN
t
VL
IL
VOUT
VM
+ VL RLOAD
SGND
COUT
VIN - VOUT
-VOUT
t
VOUT
t
VOUT Decreases with D
VOUT = DVIN
VIN
VGATE
ISW
VGATE
SIN
t
VL
IL
VOUT
VM
+ VL RLOAD
SGND
COUT
VIN - VOUT
-VOUT
t
VOUT
t
Ripple Current
VIN
• Recall, IL is the sum of the
current flowing through SIN
and SGND
ISW
SIN
IL
IL
ISW
VOUT
IL,AVG
IGND
SGND
RLOAD
COUT
time
IGND
Memo
Memo