1
EE462L, Spring 2014
DC−DC Boost Converter
1
EE462L, Spring 2014
DC−DC Boost Converter
2
V
in
+
V
out
–
C
i
C
I
out
i
in
Buck
converter
i
L
L
+ v
L
–
Boost
converter
V
in
+
V
out
–
C
i
C
I
out
i
in
i
L
L
+ v
L
–
3
Boost converter
This is a much more unforgiving circuit than the buck
converter
V
in
+
V
out
–
C
i
C
I
out
i
in
i
L
L
+ v
L
–
i
D
•If the MOSFET gate driver sticks in the “on” position,
then there is a short circuit through the MOSFET –
blow
MOSFET!
•If the load is disconnected during operation, so that I
out
= 0, then L continues to push power to the right and
very quickly charges C up to a high value (250V) –
blow diode and MOSFET!
•Before applying power, make sure that your D is at the
minimum, and that a load is solidly connected
!
4
Boost converter
V
in
+
V
out
–
C
i
C
I
out
i
in
i
L
L
+ v
L
–
i
D
•Modify your MOSFET firing circuit for Boost
Converter operation (see the MOSFET Firing Circuit
document)
•Limit your output voltage to 120V
5
Boost converter
Using KVL and KCL in the average sense, the
average values are
+ 0 V –
I
out
V
in
+
V
out
–
C
I
out
L
0 A
I
in
V
in
+
V
out
–
C
i
C
I
out
i
in
i
L
L
+ v
L
–
i
D
Find the input/output equation by examining the
voltage across the inductor
6
Switch closed for DT seconds
Reverse biased, thus the
diode is open
L
V
dt
di
in
L
for DT
seconds
V
in
+
V
out
–
C
I
out
i
in
i
L
L
I
out
Note – if the switch stays closed, the input is short circuited!
+ V
in
−
7
Switch open for (1 − D)T seconds
Diode closed. Assume
continuous conduction.
L
V
V
dt
di
out
in
L
V
in
+
V
out
–
C
I
out
i
in
i
L
L
for (1−D)T seconds
(i
L
– I
out
)
+ (V
in
− V
out
)
−
8
Since the average voltage across L is
zero
0
1
out
in
in
Lavg
V
V
D
V
D
V
in
in
in
out
V
D
V
D
V
D
V
)
1
(
D
V
V
in
out
1
The input/output equation
becomes
A realistic upper limit on boost is 5
times
!
9
Examine the inductor current
Switch
closed,
Switch open,
L
V
dt
di
V
v
in
L
in
L
,
L
V
V
dt
di
V
V
v
out
in
L
out
in
L
,
sec
/
A
L
V
in
DT
(1 − D)T
T
I
max
I
min
I
avg
= I
in
I
avg
= I
in
is half way between
I
max
and I
min
sec
/
A
L
V
V
out
in
ΔI
i
L
10
Inductor current rating
2
2
2
2
2
12
1
12
1
I
I
I
I
I
in
pp
avg
Lrms
2
2
2
2
3
4
2
12
1
in
in
in
Lrms
I
I
I
I
Max impact of ΔI on the rms current occurs at the boundary of
continuous/discontinuous conduction, where ΔI =2I
in
in
Lrms
I
I
3
2
2I
in
0
I
avg
= I
in
ΔI
i
L
Use max
11
MOSFET and diode currents and current
ratings
in
rms
I
I
3
2
Use max
2I
in
0
2I
in
0
Take worst case D for
each
V
in
+
V
out
–
C
i
C
I
out
i
in
i
L
L
+ v
L
–
i
D
12
Capacitor current and current rating
2I
in
−I
out
−I
out
0
Max rms current occurs at the boundary of
continuous/discontinuous conduction, where ΔI =2I
out
out
Crms
I
I
Use
max
i
C
= (i
D
– I
out
)
V
in
+
V
out
–
C
i
C
I
out
i
in
i
L
L
i
D
See the lab document for the
derivation
13
Worst-case load ripple voltage
Cf
I
C
T
I
C
Q
V
out
out
The worst case is where C provides I
out
for most of the period.
Then,
−I
out
0
i
C
= (i
D
– I
out
)
14
Voltage ratings
Diode sees V
out
MOSFET sees V
out
C sees V
out
• Diode and MOSFET, use 2V
out
• Capacitor, use 1.5V
out
V
in
+
V
out
–
C
I
out
i
in
i
L
L
V
in
+
V
out
–
C
I
out
i
in
i
L
L
15
Continuous current in L
sec
/
A
L
V
V
out
in
f
L
D
D
V
T
D
L
V
D
V
T
D
L
V
V
I
boundary
in
boundary
in
in
boundary
in
out
in
1
1
1
1
1
1
1
2
f
I
D
V
L
in
in
boundary
2
2I
in
0
I
avg
= I
in
i
L
(1 − D)T
f
I
V
L
in
in
2
guarantees continuous
conduction
Then, considering the worst case (i.e., D → 1),
use max
use min
,
2
f
L
D
V
I
boundary
in
in
16
Impedance matching
out
out
load
I
V
R
equiv
R
load
out
out
out
out
in
in
equiv
R
D
I
V
D
D
I
V
D
I
V
R
2
2
1
1
1
1
DC−DC Boost
Converter
+
V
in
−
+
−
I
in
+
V
in
−
I
in
Equivalent from
source perspective
Source
D
V
V
in
out
1
in
out
I
D
I
1
17
Example of drawing maximum power
from solar panel
I
sc
V
oc
P
max
is approx.
130W (occurs at
29V, 4.5A)
44
.
6
5
.
4
29
A
V
R
load
For max power from
panels, attach
I-V characteristic of 6.44Ω
resistor
But as the sun
conditions change,
the “max power
resistance” must also
change
18
Connect a 100Ω resistor directly, extract only
14W
130W
6.4
4Ω
res
ist
or
100Ω
resistor
14W
75
.
0
100
44
.
6
1
1
,
1
2
load
equiv
load
equiv
R
R
D
R
D
R
To extract maximum power (130W), connect a boost converter
between the panel and the load resistor, and use D to modify the
equivalent load resistance seen by the source so that maximum
power is transferred
So, the boost
converter reflects a
high load resistance
to a low resistance
on the source side
19
5A
10A
10A
120V
120V
Likely worst-case boost situation
5.66A
200V,
250V
16A, 20A
Our components
9A
250V
MOSFET. 250V, 20A
L. 100µH,
9A
C. 1500µF, 250V, 5.66A
p-p
Diode. 200V, 16A
BOOST DESIGN
20
5A
1500µF50kHz
0.067V
BOOST DESIGN
MOSFET. 250V, 20A
L. 100µH,
9A
C. 1500µF, 250V, 5.66A
p-p
Diode. 200V, 16A
21
40V
2A
50kHz
200µH
BOOST DESIGN
MOSFET. 250V, 20A
L. 100µH,
9A
C. 1500µF, 250V, 5.66A
p-p
Diode. 200V, 16A