boost converter ppt

1. 1
EE462L, Spring 2013
DC−DC Boost Converter

2. 2
Vin
+
Vout
–
C iC
Ioutiin
Buck converter
iL
L
+ vL –
Boost converter
Vin
+
Vout
–
C iC
Iout
iin
iL
L
+ vL –

3. 3
Boost converter
This is a much more unforgiving circuit than the buck converter
Vin
+
Vout
–
C iC
Iout
iin
iL
L
+ vL –
iD
• 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 Iout = 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. 4
Boost converter
Vin
+
Vout
–
C iC
Iout
iin
iL
L
+ vL –
iD
• Modify your MOSFET firing circuit for Boost Converter
operation (see the MOSFET Firing Circuit document)
• Limit your output voltage to 120V

5. 5
Boost converter
Using KVL and KCL in the average sense, the average
values are
+ 0 V – Iout
Vin
+
Vout
–
C
Iout
L
0 A
Iin
Vin
+
Vout
–
C iC
Iout
iin
iL
L
+ vL –
iD
Find the input/output equation by examining the voltage
across the inductor

6. 6
Switch closed for DT seconds
Reverse biased, thus the
diode is openL
V
dt
di inL =
for DT
seconds
Vin
+
Vout
–
C
Iout
iin
iL
L
Iout
Note – if the switch stays closed, the input is short circuited!
+ Vin −

7. 7
Switch open for (1 − D)T seconds
Diode closed. Assume
continuous conduction.L
VV
dt
di outinL −
=
Vin
+
Vout
–
C
Iout
iin
iL
L
for (1−D)T seconds
(iL – Iout)
+ (Vin − Vout ) −

8. 8
Since the average voltage across L is zero
( ) ( ) 01 =−•−+•= outininLavg VVDVDV
inininout VDVDVDV •−•+=−• )1(
D
V
V in
out
−
=
1
The input/output equation becomes
A realistic upper limit on boost is 5 times
!

9. 9
Examine the inductor current
Switch closed,
Switch open,
L
V
dt
di
Vv inL
inL == ,
L
VV
dt
di
VVv outinL
outinL
−
=−= ,
sec/A
L
Vin
DT (1 − D)T
T
Imax
Imin
Iavg = Iin
Iavg = Iin is half way between
Imax and Imin
sec/A
L
VV outin −
ΔI
iL

10. 10
Inductor current rating
( )22222
12
1
12
1
IIIII inppavgLrms ∆+=+=
( ) 2222
3
4
2
12
1
inininLrms IIII =+= ∆
Max impact of ΔI on the rms current occurs at the boundary of
continuous/discontinuous conduction, where ΔI =2Iin
inLrms II
3
2
=
2Iin
0
Iavg = Iin ΔI
iL
Use max

11. 11
MOSFET and diode currents and current ratings
inrms II
3
2
=
Use max
2Iin
0
2Iin
0
Take worst case D for each
Vin
+
Vout
–
C iC
Iout
iin
iL
L
+ vL –
iD

12. 12
Capacitor current and current rating
2Iin −Iout
−Iout
0
Max rms current occurs at the boundary of continuous/discontinuous
conduction, where ΔI =2Iout
outCrms II =
Use max
iC = (iD – Iout)
Vin
+
Vout
–
C iC
Iout
iin
iL
L
iD
See the lab document for the derivation

13. 13
Worst-case load ripple voltage
Cf
I
C
TI
C
Q
V outout =
•
=
∆
=∆
The worst case is where C provides Iout for most of the period. Then,
−Iout
0
iC = (iD – Iout)

14. 14
Voltage ratings
Diode sees Vout
MOSFET sees Vout
C sees Vout
• Diode and MOSFET, use 2Vout
• Capacitor, use 1.5Vout
Vin
+
Vout
–
C
Iout
iin
iL
L
Vin
+
Vout
–
C
Iout
iin
iL
L

15. 15
Continuous current in L
sec/A
L
VV outin −
( ) ( )
( )
fL
D
D
V
TD
L
V
D
V
TD
L
VV
I
boundary
in
boundary
in
in
boundary
inout
in
−





−
−
=−•
−
−=−•
−
=
11
1
1
1112
fI
DV
L
in
in
boundary
2
=
2Iin
0
Iavg = Iin
iL
(1 − D)T
fI
V
L
in
in
2
> guarantees continuous conduction
Then, considering the worst case (i.e., D → 1),
use max
use min
,2
fL
DV
I
boundary
in
in =

16. 16
Impedance matching
out
out
load
I
V
R =
equivR
( ) ( ) ( ) load
out
out
out
out
in
in
equiv RD
I
V
D
D
I
VD
I
V
R 22
11
1
1
−=−=
−
−
==
DC−DC Boost
Converter
+
Vin
−
+
−
Iin
+
Vin
−
Iin
Equivalent from
source perspective
Source D
V
V in
out
−
=
1
( ) inout IDI −= 1

17. 17
Example of drawing maximum power from
solar panel
Isc
Voc
Pmax is approx. 130W
(occurs at 29V, 4.5A)
Ω== 44.6
5.4
29
A
V
Rload
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. 18
Connect a 100Ω resistor directly, extract only 14W
130W
6.44Ω
resistor
100Ω resistor
14W
( ) 75.0
100
44.6
11,1 2
=−=−=−=
load
equiv
loadequiv
R
R
DRDR
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. 19
Worst-Case Component Ratings Comparisons
for DC-DC Converters
Converter
Type
Input Inductor
Current
(Arms)
Output
Capacitor
Voltage
Output Capacitor
Current (Arms)
Diode and
MOSFET
Voltage
Diode and
MOSFET
Current
(Arms)
Boost 1.5 2
5A 10A10A 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. 20
Comparisons of Output Capacitor Ripple Voltage
Converter Type Volts (peak-to-peak)
Boost 5A
1500µF 50kHz
0.067V
BOOST DESIGN
MOSFET. 250V, 20A
L. 100µH, 9A
C. 1500µF, 250V, 5.66A p-p
Diode. 200V, 16A

21. 21
Minimum Inductance Values Needed to
Guarantee Continuous Current
Converter Type For Continuous
Current in the Input
Inductor
For Continuous
Current in L2
Boost
–
40V
2A 50kHz
200µH
BOOST DESIGN
MOSFET. 250V, 20A
L. 100µH, 9A
C. 1500µF, 250V, 5.66A p-p
Diode. 200V, 16A