Transcript Slide 1

EE462L, Fall 2011
DC−DC Boost Converter
1
Buck converter
+ vL –
iL
iin
Iout
L
Vin
Boost converter
C
iin
+ vL –
iL
iC
Iout
L
Vin
+
Vout
–
C
iC
+
Vout
–
2
Boost converter
iin
+ vL –
iL
iD
Iout
L
Vin
C
iC
+
Vout
–
This is a much more unforgiving circuit than the buck converter
• 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
3
Boost converter
iin
+ vL –
iL
iD
Iout
L
Vin
C
iC
+
Vout
–
• Modify your MOSFET firing circuit for Boost Converter
operation (see the MOSFET Firing Circuit document)
• Limit your output voltage to 120V
4
Boost converter
iin
+ vL –
iL
iD
Iout
L
Vin
C
iC
+
Vout
–
Using KVL and KCL in the average sense, the average
values are
Iin
+0V–
Iout
L
Vin
C
Iout
+
Vout
0A
–
Find the input/output equation by examining the voltage
across the inductor
5
Switch closed for DT seconds
iin
+ Vin −
iL
Iout
L
Vin
C
di L Vin

dt
L
Iout
+
Vout
–
Reverse biased, thus the
diode is open
for DT
seconds
Note – if the switch stays closed, the input is short circuited!
6
Switch open for (1 − D)T seconds
+ (Vin − Vout ) −
iL
iin
Iout
L
Vin
C
di L Vin  Vout

dt
L
+
Vout
(iL – Iout)
–
Diode closed. Assume
continuous conduction.
for (1−D)T seconds
7
Since the average voltage across L is zero
VLavg  D Vin  1  D Vin Vout   0
Vout  (1  D)  Vin  D Vin  D Vin
The input/output equation becomes
Vin
Vout 
1 D
A realistic upper limit on boost is 5 times
8
Examine the inductor current
Switch closed,
di L Vin
v L  Vin ,

dt
L
Switch open,
di L Vin  Vout
vL  Vin  Vout,

dt
L
Vin  Vout
A / sec
L
iL
Imax
Iavg = Iin
Vin
A / sec
L
Imin
DT
Iavg = Iin is half way between
Imax and Imin
ΔI
(1 − D)T
T
9
Inductor current rating
2
2
I Lrms
 I avg

 
1 2
1
2
I pp  I in

I 2
12
12
Max impact of ΔI on the rms current occurs at the boundary of
continuous/discontinuous conduction, where ΔI =2Iin
2Iin
iL
Iavg = Iin
ΔI
0
2
2
I Lrms
 I in

I Lrms 
1
2Iin 2  4 Iin2
12
3
2
I in
3
Use max
10
MOSFET and diode currents and current ratings
iin
+ vL –
iL
iD
Iout
L
Vin
C
iC
+
Vout
–
2Iin
0
2Iin
0
Use max
Take worst case D for each
I rms 
2
I in
3
11
Capacitor current and current rating
iin
iL
iD
Iout
L
Vin
C
iC
+
Vout
–
iC = (iD – Iout)
2Iin −Iout
0
−Iout
Max rms current occurs at the boundary of continuous/discontinuous
conduction, where ΔI =2Iout
Use max
ICrms  I out
See the lab document for the derivation
12
Worst-case load ripple voltage
iC = (iD – Iout)
0
−Iout
The worst case is where C provides Iout for most of the period. Then,
Q I out  T I out
V 


C
C
Cf
13
Voltage ratings
Diode sees Vout
iin
iL
Iout
C sees Vout
+
Vout
–
L
Vin
C
iin
iL
Iout
L
Vin
C
+
Vout
–
MOSFET sees Vout
• Diode and MOSFET, use 2Vout
• Capacitor, use 1.5Vout
14
Continuous current in L
Vin  Vout
A / sec
L
iL
2Iin
Iavg = Iin
0
(1 − D)T
 1

Vin
Vin 
 11  D 
 Vin
V V
1 D 
2 I in  out in  1  D T  1  D
 1  D T 
Lboundary
Lboundary
Lboundaryf
2 I in 
VinD
Lboundaryf
,
V D
Lboundary in
2 I in f
Then, considering the worst case (i.e., D → 1),
V
L  in
2 I in f
use max
guarantees continuous conduction
15
use min
Impedance matching
I out  1  DIin
Iin
+
+
Source
DC−DC Boost
Converter
Vin
−
Vin
1 D
−
Vout 
V
Rload  out
I out
Iin
+
Vin
Equivalent from
source perspective
Requiv
−
Vin 1  DVout
2 Vout
Requiv 

 1  D
 1  D2 Rload
I out
Iin
I out
1 D
16
Example of drawing maximum power from
solar panel
PV Station 13, Bright Sun, Dec. 6, 2002
6
Isc
Pmax is approx. 130W
(occurs at 29V, 4.5A)
5
I - amps
4
For max power from
panels, attach
3
Rload 
2
1
0
0
5
10
15
20
25
V(panel) - volts
30
35
40
Voc
I-V characteristic of 6.44Ω resistor
45
29V
 6.44
4.5 A
But as the sun conditions
change, the “max power
resistance” must also
change
17
Connect a 100Ω resistor directly, extract only 14W
PV Station 13, Bright Sun, Dec. 6, 2002
6
130W
5
4
I - amps
So, the boost converter
reflects a high load
resistance to a low
resistance on the
source side
3
2
14W
1
0
0
5
10
15
20
25
30
35
40
45
V(panel) - volts
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
Requiv  1  D  Rload, D  1 
2
Requiv
Rload
 1
6.44
 0.75
100
18
BOOST DESIGN
Worst-Case Component Ratings Comparisons
for DC-DC Converters
Our components
9A
Converter
Type
Boost
Input Inductor
Current
(Arms)
2
I in
3
10A
250V
5.66A
Output
Capacitor
Voltage
Output Capacitor
Current (Arms)
1.5 Vout
I out
200V, 250V
Diode and
MOSFET
Voltage
2 Vout
120V
5A
120V
Likely worst-case boost situation
16A, 20A
Diode and
MOSFET
Current
(Arms)
2
I in
3
10A
L. 100µH, 9A
C. 1500µF, 250V, 5.66A p-p
Diode. 200V, 16A
MOSFET. 250V, 20A
19
BOOST DESIGN
Comparisons of Output Capacitor Ripple Voltage
Converter Type
Boost
Volts (peak-to-peak)
I out 5A
Cf
0.067V
1500µF 50kHz
L. 100µH, 9A
C. 1500µF, 250V, 5.66A p-p
Diode. 200V, 16A
MOSFET. 250V, 20A
20
BOOST DESIGN
Minimum Inductance Values Needed to
Guarantee Continuous Current
Converter Type
Boost
For Continuous
For Continuous
Current in the Input
Current in L2
Inductor
V
40V
L  in
–
2 I in f
200µH
2A
50kHz
L. 100µH, 9A
C. 1500µF, 250V, 5.66A p-p
Diode. 200V, 16A
MOSFET. 250V, 20A
21