boost converter class notes
TRANSCRIPT
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EE462L, Spring 2014DC−DC Boost Converter
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Vin +
Vout–
C iC
Iout iinBuck converter
iL
L
+ vL –
Boost converter
Vin +
Vout–
C iC
Iout iin
iL
L
+ vL –
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
!
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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
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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
Switch closed for DT seconds
Reverse biased, thus the diode is openL
Vdtdi 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
Switch open for (1 − D)T seconds
Diode closed. Assume continuous conduction.L
VVdtdi outinL
Vin +
Vout–
C
Iout iin
iL
L
for (1−D)T seconds
(iL – Iout)
+ (Vin − Vout ) −
8
Since the average voltage across L is zero
01 outininLavg VVDVDV
inininout VDVDVDV )1(
DVV in
out
1
The input/output equation becomes
A realistic upper limit on boost is 5 times
!
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Examine the inductor current
Switch closed,
Switch open,
LV
dtdiVv inL
inL ,
LVV
dtdiVVv outinL
outinL
,
sec/ ALVin
DT (1 − D)T
T
Imax
Imin
Iavg = Iin
Iavg = Iin is half way between
Imax and Imin
sec/ ALVV outin
ΔI
iL
10
Inductor current rating
22222121
121 IIIII inppavgLrms
2222342
121
inininLrms IIII
Max impact of ΔI on the rms current occurs at the boundary of continuous/discontinuous conduction, where ΔI =2Iin
inLrms II3
2
2Iin
0Iavg = Iin ΔI
iL
Use max
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MOSFET and diode currents and current ratings
inrms II3
2
Use max
2Iin
0
2Iin
0
Take worst case D for each
Vin +
Vout–
C iC
Iout iin
iL
L
+ vL – iD
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
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Worst-case load ripple voltage
CfI
CTI
CQV outout
The worst case is where C provides Iout for most of the period. Then,
−Iout
0
iC = (iD – Iout)
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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
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Continuous current in Lsec/ A
LVV outin
fL
DD
VTD
L
VD
V
TDL
VVIboundary
in
boundary
inin
boundary
inoutin
11
11
1112
fIDVLin
inboundary 2
2Iin
0Iavg = Iin
iL
(1 − D)T
fIVLin
in2
guarantees continuous conduction
Then, considering the worst case (i.e., D → 1),
use max
use min
, 2fL
DVIboundary
inin
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Impedance matching
outout
load IVR
equivR
loadout
outout
out
in
inequiv RD
IVD
DIVD
IVR 22 11
1
1
DC−DC Boost Converter
+
Vin
−
+
−
Iin
+
Vin
−
Iin
Equivalent from source perspective
Source DVV in
out
1
inout IDI 1
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Example of drawing maximum power from solar panel
PV Station 13, Bright Sun, Dec. 6, 2002
0
1
2
3
4
5
6
0 5 10 15 20 25 30 35 40 45
V(panel) - volts
I - a
mps
Isc
Voc
Pmax is approx. 130W (occurs at 29V, 4.5A)
44.65.429AVRload
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
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Connect a 100Ω resistor directly, extract only 14W
PV Station 13, Bright Sun, Dec. 6, 2002
0
1
2
3
4
5
6
0 5 10 15 20 25 30 35 40 45
V(panel) - volts
I - a
mps
130W
6.44Ω
resistor
100Ω resistor
14W
75.0100
44.611 ,1 2 load
equivloadequiv R
RDRDR
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
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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 inI3
2 1.5 outV outI 2 outV inI3
2
5A 10A10A 120V 120V
Likely worst-case boost situation
5.66A 200V, 250V 16A, 20AOur components
9A 250V
MOSFET. 250V, 20A
L. 100µH, 9AC. 1500µF, 250V, 5.66A p-p
Diode. 200V, 16A
BOOST DESIGN
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Comparisons of Output Capacitor Ripple Voltage
Converter Type Volts (peak-to-peak) Boost
CfIout
5A
1500µF 50kHz
0.067V
BOOST DESIGN
MOSFET. 250V, 20A
L. 100µH, 9AC. 1500µF, 250V, 5.66A p-p
Diode. 200V, 16A
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Minimum Inductance Values Needed to Guarantee Continuous Current
Converter Type For Continuous
Current in the Input Inductor
For Continuous Current in L2
Boost fI
VLin
in2
–
40V
2A 50kHz
200µH
BOOST DESIGN
MOSFET. 250V, 20A
L. 100µH, 9AC. 1500µF, 250V, 5.66A p-p
Diode. 200V, 16A