User manual | 405_Switch_Mode_Converters

Flyback Converters
Boost Converter
• Store energy in an inductor (choke) before
transferring the energy to the load and
output capacitor
• While energy is being transferred to the
inductor, load energy is provided by the
output capacitor
• Two basic versions:
– Boost (same polarity)
– Buck-Boost (output V polarity inversion)
7-1
• Essentially same components as Buck
– arranged differently
• ‘Boost’ cause V > Vg
7-2
1
Topology
BOOST CONVERTER SWITCHING ANALYSIS IN DC STEADY STATE
iL
iL
Vo
C
vL
p
q
C
vL
Vin Vin
p
q
(a)
Vo
(b)
Figure 3-7 Boost dc-dc converter.
3
7-3
• When switch closed, diode reversed, output
isolated, inductor charging
• When switch open, output receives energy from
input and the inductor – hence Boost
• Large C maintains the output V constant
7-4
2
Analysis
7-5
7-6
3
7-7
7-8
4
Steady State
Inductor Current
7-9
7-10
5
Step-Up (Boost) Converter
•
•
•
•
Output voltage always higher than the
input voltage
When the switch is ON:
diode is reversed biased
output circuit is thus isolated
inductor is charged
When the switch is OFF:
the output stage received energy from the
inductor as well as from the input
Filter capacitor is very large to ensure
constant output voltage
Applications:
• regulated switch mode power supplies
• Regenerative braking of dc motors
7-11
7-12
6
Step-Up (Boost) Converter: Continuous current conduction mode
•
•
Boost converter: operation and waveforms
Inductor current iL flows continuously
Average inductor voltage over a time
period must be zero
q
t
0
vL  Vin
Vd ton  Vd  V0 toff  0
Vin
vA  0
iL
Vo
vA
v A  Vin
Vo
vL
A
Vin
t
q 1
V0
T
1
 s 
Vd toff 1  D
(Vo  Vin )
(a)
B
t
0
iL
IL
0
Vo
Assuming a lossless circuit
Vin
Vd I d  V0 I 0
vL  Vin  Vo
v A  Vo
idiode
iL 
iL
iL ,ripple
iL
Vo  Vin 
t
0
0
Dividing both side by Ts
Vo
1

Vin 1  D
t
I diode ( I o )
t
0
Vin
V V
DTs  o in (1  D )Ts
L
L
Vin I in  Vo I o
I L  I in 
Vo
I
1 Vo
Io  o 
Vin
1 D 1 D R
iC (t ) idiode,ripple (t )  idiode  I o
iC
t
0
and
q0
I0
 1  D 
Id
( I0 )
(b)
(c)
Figure 3-8 Boost converter: operation and waveforms.
7-13
7-14
7
PSpice Modeling:
Concept Quiz
In a Boost converter operating in a continuous-conduction mode,
increasing the duty-ratio decreases the output voltage to the input
voltage ratio.
A. False (correct)
B. True
7-15
7-16
8
Simulation Results
Boost converter: voltage transfer ratio
15
10
Vo
Vin
5
0
1
1 D
-5
1
-10
IL
0
DCM
-15
1.950ms
I(L1)
1.955ms
V(L1:1,L1:2)
1.960ms
1.965ms
1.970ms
1.975ms
1.980ms
1.985ms
1.990ms
1.995ms
2.000ms
I L ,crit
CCM
Figure 3-10 Boost converter: voltage transfer ratio.
Time
7-17
7-18
9
Capacitor - Output Voltage Ripple
Step-Up (Boost) Converter: Effect of parasitic elements
•
•
•
Parasitic elements are due to the
losses associated with the inductor,
capacitor, switch and diode
Figure shows the effect of the
parasitic elements on the voltage
transfer ratio
Unlike ideal characteristics, in
practice, Vo /Vd declines as duty
ratio approaches unity
•
•
•
7-19
Choose L such that ripple
magnitude is obtained
Choose C that desired voltage
ripple is obtained
In practise, capacitor equivalent
series resistance leads to
increased voltage ripple
7-20
10
Inductor Selection
7-21
11

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