Physics 160
Lecture 18
R. Johnson
June 3, 2015
Final Exam
•
•
•
•
•
30% weight in the course grade
Monday, June 8, 2015, 4:00 to 7:00 pm in this room
Closed book
book.
You may bring one sheet of 8½ by 11 paper, front and back,
with your own notes.
Calculators are allowed, but no computers or smart phones.
•
Bring loose-leaf paper to do your work on.
•
See the eCommons course materials page for a practice final
exam
e
a p
plus
us so
solutions
ut o s ((2014
0 e
exam).
a )
June 3, 2015
Physics 160
2
723 Regulator Usage
2 to 7 volts output
June 3, 2015
Physics 160
3
723 with Vout < Vref
•
This is Figure 6.4 in the textbook (not counting the input voltage
source) and is very similar to the first section of Lab 12.
D1N4002
D2
R6
5V
V
6.8
12
+
+
Vz
Vref
1.5k
2
3
4
5
9
6
C3
RLoad
4.7uF
100
R3
2k
LM723
13
VOFF = 0
VAMPL = 10
FREQ = 60
OUT
Vcc-
V1
10
Vcc+
500uF
C1
CL
CS
- -
V
R4
U1
7
D1N4002
D1N4002
Vc
D1N4002
D7
11
D4
COMP
D8
R2
10Meg
R5
4.7k
C2
100pF
June 3, 2015
Physics 160
4
Simulation of a 5V, 50mA Supply
Before Regulator
After Regulator
June 3, 2015
Physics 160
5
Output Voltage vs Input Voltage
After Regulator
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Physics 160
6
3-Terminal Regulator LM317
This device maintains a
reference voltage of
1.25V between the ADJ
and Vout terminals.
terminals
It can be used for any
positive supply voltage
from 1.2V to 37V with a
current of up to 0.5 A to
1.5 A (depending on
version
i and
d package
k
type).
To be
T
b precise,
i
you can ttake
k into
i t
account the small current
flowing into the ADJ input.
June 3, 2015
Physics 160
7
LM317 Simulation
U2
LM317K
D2
2
IN
OUT
ADJ
D1N4002
~5V
3
V
D4
D1N4002
D7
V
1
D8
R6
D1N4002
240
700uF
D1N4002
C3
RLoad
C1
50
4.7uF
R7
750
V6
VOFF = 0
VAMPL = 10
FREQ = 60
5
June 3, 2015
750
 5  1.21
240  750
Physics 160
8
LM 317 Simulation
Before Regulator
After Regulator
June 3, 2015
Physics 160
9
Practical Linear Power Supplies
•
•
•
•
•
•
3-wire plug
Fuse (slow blow)
Transformer: important
p
for safety!
y Estimating
g the ohmic heating
g
in the primary winding is difficult, because of the uncertain
fraction of the time that the rectifiers in the secondary circuit are
conducting.
Rectifier (full wave, to minimize rms current in transformer
windings)
Filter capacitor;
p
don’t make oversized or more transformer
heating will result. A volt or so of ripple at the maximum current
load is okay; the regulator removes most of it. (Charge is the integral of I
over time, but the heat
Regulator,
g
with current limiting
g
dissipated
p
in the transformer
coil is an integral of I 2.)
– Foldback current limiting
•
•
High-current pass transistors. Heat Sinks. Bipolar transistor
issues. MOSFETs.
Voltage limiting: crowbar
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Physics 160
10
Unregulated Power Supply
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Physics 160
11
5-V Regulator with Outboard Pass Transistor
Transistor external
to the chip for high
current.
Output 5V
at 2 amps
Crowbar for over-voltage
protection, using a silicon
controlled rectifier (SCR
or “thyristor”)
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Physics 160
12
Foldback Current Limiting I
Simple system to reduce the output current
when the output is short-circuited.
max 
1
Rs

1 


R2 
R
V BE  2 Vreg 
R1 
R1

Decreasing Rload
Short circuit Vreg  0 : I SC 
1
RS

1 

R2 
VBE
R1 
Vreg
 R2  Vreg
I max
 
 1  
I SC
R

R
2  V BE
 1
June 3, 2015
Physics 160
13
Foldback Example in PSpice
This defines a
parameter named
RLD, to be used as
the variable load
resistance.
D1N4002
D2
Q2
D4
Q2N3055
R1
RLD = 5
500uF
Vc
U1
+
+
Vz
Vref
R5
V
1.5k
5
9
6
R9
2k
LM723
C2
4.7uF
7
13
5 V output
output, up
to about 1 A
R2
1.5
2
3
4
CL
CS
- -
OUT
COMP
10
2.7k
12
1
C1
12Vdc
Vcc+
D1N4002
V7
PARAMETERS:
D1N4002
Vcc-
D1N4002
D7
11
1
D8
C3
100pF
R7
g
10Meg
I want to do a DC
parametric sweep, so I
replace the sine wave
here by a simple DC
source.
June 3, 2015
R6
15k
R4
4.7k
This is the resistor
divider that produces
the current foldback.
foldback
Physics 160
14
RL
{RLD}
Example Without Foldback
Output Voltage
Output Current
Current increases as 1/R as
the resistance goes down.
Lots of power is
being dissipated
as heat in the
regulator!
Example of a parametric
sweep, in which we sweep the
load resistance from 10 ohms
down to 0
0.1
1 ohms
ohms.
June 3, 2015
Physics 160
15
Foldback Example
Output Voltage
Output Current
As the over-current protection
p
shuts down the output voltage,
the current goes down,
reducing power dissipation and
heat in the regulator
regulator.
Load Resistance (ohms)
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Physics 160
16
Parallel Bipolar Transistors
•
Resistors in series with the emitters prevent the one transistor
with higher performance from trying to carry all the current.
– If more current flows through one transistor
transistor, the increased voltage
drop in the emitter resistor will reduce VBE (negative feedback).
•
MOSFETs do not require this. They will naturally regulate
themselves from self heating.
– MOSFETs also do not suffer from “2nd breakdown.”
Q1
Q2
Q2N3055
R1
June 3, 2015
Q3
Q2N3055
R2
Q2N3055
R3
Physics 160
~0.2 V drop max
17
Lab Supply with MOSFET Pass Transistors
June 3, 2015
Physics 160
18
Switching Regulator
•
Use for
– digital circuits (e.g. computers, cell-phone charging)
– high power devices
– low power, high-efficiency DC-to-DC conversion
•
but stick with the linear supply for sensitive analog applications
with small signals (common in physics labs)
– avoids interference from high frequency switching noise
•
Switching regulator advantages:
– Hi
High
h efficiency
ffi i
(less
(l
power llostt tto heat
h t in
i th
the supply)
l )
– Input voltage level is not important (e.g. 240V vs 120V)
• Can step the voltage up or down, or even invert it!
• Efficiency
Effi i
has
h little
littl d
dependence
d
on th
the iinputt llevell
– Can safely run off of the rectified AC line without an AC power
transformer
– Light weight (no heavy transformer)
– Compact (small transformers or inductors and small capacitors)
June 3, 2015
Physics 160
19
Simplified Switcher Example
Vin
Vout<Vin
Feedback system
to control the gate
pulses is not
shown.
The control pulses
Gate voltage
will arrive at a high
frequency,
q
y, typically
yp
y
tens of kHz to MHz,
so the output ripple
Input current
is high frequency
and small ((easy
y to Inductor current
filter with small
capacitors). Also,
Point X voltage
the higher the
frequency
q
y the
smaller the inductor
Output voltage
needed.
June 3, 2015
Physics 160
20
Commercial Switcher Example
Step-down
Step
down (“buck”)
( buck ) switching regulator with feedback system shown.
June 3, 2015
Physics 160
21
Simplified Switching Configurations
• Step up the
voltage.
The value
Th
l
off the
th output
t t voltage
lt
h to
has
t be
b
determined by a feedback system that
compares the output with a reference
voltage and then applies the control
signals
i
l as needed
d d to
t match
t h it.
it
• Invert the
voltage.
g
June 3, 2015
Physics 160
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Example DC to DC Conversion
Vout=10 V
12ms
D5
L1
1
2
1
U4
220uH
V
D1N4002
DC input
voltage
lower than
desired
output
voltage.
R9
Vin=6 V
5
V1
6Vdc
C3
50uF
16k
1k
Load resistance
gets cut in
half at 12ms,
to demonstrate
the regulation.
D1N4002
Q2N3904
This is for illustration. It is not
meantt to
t be
b a practical
ti l
example!
Oscillator with ~48 kHz
20
 5.55
20  16
10 
Oscillator with
~15 kHz output.
square-wave
output.
R1
R6
U2
2
X1
VCC
8
5.6 V
zener
voltage
reference
3
OUT
D3
D1N4734
3
-
G
6
7
1
LM311
20k
R8
Comparator, to
compare the
output against
a reference
voltage.
20k
GND
TRIGGER
RESET OUTPUT
CONTROL
THRESHOLD
DISCHARGE
1k
B/S
4
R4
+
8
5
10k
V+
B
500
V-
R3
2
4
5
6
7
400
R5
Q1
V
10Vdc
RL2
400
D12
500
RL
16k
R10
V3
2
555D
0.02u
1
C1
C2
0.01u
Comparator
output turns
the oscillator
on and off.
V4
10Vdc
June 3, 2015
Physics 160
In practice,
practice one buys
such a device as an
integrated package.
23
Example DC to DC Conversion
Oscillator output
Switch at full speed
until output is fully
charged.
h
d
Switching gets turned
on and
d off
ff by
b
comparator to regulate
the output at ~10V
with 400 ohm load.
Here
e e tthe
e load
oad is
s
heavier, at 200 ohms,
so more switching is
needed.
Power supply output
June 3, 2015
Physics 160
24
Example DC to DC Conversion
Oscillator output
Power supply output
June 3, 2015
Here the oscillator is turned on full time, giving
the maximum current output, to charge the
capacitor up to the desired 10V
10V.
Physics 160
25
Example DC to DC Conversion
Oscillator output
Power supply output
June 3, 2015
With the 400 ohm load resistance, the oscillator
output is going at ~12 kHz.
Physics 160
26
Example DC to DC Conversion
Oscillator output
Power supply output
With the 200 ohm load resistance, the oscillator
output is going at ~24 kHz.
Note: it is more common to keep the frequency
constant and vary the pulse width
width.
June 3, 2015
Physics 160
27
Line-Powered Switcher
High frequency, so a large, heavy
transformer is not needed.
Commonly used for notebook
computers If you plug it into
computers.
240V ac in Europe, for example,
it will work just fine, with the
p voltage
g and
same output
without overheating.
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