physics 160 lecture 18
TRANSCRIPT
Physics 160Lecture 18
R. JohnsonJune 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• Closed 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 plus solutions (2014 exam).e a p us so ut o s ( 0 e a )
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723 Regulator Usage
2 to 7 volts output
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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.
5VV
R6
D2D1N4002
D8
D1N4002
-
+
U11211
104
5
23
Vcc
+Vc
OUT-
CLCS
RLoad
VR4
1.5k
V6.8
D7
D1N4002C1
500uF
D4
D1N4002
C3+
LM723
713
5
69
Vcc
-
CO
MP +
VrefVz
RLoad
100R3
2kV1
FREQ = 60VAMPL = 10VOFF = 0
R210Meg
R5
4.7k
C3
4.7uF
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C2
100pF
Simulation of a 5V, 50mA Supply
Before Regulator
After Regulator
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Output Voltage vs Input Voltage
After RegulatorAfter Regulator
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3-Terminal Regulator LM317
This device maintains a reference voltage of 1.25V between the ADJ and Vout terminalsand Vout 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
i d kversion and package type).
T b i t k i t
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To be precise, you can take into account the small current flowing into the ADJ input.
LM317 Simulation
U2
R6
U2LM317K
2 3
1
IN OUT
AD
J
V
D4
D1N4002
D8
D1N4002
~5VD2D1N4002
V
240
C1
700uFRLoad
50
D1N4002D7
D1N4002
D1N4002
C3
4.7uFR7
750V6
FREQ = 60VAMPL = 10
VOFF = 0750
21.15750240
7505
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750240
LM 317 Simulation
Before Regulator
After Regulator
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Practical Linear Power Supplies• 3-wire plug• Fuse (slow blow)• Transformer: important for safety! Estimating the ohmic heating p y g 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; don’t make oversized or more transformer pheating will result. A volt or so of ripple at the maximum current load is okay; the regulator removes most of it.
• Regulator, with current limiting(Charge is the integral of I over time, but the heat dissipated in the transformer g g
– Foldback current limiting• High-current pass transistors. Heat Sinks. Bipolar transistor
issues. MOSFETs.
pcoil is an integral of I 2.)
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• Voltage limiting: crowbar
Unregulated Power Supply
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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
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controlled rectifier (SCRor “thyristor”)
Foldback Current Limiting
regBE V
RR
VRR
RI
1
2
1
2max 11
Decreasing Rload
s RRR 11Simple system to reduce the output current when the output is short-circuited.
BESCreg VRIV
211:0Short circuit BE
SSCreg V
RRIV
11:0
regV
Short circuit
VRI
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reg
SC VV
RRR
II
21
2max 1
Foldback Example in PSpiceThis defines a
D2D1N4002
This defines a parameter named RLD, to be used as the variable load resistance.
U11211
R1
2.7k
PARAMETERS:RLD = 5
V7
Q2
Q2N3055
C1
500uF
D4
D1N4002D7
D1N4002
D8
D1N4002
5 V output up
-
+
U111
104
5
23
69
Vcc
+Vc
OUT
Vcc
-
CO
MP
-
+
CLCS
VrefVz
12Vdc R21.5
V
R9
R5
1.5k
5 V output, up to about 1 A
LM723
713
R615k
R44.7k
2k
RL{RLD}C3
100pF
C24.7uF
R710MegI want to do a DC gI want to do a DC
parametric sweep, so I replace the sine wave here by a simple DC source.
This is the resistor divider that produces the current foldback
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source. the current foldback.
Example Without Foldback
Output VoltageOutput CurrentOutput Current
Lots of power is being dissipated
Current increases as 1/R as the resistance goes down.
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 1 ohmsdown to 0.1 ohms.
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Foldback Example
Output Voltage
Output Current
As the over-current protection pshuts down the output voltage, the current goes down, reducing power dissipation and heat in the regulator
Load Resistance (ohms)
heat in the regulator.
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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 the increased voltageIf more current flows through one 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.themselves from self heating.– MOSFETs also do not suffer from “2nd breakdown.”
Q1
Q2N3055
Q3
Q2N3055
Q2
Q2N3055
R2
Q2N3055
R3R1
Q2N3055Q2N3055
~0.2 V drop max
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Lab Supply with MOSFET Pass Transistors
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Switching Regulator• Use for
– digital circuits (e.g. computers, cell-phone charging)– high power devices– 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)with small signals (common in physics labs)– avoids interference from high frequency switching noise
• Switching regulator advantages:Hi h ffi i (l l t t h t i th l )– High efficiency (less power lost to heat in the supply)
– Input voltage level is not important (e.g. 240V vs 120V)• Can step the voltage up or down, or even invert it!
Effi i h littl d d th i t l l• Efficiency has little dependence on the input level– Can safely run off of the rectified AC line without an AC power
transformerLight weight (no heavy transformer)
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– Light weight (no heavy transformer)– Compact (small transformers or inductors and small capacitors)
Simplified Switcher Example
Feedback system
Vin Vout<Vin
to control the gate pulses is not shown.
Gate voltageThe control pulses will arrive at a high frequency, typically
Input current
Inductor current
q y, yp ytens of kHz to MHz, so the output ripple is high frequency and small (easy to
Point X voltage
( yfilter with small capacitors). Also, the higher the frequency the
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Output voltageq y
smaller the inductor needed.
Commercial Switcher Example
Step-down (“buck”) switching regulator with feedback system shown.
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Step down ( buck ) switching regulator with feedback system shown.
Simplified Switching Configurations
• Step up theStep up the voltage.
Th l f th t t lt h t bThe value of the output voltage has to bedetermined by a feedback system thatcompares the output with a referencevoltage and then applies the controli l d d t t h it
• Invert the voltage.
signals as needed to match it.
g
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U4
12ms1 2
L1
220uH
1 2D5
Example DC to DC Conversion Vout=10 V
C3
50uF
DC inputvoltagelower thandesiredoutput
R9
5RL
400
V
V16Vdc
RL2
400
D1N4002
R5
16k
Vin=6 V 16k
D12
D1N4002
voltage.
Load resistancegets cut inhalf at 12ms,to demonstratethe regulation.
V
R10
1k
Q1
Q2N3904
This is for illustration. It is not t t b ti l
Oscillator with~15 kHz output.
meant to be a practical example! Oscillator with ~48 kHz
square-wave output.
55.51620
2010
R3
500
R6
1k
5.6 Vzenervoltagereference
X1
234
5
8
TRIGGEROUTPUTRESET
VC
C
R1
10k
D3
D1N4734V310Vdc
U2
LM311
7
2
3 1
84
6
5
OUT
+
- G
V+
V-
B/SB
R8
R4
500
Comparator, tocompare theoutput againsta referencevoltage.20k
C1
0.02u
555D
1
567
GN
D
CONTROLTHRESHOLDDISCHARGE
C2
0.01u
R8
20k
Comparatoroutput turnsthe oscillatoron and off.
20k
In practice one buys
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V410Vdc
In practice, one buys such a device as an integrated package.
Example DC to DC Conversion
Oscillator output
Switching gets turned d ff b Here the load is
Switch at full speed until output is fully h d
on and off by comparator to regulate the output at ~10V with 400 ohm load.
e e t e oad sheavier, at 200 ohms, so more switching is needed.
charged.Power supply output
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Example DC to DC Conversion
Oscillator output
Power supply outputHere the oscillator is turned on full time, giving the maximum current output, to charge the capacitor up to the desired 10Vcapacitor up to the desired 10V.
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Example DC to DC Conversion
Oscillator output
Power supply output With the 400 ohm load resistance, the oscillator output is going at ~12 kHz.
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Example DC to DC Conversion
Oscillator output
Power supply outputWith 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 widthconstant and vary the pulse width.
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Line-Powered SwitcherHigh frequency, so a large, heavy transformer is not needed.
Commonly used for notebook computers If you plug it intocomputers. If you plug it into 240V ac in Europe, for example, it will work just fine, with the same output voltage and
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p gwithout overheating.
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