physics 160 lecture 18

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

June 3, 2015 Physics 160 28

p gwithout overheating.

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