Lecture 9: Power Supplies

Lecture 9: Power Supplies

Electronic Devices, 9th edition Thomas L. Floyd

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Electronic Devices, 9th edition Thomas L. Floyd

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Electronic Devices, 9th edition Thomas L. Floyd

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Electronic Devices, 9th edition Thomas L. Floyd

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Power Supply Regulation An ideal power supply provides a constant dc voltage despite changes to the input voltage or load conditions. The output voltage of a real power supply changes under load as shown in the second plot. The output is also sensitive to input voltage changes. Voltage

Voltage

VNL

VNL VFL

Ideal power supply 0

Real power supply Current

0

Electronic Devices, 9th edition Thomas L. Floyd

0

Current

0

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Line Regulation Line regulation is a measure of how well a power supply is able to maintain the dc output voltage for a change in the ac input line voltage. The formula for line regulation is  VOUT  Line Regulation =  100%  VIN 

Line regulation can also be expressed in terms of percent change in VOUT per volt change on the VIN (%/V). VOUT / VOUT 100%  Line Regulation = VIN

Electronic Devices, 9th edition Thomas L. Floyd

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Load Regulation Load regulation is a measure of how well a power supply is able to maintain the dc output voltage between no load and full load with the input voltage constant. It can be expressed as a percentage change in load voltage:  VNL  VFL  Load Regulation =  100%  VFL 

Load regulation can also be expressed in terms of percent change in the output per mA change in load current (%/mA). Sometimes a maximum error voltage is given in the specification as illustrated in the next slide for a commercial power supply. Electronic Devices, 9th edition Thomas L. Floyd

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Load Regulation Sometimes the equivalent Thevenin resistance of a supply is specified in place of a load regulation specification. In this case, VOUT can be found by applying the voltage divider rule:   RL VOUT  VNL   R  R L   OUT In terms of resistances, load regulation can be expressed as:

 ROUT Load regulation    RFL Electronic Devices, 9th edition Thomas L. Floyd

Power Supply RTH = ROUT VOUT

VTH = VNL

RL

 100%  © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Load Regulation A power supply has an output resistance of 25 mW and a full load current of 0.50 A to a 10.0 W load. (a) What is the load regulation? (b) What is the no load output voltage?

 ROUT   0.025 W  Load regulation  100%  (a)    100% = 0.25%  10.0 W   RFL  (b) By Ohm’s law, VOUT = 5.0 V. VNL 

Electronic Devices, 9th edition Thomas L. Floyd

VOUT   RL   R  R L   OUT



5.0 V = 5.013 V 10.0 W      0.025 W + 10.0 W  © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Series Regulators Series Regulator block diagram: Control element

VIN

Reference voltage

Error detector

VOUT

Sample circuit

Basic series regulator circuit: Control element VIN

VOUT Q1

R1 +

VREF

The control element maintains a constant output voltage by varying the collector-emitter voltage across the transistor. Electronic Devices, 9th edition Thomas L. Floyd

– D1

Error detector

R2 Sample circuit R3

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Series Regulators  R  The output voltage for the series regulator circuit is: VOUT  1  2  VREF  R3  (a) What is the output voltage for the series regulator? (b) If the load current is 200 mA, what is the power dissipated by Q1?  R  (a) VOUT  1  2  VREF  R3   100 kW   1+  3.9 V  47 kW  = 12.2 V (b) P = VI = (18 V – 12.2 V)(0.2 A) = 1.16 W Electronic Devices, 9th edition Thomas L. Floyd

VIN

VOUT

18 V

Q1

R1

4.7 kW VREF

+ –

3.9 V

D1

R2

100 kW R3

47 kW

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Shunt Regulators Shunt Regulator block diagram: R1 VIN

VOUT

Reference voltage

Error detector

Control element (shunt)

Basic shunt regulator circuit: Sample circuit

VOUT

VIN R1 R2

–

VREF

The control element maintains a constant output voltage by varying the collector current in the transistor. Electronic Devices, 9th edition Thomas L. Floyd

Error detector

Control element Q1

+

RL

R3

D1

Sample circuit R4

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Summary Shunt Regulators

Shunt regulators use a parallel transistor for the control element. If the output voltage changes, the op-amp senses the change and corrects the bias on Q1 to follow. For example, a decrease in output voltage causes a decrease in V VB and an increase in VC. V

OUT

IN

R1

Although it is less efficient than the series regulator, the shunt regulator has inherent short-circuit protection. The maximum current when the output is shorted is VIN/R1. Electronic Devices, 9th edition Thomas L. Floyd

R2

Error detector –

VREF

Control element Q1

+

RL

R3

D1

Sample circuit R4

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Electronic Devices, 9th edition Thomas L. Floyd

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Electronic Devices, 9th edition Thomas L. Floyd

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Summary Switching Regulators

All switching regulators control the output voltage by rapidly switching the input voltage on and off with a duty cycle that depends on the load. Because they use high frequency switching, they tend to be electrically noisy. Andecrease A increaseininthe theduty dutycycle cycledecreases increasesthe theoutput outputvoltage. voltage. on/off control

tonton ton

tofftoff toff

tonton ton

tofftoff toff

tonton ton

tofftoff toff

tonton ton

VC VC VC

VOUT Electronic Devices, 9th edition Thomas L. Floyd

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Summary Switching Regulators

A step-down switching regulator controls the output voltage by controlling the duty cycle to a series transistor. The duty cycle changes depending on the load requirement. Because the transistor is either ON or OFF on all switching regulators, the power dissipated in the transistor is very small and the regulator is very efficient. The pulses are smoothed by an LC filter. Electronic Devices, 9th edition Thomas L. Floyd

Q1 VIN

C Lcharges reversesLpolarity

off on

+ 

+  RL

C

D1

R1

VOUT

Variable pulse-width oscillator

R2 – + R3

D2

VREF

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Summary Switching Regulators

In a step-up switching regulator, the control element operates as a rapidly pulsing switch to ground. The switch on and off times are controlled by the output voltage. Step-up action is due to the fact the inductor changes polarity during switching and adds to VIN. Thus, the output voltage is larger than the input voltage.

VIN

+

L field L field collapses builds

OUT

+

R1

L

+

Variable pulse-width oscillator

– D2

Electronic Devices, 9th edition Thomas L. Floyd

charges discharges on off C C +V D1

C

on off Q1

RL R2

+

R3

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.