## Presentation on theme: "© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Electronic Devices, 9th edition Thomas L. Floyd Lecture 9: Power Supplies."— Presentation transcript:

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Electronic Devices, 9th edition Thomas L. Floyd Power Supply Regulation An ideal power supply provides a constant dc voltage despite changes to the input voltage or load conditions. Ideal power supply 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. Real power supply V NL V FL

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Electronic Devices, 9th edition Thomas L. Floyd 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 Line regulation can also be expressed in terms of percent change in V OUT per volt change on the V IN (%/V).

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Electronic Devices, 9th edition Thomas L. Floyd Load Regulation Sometimes the equivalent Thevenin resistance of a supply is specified in place of a load regulation specification. Power Supply In this case, V OUT can be found by applying the voltage divider rule: In terms of resistances, load regulation can be expressed as:

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Electronic Devices, 9th edition Thomas L. Floyd Load Regulation A power supply has an output resistance of 25 m  and a full load current of 0.50 A to a 10.0  load. (a)What is the load regulation? (b)What is the no load output voltage? = 0.25% (a) (b) By Ohm’s law, V OUT = 5.0 V. = 5.013 V

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Electronic Devices, 9th edition Thomas L. Floyd Regulators The fundamental classes of voltage regulators are linear regulators and switching regulators. Both of these are available in integrated circuit form. Two basic types of linear regulator are:  the series regulator  the shunt regulator.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Electronic Devices, 9th edition Thomas L. Floyd Series Regulators Series Regulator block diagram: Basic series regulator circuit: The control element maintains a constant output voltage by varying the collector-emitter voltage across the transistor.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Electronic Devices, 9th edition Thomas L. Floyd Series Regulators The op-amp in the series regulator is actually connected as a noninverting amplifier where the reference voltage V REF is the input at the noninverting terminal, and the voltage divider R2/R3 forms the negative feedback circuit. The closed-loop voltage gain is

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Electronic Devices, 9th edition Thomas L. Floyd Series Regulators The output voltage for the series regulator circuit is: (a) What is the output voltage for the series regulator? (b) If the load current is 200 mA, what is the power dissipated by Q 1 ? (a) (b) 47 k  100 k  3.9 V 18 V 4.7 k  = 12.2 V P = VI = (18 V – 12.2 V)(0.2 A) = 1.16 W

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Electronic Devices, 9th edition Thomas L. Floyd Shunt Regulators Shunt Regulator block diagram: Basic shunt regulator circuit: The control element maintains a constant output voltage by varying the collector current in the transistor.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Electronic Devices, 9th edition Thomas L. Floyd Shunt Regulators 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 V IN /R 1. 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 Q 1 to follow. For example, a decrease in output voltage causes a decrease in V B and an increase in V C.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Electronic Devices, 9th edition Thomas L. Floyd 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. on/off control V OUT An increase in the duty cycle increases the output voltage.A decrease in the duty cycle decreases the output voltage.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Electronic Devices, 9th edition Thomas L. Floyd Buck Converter 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.

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Electronic Devices, 9th edition Thomas L. Floyd Buck Converter 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. on C charges +  + L reverses polarity off

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Electronic Devices, 9th edition Thomas L. Floyd Buck Converter on/off control V OUT An increase in the duty cycle increases the output voltage.A decrease in the duty cycle decreases the output voltage.