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Lecture 9: Power Supplies

Lecture 9: Power Supplies. Power Supply Regulation. An ideal power supply provides a constant dc voltage despite changes to the input voltage or load conditions.

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Lecture 9: Power Supplies

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  1. Lecture 9:Power Supplies

  2. 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. VNL VNL VFL Ideal power supply Real power supply

  3. 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 VOUT per volt change on the VIN (%/V).

  4. Load Regulation When the amount of current through a load changes due to a varying load resistance, the voltage regulator must maintain a nearly constant output voltage across the load.

  5. 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: 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.

  6. 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: Power Supply In terms of resistances, load regulation can be expressed as:

  7. Load Regulation Example: • 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. • What is the load regulation? • What is the no load output voltage? Solution: = 0.25% (a) (b) By Ohm’s law, VOUT = 5.0 V. = 5.013 V

  8. 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.

  9. 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.

  10. Series Regulators The op-amp in the series regulator is actually connected as a noninvertingamplifier where the reference voltage VREF is the input at the noninverting terminal, and the voltage divider R2/R3 forms the negative feedback circuit. The closed-loop voltage gain is

  11. Series Regulators The output voltage for the series regulator circuit is: Example: (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? Solution: (a) 18 V 4.7 kW = 12.2 V 3.9 V 100 kW (b) P = VI = (18 V – 12.2 V)(0.2 A) 47 kW = 1.16 W

  12. 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.

  13. 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 VB and an increase in VC. 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.

  14. Shunt Regulators

  15. 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. An increase in the duty cycle increases the output voltage. A decrease in the duty cycle decreases the output voltage. on/off control VOUT

  16. 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.

  17. Buck Converter C charges L reverses polarity on off +- - + 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.

  18. Buck Converter An increase in the duty cycle increases the output voltage. A decrease in the duty cycle decreases the output voltage. on/off control VOUT

  19. Buck Converter

  20. Boost Converter 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. C charges 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. L field collapses L field builds on C discharges off +- - + on off

  21. Boost Converter

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