Switching Regulator Fundamentals
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This document provides an overview of switching regulators and their topologies. It discusses pulse width modulation and how switching regulators use an inductor to average switched voltage. The main topologies covered are buck, boost, and buck-boost converters. The document analyzes the ideal operation of each topology.
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Switching Regulator Fundamentals
- 8. Inductive Switching DC State AC State
Editor's Notes
- Welcome to this module on switching regulator from National Semiconductor. This module will overview the switching regulator family and discuss the topologies of switching regulator.
- The switching regulator is increasing in popularity because it offers the advantages of higher power conversion efficiency and increased design flexibility (multiple output voltages of different polarities can be generated from a single input voltage). A fresh look at how inductor and switch placement define the basic converter topology. Pulse width modulation and inductor operation are covered.
- By opening and closing the switch, the average voltage as seen by the load resistor R is: Vo(avg) = Vin D; Where: D = t (on)/T Changing the duty cycle D changes the average voltage to the load. This method of control is known as pulse width modulation, which is used in fiexed frequency switching regulators. Usually using only a switch is fine for lighting and heater control. When a DC output voltage is needed, an additional filter must be used.
- The basic switching regulator has three terminals. This means there are only three places the inductor can connect to, which is a very important concept. All square wave inductive switchers use at least one of the following building blocks. By noting the inductor placement, it's easy to tell what the inductors DC (average) current is. A buck converter has the inductor connected to the output terminal. The DC (average) inductor current is equal to the output current. This converter will step down the input to produce a lower output voltage of the same polarity.
- A boost converter has the inductor connected to the input terminal. The DC (average) inductor current is equal to the input current. This converter will step up the input to produce a higher output voltage of the same polarity.
- A buck-boost converter has the inductor connected to the ground terminal. The DC (average) inductor current is equal to the sum of the input and output current. This converter will invert the input to produce a lower or higher output voltage of opposite polarity.
- The inductor doesn't care where it is connected. Its operation will always be the same. In steady state, the average inductor voltage equals zero. V1 and V2 are defined by the switches and the applied circuit voltages. The inductor will pass the required DC current. An AC current will ramp up and down as the inductor voltage is switched.
- The input, output and duty cycle relationships are defined for each topology. All voltages and currents are presented in terms of magnitude, not polarity. Because of it's simplicity and ease of use, the buck converter is very popular in distributed power systems. The basic practical buck converter uses a free-wheeling diode in place of Sw2. Capacitors are used to filter the ripple currents and produce a DC voltage. Since the inductor is in series with the output, the output capacitor sees low ripple current. With a switch at the input, the input capacitor sees high pulsating current. The negative buck is a mirror image of the positive buck. Note that the diode points in the direction of positive current flow at the output. For a practical buck converter, the loss elements must be accounted for.
- Because of it's ability to step up a voltage, the boost converter is popular in many battery powered applications. The basic practical boost converter uses a free-wheeling diode in place of Sw2. Since the inductor is in series with the input, the input capacitor sees low ripple current. With a diode at the output, the output capacitor sees high pulsating current. The negative boost is mirrored from the positive boost. Note that the diode points in the direction of positive current flow at the output. For a practical boost converter, the loss elements must be accounted for.
- The buck-boost converter is used when a voltage inversion is needed. For this discussion, all voltages are presented in terms of magnitude. The basic practical buck-boost converter uses a free-wheeling diode in place of Sw2. With a switch at the input and diode at the output, both the input and output capacitors see high pulsating currents. The negative-to-positive buck-boost is a mirror image of the positive-to-negative buck-boost. Note that the diode points in the direction of positive current flow at the output. For a practical buck-boost converter, the loss elements must be accounted for. A buck-boost is used to produce an output voltage of opposite polarity from the input.
- Thank you for taking the time to view this presentation on switching regulator . If you would like to learn more or go on to purchase some of these devices, you can either click on the link embedded in this presentation, or simple call our sales hotline. For more technical information you can either visit the NS site – link shown – or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility.