An example of a general-purpose regulator is Motorola's MC1723. It can be used in many different ways, for example, as a fixed positive or negative output voltage regulator, variable output voltage regulator, or switching regulator. Because of its flexibility, it has become a standard type in electronic industry. Although it is designed to deliver load current to 150 mA, the current capability can be increased to several amperes through the use of one or more external pass transistors. Figure (a) shows the connections for switching-regulator action of the 723-type regulator for positive output. The regulator requires an external transistor and a 1 mH choke. To minimize its power dissipation during switching, the external transistor used must be a switching power transistor. The 1 mH choke smooths out the current pulses delivered to the load while capacitor C holds output voltage at constant dc level.
Fixed voltage regulators such as the 78XX and 79XX and the adjustable regulators such as the LM317 are all called series dissipative regulators. This is because these regulators simulate a variable resistance between the input voltage and the load, and hence function in a linear mode. In fact, for a specified range of variation in the input voltage and load current, the linear regulator maintains a constant output voltage by dissipating the excess power as heat. In a series dissipative regulator, conversion efficiency decreases as the input/output voltage differential increases, or vice versa. For this reason, the linear series regulator is well suited for medium current applications with a small voltage differential, where the power dissipation can be handled with heat sinks.
To improve the efficiency of a regulator, the series-pass transistor is used as a switch (alternatively turned on and off) rather than as a variable resistor as in the linear mode. A regulator constructed to operate in this manner is called a series switching regulator. In such regulators the series-pass transistor is switched between cutoff and saturation at a high frequency, which produces a pulse-width-modulated (PWM) square wave output. This output is then filtered through a low pass LC filter to produce an average dc output voltage. Thus the output voltage is proportional to the pulse width and frequency. The efficiency of a series switching regulator is independent of the input/output differential and can approach 95%.
Switching regulators come in various circuit configurations including the flyback, feed-forward, push-pull, and non isolated single-ended or single-polarity types. Also, the switching regulators can operate in any of three modes: step-down, step-up, or polarity inverting.
Fixed voltage regulators such as the 78XX and 79XX and the adjustable regulators such as the LM317 are all called series dissipative regulators. This is because these regulators simulate a variable resistance between the input voltage and the load, and hence function in a linear mode. In fact, for a specified range of variation in the input voltage and load current, the linear regulator maintains a constant output voltage by dissipating the excess power as heat. In a series dissipative regulator, conversion efficiency decreases as the input/output voltage differential increases, or vice versa. For this reason, the linear series regulator is well suited for medium current applications with a small voltage differential, where the power dissipation can be handled with heat sinks.
To improve the efficiency of a regulator, the series-pass transistor is used as a switch (alternatively turned on and off) rather than as a variable resistor as in the linear mode. A regulator constructed to operate in this manner is called a series switching regulator. In such regulators the series-pass transistor is switched between cutoff and saturation at a high frequency, which produces a pulse-width-modulated (PWM) square wave output. This output is then filtered through a low pass LC filter to produce an average dc output voltage. Thus the output voltage is proportional to the pulse width and frequency. The efficiency of a series switching regulator is independent of the input/output differential and can approach 95%.
Switching regulators come in various circuit configurations including the flyback, feed-forward, push-pull, and non isolated single-ended or single-polarity types. Also, the switching regulators can operate in any of three modes: step-down, step-up, or polarity inverting.
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