Introduction to DC Regulators
There are two techniques in voltage regulation. One is linear voltage regulation and another is switching mode voltage regulation. We here talk about switching mode voltage regulators. There are several switched-mode DC to DC converters like buck converter, buck – boost converter and boost converters. In this lesson series we are going to talk about switched-mode converters which are used to efficiently convert one level of DC voltage to another level. Before go to the topic let’s get some idea about linear voltage regulators vs switched mode regulators.
You can see here the output voltage is Vo=ILRL . Load current is controlled by the transistor. By varying the base current of the transistor, the output voltage can be controlled. Because the transistor operates linear between cut off and saturation region, these type of regulators called linear voltage regulator. Here the transistor affects the circuit as a variable resister. Linear regulators are required to have an input voltage higher than the output voltage. That is called drop out voltage.
But, when compare to the switched-mode regulators, linear regulators are inefficient. Because the power loss in the transistor (Power =VCEIL). Different between input and output voltages, dissipated as heat.
Example: If input voltage in V and output voltage is V/4. Then the power absorbed by the load is PLOAD = VIL/4 . The power absorbed by the transistor is PTRANS=VIL/4 . so we can see the 75% of power is wasted. Therefore, linear regulators only use with low power devices.
The efficient way is to use switched-mode regulators. Here the transistor used as a high speed switching device. These are efficient because the transistor either fully off or fully conducting.
This is known as DC chopper. You can see when the switch is closed output voltage is same as the input voltage. And when switch is open output voltage will be zero. Periodic opening and closing of the switch results in the pulse output like above picture.
The Average output voltage is ,
Here the D is duty ratio. The average output voltage can be controlled with duty ratio. The duty ratio can be controlled by changing the switch on and off time.
f is switching frequency. Here we assumed the power absorbed by ideal switch is zero. So we can say that during transistor act as an open circuit (No current) and during it is acting as short circuit (No voltage across the transistor). All the power absorbed by the load. Therefore, energy efficiency is 100 percent.