How Linear Regulator Provides Output Regulation

In power supply the term voltage regulation is often mentioned and this is a very important thing actually. Nobody will buy a power supply with an output that has a voltage swing of 10% or even higher. Imagine for a 12V output the voltage can go as high as 13.2V and as low as 10.8V. This is ridiculous. In today’s power supply design, output regulation is normally at +/-2% of the output voltage. There are two categories of power supply; these are linear and switch mode. The former is the one usually teach in the universities. The latter on the other hand is a more complicated design and not usually teaches in colleges. In this article we are going to discuss more on linear power supply type. The term linear is coined because of the linear regulator used in the output of the power supply. This linear regulator is the one providing regulation of the output. But here is a question; how linear regulator provides output regulation? We are going to answer this one, so just on keep reading.

Analysis on How Linear Regulator Provides Output Regulation

Figure below shows a linear regulator. Actually this is the internal circuit of a linear regulator IC (plus the other protections and housekeeping).

figure 1

 

The principle of the circuit is to keep the positive and negative inputs of the comparator X1 equal to each other. As you can see the positive input is connected to a Zener diode which acts a reference. On the other hand the negative input is connected to the output via a divider resistor. When the output decreases, the level of the negative input will be less than the reference and as a result the comparator output will be equal to Vin. This will provide a strong bias to the transistor Q1 and the voltage drop on it will decrease so that the output will then become regulated.

When there is overshoot in the output, the negative input become higher than the reference and then the comparator output will be low. The transistor Q1 will then turn off and the output will decrease. The transistor Q1 will go back to operation when the level of the output decreases below the set regulation.

You may be thinking that the output will swing above and below the set regulation before the linear pass transistor Q1 can react. Actually you cannot observe it because the response is fast. The positive and negative inputs of the comparator is always tracking each other and do not let the other become higher than the other. However during dynamic loading the ripple may increase. The usual solution is to add compensation circuit. Nowadays linear regulator has good response to dynamic loading and no ripple problems.

Obviously the transistor Q1 is operating in the linear region in order to maintain the output voltage to the set regulation. This is why the term linear is coined. The output maintains regulation by varying the voltage drop across collector-emitter of Q1. This is a short answer to the question how linear regulator provides output regulation. The voltage drop on the collector-emitter of the transistor is just the difference of the input and output voltages. The power dissipation on the transistor is the product of the collector-emitter voltage and the load current. This is huge and explains why a linear power supply has a bad efficiency.

Circuit Simulation on How Linear Regulator Provides Output Regulation

Let us take a look on the simulation response to have a conclusive data how linear regulator provides output regulation. I re-draw the circuit below with some probes so that we can see the waveforms and voltage levels. The target output voltage is 10V which is happening when the voltage across R2 and the Zener diode Z1 level are equal.

figure 2

Below is the output voltage based on the simulation

figure 3

Since the input voltage is 20V and the output is only 10V, the collector-emitter of Q1 must be around 10V as well. Based on the simulation result below, yes it is.

figure 4

The 10V output will be realized when the level of the positive and the negative input of the comparator are equal. Based on the simulation results below, yes they are equal.

figure 5

To sum it up, the key on how linear regulator provides output regulation is the linear operation of the series pass transistor. Linear regulator is only recommended to be used in small power applications due to its huge power dissipation. Huge power dissipation is not acceptable in power supply industry and also it requires a physically bigger device.

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