linear and switching power supply basic concepts (1/2)

Introduction

We can divide regulators into two major types :

Linear regulators
Switching mode regulators

In this article, we will introduce you to the linear regulators and then we will get to the magical switching supplies!

Linear regulators

Fig.1

Linear regulator shown in fig.2 produces DC regulated output from a higher DC input supply by simply putting a transistor in series between the input and the output.

Fig.2

This series transistor (called “series–pass transistor” or simply “pass transistor”) is operated in the linear(ohmic) region of its VI curve.

That’s it, Transistor Q1 acts as a variable resistance that drops the excess voltage across it. So it can be represented as shown in fig.3 .

Fig.3

Using simple math, the voltage drop across the resistor is the difference between input and output voltages (this term is called the “headroom”)

V = V (in)-V(out)

If we want to calculate the dissipated (wasted) power in the resistor it would be V*I where V is the drop across it and I is the current passing through it (load current).

For example, If we have 9 V dc input supply and we need 5 V dc output, then at a load current of 200 mA :

Wasted power = voltage drop across the transistor * load current

P = (9-5) *(.2) = .8 W.

And the useful power = output voltage * load current

Po = 5 * .2 = 1 W.

With input power of 1.8 W (1+.8) , the efficiency (Output power / Input power) would be 55.5% !!

So we can see that it has very poor efficiency that even gets worse when the load draws more current or the output voltage is lower. So, basically, it’s not a good idea to use linear regulators for relatively high power applications.

Another limitation of linear regulator is that the headroom should be positive number (input voltage must be higher than output voltage) so it only steps voltage down.

In many linear voltage regulators the headroom have to be around 2v. Regulators that accepts barely higher input voltage than the output is called LDO (low dropout ) regulators.

Applications of linear regulators

There are still many situations where the linear regulators are preferable, for example, simple low power , low noise and fast response applications.

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