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Linear Voltage regulators

What do you need to know to understand this topic?

Sections

As we have seen in the voltage regulators section, regulators can be roughly divided in linear or switched-mode. Linear regulators control the voltage drop between the input and output as required to produce the desired output. These are very simple and used mainly for low power circuits and low voltage differences between input and output (otherwise a large dissipation of energy would overheat the regulator).

How do linear regulators control the voltage drop?

To understand how linear regulators work, imagine that the output voltage is the level of water in a tank and the output current is a leakage. To maintain the level in the tank, the same flux of water must enter the tank as the one that is leaking. To control the influx, we have two options: either strangle the influx to match the leakage or divert any influx that may be exceeding the leakage to another path.

Similarly, linear regulators can work in two ways.

Concept of linear regulators

The above circuit is the concept of a linear regulator, where $R_L$ is the load resistance. The equation $$ V_{out} = V_{in} - R_S (I_L + I_Z) $$ rules this circuit. The variables the regulator does not control are $V_{in}$ and $I_L$. Its objective is to keep $V_{out}$ fixed. Its knobs to do that are $R_S$ and $I_Z$. Therefore, it can either "strangle the input current" (change $R_S$) or divert any extra current from the input current to ground (change $I_Z$) to keep $V_{out}$ always at the same value. The regulators that have $R_S$ as a variable resistance, which is in series with the load are called series regulators, while the ones that fix $R_S$ but create a shunt through $I_Z$ are called shunt regulators.

Therefore, we can divide the linear voltage regulators in two categories:

Shunt regulator

The device that is used to divert the extra current of a shunt regulator is the zener diode. Every diode has a maximum reverse voltage above which it breaks down, the properly named reverse breakdown voltage. A zener diode differs from normal diodes in that its breakdown voltage is well-defined during fabrication and it can revert from the breakdown region. In this region, its dynamic resistance is very low, which means that a large change in current has a small effect on its voltage. Therefore, the zener diode has the ability to keep a reasonably steady reverse voltage at its terminals around its breakdown voltage for a wide range of reverse currents. This property makes it a voltage reference on its own. The shunt regulators can be as simple as a resistor and a zener diode in series and the load is applied to the cathode of the zener.

Concept of linear shunt regulator

Concept of linear shunt regulator

Linear shunt regulator

$$I_{out} = I_{in} - I_{z}$$ The efficiency of the regulator is $$\eta = \frac{P_{out}}{P_{in}} = \frac{V_{out} I_{out}}{V_{in} I_{in}} = \frac{V_{out}I_{out}}{V_{in}(I_{out}+I_{z})}$$ and the power loss is: $$P_{in} - P_{out} = V_{in}I_{in} - V_{out}I_{out} = (V_{in} - V_{out}) I_{in} + V_{out} I_{z}$$ i.e., the power loss is equal to the power lost in the resistance $R_S$ , plus the power consumed by the parallel resistance $R_Z$.

There is much more to say about the shunt regulators. To go deeper, read the shunt regulators page.

Series regulator

A series regulator controls the voltage drop between input and output nodes by actively controlling the value of the series resistance.

Concept of linear series regulator

Linear series regulator

To that end, an active device such as a Bipolar Junction Transistor or Field Effect Transistor (MOSFET) is used. The details vary for different implementations, but it boils down to this: the output is compared with a voltage reference and the series resistance is changed in a way that the voltage drop maintains the output voltage close to that voltage reference. Unlike the shunt regulator, all current flows to the load (ignoring any quiescent current of the control circuit): $$I_S = I_L$$ $$I_{out} \approx I_{in}$$ Hence, the efficiency is only defined by the ratio of voltages: $$\eta = \frac{P_{out}}{P_{in}} = \frac{V_{out} I_{out}}{V_{in} I_{in}} \approx \frac{V_{out}}{V_{in}}$$ and the power is lost only in the series resistance: $$P_{in} - P_{out} = (V_{in} - V_{out}) I_{out}$$ Therefore, series regulators have better efficiency than shunt regulators.

There is much more to say about the series regulators. To go deeper, read the series regulator page.

Specifications of linear regulators

You should be aware of a number of specifications related to linear regulators. Just to name a few:

Protections

Regulators can be very badly used: excessive currents, excessive temperatures, swapping input and output, etc.. To make them more robust, IC regulators already come with protections. The most notable are:

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