Switch Mode Regulator Theory: switching regulator

The switching regulator is the key element within any switch mode power supply

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Within any switch mode power supply, it is the switch mode regulator or switching regulator that provides the voltage regulation. The switching regulator can be used on its own or as part of a complete power supply.

Switch mode regulators come in a variety of forms, but each one will be able to provide voltage regulation by using a series switching element that charges up a reservoir capacitor when the voltage falls below a predetermined level.

Switching regulator basics

The basis of switch mode regulator revolves around the ability of inductors and capacitors to store energy. The capacitors and inductors are integral elements of the switch mode regulator technology.

  • Capacitance   If a current is applied to a capacitor, the capacitor gradually charges up and the voltage across it rises linearly at a rate equal to I/C where is the applied current and C is the capacitance. In this case the voltage across the capacitor cannot change instantly.

    Switch mode power supply, SPMPS basic concept
    Basic concept behind a switch mode power supply
    When an instantaneous change in current occurs, the voltage changes linearly. [This assumes a current source with an infinite voltage capability is used].

    I = C   d V d t
    Switch mode power supply V t graph
  • Inductor:  : For an inductor, it is not possible for there to be an instantaneous change in current. Instead, when a voltage is applied, the current builds up linearly over time at a rate equal to V/L where V is the applied voltage and L is the inductance.

    Switch mode power supply, SPMPS basic concept using an inductor
    Inductor format for a switch mode power supply
    Using the standard equations it is possible to determine the current and voltage profiles:

    V = L   d i d t
    The energy from the rising current is stored in the magnetic field associated with the inductor. If the current flowing through the inductor is suddenly interrupted, the magnetic field reacts against this and produces a very high "back emf" to counteract the change.

    Switch mode power supply i t graph

Having seen the fundamental or basic concepts behind switching voltages and currents to capacitors and inductors, these basic concepts can be applied to switch mode regulator solutions to provide a variety scenarios for voltage step up and step down circuits.

As the technology uses switching techniques where the series element is on or off, this approach provides much better levels of efficiency than a linear where power is dissipated.

Capacitor based switch mode regulation

The basic concept of the capacitor switched mode regulator is shown in the diagram. When the switch is closed, current is able to flow into the reservoir capacitor and provide charge. When the voltage on the capacitor is at is required level, the switch opens and the load will draw current from the capacitor.

As the voltage falls, this will be sensed by the control circuitry and the series switch will be turned on again to bring the capacitor voltage up to the required level.

Switch mode power supply: capacitor switching

This circuit is not as effective as may be thought at first sight. Although the only resistive element in the theoretical circuit is the load, this is not the only way in which energy is lost because charging a capacitor directly from a voltage source or a capacitor dissipates as much energy as is transferred to the capacitor. As a result of this, switching mode regulators cannot use capacitor switching techniques alone.

Inductor based switch mode regulation

It is also possible to use inductors as an element in switch mode regulators.

The inductor can be used to transfer energy from one voltage source to another. While a simple resistor can be used as a dropper to drop voltage when transferring from one voltage source at a higher voltage to one at a lower voltage, this is very wasteful in terms of power. If an inductor is used, then all the energy is transferred, assuming a perfect inductor.

The use of an inductor has the advantages that energy can be transferred from one source to another regardless of the respective values of voltage and their polarities. To achieve this the proper configuration is obviously required.

Switch mode power supply: inductor switching

When the switches are in the positions shown above, the voltage V1 is applied across the inductor and the current i1 builds up at a rate equal to V1/L. Therefore the peak value obtained will be proportional to the time the switches are in this position, i.e. (V1/L) x t

When the switches are reversed, the current will continue to flow at a rate i2 which is equal to -V2/L.

As an ideal inductor dissipates no energy, there is no power loss in an ideal system using an inductor in this fashion. As a result, it is this method of energy transfer that forms the basis for all switching regulators.

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