 |
78XX series of voltage regulators. E.g. 7805 and 7812 having regulated outputs of plus 5 volts and plus 12 volts respectively. Silverxxx, CC BY SA 3.0 Unported via Wikimedia Commons. |
We're surrounded by electronic gadgets and appliances, and even devices that don't appear to be electronic may use electronic circuitry internally. For electronics to work properly, it's imperative that voltage be stable and constant. That's the job of a voltage regulator. In this article, we'll explore voltage regulators in a little more detail.
 |
An L7805 linear voltage regulator on a printed circuit board (PCB). This regulator outputs 5 volts at currents up to 1 amp. © Raimond Spekking / CC BY-SA 4.0 (via Wikimedia Commons) |
What Is a Voltage Regulator?
A voltage regulator is an electronic device used to keep the voltage output of a power supply at a constant level, independent of the current drawn by a load. In general, these devices are implemented as a single integrated circuit (IC) in a variety of package formats or as separate modules, consisting of several discrete components and possibly integrated circuits. A regulator that reduces voltage is called a buck regulator, and one that increases voltage is called a boost regulator.
What Affects the Output of an Unregulated Voltage Source?
The output voltage of an unregulated voltage source depends on:
- Load current: The output of an unregulated voltage source will drop as the current increases. This is because of internal resistance, which causes a voltage drop as current flows. This voltage drop subtracts from the ideal internal voltage source and causes the output of the supply to be lower than the open circuit voltage without a load. We'll examine this in more detail later and work out the equations for output voltage versus load resistance.
- Battery charge state: If a voltage source is a battery, the voltage can vary depending on its state of charge or discharge.
- Input voltage to the unregulated voltage source: The output voltage of an unregulated power supply powered by the mains can also change as the mains voltage changes.
For more information on basic electricity and volts, amps and watts, see my guide: How to Understand Electricity: Volts, Amps and Watts Explained on Appliances
 |
Block diagram of a voltage regulator. Vout may be greater or less than Vin. © Eugene Brennan |
Why Does Voltage Vary if a Regulator Isn't Used? A Detailed Analysis
All linear electrical networks containing only voltage sources, current sources and resistances (which include power supplies and batteries) can be modeled as a Thévenin equivalent circuit with an ideal voltage and a source resistance in series, as shown in the diagram below. The ideal voltage source produces a voltage that doesn't change no matter what current is drawn from it. So, for instance, if the voltage source is 12 V and one million amps are drawn, it still outputs that voltage. Such devices don't exist in real life, and all voltage supplies have internal resistance that causes a voltage drop as the voltage source is loaded.
As current increases through the internal resistance of the source, Rint in the diagram below, it causes a potential drop across the resistor. The potential drop equals the current through the source resistance, multiplied by the resistance Rint. This potential drop subtracts from the ideal voltage V, so the voltage at the output terminals of the Thévenin equivalent circuit at the load RL, which we'll call VL is:
VL = V - IRint
 |
Any linear electrical network can be modeled as a Thévenin equivalent circuit (enclosed within the red dotted line) with an ideal voltage source V and a source resistance Rint in series. RL is a connected load. © Eugene Brennan |
On open circuit, with no load connected (RL is infinite) and I is zero, so:
VL = V - IRint = V - 0Rint = V
The output voltage on open circuit is the same as the ideal voltage source.
What is the output voltage with a load connected?
To work out the voltage with a load connected, we need to find the current I.
The total resistance of the circuit is the sum of the Thévenin equivalent source resistance Rint and load resistance RL in series.
The current through both resistors is given by Ohm's law as:
I = V/(Rint + RL)
The voltage at the load equals the current I flowing through the load, multiplied by the resistance of the load. If the load voltage is VL, then substituting for I from the equation above gives us:
VL = IRL = VRL/(Rint + RL)
Dividing the numerator and denominator of the right hand side of the equation by RL gives:
VL = V/(Rint/RL + 1)
We can see that if RL is infinite, i.e., on open circuit:
VL = V/(Rint/∞ + 1) = V/(0 + 1) = V
So, the output voltage is the same as that of the ideal voltage source.
When RL = 0, i.e. a short circuit:
VL = V/(Rint/0 + 1) = V(∞ + 1) = 0
The output voltage is zero when the load resistance is zero.
(Strictly speaking, division by 0 is undefined in math, but we can practically think of division by 0 giving an infinite result and dividing by infinity as giving 0 in the equations above.)
In between these values, as the load resistance falls, the Rint/RL + 1 term in the denominator of the equation increases, and VL decreases. This is the crux of the problem. As a supply is loaded and load resistance falls and more current is drawn, the output voltage of a non-regulated supply falls. A regulator solves this problem.
It's important to remember that the Thévenin equivalent circuit is just a model, and there aren't actually an ideal voltage source and series resistor component in, for example, an AA cell, bench power supply, lithium battery or power supply in an appliance or gadget. The model just describes how a supply behaves.
What Are the Two Types of Voltage Regulators?
There are two types of semiconductor regulators: the linear regulator and switching regulator.
Linear Regulator
There are two types:
- Series regulator.
This uses a pass transistor such as a bi-junction transistor (BJT) or
metal–oxide–semiconductor field-effect transistor (MOSFET) and
associated circuitry to control voltage. The pass transistor effectively
works as a controlled dropper resistor in series between the input
supply and the regulator output. A typical regulator has a 5-volt
output. So, if the input voltage is 14 volts, it drops that voltage from
14 to 5 volts.
The control circuitry in the regulator monitors the regulator output voltage, and if the load tries to take more current and output voltage tries to fall, the control circuit reduces the resistance of the pass element so that it drops less voltage in order to maintain the output at a constant 5 volts. Similarly, if the load takes less current, the resistance is increased. A linear regulator is a classic negative feedback control system (like the governor on an engine, keeping speed constant as the load increases/decreases).
- Shunt regulator. This uses a device such as a Zener diode in parallel with the load. A Zener diode has a characteristic such that the voltage drop across its terminals is relatively constant, independent of the current through it. By placing the diode in parallel with a load, this has the effect of stabilising voltage and keeping it the same as the Zener diode voltage.
Disadvantages of Linear Regulators
Since the pass component in the regulator is in series with the load, the current supply from the source is the same as that supplied to the load. However, since the voltage is dropped by the pass component, power is wasted as heat in the device. The higher the input voltage, the greater the wastage since P = VI, where V is the drop across the regulator and I is the current through the load. The lower the input voltage, the better, and a small or large heat sink may be needed, depending on the ambient temperature and voltage drop. Basic regulators need about a 2-volt difference between input and output voltages to work, but low-dropout regulators are available, which can work with a smaller difference between IP and OP.
Voltages and Packages
Linear voltage regulators are commonly available in the TO220 package with voltages of ± 5 V, 6 V, 9 V, 12 V, and 15 V. The 780XX series can output a current of up to 1.5A. Regulators with higher current outputs are also available in different packages. Adjustable voltage regulators, such as the LM317, are also available.
 |
Block diagram of a series linear regulator. © Eugene Brennan |
A basic regulator made from discrete components can be made using a pass transistor, Zener diode and resistor to bias the diode. The Zener diode acts as a reference voltage, the potential drop across it staying relatively stable as the voltage of the unregulated input to the power supply varies.
Taking KVL about the base circuit:
VD = VL + Vbe
So VL = VD - Vbe
If the load increases and VL tries to fall, VD remains constant. However, Vbe increases, causing an increase in base current. This increases the collector current and decreases the collector-emitter voltage, increasing VL to compensate.
 |
Schematic of a basic voltage regulator. The Zener diode sets a reference voltage and the transistor keeps the output voltage reasonably stable as current changes. © Eugene Brennan |
Switching Regulators
A switching regulator, on the other hand, works differently. Unlike a linear regulator, which can be very inefficient and waste power as heat, switching regulators can be up to 95% efficient. In buck mode (reducing voltage), they work by chopping the input voltage to the regulator into a pulsed waveform and applying this to a capacitor/inductor, which effectively works as a tank, smoothing the chopped waveform (analogous to the way an engine flywheel smooths the pulsed intermittent power from the cylinders). The duty cycle (how long the pulse is on) of the switching waveform is varied depending on the demand of the load in order to keep the op voltage constant.
Disadvantages of Switching Regulators
Since a switching regulator runs at high frequency, switching voltages on and off can generate a lot of electromagnetic interference (EMI). Most countries have regulations for the amount of EMI emitted by products sold, governing the level of both interference radiated through the air and through power cords. Most appliances have EMI filters to reduce this interference, but some still get transmitted. If you turn on a radio and switch it to the AM band and tune away from a channel and hold the radio close to a device such as a computer or a phone, you can hear the interference as noise on the radio.
Another issue with switching regulators is that switching noise can make its way to sensitive electronics in the device. So extra filtering is necessary to reduce this.
References
Boylestad, Robert L. (1968). Introductory Circuit Analysis. (6th ed. 1990) .Merrill Publishing Company, London, England.
Donald G. Fink, H. Wayne Beatty (1978). Standard Handbook for Electrical Engineers Eleventh Edition, Mc Graw Hill.
Millman, J., & Grabel, A. (1987). Microelectronics. McGraw-Hill.
Linear and switching voltage regulator fundamental part 1. Texas Instruments. https://www.ti.com/lit/an/snva558/snva558.pdf
Disclaimer
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualised advice from a qualified professional.
© 2022 Eugene Brennan
No comments:
Post a Comment