Voltage regulator

Basic design principle and circuit diagram for the rotating-coil AC voltage regulator

This is an older type of regulator used in the 1920s that uses the principle of a fixed-position field coil and a second field coil that can be rotated on an axis in parallel with the fixed coil, similar to a variocoupler.

When the movable coil is positioned perpendicular to the fixed coil, the magnetic forces acting on the movable coil balance each other out and voltage output is unchanged. Rotating the coil in one direction or the other away from the center position will increase or decrease voltage in the secondary movable coil.

This type of regulator can be automated via a servo control mechanism to advance the movable coil position in order to provide voltage increase or decrease. A braking mechanism or high-ratio gearing is used to hold the rotating coil in place against the powerful magnetic forces acting on the moving coil.

Magnetic mains regulator

Electromechanical regulators called voltage stabilizers or tap-changers, have also been used to regulate the voltage on AC power distribution lines. These regulators operate by using a servomechanism to select the appropriate tap on an autotransformer with multiple taps, or by moving the wiper on a continuously variable auto transfomer. If the output voltage is not in the acceptable range, the servomechanism switches the tap, changing the turns ratio of the transformer, to move the secondary voltage into the acceptable region. The controls provide a dead band wherein the controller will not act, preventing the controller from constantly adjusting the voltage ("hunting") as it varies by an acceptably small amount.

The ferroresonant transformer, ferroresonant regulator or constant-voltage transformer is a type of saturating transformer used as a voltage regulator. These transformers use a tank circuit composed of a high-voltage resonant winding and a capacitor to produce a nearly constant average output voltage with a varying input current or varying load. The circuit has a primary on one side of a magnet shunt and the tuned circuit coil and secondary on the other side. The regulation is due to magnetic saturation in the section around the secondary.

The ferroresonant approach is attractive due to its lack of active components, relying on the square loop saturation characteristics of the tank circuit to absorb variations in average input voltage. Saturating transformers provide a simple rugged method to stabilize an AC power supply.

Older designs of ferroresonant transformers had an output with high harmonic content, leading to a distorted output waveform. Modern devices are used to construct a perfect sine wave. The ferroresonant action is a flux limiter rather than a voltage regulator, but with a fixed supply frequency it can maintain an almost constant average output voltage even as the input voltage varies widely.

The ferroresonant transformers, which are also known as constant-voltage transformers (CVTs) or "ferros", are also good surge suppressors, as they provide high isolation and inherent short-circuit protection.

A ferroresonant transformer can operate with an input voltage range ±40% or more of the nominal voltage.

Output power factor remains in the range of 0.96 or higher from half to full load.

Because it regenerates an output voltage waveform, output distortion, which is typically less than 4%, is independent of any input voltage distortion, including notching.

Efficiency at full load is typically in the range of 89% to 93%. However, at low loads, efficiency can drop below 60%. The current-limiting capability also becomes a handicap when a CVT is used in an application with moderate to high inrush current, like motors, transformers or magnets. In this case, the CVT has to be sized to accommodate the peak current, thus forcing it to run at low loads and poor efficiency.

Minimum maintenance is required, as transformers and capacitors can be very reliable. Some units have included redundant capacitors to allow several capacitors to fail between inspections without any noticeable effect on the device's performance.

Output voltage varies about 1.2% for every 1% change in supply frequency. For example, a 2 Hz change in generator frequency, which is very large, results in an output voltage change of only 4%, which has little effect for most loads.

It accepts 100% single-phase switch-mode power-supply loading without any requirement for derating, including all neutral components.

Input current distortion remains less than 8% THD even when supplying nonlinear loads with more than 100% current THD.

Drawbacks of CVTs are their larger size, audible humming sound, and the high heat generation caused by saturation.

A three-phase bank of voltage regulators used to control the voltage on long AC power distribution lines. This bank is mounted on a wooden pole structure. Each regulator weighs about 1200 kg and is rated 576 kVA.

Voltage regulators or stabilizers are used to compensate for voltage fluctuations in mains power. Large regulators may be permanently installed on distribution lines. Small portable regulators may be plugged in between sensitive equipment and a wall outlet. Automatic voltage regulators on generator sets to maintain a constant voltage for changes in load. The voltage regulator compensates for the change in load. Commercial voltage regulators normally operate on a range of voltages, for example 150–240 V or 90–280 V.

Linear regulators are based on devices that operate in their linear region (in contrast, a switching regulator is based on a device forced to act as an on/off switch). Linear regulators are also classified in two types:

1. series regulators
2. shunt regulators

In the past, one or more vacuum tubes were commonly used as the variable resistance. Modern designs use one or more transistors instead, perhaps within an integrated circuit. Linear designs have the advantage of very "clean" output with little noise introduced into their DC output, but are most often much less efficient and unable to step-up or invert the input voltage like switched supplies. All linear regulators require a higher input than the output. If the input voltage approaches the desired output voltage, the regulator will "drop out". The input to output voltage differential at which this occurs is known as the regulator's drop-out voltage. Low-dropout regulators (LDOs) allow an input voltage that can be much lower (i.e., they waste less energy than conventional linear regulators).

Entire linear regulators are available as integrated circuits. These chips come in either fixed or adjustable voltage types. Examples of some integrated circuits are the 723 general purpose regulator and 78xx /79xx series

Switching regulator integrated circuit LM2676, 3 A step-down converter

Switching regulators rapidly switch a series device on and off. The duty cycle of the switch sets how much charge is transferred to the load. This is controlled by a similar feedback mechanism as in a linear regulator. Because the series element is either fully conducting, or switched off, it dissipates almost no power; this is what gives the switching design its efficiency. Switching regulators are also able to generate output voltages which are higher than the input, or of opposite polarity—something not possible with a linear design. In switched regulators, the pass transistor is used as a "controlled switch" and is operated at either cutoff or saturated state. Hence the power transmitted across the pass device is in discrete pulses rather than a steady current flow. Greater efficiency is achieved since the pass device is operated as a low impedance switch. When the pass device is at cutoff, there is no current and it dissipates no power. Again when the pass device is in saturation, a negligible voltage drop appears across it and thus dissipates only a small amount of average power, providing maximum current to the load. In either case, the power wasted in the pass device is very little and almost all the power is transmitted to the load. Thus the efficiency of a switched-mode power supply is remarkably high-in the range of 70–90%.

Switched mode regulators rely on pulse-width modulation to control the average value of the output voltage. The average value of a repetitive pulse waveform depends on the area under the waveform. If the duty cycle is varied, the average value of the voltage changes proportionally.

Like linear regulators, nearly complete switching regulators are also available as integrated circuits. Unlike linear regulators, these usually require an inductor that acts as the energy storage element.[2][3] The IC regulators combine the reference voltage source, error op-amp, pass transistor with short circuit current limiting and thermal overload protection.

Simple DC power supplies can be made with rectifiers. But they suffer from changes in the input or at load. This initiated the development of the linear regulator (series/shunt) circuits*. Linear regulators maintain a constant voltage by varying their resistance to counterbalance input voltage change. Still, the efficiency of this system is poor. They are heavy and not flexible enough to be chosen for every application. So Switched Mode Power Supply (SMPS) was designed.

SMPSs use a high-frequency switch with varying duty cycle to maintain the output voltage. The output voltage variation caused by the switching is filtered out by an LC filter. Switched Mode Power Supply systems owe their origin back to the days when valve or tube car radios needed a large HT supply. For example, the radio would need 150 V DC to be generated from an automobile power system of normally 12 V DC. Today we find SMPS applications in the vast majority of electronics.

Switched Mode Power Supplies are basically DC–DC converters. If the input is AC, the input is first rectified to DC voltage. So depending on the input, an SMPS may have two (DC–AC, AC–DC) or three (AC–DC, DC–AC, AC–DC) stages.

The two types of regulators have their different advantages:

• Linear regulators are best when low output noise (and low RFI radiated noise) is required
• Linear regulators are best when a fast response to input and output disturbances is required
• At low power levels, linear regulators are cheaper and occupy less printed circuit board space
• Switching regulators are best when power efficiency is critical (such as in portable computers), except that linear regulators are more efficient in a small number of cases (such as a 5 V microprocessor often in "sleep" mode fed from a 6 V battery, if the complexity of the switching circuit and the junction capacitance charging current means a high quiescent current in the switching regulator)
• Switching regulators are required when the only power supply is a DC voltage, and a higher output voltage is required.
• At power levels above a few watts, switching regulators are cheaper (for example, the cost of removing heat generated is less)

Regulators powered from AC power circuits can use silicon controlled rectifiers (SCRs) as the series device. Whenever the output voltage is below the desired value, the SCR is triggered, allowing electricity to flow into the load until the AC mains voltage passes through zero (ending the half cycle). SCR regulators have the advantages of being both very efficient and very simple, but because they can not terminate an ongoing half cycle of conduction, they are not capable of very accurate voltage regulation in response to rapidly changing loads. An alternative is the SCR shunt regulator which uses the regulator output as a trigger. Both series and shunt designs are noisy, but powerful, as the device has a low on resistance.

Many power supplies use more than one regulating method in series. For example, the output from a switching regulator can be further regulated by a linear regulator. The switching regulator accepts a wide range of input voltages and efficiently generates a (somewhat noisy) voltage slightly above the ultimately desired output. That is followed by a linear regulator that generates exactly the desired voltage and eliminates nearly all the noise generated by the switching regulator. Other designs may use an SCR regulator as the "pre-regulator", followed by another type of regulator. An efficient way of creating a variable-voltage, accurate output power supply is to combine a multi-tapped transformer with an adjustable linear post-regulator.

In the simplest case a common collector amplifier also known as emitter follower is used with the base of the regulating transistor connected directly to the voltage reference:

A simple transistor regulator will provide a relatively constant output voltage U_out for changes in the voltage U_in of the power source and for changes in load R_L, provided that U_in exceeds U_out by a sufficient margin and that the power handling capacity of the transistor is not exceeded.

The output voltage of the stabilizer is equal to the Zener diode voltage minus the base–emitter voltage of the transistor, U_Z − U_BE, where U_BE is usually about 0.7 V for a silicon transistor, depending on the load current. If the output voltage drops for any external reason, such as an increase in the current drawn by the load (causing a decrease in the collector–emitter voltage to observe KVL), the transistor's base–emitter voltage (U_BE) increases, turning the transistor on further and delivering more current to increase the load voltage again.

R_v provides a bias current for both the Zener diode and the transistor. The current in the diode is minimal when the load current is maximal. The circuit designer must choose a minimum voltage that can be tolerated across R_v, bearing in mind that the higher this voltage requirement is, the higher the required input voltage U_in, and hence the lower the efficiency of the regulator. On the other hand, lower values of R_v lead to higher power dissipation in the diode and to inferior regulator characteristics.[4]

R_v is given by

${\displaystyle R_{\text{v}}={\frac {\min V_{R}}{\min I_{\text{D}}+\max I_{\text{L}}/(h_{\text{FE}}+1)}},}$

where

min V_R is the minimum voltage to be maintained across R_v,

min I_D is the minimum current to be maintained through the Zener diode,

max I_L is the maximum design load current,

h_FE is the forward current gain of the transistor (I_C/I_B).[4]

The stability of the output voltage can be significantly increased by using an operational amplifier:

In this case, the operational amplifier drives the transistor with more current if the voltage at its inverting input drops below the output of the voltage reference at the non-inverting input. Using the voltage divider (R1, R2 and R3) allows choice of the arbitrary output voltage between U_z and U_in.