Voltage regulator

Two common integrated linear fixed voltage series regulators, left 5 V / 1 A, right 5 V / 100 mA

A voltage regulator stabilizes an electrical voltage, usually a direct voltage, as the operating voltage of a circuit in order to avoid fluctuations in the input voltage, e.g. B. compensate for a battery or the power grid in wide areas. Integrated circuits are often used for this today. These electronic voltage regulators usually fulfill other tasks, such as current limiting or switching off the output voltage via a shutdown pin .

However, alternating voltages can also be stabilized.

overview

A voltage stabilization with a Zener diode is not a series regulator but a cross regulator. From a control point of view, however, it is an actuator because there is no control loop here.

Electronic circuits almost always require a constant operating voltage that is both independent of the input voltage and stable; H. regardless of the current consumption.

The mains voltage may fluctuate by ± 10%, i.e. 207 V to 253 V. With a conventional transformer, the operating voltage must therefore be stabilized, which also largely eliminates the residual ripple behind the rectifier. Batteries and rechargeable batteries have a similarly wide voltage range from fully charged to completely discharged.

There are different principles for DC voltage regulators:

No regulators are used for the smallest outputs, such as with reference voltage sources , but stabilization circuits with Zener diodes , as shown in the schematic diagram opposite. The particularly low differential resistance of these components of about 5 Ω, which must be much smaller than R _v, is decisive for the function . Because only then does the output voltage U _a remain approximately constant even with larger fluctuations in the load current.

Linear regulators use a power transistor that works like an electronically variable resistor. If the output voltage deviates from the setpoint, the difference is amplified and fed back to the power transistor ( control circuit ). The disadvantage is the relatively low degree of efficiency with a large difference between the input and output voltage and thus usually the need to cool the power transistor. Advantages: Load changes can be balanced quickly and easily, and there are very inexpensive modules for small outputs. There are no AC voltage residues at the output, and no interference radiation ( EMC ) is generated. Sensitive analog circuits can thus be operated without any problems.

Switching regulators contain a power transistor, which is switched on and off at a high frequency (a few kHz to MHz), and an inductance or a transformer . This transistor is always completely blocked or completely open and is therefore only slightly heated. In this way, high efficiencies of over 90% can be achieved. This extends the service life of battery-operated devices such as laptops . The disadvantage is that the circuit has to be well shielded because of the powerful alternating frequency used internally ( electromagnetic compatibility ) and that a small proportion of this alternating voltage can always be measured at the output. This interferes less with digital circuits or chargers , but it does interfere with sensitive amplifiers e.g. B. in radios , hi-fi systems or EEGs .

Switching regulators are available as buck and boost converters . With the latter, the output voltage is always greater than the input voltage, which is basically not possible with linear regulators.

Linear regulators

A distinction is made between parallel stabilization ( lateral regulator ), series stabilization ( longitudinal regulator ) and a combination of both.

In the case of the cross regulator, the load (the consumer) is parallel to the regulator circuit and it always converts the entire power that is not required into heat. In addition, there are often losses in the upstream component required to limit the current (in the simplest case a resistor ). This method, also known as a shunt regulator , is only used because of the losses if z. B. the extracted power is only low or the control speed has to be high.

In the series regulator, the controlled system (so-called series transistor) is in series with the consumer. This circuit only consumes a little more than the load current and is therefore more efficient than parallel stabilization; it is therefore most frequently used in electronics.

Regardless of the type of regulator, a voltage regulator always has a reference voltage source that consists of a circuit of different complexity depending on the requirements. In the simplest case, a series connection of a Zener diode and series resistor is used, as well as the band gap reference preferred for integrated circuits and, for precision applications, a Zener diode with regulated wiring or special temperature-compensated band gap references.

Series regulator

Integrated circuits that contain the controlled system (power transistor), the controller and a reference voltage source are usually used as series regulators . One differentiates:

• Fixed voltage regulator: Output voltage is specified by the manufacturer
• Adjustable voltage regulator: output voltage can be selected using a voltage divider

Efficiency

The efficiency of a linear regulator is calculated as follows. The controller's own power consumption (“cross current”) is often neglected because it is small in relation to the output current. ${\displaystyle \eta }$

${\displaystyle \eta ={\frac {P_{\text{aus}}}{P_{\text{ein}}}}={\frac {U_{\text{aus}}\cdot I_{\text{aus}}}{U_{\text{ein}}\cdot I_{\text{ein}}}}\;{\stackrel {I_{\text{aus}}=I_{\text{ein}}}{=}}\;{\frac {U_{\text{aus}}}{U_{\text{ein}}}}}$

Thus, within the framework of the assumptions, the efficiency is only dependent on the input and output voltage.

Function example actuator

Stabilizer with Zener diode

A rectifier circuit supplies U _E and is stabilized from D _z to U _z . R _v serves to limit the current of D _z and must also supply the changing base current of Q at the same time. The transistor works as an emitter follower , which is why the output voltage U _{A is} slightly smaller than U _z . The difference U _z  -  U _A is not constant (base-emitter voltage of Q, about 0.6 V), but changes with the emitter current. Therefore the stability is worse than that of the Zener diode (the differential resistances of the Zener diode and resistor have to be added, which leads to a worse stabilization factor). The advantage is that you can draw a significantly larger current because it mainly runs through the collector-emitter path. The transistor usually has to be provided with a heat sink in order to dissipate the resulting power loss.

This circuit is an actuator in the sense of control engineering because the output is not fed back to the input.

Function example controller

Collector circuit as an ideal transistor through impedance converter via operational amplifier , can also be seen as an amplified voltage follower (emitter follower) and is the basic circuit of linear voltage regulators: U _e = U _a

Adjustable longitudinal regulator with OP

In order to obtain a more precise stabilization, z. B. used in integrated voltage regulators or laboratory power supplies operational amplifiers (OP). The following circuit shows the principle of integrated voltage regulators, only the current limitation and the overtemperature protection are missing:

The non-inverting input of the OP is permanently stabilized at U _z . With the potentiometer R ₂ , part of the output voltage is tapped and compared with U _z by the OP . The difference between the two quantities - called the control deviation - is amplified and changes the current permeability of the power transistor. In contrast to the stabilizer shown above , the OP controls the output voltage here and adjusts it if necessary. If you use a "normal" resistor for R ₂ , you have a fixed voltage regulator.

An improvement can be achieved if Q is exchanged for a pnp transistor or p- MOSFET . In addition, the inputs of the OP and the collector / emitter of Q must be swapped. If a pnp transistor is used, an effective current limitation is also possible by interposing a base resistor R _B. If this resistance is supplemented by a potentiometer in series, the maximum load current that can be drawn can be set. This is approximately U _E · β / R _B

The circuit shown is rudimentary because it is neither short-circuit nor excess temperature-proof. Integrated voltage regulators almost always have internal protective circuits, which throttle or switch off the current through the regulator in the event of overcurrent or excess temperature of the chip.

Characteristic values ​​of integrated linear series regulator

Usual fixed voltage regulators are designed for input voltages up to 40 V. Regulators for negative voltages are also available. Most of the series regulators require a minimum voltage difference between input and output of 1.5–3 V.

The picture shows the wiring of a fixed voltage regulator and an adjustable voltage regulator.

Use of voltage regulators

The fixed voltage regulator U2, here an LM7824, only requires small capacitors at the input and output. A voltage of 24 V will be set at the output.

The adjustable voltage regulator LM317 , U1 in the adjacent circuit diagram, and the following explanations relate specifically to this type, still requires a voltage divider to set the output voltage. The following must be set between the control connection and the output connection : ${\displaystyle U_{\text{Ref}}=1{,}25\,\mathrm {V} }$

${\displaystyle U_{3}=U_{\text{Ref}}\cdot \left(1+{\frac {R_{2}}{R_{1}}}\right)+I_{\text{Adj}}\cdot R_{2}}$

The output voltage results from the ratio of to . The proportion of the second term is due to the low flow with low to negligible, when the direct-axis current in the voltage divider too small, so the absolute values of the resistors are not too large. With and you get, for example, 15 V at output U3. ${\displaystyle R_{2}}$ ${\displaystyle R_{1}}$ ${\displaystyle I_{\text{Adj}}\approx 50\,{\text{µA}}}$ ${\displaystyle 0{,}055\,\mathrm {V} }$ ${\displaystyle R_{2}=1{,}1\,\mathrm {k} \Omega }$ ${\displaystyle R_{1}=100\,\Omega }$

Low-drop series regulator

Low-drop linear regulator on a CPU adapter socket ( socket 7 )

A low-drop voltage regulator (abbreviated LDO for low drop-out ) is a series regulator with a lower minimum required difference between input and output voltage (0.1 V to 1 V instead of 1.5 V to 3 V for other regulators). The LDO property is achieved in bipolar circuit technology through the use of a pnp transistor in the series branch. Alternatively, a p-channel enhancement MOSFET can also be used, e.g. B. IRF4905. This is used instead of an npn transistor as in the circuits above. The pnp transistor can then be driven to its saturation, which allows the small voltage difference of around 0.2 V between input (emitter) and output (collector). For example, 3.3V can be obtained from 4V with LDOs.

LDOs also reduce losses, and the supplying power supply unit can be made more compact. The disadvantage is that low-drop series regulators are more prone to oscillations. The reason is that the pnp transistor has a relatively large structure in the longitudinal direction because of the power dissipation, and associated therewith a relatively large base-emitter capacitance , which is additionally reinforced by the Miller effect . This creates a dominant pole in the transfer function which, without countermasses, makes the phase reserve in the control loop negative, which can lead to undesirable oscillations.

Wiring

Regulator protected by a diode

Integrated standard voltage regulators are short-circuit and overload-proof, but some are not protected against polarity reversal. In addition, the controllers can swing unintentionally . This is not only dangerous for the voltage regulator, it can also generate undesirable high-frequency oscillations and transients in the load. These often go unnoticed because they are not displayed during a DC voltage measurement.

To avoid unwanted oscillations and for better behavior in the event of load jumps, voltage regulators require external capacitors according to their data sheet . Not all capacitors shown in the adjacent circuit diagram are absolutely necessary; according to the data sheet, only C2 with at least 100 nF is required for the popular 78xx series and must be low-induction, i.e. have a low ESL value. In addition, the capacitor must be connected to the controller with the shortest possible conductor paths, since large line inductances can also lead to oscillations.

Other controllers only require C3. In the 78xx series, C3 improves the control behavior during rapid load changes.

The additional capacitors C1 and C4 are optional. C1 can act as a smoothing capacitor after a rectifier and - if its parameters match - replace C2 at the same time. A large C4 improves the behavior in the event of load changes or absorbs short load peaks and reduces the residual ripple on the output voltage. The slower voltage rise when switching on is a disadvantage.

In some applications, a reverse polarity protection diode D1 is recommended from the controller output to the input. This circuit addition is important with a large C4 and when several loads branch off the unregulated input voltage at the same time. Then it can happen that when switching off C1 is discharged faster than C4 and the output voltage is higher than the input voltage (reverse polarity regulator). A protective diode then protects the controller from being destroyed.

Use as a constant current

If a fixed resistor is connected between the output of the voltage regulator and its base point, the voltage across the resistor is kept constant and with it the current through this resistor. In this way, a simple constant current source can be implemented that can be installed in series in a circuit as a two-pole - between the input of the voltage regulator and its base point (at the same time behind the load resistor).

Type designation 78xx

Due to their age and low cost, these fixed voltage regulators are very common:

• 78xx (positive output voltages - positive regulator)
• 79xx (negative output voltages - negative regulator)

xx = output voltage, standard voltages: 5 V, 9 V, 12 V, 15 V, ...

For example 7805 = positive regulator for 5 V output voltage or 7812 = positive regulator for 12 V output voltage.

Depending on the manufacturer, the digit sequence 78 can be preceded by a prefix . ΜA78xx, MC78xx, LM78xx and L78xx are common.
Behind the designation 78Sxx there is a 2-A type, under 78Txx one for 3 A. The 5-A types usually have the designation 78Hxx. Smaller versions are the 78Mxx for 0.5 A types and 78Lxx for 0.1 A types. Often a letter after the voltage specification denotes the tolerance. An A for ± 2% and a C for ± 4% can serve as a guide. Example: MC7809A for positive regulator, 9 V, tolerance 2%, housing TO-220.

Examples of common adjustable voltage regulators are:

• LM317 (positive output voltages - positive regulator)
• LM337 (negative output voltages - negative regulator)
• L200 (positive regulator, voltage and current regulation possible)
• LM723 (positive regulator, voltage and current regulation possible, somewhat more complex wiring, often used in laboratory power supplies)

Aileron regulator

Cross regulators , also known as parallel regulators or shunt regulators , are connected in parallel to the consumer and always draw enough current to keep the voltage at their terminals constant. They must therefore be fed from a current-limited source. The current limitation usually consists of an upstream resistor, in the case of capacitor power supplies, it consists of a capacitor.

Cross regulators are only used for small powers (reference voltage sources, small DC voltage sources). The output voltage of transverse regulators is short-circuit proof if the current-limiting element can withstand the increased power loss. The transverse regulator itself has its lowest load at maximum load (and also in the event of a short circuit).

In addition to discrete circuits, Zener diodes are used as cross regulators in simple cases , and integrated circuits for higher stability requirements . Like series regulators, these integrated circuits are available for fixed voltages (e.g. 2.5 V, 4.096 V, 5 V) as well as in adjustable versions. They are also called reference voltage sources, as this is their main area of ​​application (however there are also reference voltage sources in the circuit type of a series regulator). Cross regulators behave to the outside like a highly stable, temperature-independent Zener diode.

Type examples:

• LM336, fixed voltage
• TL431, adjustable

Charge controller or protection circuit for batteries

Overcharging when the battery is fully charged is prevented by the fact that when the maximum charging voltage is reached, most of the current is branched off in a cross regulator and converted into heat. This charging voltage limitation is built into many of the particularly sensitive lithium-ion batteries , together with other monitoring electronics, such as a current fuse, end-of-charge voltage control and temperature monitoring, directly in the housing of the (laptop) battery pack or in (cell phone) individual cells. An increase in the charging voltage of more than 4.2 V per Li-ion cell would otherwise cause the electrolyte to decompose and pose a risk of fire.

The method using a cross regulator is also used for charging current sources that must not be operated without load. A typical example of this are small wind generators ; unloaded, these could, depending on the design and wind speed, reach impermissible rotor speeds and no-load voltages.

For 12 V lead batteries there are z. B. the 1.5 A fixed voltage regulator PB137, which is specially adapted to the end-of-charge voltage of approx. 13.7 V. It is comparable to the 78xx series of controllers with an additional protective circuit. After reaching the end-of-charge voltage of the battery of 13.7 V or if it was previously charged to the prescribed end-of-charge voltage of approx. 15 V, only the low operating current of the PB137 of 5 mA flows from the charging source.

Solar power supply

Shunt regulators continue to regulate the on -board voltage generated from solar cells in satellites . Series regulators would cause a power loss of several percent here, since their minimum voltage drop requires a slightly higher voltage of the solar cell, which means that the maximum current decreases slightly with the same irradiation. Depending on the structure, a distinction is made between linear shunt regulators and regulators that short-circuit parts of the solar cells: Switching Shunt Regulators (S2R), Sequential Switching Shunt Regulators (S3R) or Sequential Switching Shunt Series Regulators (S4R). The (as required or periodically) short-circuited individual solar cell segments are connected to the bus voltage via diodes and thus decoupled from one another.

Combination cross regulator / longitudinal regulator

Such circuits can be used like a series regulator, but are also able to keep the output voltage constant even when current flows into the output ( sourcing and sinking ). Examples are the reference voltage source AD158x and the somewhat more powerful LT1118.

Switching regulator

→ Main article : Switching regulators

Circuit diagram of a buck converter

In contrast to series regulators, which convert unneeded voltage into heat, switching regulators work much more efficiently: A storage choke is periodically either supplied with energy from the input voltage with the help of fast switches (and a freewheeling diode ) or has to deliver energy at the output. The voltage present at the output of the choke is smoothed by means of a capacitor.

There would be no power loss on ideal switches, storage coils and capacitors; even on real components, the power losses are significantly lower than the losses of linear regulators.

In addition to a voltage reduction, as with series regulators, a higher or an inverted output voltage can also be generated.

As with linear series regulators, there are variants with fixed and adjustable output voltage. Switching regulators for large currents (from around 5 A) require one or more external switching transistors. In the case of low to medium powers, power MOSFETs are mostly used today as switches and for the freewheeling diodes (see synchronous rectification ) in order to avoid the power loss due to the threshold voltage of bipolar transistors and semiconductor diodes.

In the past, switching frequencies were only possible just above the low frequency range (the LF range itself was largely avoided for noise protection and for reasons of audible interference in LF components), there will be switching regulators with several megahertz operating frequency in 2020. This allows electrolytic capacitors to be replaced by ceramic multilayer capacitors (MLCC) and the chokes are smaller, which increases the service life, reliability, miniaturization and working temperature.

Switching regulators with synchronous rectification are regenerative, which means that they can also transfer current from the "load" back into the supply voltage source if the load voltage exceeds the setpoint voltage. This property can significantly increase the efficiency in the drive technology of hybrid electric vehicles and electric locomotives ( four-quadrant actuator , recuperation ).

Overload behavior

If any, the following characteristics are common. In some cases the overcurrent protection is missing and a short circuit can destroy the power component. A sufficiently high internal resistance of the voltage source or the power component also represents (primitive) overload protection.

In addition to protection against overcurrent, thermal protection should not be missing, in the simple case through oversized cooling. In the case of integrated voltage regulators, one is usually built in. When designing the circuit, it should be noted that the local heating that occurs can lead to consequential damage.

Foldback behavior

If the maximum output current is exceeded, the voltage is reduced so much that only a very small current flows. The voltage does not return until the load has been removed.

Electronic security

If the maximum output current is exceeded, the output is switched off. It is started manually with a button. Depending on the circuit design, startup occurs automatically when the voltage on the input side is switched on - or manually.

Rectangle behavior

When the maximum current is reached, the controller switches to constant current mode: If the load continues to increase, the output current remains constant until the short circuit. This behavior is often found with laboratory power supplies and with integrated fixed voltage regulators.

Hiccup operation ("hiccup" operation)

In the event of an overload, the regulator (often with switching regulators) constantly tries to increase the output voltage and supplies periodically increasing current pulses up to the level of the maximum current.

AC voltage regulator

Regulation with variable transformers

Scheme of a tap changer in a power transformer with uninterrupted switchover between stages two and three

The regulation of AC mains voltage with variable transformers is very low loss and distortion-free, but slow due to the motorized actuator.

Transformers are used whose transformation ratio can be changed during operation. These can be automatically driven variable transformers or transformers with taps ( voltage regulation ) that are switched.

For power transformers in substations , so-called tap changers for power transformers are used as standard . These switch arrangements consist of a load switch with a preselector. The preselectors represent isolators , which are permanently installed in the housing of the power transformer and are arranged on the high-voltage winding due to the lower current load. The mains voltage can be adjusted in the range of typically ± 25%. The voltage change takes place with these and also with smaller devices without interruption under load by switching between winding taps. This creates a brief interturn short-circuit, the current of which must be limited by two resistors, referred to as resistors A and B in the illustration opposite . The last stage of voltage regulation in public power grids takes place when the medium-voltage grids are fed . The local transformer stations for supplying households with low voltage, usually 230/400 V, usually have a fixed transformation ratio without any setting option during operation.

In the case of indirectly working regulators, an in-phase or 180 ° phase-shifted voltage in series with the consumer is added to the input voltage or subtracted from it via additional transformers of comparatively low power. The additional transformers are in turn fed from automatically driven variable transformers. Depending on their transmission ratio, they allow a voltage regulation of z. B. ± 25%. With network control devices based on this principle and also with smaller devices with automatic variable transformers, no measures for current limitation during winding changeover are required; the graphite rollers used to make contact with the exposed windings themselves make a sufficient contribution to current distribution.

Transverse regulators, implemented for example in the form of phase shifting transformers , are used in three-phase alternating current networks at the maximum voltage level of 220 kV or 400 kV. This means that both the amount and the phase position can be changed and thus load flows can be specifically influenced. Like AC regulators, they can be implemented with additional transformers. The control winding of the additional transformer is in series with the load on phase S. The control voltage induced in it has a certain phase position that can be set with the primary voltage and its phase position. The primary winding is fed by a tertiary winding of the main transformer or by a separate excitation transformer. By selecting the vector groups of the windings of the main and the additional transformer, additional voltages of any level and phase position can be set.

Newer methods for setting the AC mains voltage and the power flows are based on power electronics and are summarized under the term Flexible AC Transmission System ( FACTS ).

Magnetic voltage stabilizers

Magnetic voltage equalizer for mains voltage consumers up to 200 W; 125 mm × 200 mm × 260 mm

In the past, magnetic voltage stabilizers were often used to operate devices equipped with tubes in order to automatically stabilize the mains voltage, which was then often more fluctuating. They consist of a transformer, a choke with taps and a capacitor that forms an oscillating circuit with the choke. The saturation behavior of the iron core of the choke is used during stabilization. These regulators work faster (20 to 30 ms) than motorized mains voltage regulators, but have a smaller control range, a residual error after stabilization (fluctuations of 20% are reduced to 3%, for example) and have greater losses. They contain no mechanically moving parts or contacts and are therefore very reliable.

The rated output of historical devices was up to 300 watts, according to their intended use, in 2020 devices with up to several kilowatts will be offered. The control properties are described with a settling time of 40 to 60 ms with a deviation of 1% for network fluctuations of + 10% and -20%. A load with a power factor of 1 is required.

Magnetic voltage stabilizer with transducer

AC voltage stabilizers that work on the basis of transducers have been around since the 1960s : this regulates the primary voltage of an autotransformer. Well-known manufacturers are Philips, Statron, Wandel and Goltermann (WS-3, WS-5, WS-6 and WS-10) and Wekafon. They are as fast as the classic "constants", but much more accurate. The effective output voltage deviation is e.g. B. for WS-5 0.03%.

See also

• Voltage stabilization
• Voltage regulator module

literature

• Adolf J. Schwab: Electrical energy systems - generation, transport, transmission and distribution of electrical energy . Springer, 2006, ISBN 3-540-29664-6
• L. Retzbach: Batteries and chargers . Neckar Verlag, 13th edition, 2002, ISBN 3-7883-2142-3

Web links

Commons : Voltage Regulators  - collection of images, videos and audio files

Wiktionary: Voltage regulator  - explanations of meanings, word origins, synonyms, translations

• Hobby electronics - basic circuits of fixed voltage regulators
• Integrated fixed and adjustable 3-pin voltage regulators
• Voltage regulator special: The 78xx, LM317 and LowDropout principle
• Calculation of the voltage divider for the LM317 with E12 / E24 resistors
• Simple laboratory power supply with NPN complementary Darlington stage and overload display
• Lowdropout power supply with the legendary "723", with complementary Darlington power stage and pulse foldback current limitation  - a practical example of hiccup operation
• Mosfet power supply circuits
• Voltage regulator in the car with animation
• Explanation of a 7805 circuit with PSpice simulation

Individual evidence

1. ↑ for example AD158x from Analog Devices
2. ↑ data sheet 78xx
3. ↑ Data sheet L200
4. ↑ Ahmed M. Kamel, Amged El-Wakeel, Usama Abouzayed: Sequential Switching Shunt Regulator for Satellite Power Control System , in researchgate , June 2017, DOI: 10.9790 / 1676-1203027883
5. ↑ ^{a b} https://www.transformatoren.buerkle-schoeck.de/wp-content/uploads/sites/8/2016/02/kon11.pdf data sheet constant holder from Bürkle + Schöck KG , accessed on July 13, 2020
6. ↑ Data of the device shown Konstanze
7. ↑ http://voltage-stabilizer-sb.com/de/magspannung.html Magnetic voltage stabilizers from Schuntermann Transformatoren GmbH , accessed on July 13, 2020
8. ↑ AC voltage stabilizer WS 220/500 Strom-V Statron (1500W). 
9. ↑ AC voltage stabilizer WS-5 equipment Wandel & Goltermann (500 / 1000W). 
10. ↑ AC voltage stabilizer WS6 Wandel & Goltermann. 
11. ↑ AC voltage stabilizer WS-10 Wandel & Goltermann. 
12. ↑ Mains voltage regulator Strom-V Wekafon, Elektro-Ger.