Voltage regulator

Voltage regulators are electronic circuits that stabilize voltages and can compensate for the fluctuations of battery or mains voltage within wide limits. Both direct and alternating voltages can be stabilized. For small DC voltages, the entire circuit is often combined to save space in one component.

Electronic voltage regulators are often designed so that they also limit the maximum current flowing.

• 2.2.1 charge controller or battery protection circuit for
• 2.2.2 Solar Power

• 4.1 foldback behavior
• 4.2 Electronic Security
• 4.3 Rectangular behavior
• 4.4 hiccup mode ( " hiccup " mode)

• 5.1 Control with Variable Transformers
• 5.2 Constant Voltage

Overview

The mains voltage may fluctuate depending on the load between 207 V and 253 V, electronic circuits such as computer may not work properly but if more than 5 % deviation from the nominal value. For this reason, the operating voltage is basically stabilized, whereby the ripple is behind the rectifier largely eliminated.

With DC voltage regulators, there are different principles:

For minimum benefits as for reference voltage sources you do not use the controller but stabilization circuits with Zener diodes, as shown in the picture. Crucial to the function is the particularly low differential resistance of these components of about 5 Ω, which must be much smaller than rv. Only then the output voltage Ua even with large fluctuations in the load current remains approximately constant.

Linear regulator using a power transistor, which operates as an electronic of variable resistance. Deviates from the output voltage from the nominal value, the difference is amplified and fed back to the power transistor ( loop). A disadvantage is the relatively low efficiency of 60%, associated with the need to cool the power transistor. Advantages: load changes can be quickly and well balanced for low power there are very inexpensive components. At the output there is no AC power residues and because the circuit does not generate interference to small-signal amplifier can be operated easily.

Switching regulator always contain an inductance whose current flow is not gradually changed by a power transistor, but with high frequency ( about 40 kHz) is turned on and off. Like any other switch that transistor hardly heated, so efficiency can be achieved by 90%. This extends the operating life of battery-powered devices such as laptops. The disadvantage is that the circuit must be well shielded because of the powerful exchange rate used internally (Electromagnetic Compatibility) and that at the output a small proportion of this AC voltage is always measurable. That bothers neither computers nor chargers, but rather sensitive amplifiers in radios and hi-fi systems, electroencephalography and atomic force microscopes.

There are different switching regulator type in boost converters, the output voltage is always greater than the input voltage - with linear regulators is in principle impossible.

Linear regulators

One distinguishes the parallel stabilization ( cross- regulator ), the series Stabilization ( series regulator ) and the combination of the two.

The transverse control the load ( the load ) is parallel to the control circuit, and it always turns the entire unnecessary power into heat. Moreover, losses often occur in the upstream component necessary to limit the current on (in the simplest case, a resistor). This is also referred to as a shunt regulator method is only used because of losses, such as when the power drawn is low, or when the control rate must be high.

For linear series regulators, the controlled system (so-called longitudinal transistor) in series with the load. This circuit always takes up only slightly more than the load current and is therefore more efficient than a parallel stabilization; It is therefore most frequently used in the electronics.

Regardless of the type of controller always belongs to a voltage regulator, a reference voltage source, which is in accordance with the requirements of a different complex circuit. Is used in the simplest case, the serial connection of a Zener diode and resistor, further preferred in the integrated circuit and the bandgap reference in precision applications, with a Zener diode circuit or any specific controlled temperature-compensated band gap reference.

Series regulator

As a series regulator usually find integrated circuits using that contains the controlled system ( power transistor ), the controller and a reference voltage source. We distinguish:

• Voltage regulator: output voltage is determined by the manufacturer
• Adjustable voltage regulator: Output voltage is selectable by means of a voltage divider

Operation

A rectifier circuit provides the UE and is stabilized by Dz in the IP. RV is used for the current limiting of Dz and simultaneously must also provide the base current of Q changing. The transistor operates as an emitter follower, and therefore the output voltage UA is slightly smaller than the IP. The difference Uz UA is constant ( base-emitter voltage of Q, about 0.6 V), but varies with the emitter current. Therefore, the stability is inferior to that of the Zener diode ( the Zener diode and differential resistances of resistance must be added, which leads to a poorer stabilizing factor). Advantage is that one can take a much larger current, because it runs mainly via the collector- emitter path. The transistor must usually be provided to dissipate the power loss with a heat sink.

The circuit shown is rarely used because it is not short circuit proof and a relatively large voltage difference between input and output is required.

Integrated voltage regulators usually have internal protection circuits that the power to throttle back or shut down if over temperature of the chip by the controller.

To obtain an accurate stabilization, for example in the integrated voltage regulators or lab power supplies operational amplifier ( OPV) are used. The following circuit shows the principle of integrated voltage regulators, only the current limit and over-temperature protection are missing:

The non-inverting input of the op amp is stabilized firmly on Uz. With the potentiometer R2 is a part of the output voltage is measured and compared with the OPV Uz. The difference of the two variables - called deviation - is amplified and changes the current transmission of the power transistor. In contrast to the previously shown here, the OPV stabilizer controls the output voltage and adjusts if necessary. If one uses for R2 a "normal" resistance, so you have a fixed-voltage regulator.

An improvement can be achieved if Q is replaced with a PNP transistor or P-MOSFET. In addition, the inputs of the op amp and collector / emitter of Q must be replaced. A PNP transistor is used, an effective current limitation can be performed by the intermediary of a base resistance RB. This resistance is accompanied by a potentiometer connected in series, the maximum load current can be drawn can be set. The maximum load current is about UE · β / RB

Characteristics of integrated linear series regulator

Conventional fixed voltage regulators are designed for input voltages from 7 to 40 volts. There are also controls for negative voltages. Most linear regulators a minimum voltage difference between the input and output of 1.5-2 V is required.

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

At an input voltage (U1 ) of 30 V at the output of Fixspannungsreglers 7824 (U2 ) set a voltage of 24 V. Man sized resistors on the principle of a voltage divider, so that adjusts between control terminal and output terminal:

The longitudinal current in the voltage divider must not fall below a certain value, in order to maintain the specified control accuracy as flowing out of a certain small current from the control input. The guide value specified by the manufacturer of the longitudinal current ( and thus the value of R1) moves 1-10 mA; in the following dimensioning example he is 1.25 mA. with

Obtained, for example, 15 V at the output of U3. specified by the manufacturer for each voltage regulator type, the type LM317, it is about 1.25 V.

Low-drop series regulator

A low drop-out voltage regulator ( LDO often called, for low drop-out ) is a series regulator having a lower minimum required difference between the input and output voltage ( 0.1 to 1 volts instead of 1.5 to 3 volts at other controllers ). LDO property is achieved in bipolar circuitry through the use of a PNP transistor in the series branch. Alternatively, a p-channel enhancement mode MOSFET can be used, for example, IRF4905. This is used instead of an NPN transistor as shown in the above circuits. The pnp transistor can then be driven to its saturation, which allows the low voltage differential of at least about 0.2 V is applied between the input (emitter ) and the output (collector). For example, can be obtained from 5 volts to 4.096 volts LDOs.

LDOs also reduce the losses, and the feeding power supply can be interpreted narrowly. The disadvantage is that low-drop series regulator greater tendency to oscillate. The reason for this is that the PNP transistor in the longitudinal direction because of the power loss, a relatively large structure, connected with a relatively large base-emitter capacitance which is enhanced by the Miller effect. This results in the transfer function of a dominant pole, which without counterweights can be negative, the phase margin of the control loop, which can lead to undesirable vibrations.

Wiring

Integrated Standard voltage regulators are short-circuit and overload protected, some are not protected against reverse polarity. In addition, the controller can swing unintentionally. This is not only dangerous for the voltage regulator, it can also unwanted high frequency oscillations and transients are generated in the load. These often go undetected because they are not displayed when a DC voltage measurement.

To avoid undesirable overshoot and better behavior at sudden load changes require external voltage regulator capacitors according to their data sheet. Not all plotted in the adjacent diagram capacitors are imperative in the popular 78xx series is the data sheet only C2 with at least 100 nF must be prescribed and inductance, so have a low ESL value. In addition, the capacitor with the shortest possible conducting paths must be connected to the controller, as well as large cable inductance can lead to oscillation.

Other controllers require mandatory only C3. The 78xx series C3 improves the control response at rapid load changes.

The additional capacitors C1 and C4 are optional. C1 can act as a smoothing capacitor in accordance with a rectifier and - if the parameters fit - C2 replace simultaneously. A large C4 improves the performance when the load changes or intercepts short load peaks and reduces the ripple on the output voltage. A disadvantage is the slower voltage rise at startup.

In some applications, a reverse voltage protection diode from the regulator output to input is recommended. Of significance of this additional circuit is at large C4 and if at the same branch more loads from the unregulated input voltage. Then it can happen that will discharge faster than C4 when you turn off C1 and the output voltage is higher than the input voltage ( reversed-polarity control). A protection diode protects the regulator then from destruction.

Type designations

• Fixspannungsregler or voltage regulator (examples) 78xx (positive output voltages - positive control)
• 79xx ( negative output voltages - negative control)

Xx = output voltage, Standard voltage: 5 V, 9 V, 12 V, 15 V, ... for example LM7805 = Positive regulator for 5 V output voltage or L78M12 = Positive regulator for 12 V output voltage.

Behind the name 78Sxx hides a 2- A type, under a 78Txx for 3 A. The 5 -A- types usually have the name 78Hxx. Smaller versions are 78Mxx for 0.5 - A types and 78Lxx for 0.1 -A types. Depending on the manufacturer, prior to the sequence of numbers 78 still stand a prefix. Are usual μA78xx, MC78xx, LM78xx and L78xx. Often referred to a letter according to the voltage specified tolerance. As a guide, an A for ± 2 % and ± 4% for C can be used. Example: MC7809A for positive regulator, 9V, tolerance 2% TO- 220.

• Adjustable voltage regulator ( examples) LM317 (positive output voltages - positive control)
• LM337 ( negative output voltages - negative control)
• L200 (positive output voltages - positive control)
• TL783 (positive output voltages - positive control)
• LM723 ( voltage and current control is possible, something more complex circuitry, often used in laboratory power supplies )
• MAX667 ( LDO regulator for battery applications: input voltage monitor, output voltage either 5 V or adjustable)

Cross regulator

Cross regulator, also called Parallel controller or shunt regulator are connected in parallel to the load and always take as much electricity in order to keep the voltage at its terminals constant. They must therefore be fed from a current-limited source. The current limit is usually from an upstream resistance. Cross regulators are used only for small capacities ( reference voltage sources, small DC voltage sources). The output voltage of cross regulators is short -circuit proof, if the current limiting element can withstand the increased power dissipation. The cross regulator itself has at maximum load (and even at short circuit) the least of his burden.

As a cross- regulator, integrated circuits are used in addition to discrete circuits in simple cases, Zener diodes, for greater stability requirements. These integrated circuits are as well as longitudinal controller for fixed voltages ( eg, 2.5 V, 4.096 V, 5 V) as well as in adjustable versions are available. They are also called reference voltage source, since that is their main area of ​​application (there are, however, reference voltage sources in the mode of a series regulator ). Cross regulators behave outwardly like a highly stable, temperature-independent zener diode.

Type examples:

• LM336, fixed voltage
• TL431, adjustable

Charge controller or battery protection circuit for

Overcharging is at full (lithium -ion ) battery prevented an accumulator to overload imminent, current is shunted in the cross controller and converted to heat. This charging voltage limitation is built into many of the for -sensitive lithium - ion batteries with other electronic monitoring system, such as power backup, Endladeschlusssspannungskontrolle and a temperature monitor directly into the housing of ( LapTop ) battery packs or in ( mobile ) single cells. A charging voltage increase above 4.2 V per cell LiIon addition would otherwise lead to a decomposition of the electrolyte and are a fire hazard.

The method by means of transverse controller is also used for charging of power source may not be operated unloaded. A typical example of this is small wind generators: unloaded can, depending on the type and wind speed, reach unacceptable rotor speeds and idle voltages.

For 12 V lead-acid batteries for example, there a specially tuned to the standby charge voltage of about 13.7 V 1.5 A Fixed Output Voltage Regulators PB137. That's roughly equivalent to the regulator 78xx series with additional protective circuit. Then flow to it for achieving the standby battery voltage of 13.7V or if previously charged to prescribed charge voltage of about 15V, only the low operating current of the PB137 to 5mA from the charging source.

Solar power

Shunt regulator regulate further in satellite power from solar cells. Series regulator would cause a loss of power here of several percent, as their minimum voltage drop a slightly higher voltage of the solar cell requires, whereby the maximum current decreases slightly with the same irradiation. Depending on the structure, a distinction is simple shunt regulator (shunt regulator ), switching shunt regulator ( S2R ), Sequential Switching Shunt Regulator ( S3R ) or Sequential Switching Shunt Series Regulator ( S4R ).

Combination of cross regulator / linear regulator

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

Switching regulator

Switching regulator generally function differently as a linear regulator: The input voltage is periodically switched on and off and a downstream throttle sufficient inductance smoothes the current charging an electrolytic capacitor. Depending on the ratio of on-to- off time and the discharge current is established at a certain output average voltage one. Should this be constant, must regulate a control circuit, the duty cycle of the pulse width modulation. Advantageous is the much lower power loss when the switch have a sufficiently small passage resistance. Disadvantages are the higher circuit complexity, EMC interference by hacking ( fast switching ) and the relatively slow response to load jumps.

There are, as with linear series regulators such Fixed and those with pre-selectable output voltage. Switching regulator (eg for processors with 1-3 volts) almost imperative, otherwise the power consumption and the heat loss of the regulator would be unreasonably high especially in the generation of low voltages at high current. Switching regulator for high currents (from about 5 A) additionally require one or more external switching transistors, which are often also replace the diode ( synchronous rectification ), to avoid the very large currents at considerable power loss of the diode.

Switching regulator with synchronous rectification have regenerative capability, that is, they can also transfer power from the " load " back to the supply voltage source when the load voltage exceeds the reference voltage. This property can in drive technology of hybrid electric vehicles and electric locomotives efficiency significantly increase ( four-quadrant ).

All modern charge controllers are switching regulator, because only provide a useful efficiency can be achieved.

Overload behavior

If any of the following characteristics are common. In some cases, the over current protection is absent, and when short-circuit the power component may be destroyed. A sufficiently high internal resistance of the voltage source or the power component also represents a ( primitive ) Overload protection

In addition to protection against overcurrent and thermal protection must not be missing, in the simple case of over-sized cooling. In integrated voltage regulators is one such built into the rule. It should be noted in the circuit design that the occurring local heatings can cause any damage.

Foldback behavior

Exceeding the maximum output current, the voltage is reduced, so that only a very small current flows. The voltage does not return again when the load is removed.

Electronic Security

When the maximum output current is exceeded, the output is switched off. The start is done manually with a button. Depending on the circuit design of the system is started automatically when switching the input side voltage - or just manually.

Rectangle behavior

Upon reaching the maximum current the controller switches to the constant current mode: If the load increases further, the output current remains constant until the short circuit. This behavior is often found in laboratory power supplies as well as integrated fixed voltage regulators.

Hiccup mode ( " hiccup " mode)

The controller (common in switching regulators ) attempts at overloading constantly anew to raise the output voltage and supplies to periodically up to the amount of the maximum current rising current pulses.

AC voltage regulator

Control with variable transformers

The control of AC line voltage with variable transformers is very low-loss and low distortion, but due to the motor actuator slowly.

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

In power transformers in substations by default, so-called tap-changers for power transformers. This switch assemblies consist of a load switch with selection. The selection represent separators, which are installed permanently in the housing of the power transformer and arranged because of the lower current load on the high voltage winding. The adjustment of the line voltage in the range of typically ± 25 % is allowed. The change of the voltage takes place without interruption and with these smaller units under load by switching between the winding taps. This gives a brief winding short, refers to its current through two resistors in the illustration as a resistance A and B, must be limited. The final stage of voltage regulation in power distribution networks is performed in the supply of medium voltage networks. The local transformer stations to supply the households with low voltage of typically 230/400 V have usually a fixed ratio adjusted without adjustment during operation.

With indirectly operating regulators comparatively low capacity is added to an in-phase or out of phase by 180 ° voltage in series with the load to the input voltage or subtracted from her about additional transformers. The auxiliary transformers are in turn fed by automatically driven variable transformers. They allow, depending on their ratio voltage regulation of eg ± 25%. For network control devices based on this principle and also in smaller devices with automatic variable transformers there is no need for measures to limit the current during the winding - switching; the case used for contacting the exposed windings graphite rolls offer itself a sufficient contribution to the current distribution.

As a cross- regulator, implemented for example in the form of phase -shifting transformers are used in three-phase AC networks at the highest voltage level of 220 kV or 400 kV. Thus, both the amount and the phase position to be changed and thus flows are specifically influenced. They can be realized as AC controller with additional transformers. The control winding of the additional transformer is connected in series with the load to the phase S. The control voltage induced in it has a certain phase position of which can be adjusted with the primary voltage and the phase. The primary winding is fed with a tertiary winding of the main transformer, or by a separate exciter transformer. By choosing the vector group of windings of the main and the additional transformer to additional stresses of any size and phase can be adjusted.

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

Constant Voltage

Constant Voltage were formerly often used to operate tube- tipped device to stabilize at that time often fluctuating mains voltage automatically. They consist of a transformer, a reactor with taps, and a capacitor that forms a resonant circuit with the inductor. In the stabilization of the saturation characteristics of the iron core of the reactor is utilized. These controllers operate faster than motor power voltage regulator, but have a smaller control range, a residual error after stabilization ( variations of 20%, for example, reduced to 3% ) and have larger losses than those. They contain no moving parts or contacts and are therefore very reliable.