Difference between voltage regulation and voltage regulator
Most electronic circuits need a certain, constant operating voltage. 5V is usual here, with some components also 3.3V. The problem therefore often arises that a fluctuating voltage must be regulated to a fixed value in order to operate the circuit. With batteries and accumulators, the voltage drops when they are discharged, and with simple power packs and transformers, the voltage fluctuates depending on the load. To solve this problem, there are so-called voltage regulators: components that are supplied with the unregulated, fluctuating voltage and generate the desired, constant voltage from it.
The regulators compare the output voltage with an internally generated reference voltage. If the output voltage is too low, a transistor through which the output current flows is driven harder so that a larger current can flow until the desired voltage is reached.
If the output voltage increases too much, the current is reduced via the transistor until the voltage has stabilized again. The transistor is used like a variable resistor, which is regulated in such a way that the "superfluous" voltage difference between the input voltage and the desired output voltage drops across it, regardless of the current flowing.
The disadvantage of these linear voltage regulators is their poor efficiency and their large power dissipation. The part of the voltage that is fed to the input and is not currently required at the output drops across the transistor and is converted into heat there.
The efficiency of a linear regulator (without taking into account its own power requirements) is calculated as follows:
Example: A linear regulator with 5V output voltage is supplied with 12V at the input. Regardless of the current drawn, the efficiency is then only (5V / 12V) * 100% = 41.7%. At higher input voltages, it gets even worse. Assume that 500mA are required at the output. The controller then has to convert 12V-5V = 7V at 500mA into heat. This corresponds to a power loss of 7V * 0.5A = 3.5Watt.
Depending on the current flow, this leads to a strong heating of the controller, so that in most cases a heat sink is necessary, which incurs additional costs and takes up a lot of space. In addition, the large losses reduce the operating time if the controller is supplied with rechargeable batteries or batteries.
Linear regulators are available as fixed voltage regulators and regulators with adjustable output voltage. Many of the regulators are short-circuit proof.
With normal voltage regulators, the input voltage must be at least 2-3 volts higher than the output voltage. With "low-drop" controllers, only an extra voltage ("dropout") of around 0.1-0.5 V is required. In return, however, more current usually flows via the ground connection. In addition, with low-drop regulators, the capacitor behind the regulator must be paid more attention to. Usually a minimum value for the capacitance (sometimes> 20 µF) is required. In addition, there is a permitted range for the apparent series resistance (ESR) of the capacitor, which must also be adhered to so that the controller does not oscillate reliably. In addition to the capacitance, the capacitor type (usually ceramic capacitor, tantalum electrolytic capacitor or low ESR electrolytic capacitor) must match. The requirements are different depending on the controller - You can hardly avoid a look at the data sheet with a low drop regulator.
- Fixed voltage regulator
- 7805 (5V)
- 78xx (xxV), 1 A positive, housing TO220
- 78Sxx (xxV), 2 A positive, housing TO220
- 79xx (-xxV), 1 A negative, housing TO220
- LP2950-xx (xxV) Low drop, dropout typ.300mV, max.100mA, housing TO92 / SO8
- LM2940-xx (xxV) Low drop, dropout typ. 500mV, max. 1 A, housing TO220, output capacitor> 22 µF with ESR 0.1 - 1 Ohm (e.g. tantalum, possibly low ESR electrolytic capacitor)
- LT1761ES5-xx (xxV) Low drop, dropout typ.300mV, max.100mA, reverse polarity protection, housing SOT23-5
- Liner regulator, adjustable
- LM317: 1A positive
- LP2951: low drop 100 mA
For the resistance calculation on an LM317 see .
Switching regulators work on a completely different principle. While in linear regulators the voltage difference is converted into heat and is thus lost, in switching regulators this energy is stored in a magnetic field and fed back into the circuit at a later point in time.
This requires a more complicated structure than the linear regulator. A switching regulator has two working phases:
- Input energy is stored in the magnetic field
- Off phase
- The energy stored in the magnetic field is released to the output
Depending on the construction principle, energy can also flow directly from the input to the output in both phases.
The main advantage of a switching regulator is its high degree of efficiency, depending on the type, around 70% to over 90% can be achieved. In addition, the output voltage can be higher than the input voltage and have a different sign. This results in longer runtimes in rechargeable battery and battery operation and significantly less heating compared to linear controllers. Therefore, you can get by without or with a comparatively small heat sink.
The disadvantage is a higher price. Usual circuits that are in the performance range of a 78xx cost around € 2 to € 5. In addition, these circuits sometimes need more space, because the regulator IC also includes the coil, capacitors and often a diode. The circuit board layout also requires a bit of care, because certain lines should be made as short and wide as possible to ensure that they function properly. Due to the high switching frequencies, the quality of the voltage is usually somewhat poorer than with a linear regulator (but it can still be filtered) and can lead to problems in the circuit such as interference from radio receivers.
The step-down converter is the easiest to understand:
- On-phase (switch S closed)
- Via S and the choke L, current flows past the diode D to the output. Voltage drops across the choke, so V isOUT less than VIN. The energy difference is stored in the choke as a magnetic field. Over time, the choke opposes an ever decreasing resistance to the current: the VOUT continues to rise. Does VOUT reaches the desired value, then S is opened and the on phase is ended.
- Off-phase (switch S open)
- The choke is an inductive component. Hence, the current through them cannot stop immediately; it must continue to flow: the choke is now a current pump fed by the energy of its magnetic field, which continues to supply the capacitor C with energy by sucking current through the diode D. In the off phase, the consumer draws its energy from the electrical field of the capacitor and the magnetic field of the choke. If the current through the choke falls below a threshold value, the next on phase follows.
- On-phase (switch S closed)
- Via S and the choke L, current flows past the diode D to GND. The current flow builds up a magnetic field in the choke. In the on-phase the consumer is supplied from the capacitor C. If the current through the throttle reaches an upper threshold value, S is opened and the on-phase ends.
- Off-phase (switch S open)
- The choke is an inductive component. Hence, the current through them cannot stop immediately; it must continue to flow: the choke pumps charge carriers via the diode D to the capacitor C. It takes the energy for this from its magnetic field. VOUT can be well above VIN climb. If the current through the choke falls below a lower threshold value, the next on phase follows.
Works in principle like the boost converter.
- On-phase (switch S closed)
- Current flows via S and the choke L past the diode D to GND. The current flow builds up a magnetic field in the choke. In the on-phase the consumer is supplied from the capacitor C. If the current through the throttle reaches an upper threshold value, S is opened and the on-phase ends.
- Off-phase (switch S open)
- The choke is an inductive component. Hence, the current through them cannot stop immediately; it must continue to flow: the choke sucks charge carriers from capacitor C via diode D. It takes the energy for this from its magnetic field. VOUT sinks below GND. If the current through the choke falls below a lower threshold value, the next on phase follows.
There are special ICs for switching regulators that contain most of the circuitry. In addition to the frequency and the pulse duty factor, the components involved (switching transistor, choke, diode, capacitors) are also decisive for the properties of the controller (efficiency, interference, power range, output voltage and current, ripple, size, etc.). Some of the controllers only work from a minimum load - with less load there is a risk of overvoltage at the output.
Examples of switching regulator ICs:
- LM2574 N5: 5 V, 0.5 A, buck converter, DIP8
- LM2576 T5: 5 V, 3 A, buck converter, TO220-5
- LM2576 T12: 12 V, 3 A, buck converter, TO220-5
- LM2576 ADJ: adjustable, 3A, TO220-5
- LM2673 T5: 5 V, 3 A, step-down converter with adjustable current limitation, TO220-5
- LM2679 T5: 5 V, 5 A, step-down converter with adjustable current limitation, TO220-5
- MC33063 / MC34063: variable approx. 0.2-1 A, DIP8, SO8, online calculation under web links
- LT1072, variable, approx. 1 A
- MAX856, upwards, e.g. 3 V -> 5 V, approx. 100 mA, SO8 (SMD)
- MCP1640 upwards, e.g. 1.2 V -> 5 V, approx. 100 mA, SOT23-6 (SMD)
- PR4401, upwards, LED driver approx. 20 mA, SOT23 (SMD)
- L4970A: adjustable 5.1-40V output, 10A, step-down converter, Multiwatt-15 housing
Notes on the components involved:
- A Schottky diode with sufficient current carrying capacity should be used as the diode, e.g. 1N5818 (1 ampere) or 1N5821 (3 ampere). It should have the lowest possible forward voltage at the desired output current. This can usually be seen from a diagram in the data sheet for the diode. A slow diode like the 1N4003 is not suitable here.
- The coil should have the lowest possible ohmic resistance. You can easily make them yourself. A toroidal core coil is useful to reduce radio interference, as it forms a closed magnetic field. The thickest possible enamelled copper wire is then wound onto the toroidal core (e.g. 0.5 ... 1mm diameter). The number of turns n required can be calculated using the formula L = Al * n² (L = required inductance, Al = constant of the core in nH). A second important point is that the core does not reach saturation. A suitable core material should be selected for this. Highly permeable ferrites with a high Al value are unfortunately mostly unsuitable - unless they have an air gap. Ferrite toroidal cores saturate prematurely - however, powder toroidal cores are possible. Fixed inductances in resistor design are not suitable for this because of their high internal resistance (poor quality). Elongated coils (axial design) can cause more interference due to the magnetic field they generate, but they hardly ever reach saturation. If you choose coils from the catalog, you will find the saturation currents, which sometimes differ from the maximum continuous current. The saturation current with the step-down converter only needs to be around 20% higher than the current drawn, with the step-up converter the ratio Ua / Ue still has to be multiplied. The required inductance can vary depending on the input voltage and output current. In the data sheet of the respective switching regulator there are diagrams in which you can read off the necessary inductance depending on the sizes mentioned.
- The output electrolytic capacitor should have a low ESR (equivalent series resistance) and a sufficient maximum ripple current (usually specified for 100 kHz). There are special low-ESR types that can be used for such applications. Alternatively, a lower ESR can also be achieved by connecting several electrolytic capacitors in parallel. However, if the ESR is too low, control oscillations can occur. There are also instructions for this from the IC manufacturer.
Buck converters place higher demands on the input electrolytic capacitor, while boost converters place higher demands on the output electrolytic capacitor. This is due to the fact that a square-wave current flows through the capacitor on the input side of the step-down regulator, but on the output side of the step-up converter.
To reduce the ripple of the output voltage, an additional LC low-pass filter should be added after the regulator. Care should be taken to ensure that the filter coil again has a low ohmic resistance, otherwise the output voltage will drop due to this resistance if the current load is higher. Fixed inductances in resistor design are not suitable for this because of their high internal resistance (too much voltage drop). A 100nF ceramic capacitor, for example, can also be connected parallel to the electrolytic capacitor.
Example for a dimensioning of the filter (according to the data sheet of the LM2576-5): 20µH, 100µF
If the regulator causes repercussions on the input voltage, an LC low-pass filter can also be used upstream of the regulator. In this case, the capacity of the input electrolytic capacitor can be chosen to be larger.
In the respective data sheet you can usually find further information on the circuit and the required parts, in particular on the dimensioning and reference points for the design of a circuit board layout.
To be added:
Constant currents are just as important as constant voltages for many tasks. For example with light emitting diodes. LEDs usually require approx. 20 mA of current, depending on the version. Series resistors are often not the best solution, especially since they no longer keep the current constant when the voltage drops and thus unnecessarily reduce the luminosity. A simple and at the same time inexpensive solution is the circuit shown here with the voltage regulator LM317K that was not used for this purpose. The LM317K is usually an adjustable voltage regulator. It also becomes a constant current source via the circuit below: The voltage regulator adjusts itself in such a way that between "Vout" and "Adj." 1.25 V. Thanks to R = U / I it is easy to calculate that a certain current flows with a certain resistance value. And we then simply let this flow through our consumers. The LM 317K can be operated with a maximum input voltage of approx. 4 to 35 volts. The current limit can be set in a range of 1.25 - 0.01 amps, R1 is therefore between 1 ohm and 120 ohm. See below under web links, there you will find an online form in the robot network with which the circuit can be precisely dimensioned.
It should be ensured that the LM317 is adequately cooled (with larger currents / power losses). Usually, the controller is glued to a heat sink with a little thermal paste (possibly also a mica washer as an insulator) and screwed to the heat sink with the help of an M3 screw.
If the voltage difference between operating voltage and load (e.g. high-power LEDs) is very large, it is advisable not to use the LM317, but a "normal" voltage regulator (e.g. 7805, 7809 or similar). The controller is connected exactly as in the circuit diagram shown. The advantage then is that relatively more power in the resistor than has to be implemented in the controller itself. A high-performance resistor can usually be cooled better than the controller in the TO-220 housing.
However, if there is a large voltage difference, you should think about the efficiency.
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