News and analytical portal "electronics time". Low Voltage Dropout Stabilizer Low Voltage Dropout Linear Stabilizers

All modern radio-electronic equipment is built on elements that are sensitive to the supply electricity. Not only the correct functioning, but also the performance of the circuits as a whole depends on it. Therefore, first of all electronic devices equipped with fixed stabilizers with low voltage drop. They are made in the form of integrated circuits, which are produced by many manufacturers around the world.

What is a low dropout voltage stabilizer?

A voltage stabilizer (SV) is understood as a device whose main task is to maintain the load voltage at a certain constant level. Any stabilizer has a certain accuracy of parameter output, which is determined by the type of circuit and the components included in it.

Internally, the CH looks like a closed system, where in automatic mode The output voltage is adjusted in proportion to the reference voltage, which is generated by a special source. This type of stabilizer is called compensatory. The regulating element (RE) in this case is a transistor - a bipolar or field-effect transistor.

The voltage regulation element can operate in two different modes (determined by the design diagram):

• active;
• key

The first mode implies continuous operation of the RE, the second - operation in a pulsed mode.

Where is a fixed stabilizer used?

Electronic equipment of the modern generation is characterized by mobility on a global scale. Device power systems are based on the use of mainly chemical current sources. The task of developers in this case is to obtain stabilizers with small overall parameters and the least possible loss of electricity on them.

Modern SVs are used in the following systems:

• mobile communications;
• portable computers;
• microcontroller power supplies;
• autonomously operating security cameras;
• autonomous security systems and sensors.

To solve power supply issues for stationary electronics, voltage stabilizers with a low voltage drop in a housing with three terminals of the KT type (KT-26, KT-28-2, etc.) are used. They are used to create simple circuits:

• chargers;
• power supplies for household electrical equipment;
• measuring equipment;
• communication systems;
• special equipment.

What are the types of fixed type SN?

All integral stabilizers (which also include fixed ones) are divided into two main groups:

• Stabilizers with a minimum low voltage drop of a hybrid design (GISN).
• Semiconductor microcircuits (SIC).

SN of the first group is performed on integrated circuits and semiconductor elements unframed type. All components of the circuit are placed on a dielectric substrate, where connecting conductors and resistors, as well as discrete elements - variable resistances, capacitors, etc., are added by applying thick or thin films.

Structurally, microcircuits are complete devices whose output voltage is fixed. These are usually stabilizers with a low voltage drop of 5 volts and up to 15 V. More powerful systems are built on powerful unpackaged transistors and a (low-power) film-based control circuit. The circuit can carry currents up to 5 amperes.

ISN microcircuits are made on a single chip, which is why they are small in size and weight. Compared to previous microcircuits, they are more reliable and cheaper to manufacture, although they are inferior in parameters to GISN.

Linear SN with three terminals are referred to as ISN. If we take the L78 or L79 series (for positive and negative voltages), then they are divided into microcircuits with:

• Low output current of about 0.1 A (L78L**).
• The average current value is around 0.5 A (L78M**).
• High current up to 1.5 A (L78).

Operating principle of a low voltage drop linear regulator

A typical stabilizer structure consists of:

• Reference voltage source.
• Error signal converter (amplifier).
• A signal divider and a regulating element, assembled on two resistors.

Since the output voltage directly depends on the resistances R1 and R2, the latter are built into the microcircuit and a CH with a fixed output voltage is obtained.

The operation of a low dropout voltage regulator is based on the process of comparing the reference voltage with the one that is supplied to the output. Depending on the level of discrepancy between these two indicators, the error amplifier acts on the gate of the power transistor at the output, covering or opening its junction. Thus, the actual level of electricity at the output of the stabilizer will differ little from the declared nominal one.

The circuit also contains sensors for protection against overheating and overload currents. Under the influence of these sensors, the output transistor's channel is completely blocked and it stops passing current. In shutdown mode, the microcircuit consumes only 50 microamps.

Low voltage drop stabilizer connection circuits

An integrated stabilizer chip is convenient because it has all the necessary elements inside. Installing it on the board requires only the inclusion of filter capacitors. The latter are designed to remove interference coming from the current source and load, as can be seen in the figure.

Regarding the 78xx series SN and the use of tantalum or ceramic input and output bypass capacitors, the capacitance of the latter should be within the range of up to 2 µF (input) and 1 µF (output) at any permissible voltage and current values. If you use aluminum capacitors, their rating should not be lower than 10 μF. The elements should be connected as close as possible to the pins of the microcircuit.

In the case where a voltage stabilizer with a low voltage drop of the required rating is not available, you can increase the rating of the MV from a smaller one to a larger one. By raising the electricity level at the common terminal, it is achieved to increase it by the same amount at the load, as shown in the diagram.

Advantages and disadvantages of linear and switching stabilizers

Continuous Continuous Circuits (CIs) have the following advantages:

1. Implemented in one small package, which allows them to be effectively placed on the working space of the printed circuit board.
2. Does not require installation of additional control elements.
3. Provide good stabilization of the output parameter.

The disadvantages include low efficiency, not exceeding 60%, associated with the voltage drop across the built-in control element. At high power microcircuits must use a chip cooling radiator.

Those with a low voltage drop across the field, the efficiency of which is approximately 85%, are considered more productive. This is achieved thanks to the operating mode of the control element, in which current passes through it in pulses.

The disadvantages of the pulsed MV circuit include:

1. Complexity of schematic execution.
2. Presence of pulsed interference.
3. Low stability of the output parameter.

Some circuits using a linear voltage regulator

In addition to the intended use of microcircuits as SN, it is possible to expand the scope of their application. Some variants of such schemes based on integrated circuit L7805.

Switching on stabilizers in parallel mode

To increase the load current, the MVs are connected in parallel to each other. To ensure the functionality of such a circuit, an additional small value resistor is installed between the load and the output of the stabilizer.

Current stabilizer based on MV

There are loads that must be powered with direct (stable) current, for example, an LED string.

Circuit diagram for regulating fan speed in a computer

This type of regulator is designed in such a way that when initially turned on, all 12 V is supplied to the cooler (for its spin-up). Then, after charging of capacitor C1 is complete, variable resistor R2 can be used to regulate the voltage value.

Conclusion

When assembling a circuit using a voltage stabilizer with a low voltage drop with your own hands, it is important to consider that some types of microcircuits (built on field-effect transistors) cannot be soldered with a regular soldering iron directly from a 220 V network without grounding the case. Their static electricity can damage the electronic element!

WITH VOLTAGE DROP 0.05 V

When powering various equipment from batteries, there is often a need to stabilize the voltage and current consumption. For example, when DVD creation laser (see article on the website) or LED flashlight. For these purposes, the industry has already developed several so-called driver microcircuits, which are a low-voltage voltage converter with a built-in stabilizer. The latest development is the LT1308A chip.

Without in any way diminishing the merits of these drivers, I would like to note that even in our large regional center, you cannot get such microcircuits. Only on order and at a price of 10 cu. Therefore, I propose a simple, cheap but effective stabilizer circuit, from radioamator 4 2007.

The stabilization factor is about 10,000, the output voltage is set with a 2.4 k* resistor within the range of 2 - 8 V. When the input voltage is less than the output, the control transistor is fully open, and the voltage drop is several millivolts. When the input voltage exceeds the output voltage, the drop across the stabilizer is only 0.05 V! This makes it possible to power light and laser diodes from two to three AA batteries. Moreover, by changing the load current within 0 - 0.5 A, Uout changes by only 1 millivolt. The board for such a simple device can not be etched, but cut out with a cutter. For those who don’t know, I’ll explain: we take a broken blade from a metal hacksaw and sharpen it using sandpaper. Next, for ease of holding in your hand, we wrap it with a thick wire.

Now with this tool we simply scratch the copper with force, like tracks.

We clean it with sandpaper, tin it, solder the parts, and you're done.

A simple circuit for regulating and stabilizing voltage is shown in the picture above; even a novice in electronics can assemble it. For example, 50 volts are supplied to the input, and at the output we get 15.7 volts or another value up to 27V.

Main radio component of this device is a field-effect (MOSFET) transistor, which can be used as IRLZ24/32/44 and others like that. They are most commonly produced by IRF and Vishay in TO-220 and D2Pak packages. It costs about $0.58 UAH at retail; on ebay 10psc can be purchased for $3 ($0.3 per piece). Such powerful transistor has three terminals: drain, source and gate, it has the following structure: metal-dielectric (silicon dioxide SiO2)-semiconductor. The TL431 stabilizer chip in the TO-92 package provides the ability to adjust the value of the output electrical voltage. I left the transistor itself on the radiator and soldered it to the board using wires.

The input voltage for this circuit can be from 6 to 50 volts. At the output we get 3-27V with the ability to regulate with a 33k substring resistor. The output current is quite large, up to 10 Amps, depending on the radiator.

Smoothing capacitors C1, C2 can have a capacity of 10-22 μF, C3 4.7 μF. Without them, the circuit will still work, but not as well as it should. Don't forget about voltage electrolytic capacitors at the input and output, I took everything designed for 50 Volts.

The power that can be dissipated by this cannot be more than 50 watts. Field effect transistor must be installed on a radiator with a recommended surface area of ​​at least 200 square centimeters (0.02 m2). Don’t forget about thermal paste or rubber backing so that the heat transfers better.

It is possible to use a 33k substring resistor like WH06-1, WH06-2; they have fairly precise resistance adjustment, this is what they look like, imported and Soviet.

For convenience, it is better to solder two pads onto the board, rather than wires, which are easily torn off.

Discuss the article VOLTAGE STABILIZER ON A FIELD TRANSISTOR

This circuit stabilizes the current through one or more LEDs, almost independently of the supply voltage. Its main advantage is the very low voltage drop, which can be less than 100 mV. The design may find application in LED strips, where the voltage can vary along the length due to resistive drop, and small changes in voltage lead to significant changes current and brightness. And also in, where every volt counts.

LED current stabilizer circuit

The voltage drop in the resistor R circuit does not exceed 40 mV. The rest depends on the parameters of Q3.

The nominal LED current here is 7.2 mA at 9 V. Increasing the voltage to 20 V causes a current change of only +15%, due to dynamic resistance.

The value of resistor R1 is selected for a blue/white LED with a voltage drop in the range of 2.9 - 3.4 volts. To maintain the desired level at a different voltage drop, change the value of R1 in proportion to the change in voltage drop.

The current through the LEDs is inversely proportional to the value of R. The current can be roughly changed by using this resistor, and fine-tuned by changing R1.

To obtain good thermal stability, Q1 and Q2 must be in thermal contact. Ideally, they should be on the same chip, but it turns out that way good results when they are pressed against each other.

The circuit works well not only with one LED. The maximum number of LEDs in a line depends only on the parameters of the circuit components.

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