Low drop stabilizer. Voltage stabilizer on a field-effect transistor - circuitry. What is a low dropout voltage stabilizer

Series Continuous Voltage Regulator - Adjustable, Low Dropout

Adjustable Series Stabilizer

To adjust the output voltage in the previous circuit, an integrated element with adjustable voltage stabilization (controlled zener diode). There is another option.

MOSFET + TL431 = Serial compensation voltage regulator with minimal fall

Ideal LDO Regulator

LDO = low dropout

For the popular three-pin integrated stabilizer LM317 (datasheet), the minimum voltage drop at which its operation is still normalized is 3 Volts. Moreover, this parameter is not explicitly indicated anywhere in the documentation, but modestly, it is mentioned in the measurement conditions. In most cases, it is assumed that the drop on the chip is 5 volts or more:
"Unless otherwise specified, VIN − VOUT = 5V".

Baba Yaga - against! It's a pity to lose 3 volts on a stupid pass transistor. And dissipate extra watts. A popular solution to the problem - switching regulators - is not discussed here due to the fact that they whistle. You can fight against interference, but, as you know: whoever does not fight is invincible! 😉

Idea
The idea of ​​this circuit goes back to one of the many datasheets on the TL431. Here, for example, is what National Semiconductor / TI offers:

Vo ~= Vref * (1+R1/R2)

By itself, such a regulator is not very interesting: in my opinion, it is no better than the usual three-pin stabilizers 7805, LM317 and the like. The minimum drop on the passing darlington is less than 2 volts here is unlikely to be obtained. And besides, there are no protections for current or overheating. Unless the transistors can be set as thick as your heart desires.

Recently, I needed to build a linear regulator with a minimum voltage drop. Of course, you can always dodge, take a transformer with a higher voltage on the secondary, put Schottky diodes in the bridge, more storage capacitors ... And with all this happiness, heat a three-pin stabilizer. But I wanted something elegant solution and with the trance that was available. What kind of feed-through regulator can provide a drop close to zero? MOSFET: Modern high-power field devices can have a channel resistance of a few milli-ohms.

Simply replacing the Darlington with an IGFET (i.e. the most common MOSFET) in the circuit above is not going to help much. Since the threshold gate-source voltage will be 3-4 Volts for ordinary ones, and everything is more than Volts for "logical" MOSFETs - this will set the minimum throughput voltage on such a stabilizer.

It could be interesting when using a field worker operating in lean mode (i.e. with a built-in channel), or with p-n junction. But unfortunately, powerful devices of these types are now practically unavailable.

Rescues an additional source of bias voltage. Such a source should not be high current at all - a few milliamps will be enough.

It all works very simply: when the voltage at the control input of the TL431, proportional to the output voltage, drops below the threshold (2.5V), the "zener diode" closes and "releases" the gate of the field switch "up". The current from an additional source through the resistor "pulls up" the voltage at the gate, and, consequently, at the output of the stabilizer.
In the opposite direction, with an increase in the output voltage, everything works in a similar way: the "zener diode" opens slightly and reduces the voltage at the gate of the field worker.
TL431 is a linear device, there are no latches in it:

Reality
In the circuit of a real device, I still added current protection, sacrificing a half-volt drop in favor of safety. In principle, low-voltage designs can often be dispensed with fuse, since field-effect transistors are available with a huge current margin and, in the presence of a radiator, are able to withstand frantic overloads. If it’s a pity even 0.5 Volt, and current protection is necessary - write, because there are ways 😉

January 30, 2012: 🙂Works great! At load currents of approximately 2A and above, it is advisable to seat powerful diodes on a small radiator. R8=0; C7=0.1 ... 10µF ceramic or film.

With the ratings R5-R6-R7 indicated in the diagram, the output voltage adjustment range is approximately from 9 to 16 volts. Naturally, the real maximum depends on how much the transformer can provide under load.
R4 must use decent power: PmaxR4 ~= 0.5 / R. In this example- dvuhvatnik will be just right.

Where it might be needed
For example: in lamp technology for supplying filament circuits with direct current.
Why a constant, and even so carefully stabilized current to power the filaments?

1. Exclude interference AC voltage into signal circuits. There are several ways for the "background" to leak from the filament circuits into the signal (a topic for a separate article!)
2. Feed the glow with a strictly specified voltage. There is evidence that exceeding the filament voltage by 10% of the nominal voltage can reduce the lamp life by an order of magnitude. The tolerance standards for the supply voltage plus the errors in the design of transformers, etc. - 10% error will easily come up.

For 6-volt incandescences, it is necessary to reduce R5: 5.6KΩ will be just right.

What can be improved
For example, to power filaments, it is useful to add a soft start. To do this, it will be enough to increase C4, say, to 1000uF and connect a 1KΩ resistor between the bridge and C4.

A bit of lamp mythology
Let me go over one persistent delusion, which claims that the power supply of the glow with a "constant" negatively affects the "sound".
The most likely source of the origin of this myth, as usual, is a lack of understanding and crooked hands. For example: one transformer feeds both anodes and heat. The rated current of the filament winding, say, 1A, which previously fed the lamps directly, and they consumed a little less than this very 1A. Everything worked well, maybe a little fonilo. If now a certain soldering rigger, imagining himself a "tube-guru", suddenly powered the same lamps from the same winding but through a rectifier/capacitor/stabilizer - that's it, khana amp! The explanation is simple, although not obvious to everyone:

1. Firstly, the transformer is now overloaded due to the pulsed nature of the charge current of the storage capacity (a separate article is needed!) In short: you need to take a trans with a rated secondary current of about 1.8 times more than the rectified load current.
2. Secondly, the shock currents of the charge of storage capacities in the power source of the glow will not add anything good to the anode power supply.

• Conclusion

• Were you interested? Write me!

Ask, suggest: in the comments, or by e-mail (available in my profile). Thank you!

All the best!
- Sergey Patrushin.

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131 thoughts on MOSFET + TL431 = Minimal Dropout Series Compensating Voltage Regulator”

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Sometimes in amateur radio practice there is a need for low voltage stabilizer on the regulating element (1.5-2V). This can be caused by insufficient voltage on the secondary winding of the transformer, dimensional restrictions when the case does not accommodate a radiator of the required size, considerations of the efficiency of the device, etc.

And if the choice of microcircuits for building "ordinary" stabilizers is wide enough (such as LM317, 78XX etc.), then microcircuits for building Low-Drop stabilizers are usually not available to everyone. Therefore, a simple scheme on available components may be very relevant.

I present a scheme that I myself have used for many years. During this time, the scheme has shown a reliable, stable work. Available components and ease of setup will allow even novice radio amateurs to repeat the design without difficulty.

zoom on click

The scheme resembles a fairly standard parametric stabilizer, which is supplemented by a GST (stable current generator) to control the base current of the regulating transistor, due to which it was possible to obtain low voltage drop.

The circuit is designed for an output voltage of 5V (set by resistor R4) and a load current of 200mA. If you want to get more current, then instead of T3 you should use composite transistor.

If you need to get a higher output voltage, you will have to recalculate the values ​​\u200b\u200bof the resistors.

When lack of transistor assemblies you can use discrete transistors. In my version, instead of assembling KR198NT5, two matched KT361 transistors were used. The KR159NT1 assembly can be replaced with two KT315 transistors, the selection of which is not required.

Since there is practically no information on the Internet on domestic components, I cite for reference the pinout of transistor assemblies.

All modern 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 a small 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 (SN) is understood as such a device, the main task of which is to maintain a constant voltage level at the load at a certain constant level. Any stabilizer has a certain parameter output accuracy, 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 (reference), which is generated by a special source. This type of stabilizer is called compensation. In this case, the regulating element (RE) is a transistor - a bipolar or a field worker.

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

• active;
• key.

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

Where is a fixed stabilizer used?

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

Modern CHs are used in the following systems:

• means of mobile communication;
• portable computers;
• batteries for microcontrollers;
• stand-alone surveillance cameras;
• autonomous security systems and sensors.

To solve the issues of powering stationary electronics, voltage stabilizers with a small voltage drop in a housing with three KT-type terminals (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 fixed type SNs?

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

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

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

Structurally, microcircuits are complete devices, the output voltage of which 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 frameless transistors and a control circuit (low power) based on films. The circuit can pass currents up to 5 amperes.

ISN microcircuits are performed on a single chip, therefore they are small in size and weight. Compared to previous microcircuits, they are more reliable and cheaper to manufacture, although they are inferior to GISN in terms of parameters.

Linear SN with three outputs belong to the 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, in the region of 0.5 A (L78M **).
• High current up to 1.5 A (L78).

The principle of operation of a linear regulator with a small voltage drop

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 value of 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 voltage regulator with a low voltage drop is based on the process of comparing the reference voltage with that which 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 transition. Thus, the actual level of electricity at the output of the stabilizer will differ little from the declared nominal.

Also in the circuit there are sensors for protection against overheating and overload currents. Under the influence of these sensors, the channel of the output transistor is completely blocked, and it ceases to pass current. In shutdown mode, the chip consumes only 50 microamps.

Schemes for switching on a stabilizer with a small voltage drop

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

For 78xx series MVs and the use of tantalum or ceramic shunt capacitors for input and output, the capacitance of the latter must be within 2 µF (input) and 1 µF (output) at any allowable voltage and current. If you use aluminum capacitors, then their value should not be lower than 10 microfarads. Connect the elements as close as possible to the pins of the microcircuit.

In the case when there is no voltage stabilizer with a small voltage drop of the desired rating, you can increase the rating of CH from a smaller one to a larger one. By raising the level of electricity at the common terminal, it is increased by the same amount at the load, as shown in the diagram.

Advantages and disadvantages of linear and switching regulators

Integrated circuits of continuous operation (SN) have the following advantages:

1. They are implemented in one small package, which allows them to be effectively located on the working space of the printed circuit board.
2. Do not require the installation of additional regulatory elements.
3. Provide good stabilization of the output parameter.

The disadvantages include low efficiency, not exceeding 60%, associated with a voltage drop across the built-in control element. At high power microchips need to apply a crystal cooling radiator.

More productive are considered with a small voltage drop across the field, the efficiency of which is approximately at the level of 85%. This is achieved due to the mode of operation of the regulating element, in which the current passes through it in pulses.

The disadvantages of the pulsed CH circuit include:

1. The complexity of the schematic execution.
2. The presence of interference of an impulse nature.
3. Low stability of the output parameter.

Some circuits using a linear voltage regulator

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

Enabling Stabilizers in Parallel Mode

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

CH based current stabilizer

There are loads that need to be powered by constant (stable) current, for example, an LED chain.

Fan speed control circuit in a computer

The regulator of this type is built in such a way that when it is initially turned on, all 12 V is supplied to the cooler (for its promotion). Further, after the charge of the capacitor C1 variable resistor R2 will be able to adjust the voltage.

Conclusion

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

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

Main radio component this device is a field-effect (MOSFET) transistor, which can be used as IRLZ24/32/44 and others like it. 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 apiece). Such powerful transistor has three outputs: drain (drain), source (source) and gate (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 possibility of regulation by a trimmer resistor 33k. The output current is quite large, up to 10 Amps, depending on the heatsink.

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

The power that this can dissipate cannot be more than 50 watts. The field-effect transistor must be installed on a radiator, the recommended surface area of ​​which is at least 200 square centimeters (0.02 m2). Do not forget about the thermal paste or rubber backing, so that the heat is better given off.

It is possible to use a 33k trimmer resistor type WH06-1, WH06-2 they have a fairly accurate resistance adjustment, this is how they look, imported and Soviet.

For convenience, it is better to solder two pads on the board, rather than wires that can be easily torn off.

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