Voltage stabilizer on a powerful field-effect transistor. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Surge Protectors

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The article describes an analog voltage regulator for a high power power supply. The author managed to significantly improve the parameters of the stabilizer by using a powerful switching field-effect transistor as a power element.

When building high-current voltage stabilizers, radio amateurs usually use specialized 142 series microcircuits and similar ones, "enhanced" by one or more bipolar transistors connected in parallel. If for these purposes a powerful switching field-effect transistor is used, then it will be possible to assemble a simpler high-current stabilizer.

A diagram of one of the options for such a stabilizer is shown in Fig. 1. It uses a powerful field-effect transistor IRLR2905 as a power transistor. Although it is designed to operate in a key (switching) mode, in this stabilizer it is used in a linear mode. The transistor in the open state has a very low channel resistance (0,027 Ohm), provides a current of up to 30 A at a case temperature of up to 100 ° C, has a high steepness and requires only 2,5 ... 3 V to control the gate voltage [1]. The power dissipated by the transistor can reach 110 watts.

The field-effect transistor is controlled by a parallel voltage regulator chip KR142EN19 (TL431). Its purpose, device and parameters are described in detail in the article [2]. The stabilizer works (Fig. 1) as follows. When the network transformer T1 is connected to the network, an alternating voltage of about 13 V (effective value) appears on its secondary winding. It is rectified by the VD1 diode bridge, and a high-capacity smoothing capacitor (usually several tens of thousands of microfarads) is allocated a constant voltage of about 16 V.

It enters the drain of a powerful transistor VT1 and through the resistor R1 to the gate, opening the transistor. Part of the output voltage through the R2R3 divider is fed to the input of the DA1 chip, closing the OOS circuit. The voltage at the output of the stabilizer increases until the voltage at the control input vu of the DA1 microcircuit reaches the threshold, about 2,5 V. At this moment, the microcircuit opens, lowering the voltage at the gate of a powerful transistor, i.e. partially closing it, and The device enters stabilization mode. Capacitor C3 accelerates the output of the stabilizer to the operating mode. The value of the output voltage can be set in the range from 2,5 to 30 V by selecting the resistor R2, its value can vary over a wide range. Capacitors C1, C2 and C4 ensure stable operation of the stabilizer.

For the described version of the stabilizer, the minimum voltage drop across the powerful regulating transistor VT1 is 2,5 ... 3 V, although this transistor can potentially operate at a drain-source voltage close to zero. This drawback is due to the fact that the control voltage to the gate comes from the drain circuit, therefore, with a lower voltage drop across it, the transistor will not open, because the gate of an open transistor must have a positive voltage relative to the source.

To reduce the voltage drop across the regulating transistor, it is advisable to power its gate circuit from a separate rectifier with a voltage of 5 ... 7 V more than the output voltage of the stabilizer.

If it is not possible to make an additional rectifier, then an additional diode and capacitor can be introduced into the device (Fig. 2).

The effect of such a simple refinement can be great. The fact is that the voltage supplied to the drain of the transistor is pulsating, has a significant variable component, which increases with increasing current consumption. Thanks to the diode VD2 and capacitor C5, the gate voltage will be approximately equal to the peak value of the pulsating, i.e. may be a few volts more than the average or minimum. Therefore, the stabilizer is efficient at a lower average drain-source voltage.

The best results can be obtained if the VD2 diode is connected to a rectifier bridge (Fig. 3).

In this case, the voltage across the capacitor C5 will increase, since the voltage drop across the VD2 diode will be less than the voltage drop across the bridge diodes, especially at maximum current. If it is necessary to smoothly adjust the output voltage, the constant resistor R2 should be replaced with a variable or trimming resistor. The value of the output voltage can be determined by the formula Uout \u2,5d 1 (2 + R3 / R840). It is permissible to use a suitable transistor from the list in the above reference sheet, preferably highlighted in yellow, in the device. If you use, for example, IRF4,5, then the minimum value of the control voltage at the gate will be 5 ... 2 V. Capacitors - small-sized tantalum, resistors - MLT, S33-1, P4-2. Diode VD1 - rectifier with a small voltage drop (germanium, Schottky diode). The parameters of the transformer, diode bridge and capacitor CXNUMX are selected based on the required output voltage and current.

Although the transistor is designed for high currents and high power dissipation, to realize its full potential, it is necessary to ensure efficient heat dissipation. The applied transistor is designed for installation on a radiator by soldering. In this case, it is advisable to use an intermediate copper plate several millimeters thick, to which the transistor is soldered and on which the remaining parts can be installed (Fig. 4). Then, after the installation is completed, the plate can be placed on the radiator. Soldering is no longer required, since the plate will have a large area of ​​​​thermal contact with the radiator.

Fig. 4

If you use a DA1 chip of the TL431C type, resistors of the P1-12 type and the corresponding chip capacitors for surface mounting, they can be placed on a printed circuit board (Fig. 5) made of one-sided foil fiberglass. The board is soldered to the terminals of the transistor and glued to the said copper plate with glue. As such a plate, you can use, for example, a case with a flange from a damaged powerful bipolar transistor, say, KT827, using a hinged mounting.

Establishing a stabilizer comes down to setting the required value of the output voltage. It is necessary to check the device for the absence of self-excitation in the entire range of operating currents. For this, the voltages at various points of the device are monitored using an oscilloscope. If self-excitation occurs, then in parallel with capacitors C1, C2 and C4, ceramic capacitors with a capacity of 0,1 μF with leads of a minimum length should be connected. These capacitors are placed as close as possible to the transistor VT1 and the DA1 chip.

Literature

1. Powerful field-effect switching transistors from International Rectifier. - Radio, 2001, No. 5, p. 45.
2. I. Nechaev. Unusual application of the KR142EN19A chip. - Radio, 2003, No. 5, p. 53, 54.

Author: I. Nechaev, Kursk

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