Powerful switch on the MIS transistor. Scheme, description

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Powerful switch on the MIS transistor. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches

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On fig. 1 shows a diagram of one of the options for a powerful electronic relay designed to switch a load current of up to 20 A at a voltage of 5 ... gate-source 20 V or no more than 2556 mΩ at 5,7 V. Such a low resistance of the open channel makes it possible to switch a large current using this device, and the installation of a transistor on a heat sink at a low switching frequency (units - tens of kilohertz) is usually not required. The device can be used, for example, as an electronic output voltage switch in a powerful power supply, powerful light sources in rechargeable flashlights, low-voltage electric motors, traction electromagnets, and for many other applications.

The use of a powerful MIS transistor as the main switching element in comparison with an electromagnetic relay makes it possible to obtain a lower resistance of "closed contacts", the absence of their burnout and spark interference, and higher speed (with electronic control). In addition, such an electronic switch will have smaller dimensions and weight than electromagnetic relays for a current of 10 ... 20 A, as well as a significantly lower current consumed by control circuits.

The electronic switch can be controlled by two small-sized non-latching buttons, for example, reed, membrane or rubber with a conductive coating.

Powerful switch on the MIS transistor
Fig. 1

On fig. 2 for comparison of dimensions shows the electromagnetic relay G7L-2A-P by Omron, the contacts of which are designed for switching current of 20 A, and the layout of the electronic relay on the MIS transistor. The electronic unit, even with a relatively spacious installation, occupies a quarter of the volume (the buttons and the LED are mounted outside the board) and is much lighter.

Powerful switch on the MIS transistor
Fig. 2

When voltage is applied to the input of the device, the field-effect transistor VT2 remains closed, the load connected to the output is de-energized, the HL1 LED is off. To apply voltage to the load, you must briefly press the button SB1. This will open the transistor VT1 followed by the transistor VT2.

The switched-on LED HL1 will inform about the voltage supplied to the load. Capacitors C3 and C4, as well as C1, C2, C5, C6 eliminate the possible impact on the state of the transistors of various interferences. Diodes VD2-VD5 are designed to force the device to turn off when the input voltage drops to about 3 V, which protects the field-effect transistor VT2 from overheating.

The fact is that such a deep decrease in the gate-source voltage of the transistor VT2 sharply increases the channel resistance and, as a result, the thermal power released in it, especially at a high load current. In order to protect the field-effect transistor from overheating, an R5VD2-VD5 circuit is provided that closes both transistors.

Varistor RU1 and zener diode VD1 protect the relatively low-voltage field-effect transistor from voltage surges, for example, from the self-induction EMF of an electric motor connected to the input or output of the device, or, for example, from accidental damage by static electricity when the gate of the transistor VT2 is touched with a screwdriver (or other metal objects) .

To turn off the device, a short-term closure of the contacts of the SB2 button is sufficient. You can control the state of the transistor VT2 not only with low-power miniature buttons, but also, for example, with two optocouplers or low-power reed relays. It should be noted that in the off state, the switch consumes practically no energy.

An experimental sample of the device was mounted on a mounting plate with dimensions of 46x27 mm made of fiberglass by surface mounting. High-current circuits are made with short pieces of mounting wire with a cross section of at least 1,2 mm.

The miniature T2556 transistor APM0252NU allows a maximum drain-to-source voltage of 25 V. With a drain current of 40 A and a gate-to-source voltage of 10 V or 20 A at a gate-to-source voltage of 4,5 V, the typical value of the open channel resistance does not exceed 4,5 and 7,5 mΩ respectively. The maximum allowable direct current of the transistor drain at a case temperature of 25 ° C is 60 A.

The transistor should be soldered to a heat sink with a usable surface area of at least 7 cm2 in case of operation at low supply voltage with high load current. When mounting the transistor, it is necessary to take measures to protect it from breakdown by static electricity.

APM2556NU transistors, designed for operation in step-down switching voltage regulators, are now widely used in modern high-performance video cards and computer motherboards. This transistor can be replaced by two miniature transistors connected in parallel, but having twice the open channel resistance, APM2510NU transistors (8,5 MΩ 10 V) or other similar low-voltage gate-source controlled transistors. When using transistors with a higher channel resistance than the APM2556NU, to keep the resistance of the switching element low, you can turn on several of the same type of field-effect transistors connected in parallel.

We can replace the 2SA733B transistor with any of the 2SA733, 2SA992, SS9015, KT3107, KT6112 series. Instead of BZV55C15, a zener diode 1 N4744A, TZMC-15, 2S215Zh, KS215ZhA is suitable, and instead of 1N4148, diode 1 N914 (or any of the KD522, KD521 series). LED - any general application, preferably with increased light output, for example, from the KIPD40, KIPD66 series. For each specific voltage on the load, resistor R6 should be selected so as not to exceed the rated current of the LED.

Oxide capacitors - K50-68, K53-19 or imported. The rest - K10-17, K10-50. The FNR-05K220 varistor can be replaced by any low-power 18...22 V varistor, such as the FNR-05K180.

Accurately assembled from serviceable parts, the device does not require adjustment.

Depending on the specific features of the application, the switch proposed for repetition can be simplified or improved. For example, if voltage surges from the power source or the connected load are excluded, you can do without the varistor RU1. You can also refuse the protective zener diode VD1 if the voltage of the power source does not exceed 15 V and any touches to the gate terminal of the transistor VT2 are excluded.

If the winding of a self-made reed relay is inserted in series into the load circuit, the open contacts of which are connected in parallel with the contacts of the SB2 button, then the load power will be automatically turned off when the current consumed by it increases above the specified one. To manufacture such a relay, it is necessary to wind several turns of a thick (0,7 ... 1,2 mm in diameter) winding wire on the KEMZ reed switch cylinder. So, for example, with a coil of seven turns of PEV-2 wire 0,68, the relay will operate at a current of about 5 A. The required number of turns for the desired protection operation current for a particular reed switch is determined experimentally.

Author: A. Butov

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