Microcircuit voltage stabilizer: protection node. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Surge Protectors

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The proposed device reliably protects the microcircuit voltage regulator without compromising its technical characteristics.

Radio amateurs widely use 1 voltage stabilizers based on three-pin microcircuits of the KR142, KR1157, KR1158, 78L, 79L series to build power supplies [1]. Although these microcircuits have built-in current and overheat protection, they often still need external protection. The fact is that during an emergency with a current overload or a short circuit in the load, these microcircuits go into the output current limiting mode. But in this case, a significant part of the input voltage is applied to the microcircuit, as a result of which it begins to warm up. Although the built-in thermal protection will reduce the output current, with a large input voltage, the microcircuit can overheat and fail, especially if it is installed on an insufficiently efficient heat sink or without it at all. What threatens such a situation, it is clear without explanation. And here a device is useful, which provides protection for the stabilizer microcircuit in some extreme operating modes and, accordingly, increases the reliability of its operation.

The scheme of the proposed device together with the stabilizer is shown in Fig. 1. The security node itself is circled with a dash-dotted line. It is assembled on two field switching transistors with channels of different types of conductivity, included in the transistor assembly IRF7309 (VT1). The main parameters of the transistors of this assembly are: open channel resistance - 0,05 ... 0,1 Ohm, maximum drain current - 3,2 ... 4 A, maximum source-drain voltage - 30 V, gate-source - 20 V, total power dissipation - 1.4 W.

Rice. 1 Schematic diagram of the device

The protective device controls the output voltage of the stabilizer. If it drops below a certain level, the device will disconnect the microcircuit from the input voltage source. Several typical emergencies are possible. Firstly, this is a short circuit in the load, in which the output voltage drops to almost zero, causing the protection device to trip. Secondly, this is an overload current higher than the maximum allowable value for the microcircuit. In this case, the microcircuit will enter the current limiting mode, the output voltage will decrease, so the protection device will work. Thirdly, a significant increase in the load current is possible, but not reaching the maximum output current of the microcircuit. For example, the load current instead of the usual 0,5 A has increased to 1,5 A. Although this mode is normal for the microcircuit, it will still heat up more. If the heat dissipation is inefficient, the case temperature will rise until it is too high. Then the thermal protection will reduce the output current, the output voltage will also decrease, as a result of which the protection device will operate, turning off the power to the microcircuit.

At the moment the device is turned on, the capacitor C1 is discharged, all the input voltage is applied to the resistor R1. Transistor VT1.1 is open until this capacitor is charged. The voltage is supplied to the input of the DA1 microcircuit, the nominal output voltage appears at its output, part of which is fed from the resistor divider R4R5 to the gate of the transistor VT1.2. This transistor opens, keeping capacitor C1 discharged, so transistor VT1.1 will remain open.

If, for some reason, the output voltage of the stabilizer decreases significantly, then the transistor VT1.2 will start to close, the capacitor C1 will charge, and the transistor VT1.1 will close. This will further reduce the output voltage. Due to the action of positive feedback, the process ends with the complete closing of transistors VT1.1 and VT1.2. The closed transistor VT1.1 opens the input circuit of the DA1 chip, providing its protection. Capacitor C1 is needed both when starting the stabilizer and for delaying the operation of the protection device, increasing its noise immunity.

To restart, you need to temporarily turn off the input voltage until the voltage across the capacitor C1 decreases by 2,5 ... 3 V due to discharging through the resistor R2. After that, the transistor VT1.1 will open and apply voltage to the input of the DA1 chip. The output voltage will start to increase. At the moment when the gate-source voltage of the transistor VT1.2 exceeds 2,5 V, it will open. Through its channel and the current-limiting resistor R3, the capacitor C1 is finally discharged. The HL1 LED will turn on - an indicator of the presence of the output voltage of the stabilizer and, accordingly, its normal operation.

Construction and details

The device is mounted on a printed circuit board made of double-sided foil fiberglass (Fig. 2). The assembled board is shown in fig. 3. The foil on the back of the board is used as a common wire. Wires are passed through the holes of the board, marked with asterisks, connecting the printed conductors on both sides. Pins 1 and 3 of the DA1 chip are soldered to the printed conductors, pin 2 is passed through the hole and soldered to the common wire foil on the reverse side. If the DA1 chip is installed on a heat sink, the board is also placed on it next to the chip.

Rice. 2 Device circuit board

The proposed protection device can be applied to any voltage stabilizer microcircuit with three outputs. If the overall output of the microcircuit is average, the pattern of the conductors of the printed circuit board is suitable without changes. Otherwise, it will require minor modification.

The proposed device is also suitable for protecting adjustable voltage stabilizers (LM317 series and similar), but in this case it is also necessary to change the pattern of the printed circuit board conductors in order to allow the installation of a resistor voltage divider and, possibly, some other elements [1, fig. 3].

Rice. 3 Appearance of the device

The device can use fixed resistors P1-4, MLT, S2-33, capacitors K50-35 or similar. The rated voltage of capacitors C1 and C2 must be at least 20% higher than the maximum input voltage, and C3 - output. LED HL1 can be any visible radiation with a rated current of 5 ... 20 mA.

Instead of the transistor assembly IRF7309 (VT1), you can use separate field-effect transistors with an insulated gate and an induced channel of the corresponding type of conductivity [2]. The transistor replacing VT1.1 must withstand the input current of the microcircuit at the maximum load current, its maximum drain-source and gate-source voltage must be greater than the maximum input voltage. For the transistor that replaces VT1.2, the maximum drain-source voltage must be greater than the maximum input.

Establishment

The adjustment comes down to the selection, if necessary, of the capacitance of the capacitor C1, so that transients in the stabilizer or load occur faster than charging the capacitor through the resistor R1. The resistance of the resistor R2 is chosen from several hundred kilo-ohms to 1 MΩ to ensure an acceptable duration of the initial discharge of the capacitor C1 - the minimum time for which the input voltage must be turned off after the protection has been triggered. Resistor R4 is selected so that the device operates when the stabilizer output voltage drops by 1 ... 3 V. With a low output voltage (3 ... 6 V), the device can be simplified by eliminating resistors R4, R5 and installing a jumper instead of R5. But in this case, the protection device will not work until the output voltage drops to about 2,5 V, since it is at this gate-source voltage that the VT1.2 field effect transistor starts to close. Therefore, at a higher output voltage (9 ... 12 V), it is still advisable to install these resistors.

Resistor R3 limits the discharge current of capacitor C1 through the channel of the transistor VT1.2 to an acceptable value. Resistor R6 and LED HL1 set if necessary. The resistance of the resistor R6 is chosen so as to obtain the required brightness of the radiation of the LED HL1, without exceeding the maximum allowable current through it.

For a voltage regulator of negative polarity (on microcircuits of the 79L series and similar), you should swap the field effect transistors VT1.1 and VT1.2, and also change the polarity of turning on all capacitors and the HL1 LED. The pattern of the PCB conductors will also have to be changed.

The input voltage, taking into account ripples, should not exceed 20 V. In conclusion, it should be noted that the proposed device will not save you from all possible emergencies, but it significantly increases the reliability of the microcircuit voltage stabilizer.

Literature

1. Biryukov S. Microcircuit stabilizers of wide application. - Radio, 1999, No. 2, p. 69-71.
2. Powerful field-effect switching transistors from International Rectifier. - Radio, 2001, No. 5, p. 45.

Author: I. Nechaev, Kursk; Publication: radioradar.net

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