A simple switching voltage regulator. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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A simple switching voltage regulator. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Surge Protectors

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The proposed switching voltage stabilizer differs from similar devices in simplicity, good repeatability and the absence of adjusting elements.

The stabilizer circuit is shown in the figure.

A simple switching voltage regulator

When the power is turned on, the voltage across the capacitor C2 is zero and current begins to flow through the resistor R1 and the emitter junctions of the transistors VT1 and VT2. Transistors VT1 and VT2, followed by transistors VT3 and VT4 open. Capacitor C2 begins to be charged by the current flowing through the inductor L1.

When the voltage on the capacitor exceeds the stabilization voltage of the Zener diode VD3, the transistors VT1 and VT2 close, as a result of which the transistors VT3, VT4 also close. Diode VD4 provides the current path for inductor L1 when transistor VT4 is closed. When the voltage across capacitor C2 becomes less than the stabilization voltage of the Zener diode VD3, the process is repeated.

With the elements indicated in the diagram, the output voltage of the stabilizer is about 5 V, and the maximum load current is 0,5 ... 0,7 A. The ripple level at an output current of 0,7 A is about 0,1 V and depends little on the load: to a greater extent it depends on the resistance of the resistors R1 and R2. The efficiency of the stabilizer is approximately 80 ... 85%. The input voltage of the device is limited by the maximum allowable voltage of the transistors VT1-VT4 and for these devices should not exceed 25 V. The negative property of the stabilizer: it is completely unstable to a short circuit at the output. However, this is easily fixable - you can introduce a protection circuit into it, for example, connect a transistor in parallel with the zener diode, which, when overloaded, "closes" it to a common wire, thereby reducing the output voltage.

If you need a stabilizer for a different output voltage, you should install a zener diode with a stabilization voltage equal to the required output. Other elements of the device do not change, it is only necessary to ensure that the operating current of the zener diode flowing through the resistor R1 is not less than the minimum allowable for this device. Otherwise, the resistance of resistors R1 and R2 should be reduced until the desired current is obtained so that their ratio remains unchanged.

Inductor L1 is wound on a K20x12x6 ring magnetic circuit made of M2000NM ferrite with a gap of 0,25 mm and contains 60 turns of PEV-2 0,6 wire. It is possible to use industrial chokes D-0,3 (if the load current does not exceed 0,3 A) with an inductance of at least 100 μH. In place of the transistor VT3, you can install any high-frequency transistor with a maximum collector current of at least 300 mA, and in place of VT4 - any of the KT802, KT805 series. We can replace the KD212A (VD4) diode with any one with a permissible operating frequency of at least 100 kHz, for example, from the KD212, KD213, KD2997-KD2999 series. The capacitance of capacitor C1 (necessarily ceramic) can be in the range of 0,33 ... 1 μF.

A properly assembled stabilizer does not require adjustment. Using an oscilloscope connected to the emitter of the transistor VT4, check the presence of rectangular pulses with a frequency of 20 ... 80 kHz. If the pulse repetition rate is higher than 80 kHz (at too high a frequency, the transistor VT4 starts to heat up), you should increase the number of turns of the inductor L1.

Author: A.Chernomyrdin, Magnitogorsk

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