Economical Switching Voltage Stabilizer

Encyclopedia of radio electronics and electrical engineering / Surge Protectors

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A distinctive feature of the switching regulator described here is the small current that its control unit consumes. This to some extent compensates for the decrease in efficiency inherent in such stabilizers at low load currents.

The pages of the magazine describe many economical DC voltage stabilizers, for example, in [1, 2]. According to the principle of operation, these devices are sequential transistor stabilizers of continuous regulation. They differ from the usual ones only in that the reference voltage generation and comparison nodes are made on elements that allow operation in the microcurrent mode, due to which some energy savings are achieved. However, the efficiency of such stabilizers is low.

Another class of these devices is switching regulators. They have a higher efficiency at medium and high load current, however, at low current, their efficiency decreases.

The described device is devoid of such a disadvantage. This allows you to use it in almost any equipment: from various digital to sound reproducing and radio receivers.

The stabilizer contains (Fig. 1) a switching composite transistor (VT1, VT2), a switching diode (VD2) and a choke (L1). The control unit includes a reference voltage source (VТЗ) and a comparator (DA1). A transistor filter (VT4, VT5) is switched on at the output of the stabilizer.

Technical specifications

The principle of operation of the device corresponds to the usual pulse regulation. It is described in detail in [3]. Therefore, it makes sense to dwell only on the distinctive features in the control unit and the transistor filter.

The basis of the control unit is a comparator made on the K140UD12 OS. A micropower exemplary voltage source is connected to its inverting input, made on a reverse-biased emitter junction of the VT3 transistor [1]. Its stabilization voltage (7 ... 7,5 V) is provided at a current of 20 ... 30 μA.

A comparison signal from a resistive divider R5-R7 is applied to the non-inverting input of the op-amp. The trimmer resistor R6 regulates the output voltage.

Capacitor C3 increases the phase shift of the feedback signal, which is necessary for the cyclic nature of the device. It also determines the frequency of work cycles and largely affects the range of pulsations.

The output of the comparator is connected to the base of the composite transistor VT1, VT2 through a resistor R3 that sets the control current, and a zener diode VD1, which provides cutoff of the control current and reliable closing of the switching transistor over the entire input voltage range. Capacitor C2 smoothes the fronts of the pulses and thereby suppresses high-frequency noise that occurs during operation.

Unlike traditional switching regulators, the output does not include an LC filter, but a transistor one. The fact is that the LC filter sharply worsens the dynamic characteristics of the device: when the load current changes, output voltage surges occur. The transistor filter is free from this disadvantage, does not require winding products and effectively suppresses ripples by at least 40 dB.

Due to the high transfer coefficient of the composite transistor VT4, VT5 (at least 1500) and the operation mode of the VT4 transistor with a low collector-emitter voltage, the filter efficiency is very high and reduces the overall efficiency of the stabilizer by only 6 ... 8%, which is a very low price for low level of pulsations.

The transistor filter has another advantage - the "soft" turn on of the stabilizer: the output voltage gradually increases over 2 ... 4 s as the capacitor C6 charges. In sound reproducing equipment, this eliminates the characteristic unpleasant clicks when the power is turned on.

The device is made on a breadboard. It uses imported compact oxide capacitors from Samsung (C1, C5-C7), ceramic KM-6 (C2-C4), fixed resistors - MLT-0,125. Choke L1 contains 28 turns of wire PEV-2 0,56, wound on armored magnetic core B14 made of ferrite 2000NM. The non-magnetic gap in the magnetic core is provided with a 0,2 mm thick paper gasket.

OU K140UD12 will be replaced by K140UD1208. Transistors VT1, VT4 must have a low saturation voltage, a permissible collector pulse current of 400 ... 500 mA and a current transfer coefficient of at least 50. These conditions are met by transistors of the KT209 or KT501 series with letter indices D, E, K.

The current transfer coefficient of transistors VT2, VT5 must be at least 300. Subject to this condition, in addition to those indicated in the diagram, transistors of the KT361 and KT315 series with letter indices B, G, E are applicable.

Transistors VT1, VT4 at rated current do not require heat dissipation. If the stabilizer is supposed to operate at the maximum load current, the transistor VT1 should be installed on a small heat sink with an area of ​​​​10 ... 15 cm 2. It is also permissible to use medium power transistors, for example, the KT639, KT644 series, while the output current of the stabilizer can be increased to 0,5 A.

A properly assembled device starts working immediately. Its establishment comes down to setting the output voltage of 6 V with a trimmer resistor R9 at a load current of 1 mA (corresponding to a load resistance of 9,1 kOhm - when there is no load, the output voltage increases). Then, by connecting a resistor with a resistance of 91 ohms and a power of at least 1 W to the output of the stabilizer, they check and, if necessary, correct the voltage drop between the emitter and collector of the VT10 transistor within 4 ... 0,9 V by selecting the resistor R1,1. After that, they finally set output voltage resistor R6.

The stabilizer can also operate at a different output voltage (8 ... 12 V), and with its growth, the efficiency of the device increases.

The ripple level is checked with an oscilloscope connected to the output of a loaded stabilizer. If the amplitude of the ripples at the maximum load current exceeds 2 mV, the capacitor C3 is selected (in the direction of decreasing capacitance), however, avoiding the breakdown of the oscillations.

It is advisable to make the stabilizer on a common board together with a rectifier in the form of a single unit, and its design depends on the characteristics of the powered device. The rectifier is a conventional full-wave (Fig. 2), capacitors C1, C2, respectively, eliminate mains interference and a multiplicative background when powering radios.

It should be noted that in a switching power supply, the power of the network transformer T1 is 20 ... 30% less than in a continuous one. In this regard, the proposed block can be made very small and built, for example, in the battery compartment of a radio or radio. Of course, it can also be used as a separate network adapter.

Literature

1. Nechaev I. Economic stabilizer. - Radio, 1984, No. 12, p. 53.
2. Fedichkin S. Micropower voltage stabilizers. - Radio, 1988, No. 2, p. 56, 57.
3. Titze U., Shenk K. Semiconductor circuitry. - M.: Mir, 1983.

Author: A.Pakhomov, Zernograd, Rostov region

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