Switching regulator capacitor power supply

Encyclopedia of radio electronics and electrical engineering / Surge Protectors

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The author of the published article shares his experience of using switching output voltage stabilizers in transformerless power supplies with a ballast capacitor.

One of the most serious disadvantages of transformerless power supplies with a quenching capacitor, for example, described in [1, 2], is that they cannot be connected to the network without load or with insufficient power load. Eliminate it by turning on the stabilizer on the zener diode parallel to the output of the rectifier bridge [3]. But at the same time, the zener diode itself can consume a current, the value of which is commensurate with the load current, if we take into account the influence of the spread in the capacitance of the quenching capacitor, the stabilization voltage of the zener diode, and fluctuations in the direction of increasing the mains voltage. Significant power is dissipated on the zener diode, so it has to be placed on a heat sink [2].

The main idea of ​​improving a transformerless power supply with a quenching capacitor is to introduce a pulse control element into it, for example, as done in [4], to reduce the power dissipated by the stabilizer.

In the proposed stabilized power supply with adjustable output voltage (see diagram), an analog of an uncontrolled four-layer diode (dinistor) [1], made on a complementary pair of transistors KT5A, KT502A, is connected in parallel to the output of the diode bridge VD503. To ensure a stable threshold for turning on the analog of the dinistor, a zener diode VD2 is connected in series with the emitter junction of the transistor VT1.

With an increase in the output voltage of the diode bridge, the capacitor C2 begins to charge. When the voltage reaches a certain value, depending on the position of the variable resistor R6 slider, the zener diode VD2 turns on and the transistor VT1 opens first, and then VT2. Due to the deep positive feedback, the transistors open like an avalanche and shunt the output of the bridge, which leads to an abrupt decrease in the voltage across it to almost zero. Diode VD3 closes, and capacitor C2 feeds the load. When the voltage at the output of the bridge drops to zero, the transistor analog of the dinistor turns off, charging of the capacitor C2 begins. The process is repeated. The total saturation voltage between the emitters of the transistors (voltage drop across the dinistor analog) is about 0,7 V.

Depending on the load resistance, the dinistor analog is turned on at different moments of the half-cycles of the mains voltage. In idle mode, the output of the diode bridge is short pulses, following with the highest duty cycle. When the load is connected, the duty cycle decreases: the open time of the transistors decreases, which leads to an increase in the duration of the voltage pulse supplied through the isolation diode VD3 to the capacitor C2.

The process of voltage stabilization is very similar to the operation of a pulse-width regulated voltage regulator known to radio amateurs. The pulse repetition rate is equal to the ripple frequency on the capacitor C2. The isolation diode VD3 prevents the discharge of the capacitor C2 through open transistors.

The amplitude of the current pulse through the zener diode VD2 does not exceed 0,5 mA in all modes of operation, which indicates the efficiency of the stabilizer with a transistor analogue of the dinistor according to the control signal. For comparison: if you use a pulse element - a trinistor, then the devices of the KU201, KU202 series require a turn-on current amplitude of up to 100 mA.

In addition, the use of a parallel stabilizer allows you to smoothly adjust the output stabilized voltage at the load, for example, with a resistance of 1 kOhm in the range from 4,7 to 46 V. At idle - from 4,84 to 46,06 V, respectively. and at idle is about one percent. This is sufficient for almost all cases.

If the output voltage adjustment is not required (a fixed value is required), the resistors R5 and R6 are removed, and the anode of the zener diode is connected to the emitter of the transistor VT2. Such a power supply with a D814G zener diode provides a fixed stabilized voltage of 9,94 V at a load with a resistance of 180 ohms. At idle, the output voltage is 10,09 V. When using the Zener diode D814A, Uout \u7,67d 7,8 V at the same load, and at idle - XNUMX V. As you can see, the difference between the voltages at the load and at idle is about one percent in this case.

You can increase the output voltage of the rectifier by using a higher-voltage zener diode in it or two low-voltage ones connected in series. With two zener diodes D814V and D814D and a capacitance of capacitor C1 of 2 μF, the output voltage at a load with a resistance of 250 ohms can be 23 ... 24 V.

The given examples illustrate the possibility to experimentally select the elements of a transformerless rectifier for the required stabilized output voltage at a given load.

When a common wire is required between the output of the stabilized rectifier and the network, the known half-wave diode-capacitor rectifier can be used. To do this, exclude the diode bridge VD1, connect the resistor R2 in series with the ballast capacitor C1, connect the lower (according to the diagram) network wire to the "negative" output and connect the rectifier diode with the anode to the emitter of the transistor VT2 between the emitters of the transistors.

Resistor R2 limits the input current during transients at the moment the device is connected to the network. Due to the inevitable "bounce" of the contacts of the mains plug and socket, the switching process is accompanied by a series of short circuits and circuit breaks.

With one of these phenomena, the quenching capacitor C1 can charge up to the full amplitude value of the mains voltage, i.e., up to about 300 V. After breaking and then closing the circuit, the voltage on the capacitor and the mains can add up and total about 600 V. This is the worst case that must be taken into account to ensure reliable operation of the device.

Therefore, in devices proposed to improve reliability, it is better to use more powerful complementary pairs of transistors, for example, KT814A and KT815A; KT816A and KT817A; KT837A and KT805A; KT973A and KT972A; 2T505A and 2T504A, etc.

The device is galvanically connected to the network. This should be remembered and care should be taken when designing and adjusting it.

Literature

1. Dorofeev M. Transformerless with a quenching capacitor. - Radio, 1995, No. 1, p. 41, 42; No. 2, p. 36, 37.
2. Khukhtikov N. Charger. - Radio, 1993, No. 5, p. 37.
3. Biryukov S. Calculation of a network power supply with a quenching capacitor. - Radio, 1997, No. 5, p. 48-50.
4. Alekseev S. Symmetrical dinistors - in power supplies. - Radio, 1998, No. 10, p. 70, 71.
5. Voitsekhovsky Ya. Radio-electronic toys. - M.: Soviet radio, 1970, p. 40.

Author: N.Tsesaruk, Tula

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