Low power capacitor rectifier with PWM stabilizer. Scheme, description

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Low power capacitor rectifier with PWM stabilizer. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Voltage converters, rectifiers, inverters

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The transformerless capacitor rectifier brought to your attention works with auto-stabilization of the output voltage in all possible operating modes (from idle to rated load). This was achieved due to a fundamental change in the principle of generating the output voltage - not due to the voltage drop from the current pulses of the rectified half-waves of the mains voltage on the resistance of the zener diode, as in other similar devices, but due to a change in the time of connecting the diode bridge to the storage capacitor.

The diagram of a stabilized capacitor rectifier is shown in fig. 6.12.

Low power capacitor rectifier with PWM stabilizer

Parallel to the output of the diode bridge, the transistor VT1 is connected, operating in the key mode. The base of the key transistor VT1 is connected through a threshold element (zener diode VD3) to a storage capacitor C2, separated by a direct current from the output of the bridge by a diode VD2 to prevent rapid discharge when VT1 is open. As long as the voltage on C2 is less than the stabilization voltage VD3, the rectifier works in a known way.

When the voltage on C2 increases and VD3 opens, the transistor VT1 also opens and shunts the output of the rectifier bridge. As a result, the voltage at the output of the bridge abruptly decreases to almost zero, which leads to a decrease in the voltage on C2 and the subsequent turning off of the zener diode and the switching transistor. Further, the voltage on the capacitor C2 increases again until the zener diode and transistor are turned on, etc. The process of auto-stabilization of the output voltage is very similar to the operation of a switching voltage regulator with pulse-width regulation.

Only in the proposed device, the pulse repetition rate is equal to the voltage ripple frequency at C2. To reduce losses, the key transistor VT1 must be with a high gain, for example, composite KT972A, KT829A, KT827A, etc. You can increase the output voltage of the rectifier by using a higher voltage zener diode or two low-voltage 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.

For a rectifier with a positive output voltage, a KT1A or KT972A npn transistor is connected in parallel with the VD829 diode, controlled from the rectifier output through the VD3 zener diode. When the capacitor C2 reaches a voltage corresponding to the moment the zener diode opens, the transistor VT1 also opens. As a result, the amplitude of the positive half-wave of the voltage supplied to C2 through the VD2 diode decreases almost to zero. When the voltage on C2 decreases, the transistor VT1, thanks to the zener diode, closes, which leads to an increase in the output voltage. The process is accompanied by pulse-width regulation of the pulse duration at the input VD2, therefore, the voltage across the capacitor C2 remains stable both at idle and under load.

In a rectifier with a negative output voltage, it is necessary to turn on the pn-p transistor KT1A or KT973A in parallel with the VD825 diode. The output stabilized voltage at a load with a resistance of 470 ohms is about 11 V, the ripple voltage is 0,3 ... 0,4 V.

In both proposed versions of the transformerless rectifier, the zener diode operates in a pulsed mode at a current of a few milliamps, which is in no way related to the load current of the rectifier, with a spread in the capacitance of the quenching capacitor and fluctuations in the mains voltage. Therefore, the losses in it are significantly reduced, and it does not require heat removal. The key transistor also does not require a radiator.

Resistors R1, R2 in these circuits limit 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 short circuits, the quenching capacitor C1 can charge up to the full amplitude value of the mains voltage, i.e. up to about 300 V. After a break and subsequent short circuit due to "bounce", this and the mains voltage can add up to a total of about 600 V. This is the worst case that must be taken into account to ensure reliable operation of the device.

Specific example: the maximum collector current of the KT972A transistor is 4 A, so the total resistance of the limiting resistors should be 600 V / 4 A = 150 ohms. In order to reduce losses, the resistance of the resistor R1 can be selected as 51 ohms, and the resistor R2 - 100 ohms. Their dissipation power is not less than 0,5 W. The allowable collector current of the KT827A transistor is 20 A, so resistor R2 is optional for it.

Author: Semyan A.P.

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