Improvement of the high-frequency power supply of a fluorescent lamp. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Lighting

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For more than four years I have been using fluorescent lamps with self-made high-frequency power supplies (electronic ballasts) based on International Rectifier microcircuits. I want to share my experience with radio amateurs. A typical circuit and design of a ballast are described in sufficient detail in the article by A. Tarazov "High-frequency power supply for a fluorescent lamp" ("Radio", 2003, No. 5, p. 42), I will only talk about some of the features that the author of the article did not give, in my opinion enough attention.

At the moment the unit is connected to the network in the L2C6 circuit (see Fig. 1 in the mentioned article), oscillations occur, the amplitude of which as a result of resonance can reach 1000 V. A cold discharge occurs in the fluorescent lamp, due to ion bombardment, its filaments are heated and the lamp switches to normal operating mode. The ignition of the lamp in this case occurs instantly, but its service life is significantly (several times) reduced. Therefore, it is advisable to use such a "cold" start only where it is important to turn on the light without delay.

To eliminate the "cold" start and its consequences, I recommend, as shown in fig. 1, connect a posistor (RTS thermistor) RK6 in parallel with the capacitor C1. The resistance of the posistor at the time of power-up is small, it shunts the capacitor, sharply reducing the quality factor of the L2C6 circuit and not allowing the voltage between the lamp electrodes to reach a value sufficient to cause a cold discharge. The current flowing through the inductor L2 heats up the filaments of the EL1 lamp and the posistor. The resistance of the latter with heating increases significantly, the voltage between the electrodes of the lamp increases. After 1 ... 2 s, it lights up, gently switching to the operating mode.

Posistors with the necessary parameters in an amount sufficient for eight electronic ballasts can be made from the widely used ST15-2-220 thesistor (Fig. 2) from the demagnetization system of the ZUSCT TV. After disassembling the plastic case, two "tablets" are removed. With a diamond file, two notches are made crosswise on each, as shown in fig. 3, and break it into four pieces along the cuts.

It is very difficult to solder leads to the metallized surfaces of a posistor made in this way. Therefore, as shown in Fig. 4, I make a rectangular hole in the printed circuit board 3 and clamp the fragment of the "tablet" 1 between the elastic contacts 2 soldered to the printed conductors. By selecting the size of the fragment, you can achieve the desired duration of the lamp warm-up.

Capacitor C6 must be designed for a voltage of at least 1000 V. The winding of the inductor L2 must have good interlayer insulation and be reliably isolated from the magnetic circuit. Since a voltage with a frequency of 5 ... 30 kHz is applied to the VD40 diode, it is better to replace the low-frequency 1N4007 with a KD258D, BW95C or another high-frequency rectifier diode. Capacitor C7 can be installed ceramic or film with a capacity of 0,1. .0,33 uF. Such a capacity is quite enough, but the reliability of the unit will increase markedly. The IR2153 chip, if necessary, without any alterations, can be replaced with the already obsolete IR2151 or IR2152.

I cannot agree with the recommendation to use IRF840 field effect transistors in a high-frequency power supply. At one time, I myself, in an attempt to increase the reliability of the block, made this mistake. Later it turned out that the main reason for overheating and failure of transistors in such blocks is not at all an increased voltage drop on the channel of an open transistor (a small current flows through it), but dynamic energy losses for recharging a relatively large output capacitance of the transistor. This effect is masked by the fact that with the correct setting of the L2C6 circuit, the reactive component of its resistance partially compensates for the capacitive part of the output resistance of the transistors. However, violation of compensation when the lamp fails or as a result of an open circuit in its circuit almost inevitably leads to overheating of the transistors. Replacing the IRF840 transistors with less powerful, but faster IRF710 transistors, which have almost an order of magnitude smaller internal capacitances, significantly improves reliability.

A few words about the establishment of the block. The equality of the frequency of the master oscillator of the DA1 microcircuit to the resonant frequency of the L2C6 circuit is most easily achieved not by changing the gap in the magnetic circuit of the inductor L2, but by selecting the frequency-setting resistor R1. To do this, it is convenient to temporarily replace it with a pair of series-connected resistors: constant (10 ... 12 kOhm) and trimmer (4,7 ... 10 kOhm). The criterion for the correct setting is reliable start-up and stable burning of the lamp.

With widely used fluorescent lamps with a length of 600 mm and a power of 18 ... 20 W, I usually use chokes with an inductance of 1,9 mH and capacitors K78-2 0,01 uF at 1000 V. The optimal operating frequency is approximately 36 kHz.

Author: V. Chulkov, Moscow

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