Modern electronic ballast on the IR2520 chip. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Electronic ballasts. Modern electronic ballast on the IR2520 chip. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Ballasts for fluorescent lamps

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At the moment, a relatively inexpensive specialized chip IR2520D. Having only eight outputs, it not only maintains the current and voltage on the lamp within the specified limits during heating, ignition and in operation, but also has a number of protective functions. The electronic ballast circuit using the IR2520D is shown in fig. 3.28.

This circuit was successfully designed using the latest version of the Ballast Designer program, and used to replace a failed 26 W CFL electronic ballast. The internal structural block diagram can be found by referring to the proprietary datasheet.

Modern electronic ballast on the IR2520 chip

The diode bridge VD1 rectifies the AC mains voltage. Capacitor C2 - smoothing. The primary inrush of the charging current of the capacitor C2 limits the resistor R1, and the impulse noise weakens the filter L1C1.

Immediately after switching on, the charging of the capacitor C4 begins with the current flowing through the resistors R2 and R4. As soon as the voltage on this capacitor and between terminals 1 and 2 of the DA1 chip reaches 12,6 V, the microcircuit will begin to generate pulses that control the field-effect transistors VT1 and VT2. The charging of capacitor C4 will continue until the voltage across it reaches 15,6 V - the stabilization voltage of the zener diode built into the microcircuit. Since the resistors R2 and R4 provide only enough current to start the microcircuit, in operating mode it is powered by an output voltage rectifier on diodes VD2, VD3 and capacitor C5.

The frequency of the generated pulses depends on the resistance of the resistor R3 and on the voltage at pin 4 of the microcircuit. Immediately after switching on, this voltage is zero (capacitor C3 is discharged), the frequency is maximum and equal to 118,5 kHz (point 1 in Fig. 3.29). The resonant frequency of the L2C7 circuit is much lower (65,3 kHz), so the amplitude of the alternating voltage on the EL1 lamp that is not yet on is small. A high frequency current flows through its filaments, heating them up.

As the capacitor C3 is charged with current, the source of which is the microcircuit itself, the frequency of the generated pulses decreases (section 1-2 on the graph, Fig. 3.29), the lamp voltage and its filament current increase. After about 1 s, when the voltage across the capacitor C3 reaches 4,8 V, the frequency will become 75,5 kHz, and the lamp voltage will be 450 V. This voltage is sufficient to ignite, as a result, a gas discharge will occur in the lamp and it will flare up.

Modern electronic ballast on the IR2520 chip
Rice. 3.29. Graph explaining how the circuit works

Since the lamp burning voltage is much lower than its breakdown voltage, the operating point on the graph (Fig. 3.29) will jump from point 2 (corresponds to an extinguished lamp and a high quality factor of the L2C7 oscillatory circuit) to point 2G (the lamp is on, the quality factor of the circuit shunted by its discharge gap sharply decreased). The charging of capacitor C3 will continue until the voltage at pin 4 of the microcircuit reaches 6 V, which corresponds to the frequency of the voltage applied to the lamp at 47,4 kHz. This is the nominal lamp burning mode (point 3 on the graph, Fig. 3.29).

The control unit built into the R2520D chip measures the voltage drop across the resistance of its open drain-source channel, proportional to the current flowing through the field-effect transistor VT2. If the transistor opens when the instantaneous value of the load current is zero, the voltage at pin 4 of the microcircuit and the oscillation frequency dependent on it remain unchanged. But as a result of aging of the elements or for other reasons, the resonant frequency of the load may change. The consequence of this will be a non-zero value of the current flowing through the transistor VT2 at the first moment after it is opened.

Having discovered this, the control unit of the microcircuit will begin to reduce the voltage at pin 4, thereby increasing the oscillation frequency. If it is not enough to reduce the voltage at pin 4 even to 0,85 V to reach zero (this can happen if the contact in the lamp holder is broken or its filament burns out), the microcircuit will go into emergency mode by closing transistors VT1 and VT2, discharging capacitor C3 and reducing the current consumption to 100 µA. In order to exit this mode, you will have to reduce the supply voltage (between pins 1 and 2 of the microcircuit) to a value less than 10 V, and then raise it again above 12,6 V.

If, upon reaching point 2 (see Fig. 3.29), the lamp did not ignite due to its malfunction or absence, the oscillation frequency will continue to decrease, the voltage on the capacitor C7 will exceed the permissible value, and it may be broken. It is also possible to saturate the magnetic circuit of the inductor L2.

It has been established that under such conditions, the crest factor (the ratio of the amplitude value to the average) of the current flowing through the open transistor VT2 increases. Using the open channel resistance of this transistor as a current sensor, the microcircuit control unit measures the crest factor. If it is averaged over 10-20 oscillation periods, the value is more than five, the microcircuit will go into the emergency mode described earlier.

Among other features of the R2520D microcircuit, it should be noted the presence of a "bootstrap" field-effect transistor, and not a diode between pins 8 and 1. The signal generated inside the microcircuit opens and closes this transistor. This provides a high switching speed and low energy losses on the resistance of the open channel of the transistor.

In the newly manufactured electronic ballast, a choke from a faulty electronic ballast KLL was used as L2, the inductance of which was measured and found to be equal to 2,5 mH. In order to reduce it to the required 1,8 mH, it was necessary to increase the non-magnetic gap in the inductor magnetic circuit. For the correct calculation of the choke and other elements when using various CFLs, the latest available version of the automatic design program Ballast Designer should be used.

As it turned out, the frame with the winding was fixed on the magnetic circuit with electrical insulating varnish. To soften the varnish, the choke was placed for about half an hour with the leads down on the bottom of a closed vessel, into which acetone was poured with a layer of 3-4 mm deep. After that, careful rocking managed to loosen the previously strong connections. Then, without any heating, the two halves of the magnetic circuit were removed from the frame with the winding, for this it was only necessary to remove the adhesive tape that fastened them.

The length of the air gap on the central rod of the magnetic circuit was 1 mm. In order to reduce the inductance of the inductor without rewinding, it was necessary to insert gaskets made of non-magnetic material 10,25 mm thick into the joints of the side rods of the halves of the magnetic circuit. The inductance of the inductor measured after assembly is 1,78 mH. As the tests and subsequent operation of the electronic ballast proved, the conversion was successful.

In the absence of an inductance meter, you can use a suitable generator and a voltmeter (or oscilloscope) to check the resonant frequency of the L2C7 circuit. It should be close to 65 kHz.

All elements of the device are mounted on a single-sided printed circuit board, shown in fig. 3.30.

For the DA1 chip, an 18-pin panel can be provided on the board. The leads of the oxide capacitor C2 are not cut off, but are insulated with a polyvinyl chloride tube for the entire length and their ends are soldered into the board. This capacitor is installed so that, relying on the transistor VT1 and the inductor L2, it rises above the board, and when assembling the lamp, it enters its hollow base.

Inductor L1 - a "dumbbell" magnetic circuit with an outer diameter of 7-10 mm, filled with PEV-2 wire with a diameter of 0,21 mm. It is insulated with heat shrink tubing. Diode bridge VD1 in surface mount version is installed on the side of printed circuit conductors of the board. It can be replaced with a conventional one in a DP package or with separate diodes with a reverse voltage of at least 400 vis with a forward current of 1 A. But for this, the printed circuit board will need to be redone.

Modern electronic ballast on the IR2520 chip
Rice. 3.30. PCB

Resistor R1 - KNP-50. Capacitors C1 and C8 - K73-17 for a voltage of 630 V, C4 - TDC (tantalum with radial leads), C5 and C7 - imported ceramic disk with a diameter of 7 mm with an operating voltage of 2 kV. There are no special requirements for other resistors and capacitors. The transistors are installed without heat sinks.

Council. After mounting the elements, it is recommended to cover the board with several layers of electrical insulating varnish.

By turning on the electronic ballast with the lamp and making sure that it works, you can determine the power consumed by the lamp. To do this, it will be necessary to temporarily connect a current-measuring resistor with a resistance of 1 ohm in series in the lamp circuit. If the power does not correspond to the nominal, it can be changed by selecting the resistor R3. With an increase in its resistance, the frequency of the voltage applied to the lamp decreases, and the power increases.

Author: Kosenko S.I.

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Comments on the article:

Basil
The first inclusion with another at 1.23mH - did the field workers burn out? lamp TLD-18. The second inclusion with others at 1.79mH - silence, lamp TLD-30.

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