Electronic ballast built on the principle of a self-excited half-bridge inverter. Scheme, description

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Electronic ballasts on discrete elements. Electronic ballast built on the principle of a self-excited half-bridge inverter. Encyclopedia of radio electronics and electrical engineering

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A schematic diagram of a variant of an electronic ballast built on the principle of a self-excited half-bridge inverter is shown in fig. 3.37.

Electronic ballast built on the principle of a self-excited half-bridge inverter
Rice. 3.37. Schematic diagram of a self-excited half-bridge inverter on MJE13003

As can be seen from the diagram, the winding I of the transformer T1 is included in the diagonal of the half-bridge formed by two series-connected power BMTs VT1 and VT2. A current-limiting inductor L2 is connected in series with winding I, which forms a resonant circuit with capacitor C5. The resonant frequency of the circuit is determined by the formula known to us

At the moment the voltage is applied to the converter and after it is started, a resonance is excited in the L2, C5, EL1 circuit, the pulse voltage value of which is about 250-300 V (depending on the lamp), which is quite enough to ignite it.

After ignition, the current that passes through the lamp sharply reduces the quality factor of the circuit, shunting C5. The converter operates at a high frequency, and the inductive reactance of the inductor L2 limits the lamp current.

Of features of the converter we can note the autorun node on the VS1 symmetrical dynistor and the current control of the switching of power transistors.

The autostart circuit is necessary because the current feedback generator will not start itself.

After the power is turned on, the capacitor C3 is charged through the resistors R2, R3. When the voltage at C3 reaches 30 V, the symmetric dinistor VS1 breaks through, and the discharge pulse of the capacitor C3 opens the transistor VT2, as a result of which the generator starts. With the help of the diode VD5, during the operation of the generator, C3 is maintained in a discharged state.

The opening of VT2 and the start of the generator leads to the fact that an EMF is induced in the windings of the transformer T1, the polarity of which is determined by the direction of their winding. The polarity of the EMF in the basic feedback windings I and II are opposite. Therefore, the opening and closing of power transistors occurs alternately and the moment of saturation of the core of the transformer T1.

When the current transformer saturates, current continues to flow through the previously open transistor. This current is the magnetizing current of the winding of the current transformer, and while it flows, the voltages on all its windings are equal to zero. The process of resorption in the transistor begins, but current still flows through it. As a result, until the resorption process is over, current flows through the transformer winding and maintains zero voltage on its windings.

When the resorption process is over, the transistor will begin to close. But now it takes time to get out of saturation of the transformer core. It's not big, but it's there. During this time, the open transistor will almost close. And when the current transformer comes out of saturation, only then the voltages on the transformer windings can reappear, but with a different polarity, causing a forced closing of the open transistor and opening the closed one. And the closed transistor still has a turn-on delay time ...

As a result, in inverters with self-excitation, and even with current feedback, there is practically no through current. Of course, subject to the correct calculation of the current transformer. If the calculation is incorrect, the through current, although there is, is not dangerous, it manifests itself in the form of a current surge when the transistor is turned on and causes only additional dynamic losses.

The higher the switching speed of the transistor, the lower the dynamic losses and heating of the transistor, while maintaining order when switching - the next one will open only when the previous one closes.

Elements C1, R1 and L1 prevent the propagation of radio interference through the electrical network that occurs during the operation of the generator. Resistor R1 also limits the initial current pulse that occurs when the electrolytic capacitor C2 is charged.

You should not be surprised at the spread of the values of the elements indicated in the diagram - it really exists for lamps of different power and different manufacturers, of course, taking into account the fact that paired elements (for example, resistors R2 and R3) have the same values.

The same applies to diodes with transistors - the diagram shows only the most common types. Inductor L2 is assembled on a miniature W-shaped ferrite magnetic core with outer dimensions of 10-15 mm, with a small gap. Its winding contains 240-350 turns of winding wire with a diameter of 0,2 mm.

Transformer T1 is made on a ring ferrite magnetic circuit with an outer diameter of 8-10 mm and a height of 3-5 mm:

primary winding (I) contains 6-10 turns;
windings II and III - 2-3 turns each, and the wire can be either a winding diameter of 0,3-0,4 mm or an ordinary mounting one.

Inductor L1 - one and a half to two dozen turns of winding wire with a diameter of 0,5 mm, wound on a small ferrite rod. The operating frequency of the generator is determined mainly by the parameters of the transformer T1 and at rated load is 40-60 kHz.

Author: Koryakin-Chernyak S.L.

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