Variants of electronic ballast, implemented according to the scheme of a single-cycle self-oscillating converter. Scheme, description

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Electronic ballasts. Variants of electronic ballast, implemented according to the scheme of a single-cycle self-oscillating converter. Encyclopedia of radio electronics and electrical engineering

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As you know, LL has different ignition voltage and combustion voltage. The first - with cold filaments - is approximately 1000-1200 V. We will consider it equal to 1200 V (to the maximum), common to all lamps. When the discharge in the lamp is ignited, the voltage on it drops (up to 55-60 V for LL with a power of up to 20 W and 100-125 V for LL with a power of 30-40 W; this is the burning voltage). In this case, the current can increase almost to the destruction of the lamp. Therefore, it must be limited.

Variants of electronic ballast implemented according to the scheme of a single-cycle self-oscillating converter
Rice. 3.76. Single-cycle converter circuits

In converters, the circuits of which are shown in fig. 3.76, the output voltage at the front of the output pulse is used quite successfully, when the voltage rises to U_з - LL ignites, then the voltage drops to U_г - the lamp burns while the energy accumulated in the transformer is transferred to it (Fig. 3.77).

Variants of electronic ballast implemented according to the scheme of a single-cycle self-oscillating converter
Rice. 3.77. Output voltage waveform

The winding of the transformer is calculated for the working (U_г) voltage. When choosing a transformer, we take into account that for the ignition voltage (U_з) you should set the voltage value per winding turn no more than 10-15 V / vit. (at 20-24 V / vit., unstable ignition is observed).

Calculation of the transformer. Given: battery voltage U \u12,6d XNUMX V. Required: ignite LL with power P_л = 20 W. For her U_з = 1200 V, U_г \u55d 3.76 V. The number of turns of the winding III (Fig. XNUMX, a) - n_з:

Let's take 110 turns. Then for U_г will be

The voltage in the primary (collector) winding will be

The number of turns in winding II (base) will be

You can take 5 ... 7 turns. The diameter d of the wire is calculated as usual:

(this, of course, is very rough - the current is pulsed, but it is suitable for evaluation).

The operating current of LL is calculated by the formula

therefore winding wire diameter III

We take the wire of the nearest standard diameter.

Transformation ratio for winding I:

The current in the winding I is

The diameter of the winding wire I is calculated by the formula

We take a wire with a diameter of 1 mm.

Winding wire diameter II d₂ can be taken equal to 0,42 mm. A ferrite core for lamps up to 30 W can be used from TDKS-19 (S = 1,1 cm2), or use a K40x25x11 ring (S = 0,825 cm2). The ring is carefully broken; windings I and II are wound on one half of it, one above the other, with insulation between them, for example, with electrical tape.

Attention! Windings can not be spread! Winding III is wound on the other half of the ring, turn to turn, without overlaps, with insulation between the layers (you can use electrical tape).

The ring is glued with epoxy resin. A notebook sheet is used as a material for magnetic gaps. If the LL is of lower power, then the ferrite can also be taken with a smaller cross section.

For LL at 30 and 40 W, you can use a ferrite core from TDKS-9-1 (S = 2,07 cm2):

The oscillator transistor is selected from the following conditions:

Better still take u_{ke max} more than 300 V. Well, if you still choose a transistor and the minimum U_HAC. You can put KT854A or KP955A in the circuit. If this is not the case, then by increasing the number of turns of the winding III, it is possible to lower the ike to an acceptable one for the existing transistor.

By power loss

the radiator for the transistor is calculated.

In the circuit (Fig. 3.74) R2 sets P_O. If you add a resistor and switch S1 (as in Fig. 3.76, b), you get two modes - P_MAX and P_MIN.

Between the collector and emitter VT1, it is imperative to put an RF diode (KD226G, KD226D), since internal protective diodes are not provided in domestic transistors.

Author: Koryakin-Chernyak S.L.

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