Electronic ballasts. Single ended converters. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Electronic ballasts. Single ended converters. Encyclopedia of radio electronics and electrical engineering

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

Quite often, to implement electronic ballasts operating from low-voltage power supplies, they use single-ended boost converters. Virtue of these converters is the low cost of implementation. Among the various circuits, the single-cycle self-oscillating converter is distinguished by its utmost simplicity, the circuit of which is shown in Fig. 3.71.

Electronic ballasts. Single ended converters
Rice. 3.71. Scheme of a single-ended converter

Consider briefly the principle of its work. Transformer T1 - linear choke; the intervals of energy accumulation in it and the transfer of accumulated energy to the load are separated in time. On fig. 3.72 shows timing diagrams explaining the operation of the converter.

When connecting the supply voltage U_Pete through the resistor R1, the base current of the transistor VT1 begins to flow.

The diode VD1 prevents the passage of current through the circuit of the base winding, and the capacitor C2 shunting it increases the positive feedback (PIC) at the stage of the formation of voltage fronts. The transistor opens slightly, the POS circuit closes through the transformer T1, in which the regenerative process of energy accumulation takes place. Transistor VT1 enters saturation. The supply voltage is applied to the primary winding of the transformer, and the current I_I (collector current I_к transistor VT1) increases linearly.

Electronic ballasts. Single ended converters
Rice. 3.72. The shape of the current in the transformer windings:

I_I- current of the primary winding of the transformer; I_II- current of the secondary winding; t_н - interval of energy accumulation in the throttle; t_п - interval of energy transfer to the load

The base current 1B of a saturated transistor is determined by the voltage on the winding III and the resistance of the resistor R2. When collector current I_к reaches the value

where h21_э - static current transfer coefficient of the transistor VT1, the transistor exits the saturation mode.

A reverse regenerative process develops: the transistor closes, and the energy stored by the transformer is transferred to the load. After reducing the current of the secondary winding, the stage of energy accumulation begins again. Time interval t_п maximum when the converter is turned on, when the capacitor C3 is discharged, and the voltage on the load is zero.

The circuit under consideration is a functional converter of the supply voltage U_Pete into the load current source tn.

It is important to note, since the stages of energy accumulation and its transfer are separated in time, the maximum collector current of the transistor does not depend on the load current, i.e. the converter is completely protected from short circuits at the output. However, when the converter is turned on without load (idle mode), a voltage surge on the transformer winding at the moment the transistor closes can exceed the maximum allowable collector-emitter voltage and disable the transistor.

Lack of the simplest converter - the dependence of the collector current I_{to max}, and hence the output voltage from the static current transfer coefficient of the transistor VT1. Therefore, the parameters of the power supply will differ significantly when using different instances of transistors.

Author: Koryakin-Chernyak S.L.

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pasha
Can anyone tell me what the approximate capacity of C2 should be, if R1=100k, u=250V DC, R2=1 ohm, you need to get about 200 watts on the secondary.

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