DC-AC converter

AN3973

10/24

Doc ID 022164 Rev 1

3

DC-AC converter

The electronic ballast consists of a self-oscillating voltage-fed half bridge inverter, which has

been developed to drive two lamps of 28 W. One of the key components is the

STD845DN40 power bipolar transistor. It is a dual NPN high voltage power transistor and is

housed in dual-island DIP-8 package. This package solution allows to save layout area and

its driving circuit has been optimized so that the thermal performance is not too different to

that of a single transistor. The second of the key components is the possibility to use, for

bipolar transistors, a DIP-8 or SOT-82 package solution.

Figure 5.

DC-AC converter section

The DC-AC converter section has been realized according to customer indications. The

ballast coil L3/L5 (3.7 mH) and the saturable core transformer L4 (20 µH) were given by the

customer and in relation to this, all the other passive and active elements have been

selected. In particular, the design has been realized so that the lamp voltage is 180 V and

the lamp current about 0.15 A. The following design procedure is suggested for resonant

components and for proper operation of the self-oscillating inverter.

ballast coil L3/L5=3.7 mH, core saturable transformer L4=20 µH and, more specifically, the

lamp resistance (R

≈1160 Ω).

The RMS input voltage can be calculated from:

Equation 2

The value of the desired characteristic impedance is extracted:

π

=

2

V

V

BUS

RMS

_

IN