EVAL6520-1421 Datasheet, PDF (4/16 Pages) STMicroelectronics – Standard form factor

Board description
2
Board description
EVAL6520-1421
The board is supplied by any AC voltage in the European mains range and does not need
any power factor correction having an input power of less than 25 W.
The half bridge voltage is obtained by filtering the rectified input voltage. This allows the use
of a cheaper bulk capacitor and bipolar transistors.
The selection of a target condition is required by the range of the input voltage together with
the necessity to drive two different kinds of lamps. In particular, the best driving condition
and the best efficiency is obtained at 240 Vac with a 14 W lamp connected.
An EMI filter is placed at the board’s input to meet IEC61000 standards.
The lamp's cathodes are current preheated to make the ballast choke more compact thanks
to the absence of auxiliary windings.
The resonant network design starts from the selection of the resonant capacitor (C10) that
corresponds to the desired ballast efficiency. The inductance (L1) can be obtained by the
following equation:
Equation 1
Lamp
1
=
R _Lamp
1
R _Lamp
+ j ωC _RES
⋅
j ωL _RES
V _HB
+
1
R _Lamp
1
+ j ωC _RES
V_HB is the effective voltage obtained across the half bridge along one mains cycle. The
lower voltage is obtained at 50 Hz and can be approximately computed as:
Equation 2
V_HB
⎛
= _{⎜⎜⎝}
2
2
⋅
V_IN
−
V_F
⎞
⎟⎟⎠
+
⎛
⎜⎜⎝
2
2
⋅
V_IN
−
V_F
⎞²
⎟⎟
⎠
−
1.54 ⋅10⁵ ⋅ P_LAMP
f_MAINS ⋅ C_OUT
where:
● V_F is the forward voltage drop of the rectifier bridge (1 V typ.)
● V_IN is the RMS value of the input voltage
● C_OUT is the value of the bulk capacitor in µF (In this case C3 = 4.7 µF has been
selected)
A resonant capacitor equal to 3.9 nF has been selected and resonant inductor equal to 3
mH has been calculated.
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Doc ID 018537 Rev 2