AN2747 datasheet(33/44 Pages) STMICROELECTRONICS

33 / 44 page

AN2747

Full bridge design criteria

33/44

Equation 10

where Rlamp can be obtained by the relation (in the hypothesis of linear value):

Equation 11

Substituting this relation in the previous relation it is possible to obtain Vlamp.The relation to

calculate the output capacitor is:

Equation 12

●

∆I= inductor current ripple = 2 A

In this case we have:

●

Ilamp =2.5 A

●

∆Ilamp = ± 5% Ilamp = ± 125 mA = 250 mA

●

Rlamp = 40 Ω

●

∆Vlamp = 10V

●

C= 625 nF. A capacitor of 680 nF 400 V was selected.

12.2

Igniter

For the design of the igniter the maximum ignition voltage must be considered. This ignition

voltage is obtained by charging a capacitor (100 nF) using a resistor. When the voltage

exceeds the gas spark gap threshold (235 V) capacitor is discharged in the primary winding

of the high voltage transformer. This voltage is transferred at the secondary winding and a

high voltage pulse is obtained, see Figure 32.

Figure 32.

Igniter circuit

lamp

∆

lamp

∆

⋅

lamp

------------------

∆

lamp

∆

⋅⋅

-----------------------------------

V > 235V

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