AN2870 datasheet(33/41 Pages) STMICROELECTRONICS

33 / 41 page

AN2870

Designing with L6585DE

33/41

The ignition time is equal to the time the capacitor Cign is charged by the RF current and the

EOI leakage current. A precise calculation of this parameter is not needed. It’s

approximately equal to:

Equation 44

Preheating time and protection time are related to Tch cycle:

Equation 45

Equation 46

Half bridge design

When resonant network works in inductive region, half bridge MOSFETs switch at zero

voltage switching condition.

This implies that high side and low side MOSFETs experience mainly conduction losses

because of their on state resistance.

Equation 47

Irms can be considered as per Eq 38 or can be calculated considering that the waveform

seen during an on time is a sinusoid having a frequency equal to resonance frequency.

From thermal consideration the maximum Rds,on can be calculated.

In order to drive correctly the high side MOSFET, a suitable boostrap capacitor is needed.

The size of the boostrap capacitor can be calculated considering the allowed Vgs drop, the

total gate capacitance of the High side MOSFET and the Ron of the integrated boostrap

diode.

In steady state, during the ON time of the low side transistor the bootstrap capacitor stores

charges:

Equation 48

During the ON time of the high side transistor, the charges stored in bootstrap capacitor are

shared with total gate capacitance, causing a voltage drop:

ign

pre

ign

⎟

⎠

⎞

⎜

⎝

⎛

TCH

pre

prot

269740

⋅

( )2

rms

cond

⋅

boot

⋅

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