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AN1902 - APPLICATION NOTE

3.2.2 PRE-HEATING FREQUENCY

For the pre-heating frequency calculation the following considerations can be done:

tdV/dt is the duration of the snubber capacitor (C14) charge during the half-bridge mid-point commutation

It can be calculated using the following relationship:

where:

s

∆V (DC bus voltage);

s

s

It can be calculated according to the following formula:

Where K=6.7µsec/nF (fixed by the VK06)

3.3 STEADY STATE

The steady state frequency is set by the parallel of the capacitors C6-C16 for the low side and C10-C15

for the high side, where C16=C15. The same formulae of the preheat can be applied:

3.4 IGNITION

The converter of figure 9 has two different resonance frequencies, the first one before the tube ignition

the second one after the tube ignition. The converter operation from preheat to steady state is shown in

figure 12.

f

1

T

--

=

1

2

---T

t

ON

t

storage

t

dVdt

++

=

t

storage

con st

300 n

≈

=

sec

(storage duration of the device Power stage)

t

ON

KC

6

C

16

+

()

=

f

1

T

--

=

1

2

---T

t

ON

t

storage

t

dVd t

++

=

t

s tor age

const

300n

≈

=

sec