L6572

Application information

Doc ID 023882 Rev 1

17/22

In ideal conditions the resonant current is almost sinusoidal, having a phase term (Θ) in

respect to the voltage applied by the half-bridge. Generally, it can be written that:

Equation 2

The term "

Θ" is here expressed in radiant, but it can be expressed in terms of time.

Moreover, it is always positive as long as the operating point is above the resonance.

Equation 3

The following figure illustrates the case of operation at resonance (red line), with a small

phase, usually seen during normal operation (blue line) and finally during preheating (green

line) where a 90° phase is experienced.

Figure 8.

Operating phases

The voltage at SENSE pin has the same shape as the resonant current and is rectified and

then integrated by the current signal processor of the L6572. It can be easily demonstrated

that the integral of the rectified waveform does not depend on the initial phase (

Θ) but only

on the waveform amplitude and frequency.

Equation 4

As previously discussed, the non linear behavior of the glow discharge characteristics

makes this result quite approximate because the resonant current is not exactly sinusoidal.

Nevertheless it is more accurate during the preheating.

Therefore, suggested starting values for the application dimension are around:

I

res t

()

I

res rms

,

22

π f

sw

t

θ

–

⋅⋅

()

sin

⋅

=

I

res t

()

I

res rms

,

22

π f

sw

t2

π f

sw

⋅

–

θ

–

⋅⋅

()

sin

⋅

=

AV

SENSE

t

2I

res rms

,

R

SENSE

⋅⋅

2

π f

SW

⋅

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

≅

d

T1/2on

∫

=