AN2654 datasheet(23/26 Pages) STMICROELECTRONICS

23 / 26 page

AN2654

Resonant tank design

23/26

The input voltage Vbus at primary side is transferred with the below equation:

Equation 8

Please note that Vs is not the real secondary voltage, but the voltage present at the

magnetizing inductance Lm transferred to the secondary side of the ideal transformer, which

is frequency independent. The factor k, known as the couple factor, is a normally around

0.5 - 0.7 in this kind of application.

The inductance L, which is the series connection of Ls2 and the parallel value of the primary

inductance transferred to the secondary side, can be described as:

Equation 9

The resonant frequency shifts before and after lamp ignition. Before ignition, the infinite

impedance of the lamp makes Cb not influence the resonant frequency. Thus the resonant

frequency is determined only by Cp.

Equation 10

This circuit displays the characteristic of a parallel-load resonant converter. While in parallel-

load resonant operation, the inverter behaves like a voltage source to generate the

necessary striking voltage. Theoretically, the output voltage of the resonant circuit increases

until the lamp ionizes or overvoltage protection is triggered when the frequency keeps

approaching the resonant frequency. The minimum working frequency is set by the L6574

pin 4 which normally is around 50 kHz, and the frequency shift starting frequency is set by

pin 2 of L6574. So fig must be fixed in the shifting range of L6574.

Once the lamp has been ignited, the Cb and lamp resistance Rlamp can't be ignored any

more. By analyzing the circuit, the following transfer function can be derived:

Equation 11

Please note that

ω in Equation 4 should be calculated using MOSFET switching frequency

f-running, not the resonant frequency.

From Equation 4 the resonant frequency fop can be deduced as follows:

Equation 12

Properly setting the fop is very important to the total performance of the solution. As a rule of

thumb, the switching frequency should stay close to the resonant frequency fop to get a nice

sine-wave lamp current. At the same time, the designer must guarantee that the half-bridge

----------------------NV

Pri

kNV

BUS

s1Lm

---------------------- N

sec

–

()

prim kk

–

()

prim

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

sec

prim

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

sec

–

()

⋅

π LC

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

lamp

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

–

ωL

bRlamp

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

ωL

lamp

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

ωC

bRlamp

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

–

⎝⎠

⎜⎟

⎛⎞

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

π LC

()

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

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