The invention relates to a method for controlling a high-frequency transformer having at least one primary and one secondary winding on a transformer core used for intermittent operation in which the transformer is alternately connected (Tn) at regular intervals to a high-frequency voltage having a constant frequency (ON interval) and again disconnected (OFF interval) (Tf) from it.
Liquid crystal (LC) screens and televisions are now in widespread use. Since liquid crystal displays do not themselves emit light, they have to be illuminated from behind by an ancillary source of light. These kinds of backlights are generally made up of a plurality of fluorescent tubes, particularly cold cathode tubes, arranged in parallel behind the LC display.
Cold cathode tubes are operated at a high-frequency AC voltage ranging between 30 kHz and 65 kHz. They are controlled, for example, using a high-frequency transformer that is supplied by a bridge circuit.
In order to regulate the brightness of the tubes and thus the brightness of the image, the arrangement is operated in an intermittent mode (burst operation). Here, a bridge circuit generates the high-frequency operating voltage as well as the pauses for the burst operation. The brightness of the tube is the result of the mean power that is transferred during an ON-OFF period.
A pulse diagram for the voltage flow V and the current flow I at the primary winding of a high-frequency transformer is illustrated in FIG. 1. A burst period comprises exactly one ON interval Tn and one OFF interval Tf. The length of the burst period thus results from the sum of Tf+Tn and is constant, i.e. the interval frequency (burst frequency) is constant. During the period Tn, the AC voltage V is applied to the primary winding of the transformer. The AC voltage V is a square wave voltage having a constant frequency, which starts, for example, with a positive half-wave 1. The magnetizing current I in the transformer does not immediately follow the voltage V but rather increases slowly until the end of the voltage half-wave. Here, the magnetizing current I increases to up to twice the normal level 7. This excessive current 8 decays with time due to ohmic losses in the transformer and the switching elements, so that several wave trains later, the current peaks are at equilibrium level 7. After the last pulse of the ON interval Tn, the current I decays to zero, where, depending on the main inductance of the transformer and the impedance at the transformer, the decay time may extend far into the subsequent OFF interval Tf.
Operation with short ON intervals Tn becomes problematic since the current I does not have enough time to reach the normal level 7. A short OFF interval also causes problems since here again the current cannot decay fully. If the current has not decayed fully before the next ON interval begins, the excessive current 8 may increase even further in successive burst periods.
Depending on the design of the transformer, this may force the transformer core into saturation, which may result in exceptionally high current peaks.
Due to magnetostriction and through magnetic forces in the transformer core, mechanical changes in the length of the core or mechanical vibrations of the core may occur. This produces broadband acoustic noises with a predominate portion in the frequency range of the burst frequency. Specifically for use in backlights for LCD screens or televisions, such noises can be highly disruptive. These two effects depend greatly on the intensity of the magnetic field or on the intensity of the magnetizing current.
In the prior art, several solutions have already been proposed for reducing these noises. One method, for example, implements a so-called “soft start”, in which the pulse length of the high-frequency voltage is slowly raised. A disadvantage of this method is the relatively long start-time until the full pulse width is achieved. Thus, particularly in the case of short ON intervals, the full pulse width may not be achieved.