1. Field of the Invention
2. Description of the Related Art
High intensity discharge (HID) lamps show superior performance over conventional halogen lamps, such as energy saving, long life, high luminous efficacy, and good color rendering. So, HID lamps will be the preferred lighting source in the near future. Unlike incandescent lamp, HID lamps do not contain filament and hence have longer life. On the other hand, the characteristics of an HID lamp are complex compared to that of a halogen lamp.
FIG. 1 shows the block diagram of conventional microprocessor-based HID ballast. The HID ballast circuit consists of DC/DC converter 101, voltage doubler 102, DC/AC inverter 103, the high voltage igniter 104, and controller 105 including microprocessor. The DC-DC converter 101, as the battery source in automotive systems use mostly 12 V, the DC/DC converter 101 should boost the battery voltage from 9-16V up to 300 V (dc-link voltage). For the voltage doubler 102, in order to step up the dc link voltage of 300V to about 600V required for the spark gap in the igniter assembly, the dc-dc converter 101 along with a voltage doubler 102 boost the dc-link voltage to 600V. For example, as taught in Lee and Cho (“Design and analysis of automotive high intensity discharge lamp ballast using micro-controller unit” (IEEE transactions on Power Electronics)), the voltage doubler 102 is formed by the additional winding on the secondary of a DC-DC converter transformer 101.
Regarding the use of the voltage doubler 102, as the breakdown voltage of the spark-gap used in the igniter assembly is about 600 V, the dc link voltage has to be of the same value. In order to reduce the dc link voltage, the circuit in Lee and Cho, uses a transformer with two secondaries with equal number of turns. The general schematic of the arrangement is shown in FIG. 3. The voltages across the two secondary windings are summed and applied to the spark gap. As a result the dc link voltage is reduced to 300 V. The limitation of this design is that two secondary windings are required resulting in larger size for the transformer.
The present invention is related to further reducing dc link voltage to 200 V from 300 V as in FIG. 1 and to eliminate the additional winding on the dc-dc converter transformer.
FIG. 2 shows an igniter assembly of the conventional HID ballast. The igniter is a high voltage transformer which converts the low voltage from the dc converter into a high-voltage pulse of 10-22 kV magnitude for initiating an arc across the lamp electrodes. The voltage from the voltage doubler is applied to the primary windings of the igniter through a capacitor and a spark gap arrangement. The spark gap conducts when the voltage across the capacitor is equal to the breakdown voltage (about 600 V) of the spark gap. The capacitor discharges its energy through the spark-gap into the primary when the spark gap conducts to create a pulse in the primary winding. This discharge pulse in the primary is amplified by the secondary winding and applied across the lamp.
The inverter of the conventional ballast is used to provide an alternating square pulses to the lamp upon ignition. The inverter has no active role in the control of power to the lamp.
The most important and complex part of the conventional ballast is the controller which controls and dictates the mode of operation depending on the stages from the start-up to steady state. The operating mode of the controller changes from constant current mode at start-up to constant power mode after the lamp voltage begins to increase. In order to satisfy the standards, the lamp is driven at twice the rated power at start-up but with the maximum load current limited to around 2.5 A.
Unlike HID lamps for other applications, the HID lamps for automobiles must develop over 70% of the light output within 2 seconds of being turned on under cold and warm start conditions. The controller of an electronic ballast ensures that the lamp receives appropriate power under all conditions. The parameter that affects the characteristics of HID lamps is the lamp temperature which influences the power required by the lamp for successful operation. As the power demanded by the lamp at different stages of operation is dependent on the lamp temperature it is vital to use the lamp temperature as one of the feedback signals to calculate the reference power signal with which to drive the lamp.
In Fiorello's “Powering a 35 W DC metal halide high intensity discharge (HID) lamp using the UCC3305 HID lamp controller” (Unitrade Corporation), a method is proposed to monitor the temperature of the lamp in which a capacitor is charged by a variable current source and the voltage of the capacitor is measured to reflect the temperature of the lamp. The lamp temperature is estimated based on the voltage of this capacitor. The current source charges the capacitor at different rates till the lamp achieves a steady state and the voltage of this capacitor is clamped at a certain value to indicate the steady state operation of the lamp. The design of the current source is complex and expensive besides it requires careful tuning of the magnitude to control the rate of charging of the capacitor.