DC-DC converter with reduced surge voltage and an electronic ballast including the converter for a discharge lamp

#The present invention is directed to a DC-DC converter with reduced surge voltage, and more particularly, a positive to negative DC converter, and an electronic ballast including the converter for operating the lamp or load.

#JP02-106165 and JP2002-101657 disclose a DC-DC converter having a switching element which is driven to interrupt an input DC voltage for providing a regulated DC voltage to a load. The converter is equipped with a snubber circuit for reducing a surge voltage developed across the switching element. The snubber circuit is arranged to absorb the surge energy to be applied across the switching element and returning to the input DC voltage source in an attempt to reduce the surge voltage across the switching element for protection of the switching element as well as to reuse the energy for the load.

#This scheme, however, is found to be insufficient for effectively feeding back the surge energy to the load and therefore not satisfactory for reducing the surge voltage being applied to the switching element, particularly when the converter is required to give a high output power to the load.

#FIG. 1 is a circuit diagram of a DC-DC converter in accordance with a first embodiment of the present invention;

#FIGS. 2 and 3 are waveform charts respectively illustrating the operation of the above converter;

#FIG. 4 is a partial circuit diagram showing a modified start-assisting circuitry that may be utilized in the above converter;

#Now referring to FIG. 1, there is shown a DC-DC converter in accordance with a preferred embodiment of the present invention. In the illustrated embodiment, the DC-DC converter is utilized for operating a high intensity discharge lamp such as a metal halide lamp for automobile's head lamp, although the present invention should not be interpreted to be limited thereto. The DC-DC converter converts an input DC voltage from a DC voltage source 10, for example, a car battery into a high output DC voltage that is applied to the lamp. The converter includes a transformer 20 having a primary winding 21 and a secondary winding 22. A switching element 30 is connected in series with the primary winding 21 across the DC voltage source 10 and is driven to turn on and off at a high frequency for repetitively interrupt a current from the DC voltage source for storing energy in the transformer 20. The secondary winding 22 is connected in series with a rectifier diode 24 across a smoothing capacitor 40 to release the energy for charging the smoothing capacitor 40 each time the switching element 30 is turned off, developing a rectified and smoothed DC voltage across the smoothing capacitor 40 as the output DC voltage. The output DC voltage can be regulated by varying the duty of the switching element 30. The lamp L is connected in series with an ignitor 90 across the smoothing capacitor 40 to be supplied with the DC voltage. The ignitor 90 includes a pulse transformer 91 and a driver 92 for generating a high breakdown voltage to ignite the lamp L from the output DC voltage of the converter. The winding sense of the transformer 20 is selected to generate the output DC voltage which is negative with respect to the circuit ground.

#The converter includes a snubber circuit for absorbing surge energy developed in the transformer 20 and acting to give a corresponding surge voltage across the switching element 30. The snubber circuit includes a snubber capacitor 51 connected in series with a snubber diode 52 across the switching element 30, a storage capacitor 53 connected in series with a reactor 54 across the switching element 30 in parallel with the snubber diode 52, and a replenishing diode 55 connected in series with the smoothing capacitor 40 across the storage capacitor 53.

#Further, the converter includes a start-assisting circuit composed of a capacitor 61 and a resistor 62 which are connected in series across said storage capacitor 53 for providing a starting current to the lamp L for starting the lamp immediately after the lamp is ignited, details of which will be explained hereinafter.