Converters for pulsing UV-diodes
2014 International Conference on Optimization of Electrical and Electronic Equipment (OPTIM)
The progress in light emitting diodes (LEDs), especially in producing ultraviolet light, leads to new applications in medical science and in the area of disinfection. Biological investigations show a greater effectiveness when the ultraviolet light is pulsed. Therefore, converters which enable a fast turn-on and turn-off of the light emitting diodes are necessary. Several concepts to improve the possibility to supply switched loads are shown, and the equations for linear and two-level control
... e derived from the model equations. Dimensioning hints are given. When the converter has an output capacitor, the mean value of the inductor current is the same as the current in the load. The capacitor is used to absorb the current ripple. Fast load changes in DC/DC converters lead to larger displacements in the output voltage. When the load is turned off, the energy which is stored in the inductor, is transferred to the output capacitor and increases its voltage. Using a large capacitor limits the rise of the output voltage at the expense of additional costs and of reduced converter dynamics. A simple way to increase the dynamics is to use an additional switch in parallel to the inductor. Fig. 1 .a shows this basic concept. Fig. 1 .b shows a splitting of the inductor into two windings N 1 and N 2 . Only one winding can be shunted by the series connection of the auxiliary switch S k and the diode D k . When the winding is shunted, the current in this winding must jump according to the winding ratio to hold the flux in the core steady. When the voltage across N 1 is low enough so that the bodydiode of the auxiliary switch cannot turn on (or a switch which can afford negative voltage is used), the circuit can be simplified. Now no diode D k is necessary. The current through the LED is important and has to be controlled. When no output capacitor is used, the ripple through the inductor leads to the same current ripple in the light emitting device. To reduce this ripple the switching frequency and/or the inductor value have to be increased. Fig. 2 .a shows the basic concept. The load LD is turned on and off by the switch S 4 . The current is controlled by a controller. This can be a PI controller or a two-level controller with hysteresis. The current ripple is determined by the hysteresis. With this circuit it is easy to control the current in the inductor in such a way that when the active switch S 4 is turned off the desired current abruptly commutates into the load. S 1 is the active switch and D 1 is the passive switch of the buck converter. Fig. 2 .b shows a modification of the basic concept. The diode D 1 is replaced by an active switch S 2 to reduce the losses. In this case it is useful to use a half-bridge driver to drive the transistors. The load can be switched off by the active switch S 5 . This is especially useful when several loads are used which can be turned on and off separately.