dt02-1| Application Note
DESIGN TIP
By
TOPICS COVERED
Commonly used method Easier method
DT02-1 rev.b
International Rectifier 233 Kansas Street, El Segundo, CA 90245 USA
Isolation method for 1-10V controlled ballast based on the IR21592
Peter Green
Requirements of a commercial design
Circuit diagram How it works
The light output of a dimmable ballast based around the IR21592 control IC, is determined from a DC control voltage between 0 and 5V that provides a reference for the closed loop lamp power control. At inputs below 0.5V the output will be at minimum, a preset power level below which the particular lamp that the ballast has been designed for, does not operate satisfactorily. This point will vary considerably between lamp types, depending on the length and diameter of the tube and also the cathode effectiveness. Above this the lamp power will increase linearly from the minimum level to the maximum. The minimum lamp power for which stable output can be achieved is typically 1% when using the IR21592 phase control technique.
REQUIREMENTS OF A COMMERCIAL DESIGN
When designing a circuit for a commercial ballast however, the control voltage used is from 1 to 10Vdc, where voltages below 1V produce minimum output and a linear response is required up to 10V. This control voltage must also be isolated from the off-line parts of the ballast circuitry, including the IR21592, which are at mains potential and would otherwise make the control voltage input terminals live which is hazardous to ballast users and would probably damage the dimming equipment the ballast is connected to. The isolation must be rated to 4kV in order to comply with safety requirements of the European low voltage directive. The control input must also be sinkable meaning that when no input is applied the control voltage will remain at 10V and the ballast will operate at maximum output. The dimming control must be able to sink this voltage down to below 1V to achieve dimming over the ballast's full range. It is therefore necessary that the voltage can be pulled down below 1V by sinking only a small curr
ent in the order of 1mA. This allows many ballast control inputs to be connected to a single dimming control that would typically be rated to be able to sink up to 200mA and therefore could run up to 200 ballasts.
