EMI reduction method based on driving high power LED

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EMI reduction method based on driving high power LED

May 29 , 2023 0

The high-power LED module required for infrared radiation in the water quality monitoring system works on the high-current square wave signal, which may cause strong electromagnetic interference (EMI) on the bioelectrode sensor of the system, resulting in inaccurate water quality data. This design example shows a method to reduce EMI and describes it in detail with examples.

In this example, we consider a 30W high power LED module, which consists of six 5W leds, requiring over 12VDC voltage and 2A current at full load. A critical voltage (VH) of about 11VDC is the minimum required for leds to shine. The DC power supply provides an initial voltage of V1=10.5VDC, which then gradually increases to just below the critical voltage VH.

In this example, a second DC voltage, V2=3.7VDC, is applied, which is in series with the first DC voltage, adding to give a voltage V1+V2 greater than 12VDC (causing the glow).

Both V1 and V2 are connected in series to a solid state relay. When the input square wave is at its minimum, the relay opens and V1 (the critical voltage just below VH) is applied to the LED; When the input square wave is at its maximum, the relay closes and a mixed voltage of V1+V2 is applied. The square wave input is effectively converted from a 12VDC amplitude to 3.7VDC, thereby reducing the EMI level of the biosensor. When a mixed V1+V2 voltage is applied, diode D1 prevents the current from flowing back into the V1 supply.

Since the LED module is directly driven by a 12VDC square wave, the average noise of the biosensor exceeds 500μV, with peaks reaching much higher levels, as shown in the image.