Reduce EMI by driving high-power LEDs
Article By : Liao Tai-Shan
In a system for monitoring water quality, EMI was overwhelming the LED sensors, so this engineer devised a means of reducing it.
High-powered LED modules used for infrared irradiation in water quality monitoring systems operate using a high current square-wave signal that can induce strong electromagnetic interference (EMI) at the bio-electrode sensor(s) of the system, resulting in compromised water quality data.
A method of reducing EMI is shown in Figure 1 and an example is described below.
Figure 1 Block diagram for method of reducing EMI induced by input square-wave signal of a water quality monitoring system
In this example, a 30W high-power LED module composed of six 5W LED packages requiring more than 12 VDC and 2A at full load conditions will be considered. A critical voltage (VH) of approximately 11 VDC is the minimum voltage required for LED light emission. A DC power supply provides an initial voltage of V1 = 10.5 VDC, which is gradually increased to just below the critical voltage VH.
A second DC voltage, V2 = 3.7 VDC for this example, is then applied in series with the first, such that the sum of the applied voltages, V1 + V2, is greater than 12 VDC (resulting in light emission).
Both V1 and V2 are operated in series with a solid state relay. When the input square wave is at its minimum, the relay opens and V1 (just below the critical voltage of VH) is applied to the LED package; when the input square wave is at its maximum, the relay closes and the combination of V1+V2 is applied. The square wave input is effectively transposed from an amplitude of 12 VDC to 3.7 VDC, resulting in reduced levels of EMI at the bio-sensor. The diode D1 serves to preclude current flowing back to the V1 source when the combined V1 + V2 voltage is applied.
With the LED module driven directly by a 12 VDC square wave, noise at the bio-sensor averaged more than 500µV with peaks at significantly higher levels, as shown in Figure 2 below.
Figure 2 Noise levels at the bio-sensor with LED module driven by a 12 VDC square wave
Figure 3 Noise levels at the bio-sensor with LED module driven using the method described above