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Fig. 3: Voltage ripple after post filter By simply decoupling the capacitors from the AC currents with a tiny inductor, the system becomes much more reliable without overstressing the electrolytic capacitors. Audible noise Ceramic capacitors allow engineers to design LED drivers for a wide operating temperature range and long lifetime. But there is a big challenge when PWM dimming has to be implemented. The PWM signal with a frequency of several hundreds of Hertz switches the LED current on and off with the same frequency. This pulsed current is seen by the ceramic capacitors at the output of a boost LED driver causing them to resonate mechanically due to the piezoelectric effect. The capacitors start to move up and down with the frequency of the PWM and stimulate the PCB, which acts as a loudspeaker. Dependent on the current and mechanical setup this noise can be very loud and annoying. There are special ceramic capacitors available with a smaller distinctive effect, but only a very good layout and mechanical setup can solve this problem effectively. It reduces the noise to an acceptable volume. If this is done, standard ceramic capacitor can be used. First of all, place two identical capacitors directly opposite each other on the top and bottom side of the PCB. If there is only a single capacitor, LED Lighting it bends its middle towards the PCB and back. In doing this, the PCB is stimulated like the membrane of a loud speaker and emits acoustic waves. If two capacitors are placed opposite one another, both bend towards and away from the PCB at the same time. Thus, the PCB cannot resonate anymore. The second action to reduce the residual noise further is to minimise the mechanical coupling of the ceramic capacitors and the PCB. By milling holes into the PCB besides the solder points, the stimulated PCB area is reduced significantly. Figure 4 shows the read marked holes in the PCB around the ceramic capacitors. By implementing the two methods mentioned above, the audible noise is heavily reduced. Without these arrangements, the noise caused by the PWM dimming and the ceramic capacitors on the output of a boost LED driver is clearly heard up to a few meters away. By limiting the movement of the capacitors and minimising the resonating area of the PCB, the noise is reduced to such a low level that you need to put your ear as close as possible to the PCB to hear a residual very low buzzing. Conclusion Using ceramic capacitors instead of electrolytic capacitors for LED drivers reduces the form factor of the system and increases the reliability. An additional post filter consisting of a tiny inductor and ceramic capacitors reduce the output voltage ripple to a very low level. The piezo ceramic effect seen with ceramic capacitors causes audible noise when PWM dimming is implemented. This noise is attenuated effectively by proper placement of the capacitors and small cut-outs on the PCB. TI Design PMP10171 is a four channel boost LED driver with an input voltage range of 10-30V and 500mA at 40V output per channel. It incorporates the techniques described in this article and all the information (schematic, bill of material, test report, layout and firmware) is available on TI.com with PMP10171 as reference. Fig. 5: Photo of PMP10171 Fig. 4: PCB cutouts around the ceramic capacitors 34 Electronic Engineering Times Europe November 2015 www.electronics-eetimes.com


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