Feature: Electronic design
Figure 4: Current waveform of Figure 3’s driver circuit
PCB layout and component placement to reduce or eliminate EMI. In PCBs, there are many closed current loops, depending
on the circuit’s complexity, but there will be at least one; see Figure 1. The current flow in the closed loop causes a magnetic field around the trace as described in Equation 1:
(1) The magnetic field affects the traces and ICs located in the
loop, making them function poorly and inducing unwanted voltages on other traces. In power-electronics applications, some measures are
commonly taken into account to reduce EMI, including allowing sufficient space between power and ground planes and avoiding slit apertures, sharp corners, narrow power tracks and overlapping power planes, among other things.
Also, long return paths with small cross sections and loop areas give better performance in cancelling or reducing EMI; see Figure 2.
LEDs and EMI LEDs are widely used in lighting applications, due to their long life and high efficiencies (Lumens/Watt). However, LED driver circuits can also be affected by EMI problems. Small LED lamps usually also contain both LEDs and
driver circuits, so high switching frequencies and poor layout can lead to EMI problems. In this project we discuss an EMI problem in a 9W LED
driver circuit, its design verification and test measurements. Te driver circuit is an IC operating at 100kHz, placed in the centre of the PCB. As seen in Figure 3, series-connected LEDs are placed around the PCB for even light distribution. Te circuit’s operating current waveforms are shown in Figure 4, which also shows some unwanted spikes, likely generated by the main loop current. To manage EMI, two different PCB layouts are studied.
In the first, the driver IC is placed outside the main current loop, as seen in Figure 5. In the revised PCB design, LED current is measured again,
as shown in Figure 6. The unwanted spikes are smaller. The EMI problem is reduced by the revised PCB design.
Figure 5: Green mask (left) and copper layer (right) of the LED driver PCB
However, this design is not suitable for mass production, since the LED array is not symmetrical, which leads to uneven light distribution. For even light distribution, the LEDs are placed symmetrically around the PCB; see Figure 3. Te second PCB is designed to eliminate the EMI problem but also allow symmetrical placement of the LEDs; see Figure 7.
www.electronicsworld.co.uk March 2022 19
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