The switch-mode power supplies LEDs require create an EMC compliance issue that needs to be addressed.
More and more automotive manufacturers are moving away from halogen light bulbs and adopting LED technology. But LED lighting systems have diverse power supply requirements, depending on the number of devices present on a string, the current needed, dimming, and other factors. A traditional linear regulator cannot always meet these needs, so designers are increasingly relying on switch-mode power supplies (SMPS). These create an electromagnetic compatibility (EMC) compliance issue that needs to be addressed.
Fortunately, there are many effective ways to improve the EMC of an LED driver design through component placement and layout, as well as helpful features that semiconductor manufacturers are adding to their products. Let’s look at some of them.
In the design of PCBs, mechanical dimensions and placement constraints (such as connector position and keep-out areas) often complicate SMPS PCB layout. Even so, there are things engineers can do to reduce EMC issues. One effective way to ensure good EMC is to place the DC/DC converter far away from the connector and on the opposite side of the PCB, if two-sided assembly is possible. This minimizes noise coupling into the wiring harness that negates the effect of input filters. At the same time, the input filter block should be placed close to the connector and away from the DC/DC, so that no noise is induced to the filter inductor.
If such placement is not possible, the designer should use a local shield for the DC/DC converter block (converter IC, inductor, and input capacitors). A shield adds cost to the design, but it can help save on filtering components, and it is often the difference between a design being compliant or not.
If the LED lighting solution requires a metallic heat sink for thermal dissipation, the heat sink can act as a shield. There are two potential scenarios:
When the LED load is not on the same PCB as the driver board, long load wires (>10 cm) can create too much noise. To mitigate this, use a common mode choke at the input. An even simpler solution is to place two ferrite beads at the VIN cable connection and the GND cable connection, as shown in Figure 1. Use an output filter on both load cables to help with radiated emissions when conducted emissions are high, between 50 MHz and 108 MHz.
After planning PCB layout, consider where to route the DC/DC convert switch node. The EDN article, “PCB design for low-EMI DC-DC converters” offers helpful instructions on how to design the DC/DC block.
Because automotive lighting is critical for safety, LED designs must maintain a certain level and stability of brightness regardless of perturbations. It is therefore important to maintain low electromagnetic interference (EMI) generation from the LED driver circuit as well as a low susceptibility to interference.
Most LED driver ICs use a constant-current approach and may have current-sensing resistors. Other advanced drivers have several traces going into the IC that can pick up noise and potentially impact performance (e.g. temperature sensing or dimming). Route these traces in the internal layers of the board whenever possible and shield them with copper on the external layers.
When using a 2-layer PCB, avoid making cuts to the GND plane by routing long traces. Such big cuts are a common mistake in many layouts, and it is especially dangerous when a big cut is near the DC/DC block as cuts in the reference GND plane can add impedance and, depending on the size, create high-frequency noise. Instead, route the traces by alternating between the top and bottom layers for short lengths, as shown in Figure 2. This method reduces the length of each trace, making the board more immune to high-frequency interference.
Also be cautious with via placement. In small boards, where components are close with many traces, it is common to use vias for routing. But, as shown in Figure 3, closely packed vias can also result in long cuts to the metal layer. Ensure there is enough space between vias that the GND copper plane can be placed between them to avoid big cuts.
Semiconductor EMC improvements
In the last several years, semiconductor manufacturers have thought of several ways to improve DC/DC power and efficiency while also improving the EMC of circuits. One approach has been to use frequency spread-spectrum (FSS) modulation, sometimes called dithering, to spread the fundamental switching frequency’s energy over a wider band with a lower peak value. This approach allows the converter to pass EMC tests while switching inside the AM band (535 kHz to 1605 kHz). It also decreases noise radiated in the FM band (88 MHz to 108 MHz) when the converter switches above AM band frequencies.
Another way that manufacturers have improved converter EMI is to include an in-package decoupling capacitor. This provides efficient decoupling by minimizing parasitic inductance between the capacitor and switches, and also reduces PCB BOM cost.
A good example of an advanced LED driver is MPS’s MPQ7200. With a factory-trimmed FSS modulation option for improved EMI performance, the device switches at 2.3 MHz in buck mode, and at 1 MHz in buck-boost inverter (BBI) mode. A low-loss current-sensing method is implemented in the silicon, eliminating the need for external current-sense resistors and their associated traces.
LED technology allows for innovations in signalling and safety, as more and more LEDs are used to help drivers communicate with their vehicles. Modern LED lighting designs need to offer great flexibility while remaining robust and free of EMC issues. Lighting solutions tend to be in small PCBs with limited BOM count, so thoughtful component placement and layout design is necessary to meet increasing EMC requirements. Functional improvements in LED driver silicon are also helping address this issue.
Francesc Estragués is an Applications Engineer and Ralf Ohmberger is a Senior Applications Engineer, both at Monolithic Power Systems.