The LED design for automotive lighting standards has certain brightness and stability

火辣西米秀

The LED design for automotive lighting standards has certain brightness and stability [Copy link]

1. PCB layout

Engineers must work around mechanical dimensions and constraints such as connector locations and keep-out areas when laying out their PCBs.

If a PCB allows double-sided assembly, an effective way to ensure good EMC performance is to place the DC/DC converter away from the connector and on the other side of the PCB. This minimizes the coupling of noise into the harness, thereby eliminating the impact of the input filter. The input filter module should be placed close to the connector and away from the DC/DC to avoid introducing noise into the filter inductor.

If this is not possible, a partial shield should be used to shield the DC/DC converter module (including the converter IC, inductor, and input capacitors). Adding a shield will increase the design cost, but it can help save filtering components and is often used to distinguish whether the design meets EMC standards. If the LED lighting solution requires a metal heat sink to dissipate heat, it can be used as a shield. There are two possible scenarios at this time:

There are many good GND connection points on the heat sink to reduce radiation and protect the device from interference.

The connection between the heat sink and GND is not good (for example, there is only one contact point), so the shielding effect is poor and it may even act as a patch antenna. In this case, a ferrite bead can be added to the connection between the heat sink and GND to enhance the design's anti-interference ability.

When the LED load is not on the same side of the PCB as the driver board, long load lines (> 10cm) will generate excessive noise. To alleviate this condition, a common mode choke can be used at the input. But a simpler solution is to place two ferrite beads at the VIN cable connection and the GND cable connection (see Figure 1). When the conducted emission frequency is between 50MHz and 108MHz, adding output filtering on both load cables will help reduce radiated emissions.

Figure 1: Good design of input filter for LED driver with remote load

2. Clever layout

After planning the PCB layout, the DC/DC converter switch node routing must be considered.

Automotive lighting is critical to safety, so it is very important to maintain low EMI and strong anti-interference of LED driver circuits. LED designs that meet standard requirements must have a certain brightness and stability without interference.

Most LED driver ICs use a constant current approach and may also use a current sensing resistor. Some advanced drivers also have multiple traces going into the IC that can pick up noise and affect performance (e.g. temperature sensing or dimming).

Place these traces on the inner layers of the board as much as possible and shield them with copper material on the outer layers. If a 2-layer PCB is used, the traces can be alternated between the top and bottom layers for short distances. This method reduces the length of each trace, making the board immune to high-frequency interference and avoiding long cuts in the GND plane on both layers. Cuts in the reference GND plane increase impedance and can generate high-frequency noise depending on the size of the cut.

Figure 2: Long-distance routing on a 2-layer board

On small boards, components are closer together and more traces are needed, so vias are often used for routing (see Figure 3). You need to make sure there is enough space between vias to place the GND copper layer while avoiding large cutouts. Many board layouts make this mistake, and it is especially dangerous when large cutouts are close to DC/DC modules.

Figure 3: Connecting different board layers using vias

3. Semiconductor manufacturers improve EMC by improving technology

In recent years, semiconductor manufacturers have begun to think about how to improve circuit EMC performance while increasing DC/DC power and efficiency.

Spread spectrum frequency modulation (FSS), sometimes called dithering, spreads the energy of the fundamental switching frequency over a wider band with lower peaks. This allows the converter to switch within the AM band while passing EMC tests. It also reduces the noise radiated in the FM band when the converter switches above the AM band.

Another way to improve converter EMI is to add decoupling capacitors to the package. This not only reduces BOM cost but also achieves effective decoupling by minimizing the parasitic inductance between the capacitor and the switch.