Automotive LED driver design: good practice to ensure compliance with EMC standards

[Guide]Automakers are increasingly adopting LED lighting technology to replace previous halogen bulbs. LED lighting systems have a variety of different power requirements, depending on the number of devices on the string, the required current, the dimming method, and other factors. Since traditional linear regulators cannot always meet these requirements, Electronic engineers are now increasingly relying on switched mode power supplies (SMPS). However, SMPS generates electromagnetic interference (EMI).

The method introduced in this article will use component placement and layout, as well as semiconductor manufacturers to add certain functions to their products, and ultimately achieve effective improvements in the EMC performance of the LED driver design.

PCB layout

The engineer’s PCB layout work must be carried out around the mechanical size and restrictions (such as the location of the connector and the restricted area).

If a PCB allows double-sided assembly, an effective way to ensure good EMC performance is to keep the DC/DC converter away from the connector and place it on the other side of the PCB. This can avoid noise coupling into the wiring harness to the greatest extent, thereby eliminating the influence of the input filter. The input filter module should be placed close to the connector and away from 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 capacitor). Adding a shielding cover will increase the design cost, but it can help save filter components, and it is usually 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:

• The heat sink has multiple good GND connection points, which can reduce radiation and protect the device from interference.

• The connection between the heat sink and GND is poor (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 at the connection point between the heat sink and GND to improve the anti-interference ability of the design.

When the LED load and the driver board are not on the same side of the PCB, a longer load line (> 10cm) will generate excessive noise. To alleviate this situation, 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 the radiated emission.

Clever layout

After planning the PCB layout, it is necessary to consider the routing of the switching nodes of the DC/DC converter. This article will explain how to design a DC/DC module.

Automotive LED driver design: good practice to ensure compliance with EMC standards

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

Automotive lighting is critical to safety, so it is important to maintain low EMI and anti-interference properties of the LED driver circuit. A standard-compliant LED design must maintain a certain level of brightness and stability without interference.

Most LED driver ICs use a constant current method, and current sampling resistors may also be used. There are also some advanced drivers that have multiple traces entering the IC, which may absorb noise and affect performance (such as temperature sensing or dimming).

Place these traces on the inner layer of the circuit board as much as possible, and shield the outer layer with copper material. If a 2-layer PCB board is used, wires can be routed alternately in short distances between the top and bottom layers. This method reduces the length of each trace, so that the circuit board is not affected by high-frequency interference, and it also avoids making long cuts to the GND plane on both layers. The notch on the reference GND plane will increase impedance, and high-frequency noise will be generated depending on the size of the notch.

Automotive LED driver design: good practice to ensure compliance with EMC standards

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

On small circuit boards, the distance between components is closer and there are more traces, so vias are usually used for routing (see Figure 3). At this time, it is necessary to ensure that there is enough space between the through holes to place the GND copper layer, while avoiding large cuts. Many board layouts have this error, and it is especially dangerous when the large cutout is close to the DC/DC module.

Semiconductor manufacturers improve EMC by improving technology

In recent years, semiconductor manufacturers have been considering how to improve the EMC performance of circuits while increasing DC/DC power and efficiency.

Spread spectrum frequency modulation technology (FSS) is sometimes called jitter, which can expand the energy of the basic switching frequency to a wider frequency band with a lower peak value. Using this technology, the converter can switch in the AM frequency band and pass the EMC test at the same time. When the converter switches above the AM frequency band, it can also reduce the noise radiated in the FM frequency band.

Another way to improve the EMI of the converter is to add a decoupling capacitor in the package. This can not only reduce the BOM cost, but also achieve effective decoupling by minimizing the parasitic inductance between the capacitor and the switch.

MPS’s MPQ7200 is an advanced LED driver that can be used as a buck converter or a buck-boost inverter (BBI). The device complies with AEC-Q100 certification and can provide 3A current when working in step-down mode, and 1.2A current at voltages up to 42V when working in BBI mode.

Automotive LED driver design: good practice to ensure compliance with EMC standards

Figure 3: Connecting different board layers through vias

MPQ7200 has factory-set FSS modulation options, which can improve EMI performance. In the buck mode, the switching frequency is 2.3MHz; in the BBI mode, the switching frequency is 1MHz. The device uses a low-loss current sampling method, so no external current sampling resistor is required.

A typical buck-boost topology consists of four MOSFET switches, and the output voltage is positively referenced to GND. And the BBI topology of MPQ7200 only contains two MOSFET switches, thereby reducing costs and reducing switching actions. The two-switch BBI imitates the buck topology, grounding the output of the inductor; the control loop switches the power reference of the MPQ7200 to the negative output voltage; the power LED can be connected between GND and the negative output voltage. With adjustable current control loop, temperature sensing and digital dimming functions, MPQ7200 can be called an advanced power LED driver.

Figure 4 shows how the MPQ7200 provides excellent linear regulation under the typical operating voltage of automotive applications. When the input voltage changes between 15V and 10V, its output voltage and current always remain constant.

Automotive LED driver design: good practice to ensure compliance with EMC standards

Figure 4: MPQ7200 startup waveform

Another interesting test is how the interruption of the input voltage affects the beam quality. The input voltage interruption sequence is: 14VDC to 9.5V, to 8.5V, to 7.5V, to 6.5V, to 5.5V and to 4.5V. The low pressure valley time rises from 200ms to 900ms. Under low input voltage, the visible light intensity remains almost constant; but under a 4.5V pulse, the LED is completely turned off.

Automotive LED driver design: good practice to ensure compliance with EMC standards

Figure 5: Power interference test

in conclusion

As more and more LEDs are used for communication between drivers and their vehicles, LED technology has enabled innovations in signaling and safety. The key to modern LED lighting design to provide greater flexibility while maintaining robustness lies in the functional improvement of the LED driver. Lighting solutions continue to develop towards smaller PCBs with lower BOM. Only careful component placement and layout design can meet the ever-increasing EMC requirements.

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