The increasing popularity of modern electronic equipment brings convenience to people, but at the same time aggravates the deterioration of electromagnetic environment. Electromagnetic interference (EMI) refers to the interference phenomenon caused by the interaction of electromagnetic waves with electronic components.

Since electronic devices produce electromagnetic waves when they work, various electronic devices interfere with each other, which can cause unexpected impact on sensitive circuits, and in serious cases can prevent a circuit from operating normally. This is why reduced EMI is critical for system stability. This article discusses how EMI performance benefits consumer and RF devices, such as home appliances, alarm systems, and garage door openers.

How to optimize EMI
When it comes to switch-mode power supplies, it is important to address EMI from both circuit design and circuit board layout. In terms of circuit design, switching speed, as well as the ringing on the switch node (Figure 1), can impact EMI performance.

Figure 1 Typical application circuit

EMI can be optimized effectively in two ways: switching speed control and anti-ringing control.

Switching speed control
EMI is reduced by extending the switching rise time and fall time at the switch node to reduce the impact of dV/dt slew rate (Figures 2-5).

Figure 2 SW rise time before switching speed control

Figure 3 SW rise time after switching speed control

Figure 4 SW fall time before switching speed control

Figure 5 SW fall time after switching speed control

Anti-ringing control at light load
Ringing on the switch node contributes to EMI. The more ringing a device has, the worse the EMI performance (Figure 6). Adding a 1kΩ resistor (R) and another switch (S1) between SW and GND mitigates ringing. At light load, when both the HS and LS switches turn off, S1 turns on to allow some of the power of L1 to release through R and S1 to GND (Figure 7).

Figure 6 More ringing on SW before anti-ringing control

(Test conditions: VIN = 12V, VOUT = 3.3V, IOUT = 10mA)

Figure 7 Less ringing on SW after anti-ringing control

(Test conditions: VIN = 12V, VOUT = 3.3V, IOUT = 10mA)

Figures 8 and 9 show the EMI reduction achieved by implementing switching speed and anti-ringing control.

Figure 8 EMI tests before switching speed control and anti-ringing control

(Test conditions: VIN = 12V, VOUT = 5V, 1.2A)

Figure 9 EMI tests after switching speed control and anti-ringing control

(Test conditions: VIN = 12V, VOUT = 5V, 1.2A)

[Continue reading on EDN US: PCB layout for better EMI]