2. Design the EMI filter with output impedance <= 10db lower than RIN: The addition of the input filter can affect the DC-DC converter’s performance. To minimize the effect, the output impedance of the filter must always be less than the input impedance of the power converter, for all frequency up to the converter’s crossover frequency.
The output impedance of the LC filter at its resonance frequency, which is the highest value, is:
In our design we have considered the effective impedance of the filter to be 10dB less than the input impedance of the buck converter, which is approximately equal to one-third of the input impedance. For the MAXM17575 example, the required Zo is < =RIN/3 = 7.6/3 = 2.5Ω for all frequencies up to the MAXM17575 circuit’s crossover frequency, which is 45kHz.
PCB Layout Best Practices
PCB layout plays a significant role in EMI compliance success. A bad PCB layout can ruin a power converter with perfect electrical design. The following are good PCB layout practices to minimize EMI noise sources using the same buck converter example:
1. Minimizing high di/dt current loops: Properly place LO, CO, and S2 close together to minimize the I2 current loop. Then, place this entire group of components close to S1 and C1 to also minimize I1 current loop. When using a buck regulator IC (i.e. a buck controller with integrated power switches S1, and S2), it’s important to choose those ICs that have good pinouts to allow this minimization. The same consideration applies to using power modules.
2. Using Faraday shield: A Faraday shield (or cage), named after the English scientist Michael Faraday, is an enclosure used to block electromagnetic fields. There are two common ways to implement a Faraday shield in a power system:
a. A cage made of conductive material (such as copper) that encloses the entire power system or equipment. The electromagnetic field is contained inside the cage. This method is usually expensive due to the material cost of the cage and additional assembly labor.
b. A layout with shielding ground planes on both the top and bottom of the PCB with a via connecting them to mimic a Faraday cage. All high di/dt loops are placed in the inner layers of the PCB so that our Faraday cage will shield the magnetic field from radiating outward. This method is lower cost and usually adequate to contain EMI. Figure 10 illustrates this technique.
Employing these PCB layout best practices provides a reasonable way to achieve EMI regulatory compliance without compromising power converter efficiency by otherwise slowing down the switching edges.
Now let’s consider Maxim’s Himalaya wide input IC, MAX17502, which operates at 4.5-60Vin, 0.9-54Vout supplying 1A current. The following is the MAX17502 EMI EVKIT PCB layout, using the Faraday shield technique (b). Figure 11a shows the top and bottom layers used as the Faraday shield. Figure 11b shows the second and third inner layers for routing. The second layer is used here as an extra shield, but it can also be used to route traces. In this layout, the high di/dt current loops I1 and I2 are routed on the third layer, which is completely enclosed in our Faraday shield.
The following is the EMI test result of this MAX17502 EMI EVKIT, which passes CISPR 22 Class B with good margin.
Low EMI Power Components
Magnetic field from the output inductor can also radiate and cause EMI issues. Using a low EMI inductor reduces radiated EMI. Shielded inductors are recommended. This type of inductor has the magnetic field shielded and contained within the inductor structure. Avoid inductor types where the magnetic energy can radiate freely. Power modules using shielded inductors and employing good PCB layout practices will exhibit good EMI performance.
Low EMI Power Regulators and Modules
Maxim’s Himalaya regulator and power module families employ low EMI power inductors and good PCB layout practices, providing inherently low EMI power solutions. Using the Himalaya solutions means you don’t need to worry about compliance, unlike with other simplistic switchers in the market. Maxim has done all the work with the ICs, modules, and example reference layouts so that you can pass CISPR 22 (EN 55022) at optimal cost. The following are EMI test results of an example, MAXM17575, together with input EMI filter information:
Test Article (EUT)
|Result||PASS-EN55022 (CISPR 22) CLASS B|
|Input Voltage||24V-Positive||Output Voltage||5.0V|
|Switching Frequency||900KHz||Output Current||1.5A|
EMI Filter Configuration – Conducted EMI test
|Filter Component||Value||Part Number||Manufacturer|
|Inductor- L1||10uH||PA4332.103NLT||Pulse Electronics|
|Capacitor- C2, C3||1uF||GRM32CR72A105KA35||Murata|
EMI Filter Configuration – Radiated EMI test
MAXM17575 has inherently very low radiated EMI. The input filter shown for the conducted EMI test is not needed and is not used for the radiated test. Using the input filter will provide additional passing margin for the radiated test result.
|Filter Component||Value||Part Number||Manufacturer|
Planning for EMI compliance of a system upfront is critical for project success. This paper discussed common techniques to minimize EMI. The paper also presented guidelines for line filtering design, good PCB layout, and shielding practices, along with practical examples. A well-planned design using a proper filter, low EMI PMICs, components, and/or power modules, along with good PCB layout techniques and shielding, will assure a high chance of first-pass success.