Common mode choke coils (CMCC) are filters that reduce common mode noise that is problematic in differential transmission lines (non-automotive use: USB, HDMI, Mipi, etc.; automotive use: CAN, CAN-FD, 100Base-T1, 1000Base-T1, SerDes, A2B®, etc.), power lines, and audio lines.
Explore some countermeasure examples for using a common mode choke coil to deal with power supply noise in this article from Murata’s Tadashi Tanaka,Product Engineering Section 3, Product Engineering Department, EMI Filter Division.
Electronic devices are generally made up of multiple semiconductor and functional blocks and both must be supplied with power at their designated voltage. In many cases, the required voltages are all different. When this occurs, the voltage needs to be converted using a DC-DC converter. This is a circuit that converts the power supply to the voltage required for the operation of electronic components.
When working with different required voltages, size and electrical performance are two main considerations. Switching DC-DC converters are often used in such cases. However, it’s important to pay attention to the noise caused by switching. If the noise regulation standards aren’t met, then some type of noise countermeasure is required.
An example of a power line noise countermeasure
This noise countermeasure example was prepared with a typical non-isolated, step-down type (5 V to 1.8 V, Pout = 27 W) DC-DC converter with a switching frequency of 500 kHz.
The goal here is to support the CISPR22 class B general device standard (devices used in ordinary homes and light industrial environments). Effective noise countermeasures were implemented after determining whether the noise mode was primarily differential mode or common mode.
(1) Noise terminal voltage (150 kHz to 30 MHz)
The common mode voltage (noise terminal voltage) was measured for one line with respect to ground using an artificial mains network (AMN). When the noise terminal voltage is measured under the initial conditions, we can see that a noise spectrum occurs at multiples (frequency is converted to n multiples) of the 500 kHz switching frequency.
Figure 1. Initial noise terminal voltage levels
Before proceeding with the noise countermeasure, mode separation was performed using Delta-LISN (Line Impedance Stabilization Network) in order to determine whether the noise mode was primarily differential mode or common mode. As shown in Figure 2, it is clear that the noise in this case is primarily differential mode noise.
Figure 2. Noise mode separation
As a result, the noise level was significantly decreased when the EMIFIL® BNX029-01 block-type filter was used, due to its highly effective countermeasures for differential mode noise.
Figure 3. Noise countering effects of the block type EMIFIL®
Radiation noise (30 MHz to 6 GHz)
After implementing the countermeasure described above, the radiation noise was evaluated at 10 m to measure the intensity of the field emitted into the space, and discovered radiation noise that significantly exceeded the CISPR22 class B noise tolerance (Figure 4).
When the radiation path was investigated, it was discovered that the input power supply cable of the DC-DC converter was acting as an antenna and radiating noise into the surrounding space.
Figure 4. Initial radiation noise levels
The BNX029-01 is able to remove differential mode noise across a wide frequency band from 100 kHz to 1 GHz, so the remaining noise was presumed to be common mode noise.
When a PLT10HH9016R0PN common mode choke coil was inserted to deal with the common mode noise, the level of radiation noise significantly improved to meet the CISPR22 class B noise tolerance. As a result, it was learned that the radiation noise was primarily common mode noise (Figure 5).
Figure 5. Noise countering effects of a common mode choke coil
Lineup of common mode choke coils for power lines
Murata Manufacturing Co., Ltd. Maintains a wide variety of component sizes and characteristics to provide the optimal components for customer applications (Table 1).
| Product name | Size | Common mode impedance | Rated current *1 | Rated voltage |
|---|---|---|---|---|
DLW5BTM***TQ2 |
5.0x5.0mm T=2.5mm max. |
100Ω to 1400Ω @100MHz |
6A to 2A | 50Vdc |
PLT5BPH******SN |
5.0x5.0mm T=5.0mm max. |
100Ω to 500Ω @100MHz |
5.6A to 3.1A | 80Vdc |
PLT10HH******PN |
12.9x6.6mm T=9.3mm max. |
45Ω to 100Ω @10MHz |
18A to 15A | 300Vdc |
| 400Ω to 1000Ω @10MHz | 10A to 6A | 100Vdc |
Table 1. Lineup of common mode choke coils
*1 With regard to the rated current, derating is configured, so please see the detailed specifications for each product.
*2 The operating temperature range differs for each product type, so please see the detailed specifications for each product.
Summary
When implementing noise countermeasures, it is extremely important to determine whether the noise mode is common mode or differential mode. Murata has maintained common mode choke coils for power lines in every size, and these have the high rated current and high insertion loss to provide the optimal noise suppression products for your power supply circuits.
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