Conducted EMI Reduction by Means of Hybrid Common Chokes
Abstract
EMI suppression solutions come in different combinations of filters, transformer coil arrangements, and even PCB layouts. This application note offers what is called a hybrid common choke, which is a magnetic hybrid consisting of a commonmode choke and a differentialmode choke. A hybrid common choke preserves the characteristic of high impedance of the commonmode choke. Its high leakage inductance can be used as the differentialmode inductance. It not only features a smaller size to reduce the filter cost, but also provides engineers with a convenient solution to conducted EMI problems.
1. Principles and Functions of a
Hybrid Common Choke
In a typical singlestage EMI filter circuit, as in Figure 1, a commonmode noise filter (L_{CM}、C_{Y1} and C_{Y2}) and a differentialmode noise filter (L_{DM}、C_{X1} and C_{X2}), in the form of LC filters, attenuate commonmode and differentialmode noises respectively. A commonmode choke is usually made of highly permeable MnZn ferrites, and the inductance can be as high as 1~50mH. A commonmode choke can be seen in Figure 2. Due to the winding polarity arrangement, even though load currents flowing through the two sets of coils respectively, the magnetic fluxes inside the core can cancel out. Henceforth, core saturation will not occur. The commonly used core types are toroid type, UU type (UU9.8、UU10.5 etc.), ET type, and UT type, as shown in Figure 3. To obtain high commonmode inductances, the two sets of coils should be coupled as good as possible. Therefore, toroid cores of high construction cost, or onepiece cores of ET type and UT type have often been used.
Figure 1. A typical EMI filer configuration
Figure 2. A commonmode choke
Figure 3. Commonly used core types: (a) Toroid type (b) ET type (c) UU type (d) UT type
From the operating principles and the equivalent circuit of a common mode choke, as in Figure 4, it can be seen that though the two sets of coils are coupled well, there still exists a leakage inductance, which results from leakage magnetic fluxes. The leakage inductance is equivalently in series connection in the circuit, and acts as a differentialmode inductance. Therefore, the leakage inductance of a commonmode choke can be used for a differentialmode filter. However, due to the mechanical structures of common mode chokes, as in Figure 3, their leakage inductances are quite small, only around a few μH to 100μH. The only way to obtain higher leakage inductances is by increasing the coil turns. In this way, the coil wire must be thinner, given the same core, and in turn the rms current will be decreased. To counteract this, the core must be enlarged, which results in larger filters and higher cost. Some applications require high commonmode inductances. It is, however, not for filtering commonmode noises, but for obtaining the high parasitic leakage inductances, intended for differentialmode filters. Such practice is often unknown to engineers.
Figure 4. An equivalent model of a commonmode choke
To increase the leakage inductance of a commonmode choke, a unique core structure and coil winding is employed. Such a commonmode choke is called an integrated commonmode choke or a hybrid commonmode choke, as shown in Figure 5. This choke structure retains high commonmode inductances to filter out commonmode noises, and can have as high as hundreds of μH differentialmode inductances, from the leakage inductances. Combined with proper X capacitors, they can effectively filter out low/midband (150kHz ~ 3MHz) differentialmode noises. By experiment, hybrid commonmode chokes prove to make excellent filters. And their greatest advantages, low cost and small size, make them outperform their counterparts.
Figure 5. Vertical and horizontal hybrid commonmode chokes
2. Major Electrical Parameters
A hybrid common choke not only keeps the characteristics of a commonmode choke but also has those of a differentialmode choke. In addition to the general specifications for commonmode chokes and differentialmode chokes, the following parameters are also specified.
(1) Commonmode Impedance, Z_{CM}: Compared to the highfrequency equivalent resistance of a Line Impedance Stabilization Network (25Ω for commonmode), the higher the commonmode impedances, the better filters they make. In addition to core materials, the ways to wind the coils (e.g. number of coils) may affect highfrequency impedances more. Figure 6 shows the setup for measuring commonmode impedances. Figure 7 shows the commonmode impedance characteristics of the ASU1200 series. Since stray capacitance, C_{S}, exists among the coil layers, it will become capacitive at high frequency. Therefore, the smaller the C_{S}, the better.
Figure 6. The measurement setup for commonmode impedances
Figure 7. The commonmode impedance characteristics of the ASU1200 series
(2) Commonmode Inductance, L_{CM}: Conventionally, commonmode inductances are characterized by an externally added voltage (V_{OSC}) and by the frequency in use. Characterizing commonmode inductances with V_{OSC} = 1Vac @100kHz usually have more stable results, though it may vary with core materials.
(3) Differentialmode Impedance, Z_{DM}: Similarly, the setup for measuring equivalent differentialmode impedances is shown in Figure 8. The differentialmode impedance characteristics plot, as seen in Figure 9, can be used to describe the differentialmode filter performance. Compared to the LISN equivalent resistance, 100Ω, higher differentialmode impedances are better. At high frequency, it still becomes capacitive. However, with great enough impedance, it can still make a good filter.
Figure 8. The measurement setup for differentialmode impedances
Figure 9. The differentialmode impedance characteristics of the ASU1200 series
(4) Differentialmode Inductance, L_{DM}: Likewise, differentialmode inductances can be specified with V_{OSC} = 1Vac @100kHz. In practical applications, differentialmode inductances of a hybrid common choke should be greater than 100μH to effectively filter out differentialmode noises, combined with X capacitors.
(5) DifferentialMode Saturation Current, I_{sat}: As said previously, because load currents flow through the equivalent differentialmode inductors, the differentialmode inductances should not be saturated at the peak load currents; otherwise, the filter performance will be degraded. Figure 10 depicts a bridge rectifier filter circuit and the input current waveform. It is required that at peak load currents, the differentialmode inductances will not decrease because of core saturation. Conventionally, I_{sat} is defined as the current at which the inductance drops by 20% (compared to the value at no DC bias current).
(a)
(b)
Figure 10. (a) A fullbridge filter circuit, (b) the input current waveform
(6) Root Mean Square Current, I_{rms}: Equivalently, this rating is to define wire width. The input current waveform of Figure 10, I_{rms}, is not high, which can usually be estimated by that the minimum input voltage divides twice the output power. For example, with a full inputvoltage range 25W power adapter, its I_{rms} can be calculated as 2 x 25W / 90Vac = 0.55A.
Table 1: Electrical parameters of the ASU1200 series

L_{CM }(mH) ±20%

L_{DM }(μH) ±10%

I_{sat }(A)

I_{rms }(A)

ASU1201

4.0

143

3.2

1.00

ASU1202

6.0

220

2.9

0.80

ASU1203

9.0

310

2.4

0.75

ASU1204

12.0

410

2.2

0.75

ASU1205

16.0

530

1.9

0.60

ASU1206

20.0

670

1.8

0.55

3. Application Circuits
Simply put, a hybrid common choke integrates a conventional commonmode choke and a (or two) differentialmode choke(s). For different applications, EMI engineers must decide on commonmode chokes, differentialmode chokes, and also differentialmode saturation currents, I_{sat}, and root mean square current I_{rms}. The ASU1200 series hybrid common chokes are suitable for the applications of 25W50W Flyback circuits or <120W PFC circuits. Figure 11 presents the examples of two flyback circuits, using a hybrid common choke.
(a)
(b)
Figure 11. Two flyback circuits with hybrid common chokes (a) conventional filter with an X capacitor (b) a conventional filter with an X capacitor to the output of a bridge rectifier
Figure 12 shows that a hybrid common choke is used in an active filter for power factor correction (PFC) in boundary conduction mode.
Figure 12. The application circuit of a hybrid common choke in a PFC circuit
Figure 13 to Figure 15 show the EMI performance of a 24W (12V/2A) offline flyback power supply using an ASU1203 hybrid common choke. It can be clearly seen that not only commonmode noises are effectively reduced by such hybrid common choke, but also differentialmode noises by its differentialmode inductance. Overall, the EMI performances show that noises can be attenuated by about 30dB at low/midband frequency with an ASU1203.
Figure 13. Commonmode noise attenuation (The blue line depicts the measurement of commonmode noises with an ASU1203)
Figure 14. Differentialmode noise attenuation (The blue line depicts the measurement of differentialmode noises with an ASU1203)
Figure 15. Total noise attenuation (The blue line shows the measurement of total noises with an ASU1203)