When designing electronic products, in order to prevent the various sub-systems sharing the main power from interfering with each other and maintaining high-frequency isolation, one of our commonly used effective methods is to use ferrite beads on the power side. Ferrite beads can filter high frequency noise in a wide range of target frequencies. It has resistance characteristics in the target frequency range and dissipates noise energy in the form of heat. The ferrite bead is connected in series with the power rail, if both sides of the bead are grounded together with the capacitor. This forms a low-pass filter network to further reduce high-frequency power supply noise.
Of course, improper use of ferrite beads in the system design can also be harmful. There are some examples to illustrate: the use of magnetic beads and decoupling capacitors for low-pass filtering causes interference resonance; the dependence of the DC bias current leads to a decrease in the EMI suppression capability of the magnetic beads. Only after correctly understanding and fully considering the characteristics of ferrite beads can these problems be solved and avoided.
The following will discuss the precautions of system designers when using ferrite beads in power systems, such as impedance and frequency characteristics when the DC bias current changes, and interference LC resonance effects. Finally, in order to solve the problem of interference resonance, damping technology is introduced and the effectiveness of various damping methods are compared.
The device used to demonstrate the effect of ferrite beads as an output filter is a 2 A/1.2 A DC-DC switching regulator with independent positive and negative outputs (ADP5071). The ferrite beads used in this article are mainly chip-type surface mount packages.
Simplified model and simulation of ferrite beads
Ferrite beads can be modeled as a simplified circuit composed of resistance, inductance and capacitance, as shown in Figure 1a. RDC corresponds to the DC resistance of the magnetic beads. CPAR, LBEAD, and RAC represent parasitic capacitance, magnetic bead inductance, and AC resistance (AC core loss) related to magnetic beads, respectively.
Ferrite beads can be classified according to three response areas: inductive, resistive and capacitive. These areas can be determined by looking at the ZRX curve (as shown in Figure 1b), where Z is the impedance, R is the resistance, and X is the reactance of the magnetic bead. In order to reduce high-frequency noise, the component needs to be used as a resistor to dissipate high-frequency noise and dissipate it in the form of heat. The magnetic beads must be in the resistive area; this is especially important for electromagnetic interference (EMI) filtering applications. This resistive region appears after the bead crossover frequency (X = R) until the point where the bead becomes capacitive. This capacitive point is at the frequency where the absolute value of the capacitive reactance (–X) is equal to R.
In some cases, simplified circuit models can be used to approximate the impedance characteristics of ferrite beads up to the sub-GHz range.
This article takes Tyco Electronics BMB2A1000LN2 multilayer ferrite beads as an example. Figure 1b shows the BMB2A1000LN2 ZRX response measured with an impedance analyzer under the condition of zero DC bias current.
On the measured ZRX curve, in the region where the magnetic bead exhibits the greatest inductive characteristics (Z ≈ XL; LBEAD), the inductance of the magnetic bead can be calculated according to the following formula:
f is any frequency point in the area where the magnetic beads behave as inductive. In this example, f = 30.7 MHz. XL is the reactance at 30.7 MHz and the value is 233 Ω.
The inductance value (LBEAD) derived from Equation 1 is equal to 1.208 μH.
In the area where the magnetic bead exhibits the largest capacitive characteristic (Z ≈ | XC|; CPAR), the parasitic capacitance can be calculated according to the following formula:
f is any frequency point in the area where the magnetic bead behaves as capacitive. In this example, f = 803 MHz |XC| is the reactance at 803 MHz, and the value is 118.1 Ω.
The parasitic capacitance value (CPAR) derived from Equation 2 is equal to 1.678 pF.
According to the manufacturer's data sheet, the DC resistance (RDC) is equal to 300 mΩ. Alternating current resistance (RAC) is the peak impedance of the magnetic beads when they are purely resistive. Subtract RDC from Z to get RAC. Since the RDC is extremely small compared to the peak impedance, it can be ignored. Therefore, RAC is equal to 1.082 kΩ in this example. Use the ADIsimPE circuit simulation tool (powered by SIMetrix/SIMPLIS) to generate the relationship between impedance and frequency response. Figure 2a shows the circuit simulation model and provides calculated values; Figure 2b shows the actual measurement results and the simulation results. In this example, the impedance curve derived from the circuit simulation model strictly matches the measurement curve.
DC bias current considerations
In the design and analysis of the noise filter circuit, it is very helpful to adopt the ferrite bead model. For example, when forming a low-pass filter network together with decoupling capacitors, it is helpful to approximate the inductance to determine the resonant frequency cutoff. However, the circuit model in this article is an approximation in the case of zero DC bias current. This model may change with changes in the DC bias current, and in other cases a more complex model may be required.
Choosing the correct ferrite bead for the power application requires not only the filter bandwidth, but also the impedance characteristics of the bead relative to the DC bias current. In most cases, the manufacturer only specifies the impedance of the magnetic bead at 100 MHz and publishes the frequency response curve data sheet at zero DC bias current. However, when ferrite beads are used as power filters, the load current through the beads is always non-zero, and as the DC bias current increases from zero, these parameters will also change rapidly.
As the DC bias current increases, the core material begins to saturate, causing the ferrite bead inductance to drop significantly. Inductance saturation varies according to the material used in the component core. Figure 3a shows the typical DC bias dependence of two ferrite beads. When the rated current is 50%, the inductance drops by up to 90%.
If you need to filter the power supply noise efficiently, in terms of design principles, ferrite beads should be used at about 20% of the rated DC current. As shown in these two examples, at 20% of the rated current, the inductance drops to about 30% (6 A beads) and about 15% (3 A beads). The current rating of the ferrite bead is the maximum current value that the device can withstand under specified heating conditions, not the actual operating point for filtering.
In addition, the effect of the DC bias current can be observed by the reduction of the impedance value in the frequency range, which in turn reduces the effectiveness of the ferrite beads and the ability to eliminate EMI. Figure 3b and Figure 3c show how the ferrite bead impedance changes with the DC bias current. With only 50% of the rated current applied, the effective impedance at 100 MHz will drop significantly from 100 Ω to 10 Ω (TDK MPZ1608S101A, 100 Ω, 3 A, 0603), and from 70 Ω to 15 Ω (Würth Elektronik 742 792 510, 70 Ω, 6 A, 1812).
The system designer must fully understand the effect of the DC bias current on the bead inductance and effective impedance, as this may be important for applications that require high supply currents.
LC resonance effect
When ferrite beads are used together with decoupling capacitors, resonance spikes may occur. This often overlooked effect can hurt performance because it can amplify the ripple and noise of a given system rather than attenuate them. In many cases, this spike occurs near the usual switching frequency of DC-DC converters.
A spike occurs when the resonant frequency of the low-pass filter network (composed of ferrite bead inductance and high-Q decoupling capacitors) is lower than the crossover frequency of the magnetic beads. The filtering result is underdamped. Figure 4a shows the relationship between impedance and frequency measured by TDK MPZ1608S101A. Resistive elements (related to the dissipation of interference energy) have little effect until they reach the range of approximately 20 MHz to 30 MHz. Below this frequency, the ferrite bead still has a very high Q value and is used as an ideal inductor. The LC resonance frequency of a typical ferrite bead filter is generally in the range of 0.1 MHz to 10 MHz. For typical switching frequencies in the range of 300 kHz to 5 MHz, more damping is required to reduce the filter Q value.
Figure 4b shows an example of this effect; in the figure, the S21 frequency response of the magnetic bead and the capacitive low-pass filter show the peaking effect. The ferrite bead used in this example is TDK MPZ1608S101A (100 Ω, 3 A, 0603), and the decoupling capacitor used is Murata GRM188R71H103KA01 low ESR ceramic capacitor (10 nF, X7R, 0603). The load current is microampere level.
The undamped ferrite bead filter may exhibit spikes from about 10 dB to about 15 dB, depending on the Q value of the filter circuit. In Figure 4b, the spike appears at around 2.5 MHz and the gain is as high as 10 dB.
In addition, the signal gain is visible in the range of 1 MHz to 3.5 MHz. If this spike occurs in the operating frequency band of the switching regulator, there may be a problem. It can amplify interference switching artifacts and severely affect the performance of sensitive loads such as phase-locked loops (PLL), voltage-controlled oscillators (VCO), and high-resolution analog-to-digital converters (ADC). The result shown in Figure 4b is the use of extremely light loads (microampere level), but for the part of the circuit that only needs a few microamperes to 1 mA of load current or the part that is turned off in certain operating modes to save power consumption, this It is a practical application. This potential spike creates additional noise in the system, which may cause undesirable crosstalk.
Due to the inductance of the magnetic beads and the 10 nF ceramic capacitor, the resonance spike appears at about 2.5 MHz. There is a 10 dB gain instead of attenuating the fundamental ripple frequency at 2.4 MHz.
Other factors that affect resonance spikes are the series impedance and load impedance of the ferrite bead filter. Under the internal resistance of the power supply, the spike drops sharply and is weakened by damping. However, the use of this method will result in a decrease in load regulation, thus losing practicality. As the series resistance drops, the output voltage drops with the load current. The load impedance also affects the peak response. The spikes under light load conditions are more severe.
This section introduces three damping methods that the system engineer can use to greatly reduce the resonant spike level (see Figure 7).
Method A is to add a series resistor in the decoupling capacitor path, which can suppress the system resonance, but will reduce the effectiveness of the high-frequency bypass. Method B is to add a small value parallel resistance at both ends of the ferrite bead, which will also suppress the system resonance. However, the attenuation characteristics of the filter will decrease at high frequencies. Figure 8 shows the relationship between impedance and frequency of MPZ1608S101A with and without a 10 Ω parallel resistor. The light green dotted line indicates the total impedance of the magnetic beads using 10 Ω parallel resistance. The combination of magnetic bead impedance and resistance is greatly reduced and is mainly determined by the 10 Ω resistance. However, the 3.8 MHz crossover frequency when a 10 Ω parallel resistor is used is much lower than the crossover frequency of the magnetic bead itself at 40.3 MHz. In a much lower frequency range, the magnetic beads exhibit resistance, which can reduce the Q value and improve the damping performance.
Method C is to add a combination of a large capacitor (CDAMP) and a series damping resistor (RDAMP), usually this method is the best.
Adding capacitors and resistors can suppress system resonance without reducing the effectiveness of bypassing at high frequencies. Using this method can avoid the large DC blocking capacitor causing excessive power dissipation in the resistor. This capacitor must be much larger than the sum of all decoupling capacitors, which reduces the required damping resistance. At the resonance frequency, the impedance of the capacitor must be much smaller than the damping resistance in order to reduce spikes.
Figure 9. ADP5071 spectrum output with damping method C and a ferrite bead and capacitive low-pass filter. Generally speaking, Method C is the most elegant method. It is realized by adding a series combination of a resistor and a ceramic capacitor without purchasing expensive special damping capacitors. More reliable designs always include resistors, which can be easily debugged during prototyping and can be removed if they are not needed. The only disadvantage is the additional component cost and more board space. Conclusion This article discussed the key factors that must be considered when using ferrite beads. This article also details a simple circuit model, which represents a magnetic bead. The simulation results show a good correlation between actual measured impedance and frequency response at zero DC bias current. This article also discusses the influence of DC bias current on the characteristics of ferrite beads. The results show that a DC bias current exceeding 20% of the rated current may cause a large drop in the inductance of the magnetic beads. Such current will also reduce the effective impedance of the magnetic beads and weaken the EMI filtering ability. When using a ferrite bead with a DC bias current on the power supply rail, make sure that the current does not cause the ferrite material to saturate and cause a large change in inductance. Since ferrite beads are inductive, care should be taken when using them with high-Q decoupling capacitors. If you are not careful, it will cause interference resonance in the circuit, which will do more harm than good. The damping method proposed in this paper uses a series combination of a large decoupling capacitor and a damping resistor on the load, thereby avoiding interference resonance. The correct use of ferrite beads can efficiently and inexpensively reduce high-frequency noise and switching transients.