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Understanding RF inductor specifications BY CHRIS HARE Applications Engineer Coilcraft, R F inductor selection involves these key parameters: mounting (surface mount or through-hole), inductance value, current rating, DC resistance (DCR), self-reso­nant frequency (SRF), Q factor, and temperature rating. While small size is typically desired, the laws of physics limit how small an inductor can be for a given application. Inductance value and current rating are the chief deter­minants of size; other parameters can then be optimized. What determines my required inductance value? If the inductor is to be used as a simple one-element (first-order) high-frequency choke, the selection is based on the frequency of peak noise to be choked out. At the self-resonant frequency (SRF) of an inductor, the series impedance is at its maximum.

So for a simple RF choke the selection is based on finding an inductor whose SRF is near the frequency where choking is needed. For higher-order filters, the inductance value for each element must be calculated based on the filter cutoff fre­quency (for low-pass and high-pass filters) or bandwidth (for bandpass filters). These calculations are frequently performed using commercial circuit simulation programs, such as SPICE, AWR’s Microwave Office, and Agilent’s Genesys or ADS. For tuned circuits or impedance matching, a tight inductance tolerance is desirable. As shown in Table 1, wirewound inductors typically achieve tighter tolerances than multilayer or thick-film type inductors. Table 1 Inductance (nH) Inductance tolerance Q at 1.8 GHz Current rating (mA) Wirewound (Coilcraft 0402HP-2N7XGL) 2.7 2% 85 1,500 Multilayer (TDK MLK1005S2N7ST) 2.7 11% 31 500 Inductance (nH) Inductance tolerance Q at 900 MHz Current rating (mA) Wirewound (Coilcraft 0402HP-68NXGL) 68 2% 50 310 Multilayer (TDK MLK1005S68NJT) 68 5% 20 150 How does my current requirement affect my selection? Higher current requires larger wire or more strands of the same wire size to keep losses and temperature rise to a minimum.

Larger wire lowers the DCR and increases the Q, but at the expense of larger part size and possibly lower SRF. The current ratings shown in Table 1 indicate that wirewound inductors are superior to multilayer inductors of the same size and inductance value.

Higher current capacity and lower DCR can be achieved by using a ferrite core inductor with a lower turn count. Fer­rite, however, may introduce new limitations such as larger variation of inductance with temperature, looser tolerances, lower Q, and reduced saturation current ratings. Ferrite inductors with open magnetic structures, like Coilcraft’s LS series, will not saturate, even at full rated current. It looks like my current requirement determines the DC resistance. Current ratings and DCR are closely related.

Knockout 2 Photoshop Free Download. In most cases, if all other parameters are held equal, a larger part size is needed to decrease the DCR. How high must the self-resonant frequency be for the inductor to work? S elf-resonant frequency (SRF) is defined by the equation: SRF = f 0 = 1/2π√LC For choke applications, the SRF is the frequency that provides the best signal blocking.

At frequencies below the SRF, impedance increases with frequency. At the SRF, impedance is at its maximum. At frequencies above the SRF, impedance decreases with frequency. For higher-order filter or impedance-matching applications, it is more important to have a relatively flat inductance curve (constant inductance vs.

Frequency) near the required frequency. This suggests selecting an inductor with an SRF well above the design frequency. A rule of thumb is to select an inductor with an SRF that is a decade (10×) higher than the operating frequency. In general, the choice of inductance value typically determines the SRF and vice versa.

The higher the inductance value, the lower the SRF, due to increased winding capacitance. Inductance and impedance vs. Frequency Inductance and impedance rise sharply near an inductor’s self resonant frequency (SRF) as shown in Fig. For choking applications, chose an inductor whose SRF is at or near the frequency to be attenuated. For other applica­tions, the SRF should be at least 10 times higher than the operating frequency. 1: Inductance and impedance for a 100-nH wirewound inductor. When is Q factor important?

A high Q value results in a narrow bandwidth, which is important if the inductor is to be used as part of an LC tank (oscillator) circuit or in narrow bandpass applications. Refer to Fig. High Q also leads to low insertion loss, minimizing power consumption. 2: High Q gives narrow bandwidth and low insertion loss. Q factor of an inductor is calculated from: Q = Im[Z]/Re[Z] All frequency dependent real and imaginary losses are included in the measurement of Q, including inductance, capacitance, skin effect of the conductor, and core losses from the magnetic material.

As indicated in Table 1, wirewound inductors have much higher Q values than multilayer inductors of the same size and value. How do I select the temperature rating I need?

Power loss increases with current and DC resistance, causing the component’s temperature to rise. Inductors are typically rated for a specific ambient temperature and a temperature rise above ambient due to current through the inductor. For example, a part rated for 125°C ambi­ent and a 15°C rise due to the full current rating (I rms or I dc) will have approximately a 140°C maximum part temperature. You only need to verify that your application ambient temperature and current draw do not exceed the inductor ratings. How can I quickly search for inductors that have all of the characteristics I need? Comparing data sheets of inductor manufacturers can be time consuming.

Coilcraft’s Inductor Finder tool ( www.coilcraft.com/apps/finder_apps/finder.cfm) allows selection of inductors based on six different parameters. By entering just the inductance value and operating frequency, a list of all inductors that meet your requirements is presented. Download Cms Software For Avtech Dvr.

This application note presents the types of core materials used in power inductors. It also briefly discusses the core saturation, core loss, as well as the Equivalent Series Resistance (ESR).Power inductors come in all shapes and sizes and are made from a variety of core materials. For example, the Coilcraft SLC series uses a gapped ferrite core, whereas the MLC series uses a powder core. The MVR series is offered in two different materials (the MVRT uses ferrite and the MVRC uses powder).*Core Saturation*All soft magnetic materials require an air gap in the core material in order to withstand the current without core saturation that is required for power applications.

Creating a space in the core magnetic flux path gaps ferrite materials. Grinding the gap in one-piece core or placing a non-magnetic spacer between halves of assembled cores achieves these gaps. On the other hand, powder cores such as powdered iron, have by their very nature miniature air gaps distributed throughout the material. It is typically not necessary to add an actual air gap or space in powder cores.The physical difference in the way the air gap is accomplished helps explain the performance difference.

It is easy to imagine that in the powder cores the distribution of a large number of tiny air gaps do not all saturate at exactly the same flux level (applied current), whereas in the ferrite air core gap, saturation does happen in a much more all-at-once fashion.The result is that powder cores have a more gradual saturation characteristic than ferrite. Figure 2 demonstrates the difference. The curves illustrate how the 0.36 µH MVRC (powder core) inductors saturate. The MVRT (blue curve) stays flat longer, but then drops off more abruptly than the MVRC (red curve). These inductors have exactly the same winding and core size, differing only in the core material.

If the peak inductor current is less than 20 A, the ferrite core is likely to be preferred core choice, whereas the powdered iron core would be preferred at higher peak currents. This is an especially important characteristic in late generation VRM/VRD power supplies with high transient current requirements where inductor current is likely to spike well above the operating waveform.*Core Loss*At first glance, core loss would also seem be a relatively simply characteristic that could be considered as a trade-off against core saturation. The vast (order of magnitude) difference in core loss between different core material types certainly suggests that ferrite would be preferred material to powder cores. If core loss alone were considered, that would certainly be true.*ESR*In order to predict inductor performance it is helpful to have one figure of merit that includes all the losses one figure of merit that includes all the losses, regardless of their physical cause. One useful tool is the Equivalent Series Resistance (ESR), which can be measured directly and includes the effective ac and dc resistances of an inductor.Consider the following ESR curves for the same two MVR series inductor shown above. The curves indicate the total loss does not differ as much as the core loss alone would indicate.

For example, the core loss at 100 kHz is vastly different (100x), but the overall ESR shows the parts indistinguishable. The fact that the curve is still flat at that point indicates that the DCR is the dominant loss mechanism for these inductors in that range. At 1 MHz to 5 MHz, a popular range for today’s converters, the ESR is substantially higher for the powdered iron core compared to the ferrite.*Using the ESR*Even knowing the ESR of an inductor alone does not answer the question of how much loss there will be in any given application. Much still depends on the wave shape.

For example, Example 1Assume a converter is needed to provide an output of 3.3 V at 20 A (66 W). The ESRs at 500 kHz are as follows:ESR (MVR1247C-361) = 0.035 ESR (MVR1251T-361) = 0.017Since a buck converter inductor current average is equal to the DC load current, we might try to calculate the loss by I2R = I2 x ESR. By this method we can calculate the loss for each inductor.Power loss for MVR1247C-361 = 14 W Power loss for MVR1251T-361 = 6.8 WBased on this simple example it would seem that a designer might not choose to use either of these inductors. The powdered iron part would have losses of about 21% of the output power, and even the ferrite part about half of that. However, we have not properly considered the wave shape of the inductor current. Performance will (probably) be much better than the ESR curve might predict. Figure 3 shows a very simplified version of a possible buck converter waverform.

In this case it is assumed that the inductor current is continuous and that the ripple current is relatively small compared to the average current. Let’s assume that the ripple current peak-to-peak is about 10% of the average current.

From the example, this means:Idc = 20 A Ip-p = 2 AIn order to predict the inductor losses correctly, the high frequency loss and the low frequency loss are individually calculated and then combined. The low frequency loss is calculated by I2R, where R is the DCR and I is the rms value of the dc load and the ripple current combined.