Consider self-resonant frequency when selecting components

qwqwqw2088

Consider self-resonant frequency when selecting components [Copy link]

When designing a simple DCDC circuit, after preliminary calculation, you can start selecting the inductor based on the parameters of the inductor DC resistance (DCR), capacitor rated ripple current and ESR, which are the first parameters to pay attention to. Similarly, in the RF LNA circuit, the first thing to care about is the Q value of the RF choke. It is a consensus that the Q value of the multilayer ferrite bead is too low and should not be used. However, the self-resonant frequency (Self-Resonant Frequency) of the inductor and capacitor components is a parameter that is easy to be overlooked. In MHz DCDC and RF LNA circuits, the self-resonant frequency of passive components needs to be paid proper attention. The datasheet of C0G/NP0 low-loss capacitors and high-Q RF winding inductors generally actively marks the specific value and test method of the self-resonant frequency. Simply put, capacitors are only useful as capacitive components in the range below the self-resonant frequency, and inductors are only useful as inductors in the self-resonant frequency.

Figure 1 SRF information in the Murata LQW18AS series spec

In the RLC circuit, when the attenuation provided by the system damping R is insufficient, the capacitive reactance and the inductive reactance cancel each other out, and energy is transferred back and forth between the LCs. This is "resonance". The leads of the plug-in capacitor, the high-density metal electrodes inside the MLCC, and the welding terminals can all provide a small amount of parasitic inductance, which is the fundamental reason for the "self" resonance of discrete capacitor components.

Figure 2 Parasitic inductance caused by capacitor leads

MLCC has a classic V-shaped impedance-frequency curve. As the frequency increases, the influence of parasitic inductance begins to become prominent, and the impedance first decreases and then increases. This is an inherent characteristic of MLCC. The lowest point in the curve is the self-resonant frequency of MLCC.

Figure 3 The position of the self-resonant frequency in the curve

Generally speaking, the leads of straight plug-in are longer, and their parasitic inductance is larger than that of SMD; electrolytic capacitors have a large amount of aluminum foil with a winding structure inside, and their parasitic inductance is larger than that of capacitors with other processes. The internal electrode area and solder terminal cross-section of 1206-size MLCC are obviously larger than those of 0805, so the larger the size of the MLCC, the lower the self-resonant frequency.

Figure 4 Relationship between self-resonant frequency and size

In fact, the self-resonant frequencies of large-size and high-capacitance MLCCs from different manufacturers are mostly distributed in the range of 1-3MHz, which happens to be the switching frequency range of the current embedded system pursuing small solution size DCDC chip solutions. When designing a MHz DCDC circuit, add the capacitor self-resonant frequency to the checklist to calculate whether the effective capacitance of the capacitor is sufficient at high frequencies. An efficient filter bank is beneficial for reducing the BOM cost of onboard DCDC in embedded systems and improving the system's power density and PCB component density.

Figure 5 Samsung SEM CL31A106KPHNNNE

Figure 6 Murata GRM188R61A106KAAL

The RF choke on the DC bias path of the LNA circuit provides effective isolation between different amplifiers to avoid oscillation - provided that your inductor does not work beyond the self-resonant frequency. The highest Q value is the self-resonant frequency point. As the frequency rises, the parasitic capacitance between the windings begins to cause trouble, and the Q value drops rapidly, which will reduce the working efficiency of the circuit. There are many manufacturers that can produce SMD RF winding inductors, and you can choose according to your needs.

Figure 7 Murata LQW18AS56NJ00

The signal chain of the RF circuit often requires some small capacitors to block DC. The capacitors are connected in series in the signal path. The magnitude of the parasitic inductance and capacitance value generally does not have a significant impact on the signal quality, but it is necessary to confirm whether the withstand voltage of the capacitor meets the signal power requirements. At the same time, try to choose models with excellent S21 performance to reduce insertion loss.

我喜欢玩游戏

This is amazing. I have studied it for a long time but still don't understand it.

电容器

Nice post, learned a lot