What Is Parasitic Capacitance?

The meaning of parasitics is that capacitors were not designed in that place, but because there is always mutual capacitance between wirings, mutual capacitance is like parasitic between wirings, so it is called parasitic capacitance, also called stray capacitance.

Parasitic capacitance

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The meaning of parasitics is that capacitors were not designed in that place, but because there is always mutual capacitance between wirings, mutual capacitance is like parasitic between wirings, so it is called parasitic capacitance, also called stray capacitance.

Chinese name

Parasitic capacitance

Foreign name

stray capacitance

Alias

Stray capacitance

Application

Dynamic Read-Write Memory (DRAM)

parasitic capacitance;

stray capacitance

Parasitic capacitance generally refers to inductance,

Explanation of "parasitic capacitance" in academic literature

1. On the other hand, in addition to the capacitance between the electrodes, the electrode also has a capacitive connection with the surrounding body (various components and even the human body). This capacitance is called parasitic capacitance. It not only changes the capacitance of the capacitive sensor, but because the sensor itself has a small capacitance, the parasitic capacitance is extremely unstable, which also causes the sensor characteristics to be unstable and cause serious interference to the sensor.

2. Distributed capacitances distributed between wires, coils and casings, and between certain components. These capacitances are called parasitic capacitances. Although their values are small, they are an important cause of interference.

Dynamic read-write memory (DRAM) is widely used in microcomputers because of its fast speed, high integration, low power consumption, and low price. But dynamic memory has a different working principle than static memory. It relies on internal parasitic capacitors to charge and discharge to store information. The capacitors are charged with logic 1 and uncharged with logic 0.

In fact, due to the continuous increase in frequency, the effects of lead parasitic inductance and parasitic capacitance become more serious, causing greater electrical stress on the device (expressed as overvoltage, overcurrent glitches). In order to improve the reliability of the system, some manufacturers have developed "user-specific" power modules (ASPM), which installs almost all the hardware of a complete machine into a module in the form of chips, so that components are no longer There are traditional lead connections. Such modules have been designed strictly and reasonably in terms of heat, electricity, and machinery to achieve the perfect state of optimization. It's similar to users in microelectronics

Academic pictures related to "Parasitic Capacitance"

Simulation curve of parasitic capacitance

Annual change of the total literature

Power supply ripple and transient specifications will determine the size of the required capacitors and will also limit the capacitor's parasitic composition settings. Figure 1 shows the basic parasitic composition of a capacitor, which consists of equivalent series resistance (ESR) and equivalent series inductance (ESL), and shows three types of capacitors (ceramic capacitors, aluminum electrolytic capacitors, and aluminum polymer) in a graph. (Capacitor, capacitor). Table 1 shows the various values used to generate these curves. These values are typical of low voltage (1V 2.5V), medium current (5A) synchronous step-down power supplies.

Table 1: Comparison of three capacitors, each with advantages.

Comparison of three capacitors

At low frequencies, all three capacitors do not exhibit parasitic components because the impedance is obviously only related to capacitance. However, aluminum electrolytic capacitors stop decreasing in impedance and begin to exhibit resistance characteristics at relatively low frequencies. This resistive characteristic increases until it reaches a certain relatively high frequency (capacitor inductance). Aluminum polymer capacitors are another type of capacitor that is not ideal. Interestingly, it has a low ESR and the ESL is obvious. Ceramic capacitors also have low ESR, but because of their smaller case size, their ESL is smaller than aluminum polymer and aluminum electrolytic capacitors.

Change in impedance

Figure 1 Parasitic changes in impedance of ceramic, aluminum, and aluminum polymer capacitors

Figure 2 shows the power supply output capacitor waveform of a continuous synchronous regulator operating at 500kHz. It uses the main impedances of the three capacitors shown in Figure 1: ceramic capacitors; aluminum ESR; aluminum polymer ESL.

The red lines are aluminum electrolytic capacitors, which are dominated by ESR. Therefore, the ripple voltage is directly related to the inductor ripple current. The blue line represents the ripple voltage of the ceramic capacitor, which has a small ESL and ESR. The ripple voltage in this case is a component of the output inductor ripple current. Because the ripple current is linear, this results in a series of time-squared parts and the shape looks like a sinusoid.

Finally, the green line represents the ripple voltage, and its capacitor impedance is dominated by its ESL, such as aluminum polymer capacitors. In this case, the output filter inductor and ESL form a voltage divider. The relative phases of these waveforms are the same as we expected. When ESL is dominant, the ripple voltage guides the output filter inductor current. When ESR is dominant, the ripple is in phase with current, and when capacitor is dominant, its delay is. In reality, the output ripple voltage does not include voltage from only one of these components. Instead, it is the sum of the voltages of all three components. Therefore, some parts of the ripple voltage waveform can be seen.

Ripple voltage

Figure 2 Capacitors and their parasitic elements form different ripple voltages in a continuous synchronous buck regulator

Figure 3 shows the waveform of a deep continuous flyback or buck regulator. The output capacitor current can be positive and negative, and the specific state will change rapidly and continuously. The red line clearly shows the situation, the voltage of which is multiplied by the ESR and the result is a square wave. The voltage of the capacitor element is part of a square wave. It results in linear charging and discharging, as shown by the blue triangle waveform. Finally, the voltage of the capacitor ESL becomes apparent only when the current changes during the transition. This voltage can be very high, depending on the output current rise time. Note that in this case, the green line needs to be divided by 10 (assuming a 25 nS current transition). These large inductor spikes are one of the many reasons why two-stage filters often appear in flyback or step-down power supplies.

Waveform changes with continuous flyback or step-down output current

Figure 3 Waveform changes with continuous flyback or step-down output current

In summary, the impedance of the output capacitor helps improve ripple and transient performance. As the frequency of the power supply rises, the impact of parasitic problems is greater and should not be ignored. Around 20kHz, the ESR of aluminum electrolytic capacitors is large enough to dominate the capacitance impedance. At 100 kHz, some aluminum polymer capacitors exhibit inductance. As the power supplies enter the megahertz switching frequency, pay attention to the ESL of all three capacitors. ^[1]