Parasitic means that there is no capacitance designed in that place, but there is always mutual capacitance between the wiring, which is like parasitic between the wiring, so it is called parasitic capacitance, also called stray capacitance.
Parasitic capacitance generally refers to the capacitance characteristics of inductance, resistance, chip pin, etc. under high frequency.In fact, a resistance is equivalent to a capacitor, and the series connection of an inductance and a resistance is not obvious at low frequencies, while at high frequencies, the equivalent value will increase and cannot be ignored.We should take it into account in the calculation.ESL is equivalent inductance, ESR isEquivalent resistance。Whether it's resistance, capacitance, inductance, ordiode，triode，MOS tube, andICIn the case of high frequency, we should consider their equivalenceCapacitance value, inductance value.
Academic explanation
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Explanation of "Parasitic Capacitance" in Academic Literature
1. On the other hand, except between the platescapacitanceIn addition, the electrode plate is also connected with the surrounding body (various components and even human body), which is called parasitic capacitance.It not only changedCapacitive sensorBecause the capacitance of the sensor itself is very small, the parasitic capacitance is extremely unstable, which also leads to the instability of the sensor characteristics and serious interference to the sensor.
2. Distributed between wires, coils andCasingBetween and between some elementsdistributed capacitance These capacitors are called parasitic capacitors. Although their values are small, they are an important cause of interference.
application
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Dynamic read and writememory（DRAM）, with its high speedIntegrationHigh power consumption, low priceMicrocomputerIt is widely used in.butDynamic memorywithStatic memoryThere are different working principles.It is charged and discharged by the internal parasitic capacitorMemory informationIf the capacitor is charged, it is logic 1; if it is not charged, it is logic 0.
In fact, due to the continuous increase of frequency, the influence of parasitic inductance and capacitance of the lead wire becomes more serious, resulting in greater electrical stress on the device (shown asOvervoltage、OvercurrentBurr).To improveSystem reliability, some manufacturers have developed "user specific"Power module(ASPM), which installs almost all the hardware of a complete machine into a module in the form of a chip, so that there is no traditional lead wire connection between components. Such modules are optimized through strict and reasonable thermal, electrical and mechanical design.It is similar tomicroelectronicsUsers inASIC(ASIC)。Just putcontrol software Write themicroprocessorChip, and then fix the whole module on the corresponding heat sink to form a new switching power supply device.It can be seen that the purpose of modularization is not only to make it easy to use and reduce the volume of the whole machine, but also to cancel the traditional wiring and minimize the parasitic parameters, so as to minimize the electrical stress on the device and improve the reliability of the system.
Relevant interpretation
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Academic pictures related to "parasitic capacitance"
Simulation curve of parasitic capacitance
Annual change pattern of total amount of relevant documents
Power Ripple And transient specifications will determine the requiredcapacitorAt the same time, it will limit the parasitic composition setting of the capacitor.Figure 1 shows the basic parasitic composition of a capacitor, which consists ofEquivalent series resistance(ESR）And equivalent series inductance（ESL）Composed withdiagramThere are three types of capacitors（Ceramic capacitorAluminumElectrolytic capacitorAnd $aluminum polymer capacitor).Table 1 shows the values used to generate these curves.These values are for low voltage (1V – 2.5V), medium current (5A) synchronous step-down power supplyTypical value。
Table 1: Comparison of three capacitors, each with advantages.
Comparison of three capacitors
At low frequency, all three capacitors do not showparasitic components , becauseimpedanceIt is obviously only related to capacitance.But,Aluminum electrolytic capacitorThe impedance stops decreasing and starts to show resistance characteristics at relatively low frequencies.This resistance characteristic continues to increase until a relatively high frequency is reached (the capacitor appears inductance).Aluminum polymer capacitor is another kind of capacitor that does not conform to the ideal condition.Interestingly, it has a low ESR, and the ESL is obvious.Ceramic capacitorIt also has low ESR, but because of its smaller housing size, its ESL is smaller than that of aluminum polymer and aluminum electrolytic capacitor.
Change of impedance
Fig. 1 Different Impedance Changes of Parasitics on Ceramic, Aluminum and Aluminum Polymer Capacitors
Figure 2 shows continuous synchronization operating at 500kHzRegulatorSimulated power output capacitor waveform.It uses the main impedances of the three capacitors shown in Figure 1:Ceramic capacitor;Aluminum ESR;Aluminum polymer ESL.
The red line isAluminum electrolytic capacitor, which is dominated by ESR.Therefore,Ripple voltageAnd inductanceRipple currentDirectly related.The blue line represents the ripple voltage of the ceramic capacitor, which has small ESL and ESR.The ripple voltage in this case is part of the ripple current of the output inductor.Since the ripple current is linear, this results in a series of time squared parts, and the shape looks likeSine curve。
Finally, the green line representsRippleVoltage, and its capacitor impedance is dominated by its ESL, such as aluminum polymer capacitor, etc.under these circumstances,Output filterInductance and ESL form aVoltage divider。The relative phase of these waveforms is the same as we expected.When the ESL is dominant, the ripple voltage guides the inductance current of the output filter.Ripple and current when ESR is dominantin-phaseWhen the capacitor dominates, the delay occurs.In reality, the output ripple voltage does not contain only the voltage from one of these elements.Instead, it is the sum of all three component voltages.Therefore, some parts can be seen in the ripple voltage waveform.
Ripple voltage
Fig. 2 Capacitor and its parasitic elements form different ripple voltages in the continuous synchronous step-down regulator
Figure 3 shows a depth continuousFlybackOr the waveform of the step-down regulator, whose output capacitor current can be positive and negative, and the specific state will continueRapid change。The red line clearly indicates this situation, and its voltage is obtained by multiplying this current by ESR, and the result is asquare wave。Capacitor elementThe voltage of is part of the square wave.It causes linear charging and discharging, as shown in the blue triangle waveform.Finally, the voltage of capacitor ESL is obvious only when the current changes during the transition period.This voltage can be very high, depending on the rise time of the output current.Note that in this case, the green line needs to be divided by 10 (assuming a 25 nS current transition).These large inductance spikes are often double stage in flyback or step-down power supplywave filterOne of the many reasons.
The waveform changes with the continuous flyback or step-down output current
Figure 3 Waveform changes with continuous flyback or step-down output current
In conclusion, the impedance of the output capacitor helps to improve the ripple andTransient performance。With the increase of power supply frequency, the influence of parasitic problems is greater and should not be ignored.Near 20kHz,Aluminum electrolytic capacitorThe ESR of is large enough to dominate the capacitive impedance.At 100kHz, some aluminumPolymer capacitorShows inductance.Power supply enters megahertzswitching frequency Please pay attention to the ESL of all three capacitors.[1]
