In-depth understanding and application examples of decoupling capacitors in DC-DC power supply design

Many beginners in circuit design often underestimate the complexity of DC-DC power supply design, assuming it involves merely linear and switching power supplies. They may resort to finding a reference design and copying it without much consideration.

However, the power supply is a crucial aspect of circuit design, impacting the stability, performance, and overall cost of the system. Although it is a fundamental component, an excellent design places great importance on the power supply.

To illustrate this concept visually, I have created a comparison image where human organs correspond to the functional modules of common circuits. For instance, the brain represents the CPU, while the stomach symbolizes the storage part. Just like our heart continuously beats and pumps blood, the power supply plays an essential role in a circuit system.

Certainly, there are numerous aspects to discuss regarding power supplies. For example, when designing a switching power supply, it is necessary to consider the following:

1. Understand the various components that constitute a switching power supply.
2. Familiarize yourself with the electrical characteristics and circuit symbols of different components.
3. Gain knowledge of how to construct magnetic components.
4. Ensure correct assembly of all power supply components.
5. Comprehend the significance of different power supply indicators and learn how to perform relevant tests.
6. Skillfully use instruments to debug, optimize, and fine-tune the assembled power supply.
7. Analyze and summarize the results obtained from experimentation.
8. Continuously gather and absorb power-related knowledge from diverse sources.

Therefore, this article will not talk about it, but only pay attention to a small detail, which is the decoupling capacitor in the power supply. This is often the most neglected place in DC-DC power supply design. Many people are engaged in ARM, DSP, and FPGA. At first glance, they seem to be very sophisticated, but they may not be able to provide a cheap and reliable power supply solution for their own systems. The decoupling capacitor is an indispensable part of the power supply circuit. Let’s start from one point of action of the decoupling capacitor—the small pond function of the decoupling capacitor.

Also Read: How to judge whether the IC in the electronic circuit is working normally

In an electronic circuit system, the power supply module can be likened to a reservoir, while the decoupling capacitor corresponds to a small pond. The working chip or processor represents the cornfield. For the chip to operate reliably, it requires a stable power supply. However, if the voltage demand of the chip fluctuates and the power supply is unable to respond quickly enough, the decoupling capacitor acts as a temporary power source, providing the necessary power to maintain system stability.

It is common for many people to have a limited understanding of capacitance, often perceiving it as a simple “C.” However, capacitors possess several significant parameters. In reality, capacitors can be represented by the following circuit form:

C: capacitance value. Generally, it refers to the measurement under the condition of 1kHz, 1V equivalent AC voltage, and DC bias voltage of 0V, but there are also many different environments for capacitance measurement. But one thing to note is that the capacitance C itself will change with the environment.

ESL: Capacitive Equivalent Series Inductance. Capacitor pins have inductance. Inductive reactance is small in low frequency applications, so it can be ignored. When the frequency is high, this inductance must be considered. For example, a 0.1uF chip capacitor packaged in 0805 has an inductance of 1.2nH per pin, so the ESL is 2.4nH. It can be calculated that the resonant frequency of C and ESL is about 10MHz. When the frequency is higher than 10MHz, the capacitance is inductive. ESR: Capacitive Equivalent Series Resistance. No matter what kind of capacitor there is an equivalent series resistance, when the capacitor works at the resonance point frequency, the capacitive reactance and inductive reactance of the capacitor are equal in size, so it is equivalent to a resistor, which is ESR. There are great differences due to different capacitor structures. The ESR of aluminum electrolytic capacitors is generally several hundred milliohms to several ohms, ceramic capacitors are generally tens of milliohms, and tantalum capacitors are between aluminum electrolytic capacitors and ceramic capacitors. Common capacitors include aluminum electrolytic capacitors, ceramic capacitors and tantalum capacitors.

Aluminum capacitors are manufactured by crafting grooved and oxidized aluminum foil, which is then rolled with an insulating layer and soaked in an electrolyte solution. These capacitors operate based on chemical principles, as their charging and discharging processes rely on chemical reactions. The speed at which capacitors respond to signals is limited by the movement rate of charged ions within the electrolyte. Therefore, aluminum capacitors are commonly used in low-frequency (below 1MHz) filtering applications.

The Equivalent Series Resistance (ESR) of aluminum capacitors primarily consists of the sum of the aluminum foil resistance and the equivalent resistance of the electrolyte. This value tends to be relatively large in aluminum capacitors. Over time, the electrolyte of aluminum capacitors may gradually volatilize, resulting in a decrease in capacitance or even failure. Moreover, the rate of volatilization increases with rising temperatures.

2) Ceramic capacitors rely on physical reactions to store electricity, so they have a high response speed and can be applied to high-G occasions. However, ceramic capacitors also show great differences due to different media. The best performance is the capacitor made of C0G material, which has a small temperature coefficient, but the dielectric constant of the material is small, so the capacitance cannot be made too large. The worst performance is the Z5U/Y5V material, which has a large dielectric constant, so the capacitance can reach tens of microfarads.

3) A tantalum capacitor is like a battery both in principle and in structure.

Tantalum capacitors are indeed a type of electrolytic capacitors. They offer several advantages such as small size, high capacitance, fast response speed, and low Equivalent Series Resistance (ESR). However, they tend to be relatively expensive compared to other types of capacitors.

The capacity and withstand voltage of tantalum capacitors are determined by the size of the raw material tantalum powder particles. Finer particles result in larger capacitance values, while achieving a higher withstand voltage requires thicker Ta2O5 layers, which, in turn, necessitates the use of larger particles of tantalum powder. This makes it challenging to obtain a tantalum capacitor with both high withstand voltage and large capacity within the same volume.

Another point that needs to be paid attention to with tantalum capacitors is that tantalum capacitors are relatively easy to break down and have short-circuit characteristics, and their anti-surge capability is poor. Therefore, although tantalum capacitors have many advantages, they are easy to be broken down in some occasions, so more consideration must be given when applying them. Many times, we avoid tantalum capacitors. Current Logic has been manufacturing DC DC Power Supplies and Convertors.