table of contents

・ Role of each capacitor
・ input capacitor
・ output capacitor

・ Characteristics required for capacitors

・ Characteristics important for selection
・ Rated voltage
・ DC bias characteristics
・ Equivalent series resistance: ESR
・ Equivalent Series Inductor: ESL

・ transient response
・ Load fluctuation: 1A current rises sharply, flows for 100μs, and falls sharply
・ Load change: 1A current slowly rises, flows for 100μs, then slowly falls

・ Position to place the capacitor
・ Influence of resistance
・ Effect of inductor

1. Role of each condenser

Ideally, a power supply line should contain only a pure DC component, but in reality, the input of the power supply line generates ripple current due to charging and discharging due to switching of the power supply IC and fluctuations in the input voltage. Output voltage ripple noise also occurs in the output section. The input capacitor has the role of allowing ripple current and stabilizing the input voltage, as well as reducing noise caused by ripple current. In addition, the output capacitor forms an inductor and an LC filter to smooth the output voltage and reduce the ripple noise of the output voltage. On the other hand, it has the role of responding to sudden changes (transient response) on the load side.

This time, we will divide the circuit diagram of "LT8609 Synchronous Buck Regulator Evaluation Board: DC2958A" into two parts, the input circuit and the output circuit. The noise suppression capacitor for the input circuit is C6, and the noise suppression capacitor for the output circuit is C4.

(1) Input capacitor

When a power supply IC performs switching operation, a ripple current is generated during charging and discharging. Ripple current is also generated due to input voltage fluctuations. This ripple current becomes noise in the input voltage of the power supply IC due to the parasitic inductor and resistance of the wiring. The input capacitor should be connected to the power supply line in the direction of branching the transient current that causes noise to the ground line. In this way, the input capacitor reduces these noises and stabilizes the input voltage of the power supply IC.

(2) Output capacitor

The SW output of the power supply IC must be smoothed by forming an LC filter with an inductor and a capacitor. This is the most important role of the output capacitor. In addition, it also has the role of reducing the ripple noise emitted from VOUT. Also, even if the load fluctuates transiently, it is necessary to supply a stable voltage in response to voltage fluctuations. This is also an important role of the output capacitor.

2. Characteristics required for capacitors

When dealing with capacitors in electronic circuits, we only consider capacitance, but this is the ideal case, and real capacitors have parasitic resistance and inductor (coil) components. The resistance component is called Equivalent Series Resistance (ESR), and the inductor component is called Equivalent Series Inductance (ESL). The reason why the abbreviation is ESL instead of ESI is that inductors are represented by L in general electronic circuits. The equivalent circuit looks like this:

Since ESR and ESL are parasitic elements in the first place, smaller ones are ideal. Therefore, when choosing an actual capacitor, choose one with low ESR and low ESL. The ESR and ESL values are listed in the datasheet.

​Multi Layered Ceramic Capacitor (MLCC) is the low ESR and low ESL capacitor. MLCCs are structurally able to achieve lower ESR and lower ESL compared to other capacitors, so they can continue to function as capacitors even at high frequencies. In other words, MLCCs are more effective at removing noise at higher frequencies, forming a high-performance filter.

However, the capacitance of MLCC changes with ambient temperature. This is due to the materials used in the MLCC, and is a phenomenon that occurs in all MLCCs, regardless of manufacturer. Therefore, the temperature characteristics of MLCCs are based on the standards established by the Japanese Industrial Standards (JIS) and the Electronic Industries Association of America (EIA). These standards are divided into temperature compensated and high dielectric constant types. There is a difference in the change in capacitance due to temperature, and it is necessary to use them properly according to each feature. For temperature compensation, the unit of temperature coefficient is (ppm/ °C), and for high dielectric constant type it is (%). A high dielectric constant system is sufficient for synchronous rectification converter circuits. For example, if the temperature characteristic symbol is at the level represented by X5R or X7R, it can be used sufficiently. X5R is-55 to +85°C, capacitance change rate ±15%, X7R is-55 to +125°C, capacitance change rate ±15%.

3.Characteristics important for selection

(1) Rated voltage

A capacitor has a rated voltage. Ensure that the voltage applied across the capacitor terminals is less than or equal to the rated voltage. "Applied voltage" includes not only the applied voltage under normal operating conditions, but also abnormal voltage such as surge voltage, static electricity, pulse at switch ON/OFF, and ripple voltage. When selecting the rated voltage, consider derating and select it on the premise that it will be used at 70 to 80% or less of the rated voltage.

(2) DC bias characteristics

The phenomenon in which the effective capacitance value of a capacitor changes according to the applied voltage is called voltage characteristics. Voltage characteristics when DC voltage is applied are called DC bias characteristics, and voltage characteristics when AC voltage is applied are called AC bias characteristics. In a synchronous converter circuit, DC voltage is applied to the capacitor, so DC bias characteristics are an important consideration.

The DC bias characteristic is a phenomenon in which the effective capacitance changes (decreases) when a DC voltage is applied to the capacitor. This phenomenon is unique to high-permittivity MLCCs that use barium titanate-based ferroelectrics, and rarely occurs in capacitors with dielectrics other than ceramics and temperature-compensating MLCCs. The image of characteristic change is shown in the figure below. Even among high-dielectric-constant MLCCs, if the temperature characteristics (X5R, Y5V, etc.) differ, the DC bias characteristics will also change. DC bias characteristics differ depending on the model number, so please check the data sheets, specifications, and technical notes of each manufacturer for specific characteristics.

Let's consider what kind of effect will occur if the capacitance changes due to the DC bias characteristics, etc.

Considering only the capacitance C, the impedance decreases as the frequency increases. Also, the noise removal and smoothing effects differ depending on the capacitance. Therefore, using a simulation, try changing the capacitance of the input capacitor C6 and check the state of the input voltage.

The circuit to be simulated is shown in Figure 1. In order to bring the simulation model closer to the actual circuit, consider wiring (cables, etc.) and add a pseudo resistor (R4=0.01 Ω) and inductor (L2=100nH). Also, considering the effect of wiring from the capacitor to the IC, add a resistor (R5=0.001 Ω) and an inductor (L3=0.01nH) in series with capacitor C6.

Comparing when the capacitance of the input capacitor C6 is 1​ ​μF and when it is 10​ ​μF. It can be clearly seen that 10​ ​μF is more stable.

Next, we turn our attention to the output capacitor. Try changing the capacitance of the output capacitor C4 and observe the variation of the output voltage. The circuit to be simulated is shown in Figure 2. Again, in order to bring the simulation model closer to the actual circuit, consider the effect of the wiring from the capacitor to the IC, and add a resistor (R6 = 0.001Ω) and an inductor (L4 = 0.01nH) in series with the capacitor C4. .

Comparing when the capacitance of the output capacitor C4 is 4.7​ ​μF and when it is 47​ ​μF, the voltage undulates in the case of 4.7​ ​μF, but is relatively stable in the case of 47​ ​μF. I understand this.

(3) Equivalent series resistance: ESR

ESR is listed in each product's datasheet. Although it is a constant value on the equivalent circuit, it actually changes depending on the frequency. An image of the characteristics is shown below. The selection criteria refer to the minimum ESR value. The ESR characteristics also differ depending on the model number, so please check the data sheets, specifications, and technical notes of each manufacturer for specific characteristics.

On the other hand, since the equivalent circuit of a capacitor is a series circuit of R, L, and C, it has a resonance point in its frequency characteristics. The resonance point is the frequency at which the L and C components cancel each other out, leaving only the R component.
Since only the R component appears as impedance at the resonance point, the impedance at the resonance point is considered to be equal to the ESR. If the ESR value or ESR characteristics are not listed in a data sheet, etc., the ESR can be obtained from the frequency characteristics. An image of the frequency characteristics is shown below. This also varies depending on the model number, so please check the datasheets, specifications, and technical notes of each manufacturer for details.

Using a simulation, let's check how ESR affects an actual power supply circuit.

First check the ESR effect of the input capacitor C6. Compare ESRs of 1.5mΩ and 15mΩ with a capacitance of 10​ ​μF. The simulation circuit uses Figure 1. From the simulation results, it can be seen that the ESR of 1.5mΩ has a smaller voltage fluctuation range.

Next, let's look at the output capacitor. The output capacitor C4 has a capacitance of 47​ ​μF and compares ESRs of 1.0mΩ and 10mΩ. The simulation circuit is shown in Figure 2. From the simulation results, we can see that the voltage fluctuation range is smaller when the ESR is 1.0mΩ.

(4) Equivalent Series Inductor: ESL

As mentioned earlier, the equivalent circuit of a capacitor is a series circuit of R, L and C, so the resonance frequency ​_f0 is

is obtained by C is the capacitance and L is the ESL. The resonance frequency can be found by looking at the frequency characteristics, so the ESL can be back calculated from this formula.

Let's use a simulation to check how ESL affects an actual power supply circuit. First, check the ESL effect of the input capacitor C6. Compare 0.1nH and 1.0nH​ ​ESL with a capacitance of 10​ ​μF. The simulation circuit uses Figure 1. From the simulation results, it is clear that 0.1nH has a smaller voltage fluctuation width (ripple, etc.).

Next, we turn our attention to the output capacitor. Compare 1.0nH and 10nH​ ​ESL with an output capacitor capacitance of 47​ ​μF. The simulation circuit is shown in Figure 2. From the simulation results, we can see that the voltage fluctuation range is smaller when the ESL is 1.0nH.

Now let's compare the ESL effects of the input and output capacitors.

When a current flows through an inductor in a circuit, a voltage fluctuation represented by L(di/dt) occurs. The greater the change in current (di/dt), the greater the voltage fluctuation. In order to minimize voltage fluctuations, the inductor components (capacitor ESL and wiring parasitic inductor) in the circuit must be minimized. To do this, use low-ESL capacitors, shorten capacitor mounting patterns, and minimize wiring parasitic inductors.

The current Iin (=I(L2)+I(R5)) flowing to the input side becomes a pulsed current as shown in the figure below.

On the other hand, the SW pin output of the power supply IC becomes a PWM waveform. Smooth that PWM waveform with an inductor and a capacitor. At this time, the current IL flowing through L1 becomes a triangular wave. The change in current is small compared to the current flowing on the input side, but even if such a triangular current flows, it must be supplied to the load with a voltage that fluctuates as little as possible with the output capacitor.

Comparing the current waveform on the input side and the current waveform on the output side, the current on the input side has a pulse-like current change, and the current on the output side has a triangular wave. Since the rate of change in current value (amount of current change per unit time) on the input side is much larger than that on the output side, the input side is less affected by ESL and the parasitic inductor of the capacitor mounting pattern than the output side. You can see that it comes out big.

4. transient response

Next, consider the role of the output capacitor when the load changes abruptly.

The output capacitor also plays a role in suppressing voltage fluctuations that occur when the load fluctuates sharply. Therefore, the transient response when the load changes sharply is checked by simulation with three types of C4 capacitance: 4.7 μF /​ ​47μF /​ ​100​ ​μF. The load adds a current source load and steeps the load current. The simulation circuit is shown in Figure 3.

(1) Load fluctuation: 1A current rises sharply, flows for 100μs, and falls sharply

Load regulation is 1A. Both the rise time and fall time are set to 0 s to change sharply. The load current flow time is 100​ ​μs. The simulation results figure shows the load current in green. From this result, it can be seen that the larger the capacitance, the smaller the voltage fluctuation range, but it takes longer for the fluctuation to settle down.

(2) Load fluctuation: 1A current slowly rises, flows for 100μs, then slowly falls

Under the same conditions for the capacitance of C4, we also simulated the case where the load current rises in 100​ ​μs and falls in 100 us. The load current flow time is also 100​ ​μs.
In this case as well, the larger the capacitance, the slower the voltage fluctuation.

Five. Position to place the input capacitor

Wiring from the capacitor to the IC also affects voltage fluctuations, so place the capacitor close to the power supply IC. Figure​ ​1 also considers the effect of the wiring from the capacitor to the IC​. Try it and check the effect of voltage fluctuation.

(1) Effect of resistance

When the value of R5 is changed to 0.001Ω, 0.01Ω, and 0.1Ω, and the simulation is performed, it becomes as shown in the figure below. There is no visible difference between 0.001Ω and 0.01Ω, but you can clearly see the voltage fluctuate with 0.1Ω.

(2) Effect of inductor

If you change the value of L3 to 0.01nH and 0.1nH and run a simulation, it will look like the figure below. Compared to 0.01nH, it can be seen that the voltage of 0.1nH clearly fluctuates.