Applications of Solid State Capacitors
Time：2022.04.24
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Applications of Solid State Capacitors
1. Ripple cancellation capability of solid-state capacitors
The development direction of switching power supplies during miniaturization, but capacitors are one of the components that occupy a large area in the components of the circuit board. In addition, the characteristics of capacitors generally vary greatly with the fluctuation of operating temperature, so capacitors need to be selected according to the operating temperature range.
The following experiments show the ability of solid-state capacitors to eliminate ripple at high frequencies over a wide operating temperature range.
1-1. The number of capacitors is different at the same ripple voltage
1) Experimental content
Use a general step-down switching power supply, set the operating temperature to 25°C, -20°C, and 70°C, respectively, and use solid capacitors, low-impedance aluminum electrolytic capacitors, and low-equivalent series tantalum capacitors for the output smoothing capacitors. ripple voltage for comparison.
The output smoothing capacitor (C) of the test circuit shown in the figure above uses a solid capacitor 100uF/6.3WV (6.3mmx6mm), and the ripple voltage values at ambient temperatures of 25°C, -20°C, and 70°C were measured.
Next, the residual ripple voltage equivalent to that when using a solid capacitor of 100uF/6.3WV was measured at each temperature (25°C, -20°C, 70°C) by selecting a low-impedance aluminum electrolytic capacitor and a low-equivalent series resistance tantalum capacitor. .
Finally, at 25°C, using the same number of output smoothing capacitors, measure the residual ripple voltage at each temperature (-20°C, 70°C), and obtain the equivalent series resistance of the smoothing capacitor from the variation. rate of change.
1-2. The difference between the ripple voltage before and after the durability test qi
1) Experimental content
The step-down switching power supply is used, and the output smoothing capacitors are solid capacitors and low-impedance aluminum electrolytic capacitors. Compare the change of output ripple voltage before and after the endurance test (125℃×rated voltage×100h). Ripple voltage measurements were performed in an ambient environment of 25°C, 0°C, and -20°C.
2. High-speed protection capability of solid capacitors (protective capacitors for load fluctuations)
The processing speed of ICs, especially MPUs, used in the latest electronic equipment is increasing. At the same time, in order to increase the density, the operating voltage is reduced and the circuit density is reduced. With the reduction in voltage, the load current of future MPUs will tend to increase. Rapid changes in load current with high-speed and large-load fluctuations will directly cause MPU malfunctions due to fluctuations in the power supply line. The excellent protective ability of solid-state capacitors is compared with other capacitors and the evaluation results are as follows:
2 -2. Test results
1) Comparison of the same capacity
When compared with the same capacity as shown in the figure below, the voltage drop of the power supply line is 104mV relative to the solid capacitor, the low impedance aluminum electrolytic capacitor is 548mV (about 5.3 times that of the solid capacitor), and the low equivalent series resistance capacitor is 212 mV (about 2 times that of solid-state capacitors)
2) A: Select a capacitor that can obtain the same degree of load change
As shown in the figure below, to obtain the same level of voltage drop as 1OSP100M, the use of low-impedance electrolytic capacitors requires a capacity of more than 1500UF, and the use of low-equivalent series resistance Tan capacitors requires more than 22OUFX2.
B: A capacitor is used at low temperature (-20 degrees Celsius for comparison)
As shown in the figure below, there is no change in solid capacitors when compared at low temperature, the voltage drop of low-impedance electrolytic capacitors is about 3.2 times, and the voltage of low-ESR capacitors is about 1.2 times.
3. Application in low-pass filter circuit
In order to remove the noise of the power supply line, low-pass filtering as shown in the figure below can be used.
In recent years, switching power supplies featuring miniaturization and high efficiency have been widely used, but on the other hand, switching power supplies will become a large noise source. Digital lines are also a source of noise, so almost all of the devices with analog lines that are not resistant to noise are connected to such low-pass filters on the power supply lines of the analog lines to prevent high-frequency noise from entering the analog line. line.
The equivalent series resistance of the capacitor can affect the attenuation rate of the filter. The lower the equivalent series resistance, the better the attenuation effect. This is because the electrostatic capacitance of the capacitor and the equivalent series resistance component produce a zero point, and high frequencies higher than the zero point frequency will resist the attenuation effect at +20dB/dec. That is, when the LC filter is from -40dB/dec to -20dB/dec, and the RC filter is from -20dB/dec to 0, the attenuation effect disappears.
Therefore, even if the electrostatic capacity of the capacitor is increased, there is no noise reduction effect, which is mostly affected by this zero-point phenomenon. The equivalent series resistance of solid-state capacitors is small, so they are most effective in low-pass filtering.
Next, let's take a look at the actual attenuation effect by comparing it with aluminum electrolytic capacitors.
The capacitors used for, with comparison are as follows:
•Solid capacitor: 16V/33UF, equivalent series resistance = 37mΩ (16SA33M) ※The equivalent series resistance is the measured value.
• Aluminum electrolytic capacitor: 10V/33UF, equivalent series resistance = 1410mΩ
In any case, the attenuation effect of solid-state capacitors is maintained to the high frequency range.
This is only measured at room temperature, and the difference will be more obvious if the test is performed at low temperature (especially below 0 °C). This is because the equivalent series resistance of aluminum electrolytic capacitors increases extremely at low temperatures, while the equivalent series resistance of solid-state capacitors hardly changes at low temperatures, so it does not affect the attenuation effect of filtering.
4. Smoothing capacitors for switching power supplies
When selecting the output smoothing capacitor of the switching power supply, in order to suppress the output ripple voltage, the equivalent series resistance (ESR) of the selected capacitor should be as small as possible. However, if the capacitor with small ESR is used, abnormal oscillation of the output voltage may occur.
The possibility of abnormal oscillation of the output voltage varies depending on the control method used and the so-called circuit methods such as buck type and boost type. The switch controller is used as an example to illustrate.
1. Abnormal oscillation of output voltage
In order to stabilize the output voltage, there is usually a negative feedback circuit on the switching power supply.
Figure 1 is a schematic diagram of the control part.
After the error between the output voltage and the reference voltage Vref is amplified by the error amplifier, it is converted into a digital signal through PWM comparison to open and close the switch Q1, the input voltage Vin is changed into a rectangular wave through the switch Q1, and then the DC output is obtained by smoothing the inductor L and the capacitor Gout. VoltageVouto
Therefore, L and Cout form a 2-order low-pass filter.
The frequency change of the output LC filter is shown in the network diagram in Figure 2. In addition, the phase lag of the error amplifier is originally 180 degrees due to the negative feedback circuit, so when the phase lag of the output LC filter and the phase lag of the error amplifier repeat 360 degrees, the output voltage will oscillate. Here we consider what is the most ideal LC filter. The attenuation rate of the LC filter is -40dB/dec, and the cutoff frequency of the filter is the gain (Gain) and phase (Phase) shown in Figure 2.
An ideal LC filter has a phase lag of 180 degrees, and as such, oscillations occur. In fact, the frequency characteristics are shown in practice, and the decay rate of the gain varies from -40dB/dec to -20dB/dec above a certain number of frequencies, and the phase lags to 90 degrees. This is because Gout
The capacity value and ESR form a lead line at a frequency of O
Afterwards, the attenuation rate of the gain is +20dB, and a phase advance of +90 degrees is added.
However, LC filtering using capacitors with small ESR reaches a higher frequency band, and the LC filtering phase lags close to 180 degrees and is prone to oscillation. In order to prevent the output voltage from oscillating, a margin of more than 30-40 degrees should be left in the phase on a general negative feedback circuit. The phase margin refers to the value from the lower limit of the phase to -180 degrees. The smaller the phase margin, the greater the possibility of vibration caused by poor performance of components and temperature changes.
2. Methods to prevent oscillation from occurring
Oscillation of the output voltage can be prevented by utilizing phase correction of the error amplifier feedback circuit. There are many types of phase correction routes, and the following phase correction routes are the most effective for voltage control switching power supplies.
In Fig. 3, 2 and 4 form a first-order lead line, and 1 and 3 form a first-order lag line. By adjusting these lines, the phase correction in which the phase lead occurs in the frequency band with the lowest phase limit in the frequency characteristics of the LC filter is performed to improve the negative. The phase lag of the feedback circuit as a whole. Figure 4 is an example of its adjustment.
The phase of the output filter of Figure 2 reaches its lowest point around 10KHz, at which frequency the phase lead is about 30 degrees. Therefore, even if the phase lag of the LC filter is close to 180 degrees, there is a margin of about 30 degrees, so that the oscillation of the output voltage can be prevented.
1. Specific example of preventing oscillation
Next, the following specific design examples are introduced.
Figure 5 is an example of a step-down DC-DC converter design using a power control IC manufactured by ROHM.
Calculate the equivalent series resistance of the output capacitor required to adjust the output ripple voltage to 20mVp-p as follows. Equivalent Series Resistance (ESR) Choose the following capacitors:
1) Solid State Capacitors
6V/1 0 OMf 1 P C 6.3ØX6L Equivalent series resistance (ESR) =32 mΩ※The equivalent series resistance is the measured value
2) Aluminum electrolytic capacitors
6V/180UF 3P C S Parallel 10ØX8L Equivalent Series Resistance (ESR) =128 mΩ/piece→Total Equivalent Series Resistance (ESR) =43mΩ
Photo 1 (a) and (b) are the measurement lines using the above capacitors. The following will examine the reasons for the miniaturization that can be achieved using solid capacitors compared to aluminum electrolytic capacitors with the addition of optimal phase compensation circuits.
4. Design example when using aluminum electrolytic capacitors
When aluminum electrolytic capacitors are used, the frequency characteristics of the output LC filter are shown in Figure 6, and there is sufficient leeway for the phase, even without phase compensation. Therefore, it is sufficient to use the circuit shown in Figure 7 for the phase compensation circuit.
Using the phase compensation circuit shown in Figure 7 (actually no phase compensation is performed), the overall frequency characteristics are shown in Figure 8, and it can be said that there is sufficient phase margin.
Figure 9 lists the output ripple voltage waveform.
5. Design example when using solid capacitors
If the power supply line using aluminum electrolytic capacitors is replaced with a solid capacitor without changing the phase compensation line, the output voltage will oscillate (Figure 10). This is because the frequency characteristics of the output LC filter will change from the state of Figure 6 when using aluminum electrolytic capacitors to the state of Figure 11 after switching to a solid capacitor with a low equivalent series resistance, but since the phase compensation new circuit is not changed, the phase The remainder disappears.
As shown in Figure 11, when the LC filter base has no phase margin, the phase compensation circuit shown in Figure 12 can be used to perform appropriate phase compensation.
This is because the part where the phase is slowed down in Zi, Zc in Fig. 12 forms the phase to be accelerated, thereby eliminating the phase slowdown. Therefore, the green frequency characteristic is shown in Figure 13, and its phase margin is also sufficient, and the output ripple voltage waveform (14) is also basically the same as that of the aluminum electrolytic capacitor.
6. The influence of the output waveform of the switching power supply on the actual image
