“The output voltage waveform when the output capacitor of the DC-DC converter is changed from a tantalum electrolytic capacitor to a chip multilayer ceramic capacitor. The switching frequency of the DC-DC converter is 300MHz. The capacity of the tantalum electrolytic capacitor is 100μF. Use three 22μF chip multilayer ceramic capacitor products. The ripple voltage before the replacement was 56mV, but after replacing it with a chip multilayer ceramic capacitor, it dropped to 7mV.
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There are two main reasons for replacing tantalum electrolytic capacitors with chip multilayer ceramic capacitors.
It is a reliability issue. Tantalum electrolytic capacitors may cause smoke and fire in the event of a short-circuit failure. Smoke and fire are fatal to Electronic products equipped with tantalum electrolytic capacitors.
The other is the problem of raw material tantalum. Tantalum is a rare metal, and its origin is one of the few in the world. Therefore, if there is political turmoil in the production area, it will fall into a situation of skyrocketing prices and unstable supply. As long as the raw materials are rare metals, it is impossible for users of tantalum electrolytic capacitors to completely avoid such risks.
The solution to these problems is to replace tantalum electrolytic capacitors with chip multilayer ceramic capacitors. Chip multilayer ceramic capacitors are much less likely to smoke and catch fire than tantalum electrolytic capacitors. In addition, since rare metals are not used, prices and supply are more stable. And there are some advantages that tantalum electrolytic capacitors do not have.
There are two main advantages. One is to reduce the mounting area. This is because the electrostatic capacitance per unit volume of chip multilayer ceramic capacitors is relatively large. The other is to reduce the output ripple voltage when used in output smoothing circuits such as DC-DC converters. The reason is that the equivalent series resistance (ESR: Equivalent Series Resistance) of chip multilayer ceramic capacitors is low. As shown in Figure 1, when a tantalum electrolytic capacitor is used, the output ripple voltage is 56mV, while it is reduced to 7mV when a chip multilayer ceramic capacitor is used.
Note when used for output smoothing purposes
However, the work is not over simply by replacing the tantalum electrolytic capacitors with chip multilayer ceramic capacitors. Depending on the purpose, there are sometimes some issues that need to be paid attention to.
Replacement with chip multilayer ceramic capacitors can suppress ripple voltage Figure 1: Replacement with chip multilayer ceramic capacitors can suppress ripple voltage
The output voltage waveform when the output capacitor of the DC-DC converter is changed from a tantalum electrolytic capacitor to a chip multilayer ceramic capacitor. The switching frequency of the DC-DC converter is 300MHz. The capacity of the tantalum electrolytic capacitor is 100μF. Use three 22μF chip multilayer ceramic capacitor products. The ripple voltage before the replacement was 56mV, but after replacing it with a chip multilayer ceramic capacitor, it dropped to 7mV.
A typical example is when it is used in an output smoothing circuit such as the above-mentioned DC-DC converter. Although chip multilayer ceramic capacitors do have the advantage of lowering the output ripple voltage due to their relatively low ESR, the low ESR characteristic is sometimes a “double-edged sword.” The reason is that after replacing with a chip multilayer ceramic capacitor, the phase of the feedback loop response characteristic of the DC-DC converter will be greatly shifted. In bad cases, a 180-degree offset will occur, causing the output of the DC-DC converter to oscillate abnormally. In this case, the DC-DC converter will not function.
For this reason, when chip multilayer ceramic capacitors are used in output smoothing circuits, it is necessary to adjust the constants of the phase compensation circuit. If the phase compensation circuit is integrated inside the DC-DC converter IC, it is necessary to measure the ESR of the replaced tantalum electrolytic capacitor, and then insert the same resistor as the ESR of the tantalum electrolytic capacitor in series. This can suppress abnormal oscillations.
The output voltage waveform after simply replacing the capacitor that constitutes the output smoothing circuit of the DC-DC converter from a tantalum electrolytic capacitor to a chip multilayer ceramic capacitor. The waveform oscillated abnormally, and the output voltage of the DC-DC converter also changed drastically. Figure 2 (b) is the waveform after adjusting the constants of the feedback loop phase compensation circuit. The abnormal oscillation of the waveform has been suppressed, and the output voltage has dropped to a level where there is no negative effect. If this output voltage waveform is reached in actual use, it can be used with confidence.
The output voltage waveform during abnormal oscillation and normal operation (a) is the output voltage waveform when abnormal oscillation occurs. (B) is the output voltage waveform during normal operation. By adjusting the constants of the feedback loop phase compensation circuit, abnormal oscillation can be prevented.
Generally speaking, the phase margin of the feedback loop response characteristic of the DC-DC converter is guaranteed to be 45 degrees or more. If you want to meet this requirement, you must adjust the constants of the phase compensation circuit.
However, recent improvements have been made on the side of the DC-DC converter IC, and there have been many products that do not cause problems such as abnormal oscillation even if they directly use chip multilayer ceramic capacitors with low ESR. When selecting a DC-DC converter IC, please check its data sheet (Data Sheet), etc. You can use it with confidence if it says “Stability working with chip multilayer ceramic capacitors”. On the contrary, if there is no indication, the above-mentioned countermeasures need to be taken.
The Links: LQ064A5CG01T 6MBI60FA-060
