Study on ESR curves of different types of capacitors

When the current passing through the capacitor is getting higher and higher, if the ESR value of the capacitor cannot be kept in a small range, a higher ripple voltage will be generated than before (the ideal output DC voltage should be a horizontal line, and the ripple voltage is the peaks and valleys on the horizontal line). In addition, even if the ripple voltage is the same, the impact on the low-voltage circuit is greater than that in the high-voltage case. For example, for a 3.3V CPU, the proportion of the ripple voltage of 0.2V is small and not enough to form a fatal impact, but for a 1.8V CPU, the proportion of the ripple voltage of 0.2V is enough to cause the digital circuit to misjudge. So what is the relationship between the ESR value and the ripple voltage? We can express it with the following formula: V=R(ESR)×I In this formula, V represents the ripple voltage, R represents the ESR of the capacitor, and I represents the current. It can be seen that when the current increases, even if the ESR remains unchanged, the ripple voltage will increase exponentially, and it is imperative to use capacitors with lower ESR values. This is why the capacitors used in hardware devices such as boards today are increasingly emphasizing low ESR.

The above figure is a typical filter circuit, which is also used in today's graphics cards. The SW IC is equivalent to the switching power supply on the graphics card , which converts the input 5V DC power into 3.3V DC power required by the core or video memory. The L/C part of the circuit constitutes the low-pass filter of the circuit , the purpose of which is to filter out the ripple voltage in the DC power as much as possible.

The table above shows the performance of the ripple voltage in this circuit when different types of capacitors are used in the L/C part. It can be seen that the aluminum solid polymer conductor capacitor with low ESR performance (left) has the best performance in eliminating ripple voltage, the tantalum manganese dioxide capacitor (right) has the second best performance, and the aluminum electrolyte capacitor (middle) has the worst performance. At the same time, the final value will also be affected by temperature, which we will explain in detail later. Page 5: Pay attention to the close relationship between your room temperature and capacitor performance. The performance of capacitors is not static, but will be affected by the environment, and the biggest influence on capacitors is temperature. Among the different types of capacitors, capacitors using electrolyte as cathode material, such as aluminum electrolyte capacitors, are most significantly affected by temperature. Because among the different types of cathodes, such as electrolyte, manganese dioxide, and solid polymer conductors, only electrolyte uses ion conduction, while the other types use electronic conduction. For ion conduction, the higher the temperature, the stronger the ion activity and the stronger the degree of ionization. Therefore, under the premise that the temperature does not exceed the rated limit, the performance of electrolyte capacitors at high temperature is better than that at low temperature.

The above figure shows the ability of three capacitors to reduce ripple voltage at 25 degrees Celsius (the circuit can be referenced by the circuit diagram in the previous chapter). The first table uses OSCON SVP aluminum solid polymer conductor capacitors (1 piece, 100μF, ESR=40 milliohms), the second table uses low-impedance aluminum electrolyte capacitors (3 pieces in parallel), and the third table uses low-impedance tantalum capacitors (2 pieces in parallel). It can be seen from the table that at a normal temperature of 25 degrees Celsius, the ripple voltages generated by the three are 22.8/23.8/24.8mV respectively. In other words, the ability of 1 aluminum solid polymer conductor capacitor to reduce ripple voltage at 25 degrees Celsius is roughly equivalent to that of 2 tantalum capacitors and 3 aluminum electrolyte capacitors.

The above figure also shows the same three types of capacitors, the same circuit, and the performance of reducing ripple voltage at 70 degrees Celsius. It can be seen that the performance of aluminum solid polymer conductor capacitors and tantalum capacitors has not changed much, still maintaining at around 24~25mV, but the ripple voltage of three aluminum electrolyte capacitors in parallel is reduced to 16.4mV. At this time, only two such capacitors are needed in parallel to reach a level of about 25mV at 25 degrees Celsius, and its performance is greatly improved.

Next, we will look at the performance of these three capacitors in low temperature environments. The above figure shows the performance of the three capacitors at -20 degrees Celsius. It can be seen that in low temperature environments, the performance of aluminum electrolyte capacitors is greatly reduced. The ripple voltage of 3 capacitors in parallel increased sharply from 23.8mV at 25 degrees Celsius to 57.6mV. To reduce the ripple voltage to the same value as 25 degrees Celsius, 7 such capacitors need to be connected in parallel. In comparison, we can also see that the performance of aluminum solid polymer conductor capacitors and tantalum capacitors does not fluctuate much, whether in an environment of 25 degrees, 70 degrees or -20 degrees.

From the above analysis, it is not difficult to see that the ESR value of aluminum electrolyte capacitors is extremely affected by temperature. The above chart directly draws the ESR curves of different types of capacitors under different temperature conditions. Among them, the ESR value (X axis) of aluminum electrolyte capacitors (blue line) decreases significantly with the increase of temperature (Y axis). The aluminum solid polymer conductor capacitors (purple line) and tantalum capacitors (green line) and high-end ceramic capacitors (red line) are close to straight lines, and their ESR values ​​are not greatly affected by temperature. Ordinary ceramic capacitors (pink line) are greatly affected by temperature. It should be noted here that the aluminum solid polymer conductor capacitors used for comparison in the above table have a small capacity (only 100μF) and a not too low ESR (40 milliohms). If replaced with similar products with larger capacity and lower ESR, the final performance will be more outstanding.