Where and when does the ESR rise rapidly

Low ESR electrolytic capacitors

  • Repeat: what is an electrolytic capacitor?
  • What does ESR mean?
  • Advantages of Low ESR Electrolytic Capacitors
  • Problems with Low ESR Electrolytic Capacitors

Repeat: what is an electrolytic capacitor?

A capacitor consists of two electrically conductive plates. The plates are called anode and cathode. Both plates are electrically isolated from one another by a dielectric. In the case of an electrolytic capacitor, or electrolytic capacitor for short, the cathode is a conductive liquid. They are called electrolytes. The dielectric is a very thin oxide layer that is vapor-deposited on the roughened aluminum surface of the anode.
The electrolytic capacitor is manufactured as a winding that is soaked with the electrolyte. The structure is built into an aluminum cup and tightly closed with a rubber stopper.
Radial electrolytic capacitors have visible predetermined breaking points on the top of the cup, which cause the cup to break open when there is overpressure. Because the electrolytic capacitor z. B. explodes with wrong polarity, the predetermined breaking point prevents the worst.
The main area of ​​application of electrolytic capacitors is in voltage supply circuits, where they cover disruptive AC voltage components seven or short current peaks.

What does ESR mean?

ESR is the abbreviation for Equivalent Series Resistance. R.ESR denotes the internal loss resistance of a capacitor. The internal resistance is undesirable because of its losses.
But every capacitor has a certain ESR value. It is shown in the equivalent circuit diagram (of a capacitor).

The resistance RESR (ESR value) is influenced by the material, construction and the conductivity of the liquid electrolyte (cathode). The power loss and the interference suppression depend on this value.
In the case of electrolytic capacitors, the cathode contact makes the largest contribution to the ESR value. Therefore, when developing low-ESR electrolytic capacitors, attempts are made to improve the material in order to reduce the resistance of the cathode contact. On the other hand, attempts are being made to improve the wet electrolyte or to replace it with a dry electrolyte with a lower resistance.
In addition to conductivity, the wet electrolyte also has all the other properties of a liquid. This allows the electrolyte to freeze, boil and evaporate. Especially the latter during operation or simply during storage. Electrolytic capacitors are therefore to be regarded as wearing parts within an electronic circuit. The evaporation of the electrolytes in the electrolytic capacitor is a problem especially in warm or hot circuits. Not only semiconductor components need cooling. Electrolytic capacitors also need to be cooled.
The heat does not only penetrate the condenser from the outside. The heat is also generated inside the housing, at the power loss resistor R.ESR. The intrinsic heat increases due to the evaporation of the electrolyte, which increases the resistance.
For electrolytic capacitors, the rule applies that the service life is halved by a 10 ° C increase in temperature.

Advantages of Low ESR Electrolytic Capacitors

The main area of ​​application for electrolytic capacitors is in circuits for supplying power to other circuit components. There is hardly an electronics technician who has not installed a fixed voltage regulator of the 78 series with an electrolytic capacitor. Within this area of ​​application, the electrolytic capacitor sifts AC voltage components out of the signals or serves as a backup capacitor if a consumer draws a lot of current for a short time until the voltage regulator has readjusted. You are therefore on electrolytic capacitors with the lowest possible loss resistance RESR Interested. These special types are low ESR electrolytic capacitors. Due to their low internal resistance, low ESR electrolytic capacitors contribute to the high efficiency of voltage converters. You can find them e.g. B. in voltage converter circuits that supply the power of microprocessors. No motherboard can do without low ESR capacitors.
If you take a look at switched-mode power supplies or motherboards, you can see that instead of one low-ESR capacitor with a lot of capacity, often very many with a lower capacity are connected in parallel. The reason is that the resistance RESR and the inductance LESL can be reduced by the parallel connection.

Problems with Low ESR Electrolytic Capacitors

Electrolytes must have good conductivity, especially with low ESR capacitors. To improve conductivity, they consist of solvents and various additives. One of the additives is water. The water increases the number of free ions and thus the conductivity of the electrolyte. Impure water attacks aluminum. In an exothermic reaction, aluminum (Al) changes into its hydroxide (AL (HO)3) around. This process is called corrosion. The resulting gas leads to an increase in pressure in the condenser housing. The housing bulges. Then the predetermined breaking point should open. If it doesn't, the capacitor can blow apart with a loud bang. It tears the aluminum foil, which is spread generously around the capacitor. Electrolyte that has not yet dissolved may contaminate the circuit board. This can lead to short circuits and other defects. Fires are said to have started this way.
The manufacturers of electrolytic capacitors usually have the aggressiveness of the water towards the aluminum under control. The formation of hydroxides can therefore be controlled. So it is possible to manufacture electrolytic capacitors in large series. If electrolytic capacitors are polarized correctly and operated within their limit values, they will run without problems for a long time.
Reasons for failure of electrolytic capacitors are therefore to be found in the wrong dimensioning of circuits or in the wrong composition of the electrolyte by the manufacturer. The latter occurred in the past with electrolytic capacitors on motherboards.

Overview: Low ESR electrolytic capacitors

The ESR value depends on the frequency. Therefore, the consideration and comparison with different capacitors must always be made at the same frequency. The ESR value is usually given at 100 kHz and 25 ° C.

descriptioncapacityESR (100 kHz, 25 ° C)Ripple current (100 kHz)miscellaneous
radial cup electrolytic capacitors with wet electrolyteup to 6800 µFfrom 0.028 ohmsup to 2.5 A.RMSvery low cost
radial cup with hybrid electrolyteup to 820 µFfrom 0.023 ohmsup to 2.2 A.RMSlow costs
radial cup with dry electrolyteup to 680 µFfrom 0.025 ohmsup to 2.5 A.RMSDry elco
Solid Electrolyte, Resin Encapsulated Flat Chip Aluminum Electrolytic Capacitorsup to 270 µFfrom 0.01 ohmsup to 3.5 A.RMSDry elco
Solid Electrolyte, Resin Encapsulated Flat Chip MnO2 Cathode Tantalum Electrolytic Capacitorsup to 330 µFfrom 0.1 ohmsup to 1.1 A.RMSMnO2, Tantalum cathode
Solid Electrolyte, Resin Encapsulated Flat Chip Polymer Cathode Tantalum Electrolytic Capacitorsup to 470 µFfrom 0.04 ohmsup to 1.83 A.RMSPolymer, tantalum cathode

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