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Understanding ESR in Capacitors - Patriot...

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Understanding ESR in Capacitors The need to test equivalent series resistance (ESR) increases daily because more electrolytic capacitors are finding their way into circuits where resistance causes problems. The patented ESR test provides a new test for this capacitor failure, which reads the series resistance directly in ohms. This Tech Tip answers the most common questions about ESR. 1. What Is ESR? Every capacitor has some ESR. ESR is the electrical resistances in series with the capacitor plates. This includes the resistance of the metal leads and plates and the connections between them. An aluminum electrolytic capacitor also has resistance in the wet electrolyte solution, and in the layer of aluminum oxide which contains high levels of water (called the “hydrated oxide”). 2. What Causes ESR To Change? First, we can ignore the resistance contributed by the metal leads and plates, because it’s so small. There are two common causes of high ESR: 1) Bad electrical connections, and 2) Drying of the electrolyte solution. Electrical connection problems can happen in old or new capacitors, while drying is usually only a problem in old ones. Connection problems happen because the leads coming into the capacitor cannot be made of aluminum, since aluminum cannot be soldered. The electrical connection between the aluminum plates and the copper leads calls for a weld or a mechanical crimp. Problems with either method produces high series resistance. Drying problems occur because of the importance of water in the electrolyte solution. The solution soaks the paper spacer between the two aluminum plates. The water carries the electrical charge from the negative aluminum plate to the surface of the insulating oxide on the positive plate. The oxide forms the capacitor’s dielectric and the negative charge on the surface of the oxide forms the negative capacitor plate. As the water evaporates, the electrical resistance increases. 3. Is ESR responsible for leakage? No, leakage is a PARALLEL resistance path which connects the two plates. ESR is just the opposite, since it’s all the resistance EXCEPT for the leakage path. If ESR is Fig. 1: The equivalent series resistance (ESR) is the combination of all electrical large enough, it can reduce the leakage resistances, including the leads, plates, connections, and water in the electrolyte. current.
Transcript

Understanding ESR in Capacitors

The need to tes t equ iva len t ser iesresistance (ESR) increases daily becausemore electrolytic capacitors are findingtheir way into circuits where resistancecauses problems. The patented ESR testprovides a new test for this capacitorfailure, which reads the series resistancedirectly in ohms. This Tech Tip answersthe most common questions about ESR.

1. What Is ESR?

Every capacitor has some ESR. ESR is theelectrical resistances in series with thecapac i to r p la tes . Th is inc ludes theresistance of the metal leads and platesand the connections between them. Analuminum electrolytic capacitor also hasresistance in the wet electrolyte solution,and in the layer of aluminum oxide which

contains high levels of water (called the“hydrated oxide”).

2. What Causes ESR To Change?

Fi rs t , we can ignore the res is tancecontributed by the metal leads and plates,because i t ’s so small . There are twocommon causes of high ESR: 1) Badelectrical connections, and 2) Drying of theelectrolyte solution. Electrical connectionprob lems can happen in o ld o r newcapacitors, while drying is usually only aproblem in old ones.

Connection problems happen because theleads coming into the capacitor cannot bemade of aluminum, since aluminum cannotbe soldered. The electrical connectionbetween the aluminum plates and thecopper leads calls for a weld or amechanical crimp. Problems with eithermethod produces high series resistance.

Drying problems occur because of theimportance of water in the electrolytesolution. The solution soaks the paperspacer between the two aluminum plates.The water carries the electrical charge fromthe negative aluminum plate to the surfaceof the insulating oxide on the positive plate.The oxide forms the capacitor’s dielectricand the negative charge on the surface ofthe oxide forms the negative capacitorp la te . As the water evapora tes , theelectrical resistance increases.

3. Is ESR responsible for leakage?

No, leakage is a PARALLEL resistance pathwhich connects the two plates. ESR is justthe opposite, since it’s all the resistanceEXCEPT for the leakage path. If ESR is

Fig. 1: The equivalent series resistance (ESR) is the combination of all electrical large enough, it can reduce the leakageresistances, including the leads, plates, connections, and water in the electrolyte. current.

ESR has little effect if it’s smaller than theXc. At the frequency where the ESRbecomes equal to Xc, the value of thecapacitor is effectively cut in half. Whenthe ESR is much larger than the Xc, itbecomes the dominant part of the twoseries components, which prevents thecircuit from seeing the capacitor at all.

ESR v s . P o w e r : E q u i v a l e n t s e r i e sresistance can also cause problems at low

operating frequencies if the capacitor musthandle high levels of current. This oftenoccurs in high current power supplies,such as those used to power banks ofd i g i t a l l o g i c c i r c u i t s . C o n s i d e r , f o rexample, a 20,000 µF capacitor used tofilter a 5 ampere, 60 Hz power supply. Thecapacitor must alternately charge to thevoltage supplied by the rectifier, and thendischarge into the circuit load during thetime the rectifier isn’t conducting. This 5

Fig. 2: In effect, all of the ESR componentsadd to equal one resistor placed in serieswith an ideal capacitor.

4. Why does ESR affect somecircuits worse than others?

Two types of circuits are more severelyaffected by ESR than others; those usinghigh frequencies or high currents. Circuitswhich don’t have either condit ion cantolerate much higher levels of ESR.

ESR vs. Frequency: First, let’s see howESR affects circuits with a high operatingfrequency. A typical example is the DCblocking capacitor at the output of theaudio amplifier shown in the schematic inFig. 3. The graph in Fig. 4 shows how theo u t p u t p o w e r c h a n g e s w i t h o u t p u tfrequency for two 100 µF capacitors, onewith no ESR and one with 6 ohms.

Fig. 3: A typical audio output stage with a series DC blocking capacitor can be affected byhigh ESR.

Notice that the output at 60 Hz is nearly thesame fo r bo th capac i to rs . The goodcapacitor produces 1.7 watts, and the badone produces 1.3 watts. This shows thatthe capacitor with the high ESR will stillwork well as a power supply filter. But,notice the results at 1000 Hz. The goodcapacitor allows 10 watts to pass to thespeaker, while the bad capacitor restrictsthe power to only 5.6 watts. Thedifferences become even more pronouncedat higher frequencies.

The reason for this effect is the relationshipbetween the series resistance and thecapacitive reactance (Xc) of the capacitor.Remember tha t the Xc o f a per fec tcapacitor can be thought of as its electricalresistance at a certain frequency. Xc islarge at low frequencies (infinite at DC) andlower as the frequency increases. Thecapacitor should act as a dead short tohigher frequencies.

Fig. 4: The output from the circuit in Fig. 3 with a good capacitor and one with 6 ohms ofSeries resistance causes a voltage divider ESR. Notice that this capacitor would still serve as a power supply filter, since there isaction between the Xc and the ESR. The virtually no difference at 60 or 120 Hz.

amps o f “ r ipp le cur rent ” a l te rnate ly affect the value test. Therefore, it must bepasses in and out of the capacitor. measured separately.

If the capacitor has an ESR of only 0.5ohms, Ohm’s Law te l l s us tha t th isgenerates 12.5 watts of heat inside thecapacitor. This heat causes the capacitor toquickly dry out and fail. Also, the seriesresistance can reduce the filtering. Forexample, a resistance of 0.5 ohms in a 5volt TTL power supply may cause ripple ashigh as 2 volts. This equates to 40 percentr ipp le , wh ich w i l l cause most d ig i ta lcircuits to perform incorrectly.

7. Why is ESR so difficult to detect?

The circuits that measure ESR must beable to ignore other capacitor parameters,including the capacitive reactance at agiven frequency. It would be handy if wecould just reach inside with a DC ohmmeterand measure the resistance in series witheach meta l p la te . We can ’ t do tha t ,because the capacitor blocks any attemptto measure with DC. Therefore, we mustuse an AC signal to isolate a DC failuremode.

High frequency power supplies (such asswitchers or flyback-powered TV circuits)involve both high operating frequenciesand high currents. ESR is even more of aproblem in these circuits.

Fig. 5: If a capacitor with high ESR is usedto filter a high current supply, internalheat ing may resul t . The highertemperatures may cause the capacitor todry out or rupture.

5. Should I throw a capacitor withhigh ESR away if it doesn’t affectthe circuit?

Generally, yes. Although high ESR may notaffect some circuits, it’s still a signal thatsomething isn’t right inside the capacitor.Either the connections are coming loose, orelse the electrolyte is drying out. If theprob lem cont inues to worsen, i t w i l leventually cause a circuit failure.

6. Why does ESR only apply toelectrolytics over 1 µF?

8. How do the Z METER circuitsdetect ESR?

The Sencore engineers struggled with thisproblem for a long time before coming upwith a good answer. We finally found thatwaveshaping theory supplied an answer.

First, let’s see how an ideal capacitorshould react if we apply a voltage to it froma power supply with a known outputimpedance.

The capacitor should immediately begincharging at the R/C time constant made upof the capacitance value and the outputimpedance of the supply. The voltageshould begin at zero, and then build in then o r m a l f a s h i o n u n t i l f u l l y c h a r g e d(generally in 5 time-constants) as Fig. 6shows. If we charge and discharge thiscapacitor repeatedly, we end up with asawtooth waveform.

Now, consider what happens if we place aresistor in series with the capacitor andobserve the charging curve on the side ofthe resistor farthest from the capacitor. Atthe instant voltage is applied, the capacitoracts like a short circuit. Fig. 7 shows howthe voltage at the resistor instantly rises toa DC level based on the ratio of the powersupply impedance and the resistor’s value.The capacitor then begins to chargethrough the combined resistance of the

High levels of ESR on smaller valuecapacitors will cause them to read low onthe value test. ESR on a good electrolyticcapacitor, however, is so low that it won’t

Fig. 6: When current is applied to a normal capacitor, it begins recharging from zero,building in voltage according to the normal R-C charging curve.

power supply and the series resistor. Thelarger the series resistance, the higherstep before the charging curve. If wedischarge the capacitor and repeat theprocess, we learn that the waveshapechanges from a sawtooth to a trapezoidalwaveform.

T h e Z M E T E R S s i m p l y c h a r g e t h ecapacitor, while measuring the rise involtage during the first microsecond afterapplying the voltage. The instantaneousv o l t a g e s t e p c a l c u l a t e s d i r e c t l y t oresistance using normal voltage-dividingformulas. Sencore has been granted apatent on this test.

9. How does the Z METER ESR testrelate to frequency?

A number of customers have been told thatESR must be tes ted a t a par t i cu la rfrequency. This is not the case, becausetrue ESR does not depend on frequency.ESR represents pure resistive losses, andresistance has the same impedance at anyfrequency. The Z METER only measuresresistance.

Fig. 7: When there is series resistance, the charging voltage instantly rises to a voltage/eve/ (proportional to the resistance in the capacitor and the output impedance of the powersupply) before beginning the normal R-C curve.

The confusion appears to come fromformulas which attempt to convert the "D"(dissipation factor) reading from an ACimpedance bridge to an ESR value. ESR,however, is only one of the many capacitorimperfections that causes poor D readings.D also includes the effects of leakageresistance, equivalent series inductance,dielectric absorption, dielectric stress, andlosses (such as water molecule resonance)in the dielectric. Most of these other lossesare frequency selective, so any attempt tocalculate ESR from D will make it seem thatESR varies with frequency.

Form 4416 Printed in U.S.A.


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