DC-to-DC Converter Noise Reduction

1 Application Bulletin Number 162 APPLICATION BULLETIN Mailing Address: PO Box 11400 Tucson, AZ 85734 Street Address: 6730 S. Tucson Blvd. Tucson, AZ 85706 Tel: (602) 746-1111 Twx: 910-952-111 Telex: 066-6491 FAX (602) 889-1510 Immediate Product Info: (800) 548-6132 The inherent switching inside the DC-to-DC Converter givesrise to potential sources of Noise . This Noise manifests itselfon the output voltage as spikes at the switching to size and cost requirements, internal filtering islimited but usually adequate for most applications. Whenexcessive Noise is suspected, you must rule out extraneousnoise sources. Figure 1 illustrates two recommended meth-ods for testing output voltage ripple and your circuit requires less Noise than the supply is capableof, there are two preferred filter techniques LC filters or anoutput filter is important that the inductor wire size can carry the loadcurrent, including a safety factor, and that the core does notsaturate.

2 Also notice that the DC resistance of the inductoris outside the feedback loop for regulated units and subse-quently degrades that filters are generally used only where very accurateanalog measurements are being taken, and the power supplyrejection is poor at the ripple frequency. A much morecommon filtering technique is the output filter AN OUTPUT CAPACITORThe saying the more the better is definitely not applicableto output capacitors of switching type power supplies. Thebasic design equations of the supply rules out any brute forceapproach. The parameter of major concern is the EffectiveSeries Resistance (ESR). ESR is due to stray resistanceinside the electrolytic capacitor that becomes significant atswitching power supply frequencies and higher.

3 Togetherwith Effective Series Inductance (ESL), ESR can be mod-eled as the equivalent circuit shown in Figure 2. Output PI FILTERINGFor applications requiring higher accuracy, such as analogmeasurements, LC filters should be used on each channel toattenuate high-frequency Noise . Because of the output filtercapacitor already present in the DC-to-DC Converter , addingan inductor and capacitor to the output creates a pi filter athalf the use-capacitor cost (Figure 2).FIGURE 1. Recommended Noise Measurement 1 F 50 Output Pins 50 Coaxial Cable Scope (b) Alternate Method (a) Preferred Method Output Pins Grounded Probe Tip To Scope FIGURE 3. Typical Log/Log Impedance Plot for AluminumElectrolytic is also a function of temperature and actually decreasesas temperature is increased.

4 This temperature dependency isparticularly strong in the below zero range as shown inFigure 4. This, together with decreasing capacitance (Figure5), can prove fatal to a switcher design that performed wellon the bench.+VOUT VOUTC ommonInternal Output CapacitorsC+ PWRXXXESRESLE quivalent CircuitNOTE: (1) Improvement from paralleling several capacitors with equal total (Hz)Impedance ( )10010M1k10k100k1M(1)(ESR)(ESL)XLXCDC-TO -DC Converter Noise Reduction 1987 Burr-Brown CorporationAB-162 Printed in March, 1994 SBVA0122 The ESR becomes, in effect, a voltage divider with theinternal output resistance of the supply. Therefore, the lowerthe ESR, the better suited the capacitor is for filtering theoutput of a switching type power supply.

5 As the loadincreases, the capacitors ripple current increases, causing alarger drop across the ESR, and consequently, a largeroutput ripple voltage (seen as Noise ). A commonly usedmethod for estimating ESR is by means of the equation:ESR = DF x fCWhere DF is given in %, f is given in hertz, and C is givenin farads. Dissipation factor (DF) is a function of frequencyand is useless if given at less than 1kHz. (If given at lowerfrequencies, it is more indicative of equivalent parallelresistance). If given a 1kHz or greater, the above equationapplies. Typical values are 8% to 24% at 1kHz for solidtantalums, and to 3% at 1kHz for ceramics, and 8% at120Hz for aluminum electrolytics. Most tantalum and alu-minum electrolytic capacitor manufacturers do not specifythe ESR, probably because it is not worth bragging way of reducing ESR in design is to put two or moreFIGURE 4.

6 ESR Ratio Relative to Room 5. Capacitive Ratio Relative to Room in parallel that add to the needed capacitance andreduce the ESR by the parallel resistance relation (see notein Figure 3). The second capacitor may be 1/10 or 1/100 thecapacitance of the first, since it will provide bypassing forhigher are two basic families of electrolytic capacitors fromwhich to choose: aluminum and tantalum. Aluminum typesare available in many quality grades and fabrication tech-niques. Tantalum types come in foil, solid, and wet-slugsubtypes. At first, this may present a bewildering array ofchoices, but these can quickly be reduced by your particularneed. Cheap aluminum electrolytics of questionable qualityare often used but all too frequently this leads to actual noisegeneration, with poor reliability, and, in general, criticismsof high frequency switching type supplies.

7 Good qualityaluminum electrolytics probably provide the best compro-mise, if one must be made, between cost and are computer grade and other types made especiallyfor switchers. Some have specialized construction that pro-duce very low ESR and ESL rather than low cost. Theoutstanding feature of all tantalum types is their high capaci-tance to volume efficiency. This is particularly seen in thewet-slug tantalum, an undisputed winner in the capacitanceversus volume contest. Solid tantalums appeal where there isa great emphasis on longevity, both shelf life and operatinglife. The foil-type tantalum is a very good capacitor forswitchers but is not cost competitive with aluminum switching frequencies are very high (1 MHz andgreater) and current demands are low, nonelectrolytic ca-pacitors will probably replace the previously discussedelectrolytics, providing bypassing at high frequencies wherethe ESL of electrolytics becomes too addition to considering the capacitance, ESR, ESL, andappropriate voltage derating for the application, most ca-pacitors have a maximum rms ripple current (or max rmsripple voltage) rating which should not be exceeded.

8 Ripplecurrent can be estimated by iRMS = ip (or VRMS =VP- ). These ratings are too often ignored, but the stressproduced by exceeded ripple will adversely effect the lifespanof the output filter capacitor. The effect of ripple currentflowing in the output capacitor is the dissipation of heat inthe capacitor s ESR. Heat kills electrolytics in two ways electrolytic depletion and electrolytic evaporation. The rateat which the electrolyte is lost depends on the electrolyteitself and the capacitor s internal structure. But whateverthat rate, tests by aluminum capacitor makers indicate itnearly doubles for each 10 C rise in temperature. Thus, each10 C rise nearly cuts the usable life of an aluminum capaci-tor in half.

9 The best approach to counter this problem is touse capacitors designed and rated for higher capacitor is selected, its effective use dependsgreatly upon wiring techniques. For example, inductancecan become dominant if good wiring practices are notfollowed and a low ESL requirement can be reduced throughvery careful wiring. Place the capacitor as close as possibleto the load rather than the power supply. The reason for this20 15 10 50 Temperature ( C)ESR Ratio 4040 30 20 10010203011 ( C)Capacitance Ratio 4040 30 20 1001020303is the presence of inductance in the wire or PCB trace andthe existing output capacitor inside the supply. A small pifilter is formed furthering the Reduction of Noise . Capacitorlead length including circuit wiring on both sides of thecapacitor should be minimized.

10 Short, wide straps are thebest and these can be paralleled for further Reduction in s true that good capacitors have played a big role inmaking switching type power supplies a technological andcommercial success, but this success has inspired the capaci-tor makers to devise even better suited types. Because of thisimpetus and the versatility of the switching power supply, itis difficult to say that one type of capacitor is inherentlybetter than another. Most capacitor manufacturers are will-ing and able to provide advice that would help you in : FILTERING THE PWR623 The circuit in Figure 6, developed for and in use by acustomer, is representative of the necessary considerationsin filtering high frequency switching power supplies.