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CU-Boulder ECEN 5807 - Average Current Mode Control of Switching Power Supplies

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Data SheetsUC3848UC3849APPLICATION NOTEAverage Current Mode Controlof Switching Power SuppliesLloyd DixonAbstractCurrent mode control as usually implementedin switching power supplies actually senses andcontrols peak inductor current. This gives rise tomany serious problems, including poor noiseimmunity, a need for slope compensation, andpeak-to-average current errors which the inherent-ly low current loop gain cannot correct. Averagecurrent mode control eliminates these problemsand may be used effectively to control currentsother than inductor current, allowing a muchbroader range of topological application.General PerspectiveCurrent mode control is a two-loop systemas shown in the simple example of Fig. 1. Theswitching power supply inductor is “hidden”within the inner current control loop. Thissimplifies the design of the outer voltage con-trol loop and improves power supply perfor-mance in many ways, including better dynamics.The objective of this inner loop is to controlthe state-space averaged inductor current, butin practice theinstantaneous peak inductorcurrent is the basis for control. (Switch current--equal to inductor current during the “on”time--is often sensed.) If the inductor ripplecurrent is small, peak inductor current controlU-140is nearly equivalent to average inductor currentcontrol.In a conventional switching power supplyemploying a buck derived topology, the induc-tor is in the output. Current mode control thenis actually output current control, resulting inmany performance advantages. On the otherhand, in a high power factor preregulator usingthe boost topology, the inductor is in the input.Current mode control then controls inputcurrent, allowing it to be easily conformed tothe desired sinusoidal waveshape.Peak Current Mode Control ProblemsPoor noise immunity. The peak method ofinductor current control functions by comparingthe upslope of inductor current (or switchcurrent) to a current program level set by theouter loop-see Fig. 1. The comparator turnsthe power switch off when the instantaneouscurrent reaches the desired level. The currentramp is usually quite small compared to theprogramming level, especially when V,,,, is low.As a result, this method is extremely suscepti-ble to noise. A noise spike is generated eachtime the switch turns on. A fraction of a voltcoupled into the control circuit can cause it toturn off immediately, resulting in a subhar-Fig. 1 - Peak Current Mode Control Circuit and Waveforms3-356monicoperatingmodewith much greater ripple.Circuit layout and bypass-ing are critically importantto successful operation.Slope compensationrequired. The peak cur-rent mode control methodis inherently unstable atduty ratios exceeding 0.5,APPLICATION NOTEU-140resulting in sub-harmonic oscillation. A com-pensating ramp (with slope equal to the induc-tor current downslope) is usually applied to thecomparator input to eliminate this instability. Ina buck regulator the inductor current down-slope equals VJL. With V. constant, as itusually is, the compensating ramp is fixed andeasy to calculate-but it does complicate thedesign. With a boost regulator in a high powerfactor application, the downslope of inductorcurrent equals (V,&$J/L and thus varies con-siderably as the input voltage follows the recti-fied sine waveform. A fixed ramp providingadequate compensation will overcompensatemuch of the time, with resulting performancedegradation and increased distortion.Peak to average current error. The peak toaverage current error inherent in the peakmethod of inductor current control is usuallynot a serious problem in conventional buck-derived power supplies. This is because induc-tor ripple current is usually much smaller thanthe average full load inductor current, and be-cause the outer voltage control loop soon elimi-nates this error.In high power factor boost preregulators thepeak/avg error is very serious because it causesdistortion of the input current waveform. Whilethe peak current follows the desired sine wavecurrent program, the average current does not.The peak/avg error becomes much worse atlower current levels, especially when the induc-tor current becomes discontinuous as the sinewave approaches zero every half cycle. Toachieve low distortion, the peak/avg error mustbe small. This requires a large inductor tomake the ripple current small. The resultingshallow inductor current ramp makes thealready poor noise immunity much worse.Topology problems. Conventional peakcurrent mode control actually controls inductorcurrent. As normally used for output currentcontrol, it is most effective when applied to abuck regulator where the inductor is in theoutput. But for flyback or boost topologies theinductor is not in the output, the wrong currentis controlled, and much of the advantage ofcurrent mode control is lost.Likewise, the boost topology with its induc-tor at the input is well suited for input currentcontrol in a high power factor preregulator, butbuck and flyback topologies are not well suitedbecause the inductor is not in the input and thewrong current is controlled.Average Current Mode ControlPeak current mode control operates bydirectly comparing the actual inductor currentwaveform to the current program level (set bythe outer loop) at the two inputs of the PWMcomparator. This current loop has low gain andso cannot correct for the deficiencies notedabove.Referring to Fig. 2, the technique of averagecurrent mode control overcomes these prob-lems by introducing a high gain integratingcurrent error amplifier (CA) into the currentloop. A voltage across R, (set by the outerloop) represents the desired current programlevel. The voltage across current sense resistorR, represents actual inductor current. Thedifference, or current error, is amplified andcompared to a large amplitude sawtooth (oscil-lator ramp) at the PWM comparator inputs.The gain-bandwidth characteristic of the cur-rent loop can be tailored for optimum perfor-mance by the compensation network aroundthe CA. Compared with peak current modecontrol, the current loop gain crossover fre-quency, fc, can be made approximately thesame, but the gain will be much greater atlower frequencies.The result is:1) Average current tracks the current pro-gram with a high degree of accuracy. This isespecially important in high power factorpreregulators, enabling less than 3% harmonicdistortion to be achieved with a relatively smallinductor. In fact, average current mode controlfunctions well even


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