SWITCHMODE™ Power Supply Reference Manual

SMPSRMWithin all switching power supplies, there are fourmajor current loops. Two of the loops conduct thehigh–level AC currents needed by the supply. These arethe power switch AC current loop and the output rectifierAC current loop. The currents are the typical trapezoidalcurrent pulses with very high peak currents and veryrapid di/dts. The other two current loops are the inputsource and the output load current loops, which carry lowfrequency current being supplied from the voltage sourceand to the load respectively.For the power switch AC current loop, current flowsfrom the input filter capacitor through the inductor ortransformer winding, through the power switch and backto the negative pin of the input capacitor. Similarly, theoutput rectifier current loop’s current flows from theinductor or secondary transformer winding, through theInput CurrentLoop+V in–C inPower SwitchCurrent LoopSWControlAnalogrectifier to the output filter capacitor and back to theinductor or winding. The filter capacitors are the onlycomponents that can source and sink the large levels ofAC current in the time needed by the switching powersupply. The PCB traces should be made as wide and asshort as possible, to minimize resistive and inductiveeffects. These traces should be the first to be laid out.Turning to the input source and output load currentloops, both of these loops must be connected directly totheir respective filter capacitor’s terminals, otherwiseswitching noise could bypass the filtering action of thecapacitor and escape into the environment. This noise iscalled conducted interference. These loops can be seenin Figure 21 for the two major forms of switchingpower supplies, non–isolated (Figure 21a) andtransformer–isolated (Figure 21b).LGNDOutput RectifierCurrent LoopV FBCC outV outOutput LoadCurrent LoopInput SourceGroundAJoinPowerSwitch GroundJoinOutput RectifierGround(a) The Non–Isolated DC/DC ConverterBJoinOutput LoadGroundInput CurrentLoopPower SwitchCurrent LoopOutput RectifierCurrent LoopOutput LoadCurrent LoopV outV FB+SWC outV in–C inFBControlAnalogGNDCR CSOutput Rectifier BGroundJoinJoinOutput LoadGroundInput Source AGroundJoinPower Switch Ground(b) The Transformer–Isolated ConverterFigure 21. The Current Loops and Grounds for the Major Converter Topologieshttp://onsemi.com22