AN1327 - Very Wide Input Voltage Range, Off-Line Flyback ...

1 Semiconductor Components Industries, LLC, 2012 December, 2012 Rev. 21 Publication Order Number: AN1327 /DAN1327/DVery wide Input VoltageRange, Off-Line FlybackSwitching Power SupplyOne of the many problems besetting the power supplydesigner today is being able to design a switching powersupply that is able to operate in all the power systems withintheir international marketplaces. Forward mode switchingpower supplies typically operate over a single powersystem s range of Voltage , that is, 90 to 130 VAC or200 to 270 VAC. Boost mode converters can just make therange of 90 to 270 VAC. Any higher Input voltages wouldthen require a different leads companies to create products targeted atspecific marketplaces, which can be costly, or to have theircustomers arrange jumpers to accommodate their powersystem which can be annoying or lead to costly to this are those industrial companies which may notonly have their products reside on residential power systemsbut also have the varied international industrial powersystems.

2 This means that a single product family might haveto operate from an Input Voltage of 90 to 600 VAC, wellbeyond the residential limits of 90 to 270 paper reviews one method of enabling adiscontinuous mode Flyback converter to operate beyondits traditional range of Input Voltage of 3:1 to a range of morethan :1 without affecting the reliability of its is done by changing its mode of operation and the useof recently available power MOSFETs with breakdownvoltage ratings of 1,200 1. The wide Input Range Flyback Power Supply Demonstration BoardThis document may contain references to devices which areno longer offered. Please contact your ON Semiconductorrepresentative for information on possible replacement Summary of the Operation of Fixed FrequencyFlyback ConvertersThe most common topology for those applications lessthan 150 W has been the fixed frequency, current modecontrolled, Flyback converter.

3 Its block diagram can be seenin Figure 2. Block Diagram of a Fixed Frequency Current Mode, Flyback Converter+Vin(DC)STARTUPCKTVOLTAGEREF+-P EAK CURRENTCOMPARATOROSCILLATORCLOCKSRQLEADI NG EDGESPIKESUPPRESSORFEEDBACK VOLTAGEERRORAMPINPUT RETURNOUTPUTDRIVERPOWERSWITCHCURRENTSENS EELEMENTFLYBACKTRANSFORMEROUTPUTRECTIFIE RANDFILTERSTAGEVOLTAGEFEEDBACKCIRCUITOUT PUTVOLTAGEHere a fixed frequency oscillator initiates a power switchconduction period which is terminated by either the currentwithin the power switch reaching a predetermined limit asset by the error amplifier or the oscillator terminating theperiod and initiating the next power switch representative Flyback converter can be seen inFigure 3. A Simplified Schematic of aFlyback ConverterCONTROLVin+ TQIpriVoutDThe power switch essentially places the primaryinductance of the Flyback transformer across the inputvoltage source when it is turned on.

4 The secondary isdisconnected because the output rectifier (D) is reversebiased. The primary winding s current takes the form of alinear ramp starting from zero amps and whose peak valueis given by:ipk+Vin TonLpri(eq. 1)The slope of the current ramp is Flyback topology, as with all boost mode converters,operate under the principle of storing energy within the corematerial of the transformer. The energy stored during eachconduction period is given by:Esto+Lpri i2pk2(eq. 2)To meet the short term steady state power demands ofthe load(s), the following relationship must be met: AN1327 / Lpri i2pk2(eq. 3)In reality, for any one output power, the current modecontroller strives to maintain a constant value for Ipk over theentire range of Input voltages as visualized in Figure 4. Peak Currents at Differing Input VoltagesPRIMARY CURRENTTIMEHI Input VOLTAGELOW INPUTVOLTAGEIpkThe shortcoming arises in the output drivers of the typicalcurrent mode control IC and the power switch.

5 Typically apower MOSFET is used as the power switch in most modernflyback power supplies. At high Input voltages, the on timeof the power switch becomes so short (300 600 nS) thatthe output driver cannot source enough instantaneouscurrent to drive the MOSFET into a saturated conditionbefore turning it back off. The effect is the power switchoperates in the linear conduction mode during these short on pulses. This causes a drastic drop in power switchoperating efficiency and jeopardizes the power supply New Method of ControlBy a very simple modification to the traditional fixedfrequency current mode controlled Flyback converterdesign, one can greatly extend its operational Input voltagerange. The modifications make the control method one ofvariable on time, and variable frequency. Figure 5illustrates the newly added and redefined functional blocksof this new method of 5.

6 Block Diagram of the wide Input Range, Flyback Converter+Vin(DC)STARTUPCKTVOLTAGEREF+-P EAKCURRENTDETECTORVCOCLOCKSRQCURRENTRAMP TIME DELAYFEEDBACK VOLTAGEERRORAMPINPUT RETURNOUTPUTDRIVERPOWERSWITCHCURRENTSENS EELEMENTFLYBACKTRANSFORMEROUTPUTRECTIFIE RANDFILTERSTAGEVOLTAGEFEEDBACKCIRCUITOUT PUTVOLTAGEVOLTAGETRANSLATORCONTROLA VCO ( Voltage controlled oscillator) is created byremoving the timing capacitor s charging circuit from afixed Voltage or current source and placing it under thecontrol of the error Voltage . In the design example shownlater, it means simply removing the timing resistor from thevoltage reference and wiring it to a variable Voltage createdby the output of the error amplifier. A Voltage translator isplaced between the output of the error amplifier and thecontrol Input to the VCO. It consists of a simple biased Vzener diode so that the error amplifier may make use of itsentire output Voltage swing.

7 The other new block is really aredefinition of an old familiar function the leading edgespike filter from the current sensing element. Here, theformerly annoying parasitic of time lag serves an importantfunction within the control algorithm. It now delays theactual current ramp prior to being sensed by the control allows the actual peak current to increase with increasinginput voltages while the controller sees a lowering peakcurrent needed by this control strategy. This will bediscussed ultimate goal of the new control methodology is toforce the on time of the power switch to be greater than thisminimum effective on time over the power supply s entireline/load operating range. Its operation can be bestunderstood by examining equation 3. The erroramplifier/VCO section of the circuit lowers the operatingfrequency as the Input Voltage is increased.

8 This requires theenergy stored per conduction period to increase to meet theshort term power requirement of the output. This is done byextending the on time of the power switch. If the keycomponent parameters such as maximum operating fluxdensity (Bmax) of the transformer, the avalanche ratings ofthe diodes and power switch, and the current ratings of theoutput rectifiers are adequate, then no degradation in thereliable operation of the supply is operation can be better defined by rearrangingequation 3 and neglecting any power loss due to theinefficiency of the supply one gets:ipk+2 PoutLpri f(f(Ve)) (eq. 4)where f(f(Ve)) is the controlled frequency of the power one can see, the peak current is inversely proportionalto the square root of the frequency of operation, since all theother terms are fixed in the short term operation and by thecircuit substituting equation 1 into equation 4 one further gets:ton+1 Vin 2 Pout Lprif(f(Ve)) (eq.)

9 5)There are more unknowns than there are independentequations, but at the low Input line Voltage and at the ratedoutput load, one can solve equation 5. The Input Voltage isknown to be 125 VDC (90 VAC), the frequency will be at itshighest point as designated by the designer, the on time willbe one half of the entire operating period and the peakcurrent will be calculated as it is in a commonfixed frequency Flyback converter. This will allow us todetermine the appropriate value for the primary the sample design, the frequency of operation at thehighest Input Voltage will drop to one half from that at thelowest Input Voltage . Equation 4 then dictates:ipk(hi)[2 ipk(lo)(eq. 6)If the desired maximum operating flux density (Bmax) isone half the core material s saturation flux density at 100 Cand at the high Input line, then the operating flux density atthe low Input Voltage should be:Bmax(lo)[Bsat(min)22 (eq.]]

10 7)For most common ferrite materials such as 3C8, N27, orF, the operating flux density at low line will be atapproximately 1,300 gauss. The Bmax at the high inputvoltage will be no more than one half the saturation fluxdensity at 100 inductance can now be calculated by using equation 1at the low Input Voltage , and an air gap calculated using anyone of the common is important to determine the secondary inductance suchthat the core s energy can be emptied as close to 50 percentduty cycle (1/fop(hi)) as possible. This will minimize theRMS currents to their lowest possible point over the entireoperating range. The output peak current at any operatingpoint is described as:ipk(out)[2 Iout(av) Tdisch fop(eq. 8)This would describe both the peak currents flowingthrough the output rectifiers and the peak ripple currentsflowing into and out of the output filter capacitors.]