-Chapter 2 / E lectron ics
APPLICATIONNOTE
8
•
- - - -- - ,
Figure2:Direc t voltagedrive.
A -low speed;
B•too highspeed generates fallof torque.
highervoltageis used andthe current limit is set by an externalresistor in serieswith the motor winding such that the sum oftheextern al resis-tance and the internalwinding resistance limits the currenttotheallowed value .This drive tech-niqueincreases thecurrent slew rate and typically provides bettertorqueat high rotational
speed .
Howe verthere isa significa ntpenaltypaidIn ad-ditional dissipationin the extemal resistances.
To avoid the additional dissipation a chopping controlled current drive may be employed, as shown in Figure 3. In thistechn iquethe current throughthe motor issensed and controlledbya choppi ng control circuit so that itis maintained within the rated level. Devices like the L297.
L6506andPBL3717Aimplement thistypeof con-trol.This technique improvesthe current rise time in the motor and improves the torque at high speeds whilemaintainingahigh efficiencyin the drive {2].Figure 4 shows a comparison between Onesolution isto.use~hatiscom~nlyreferred the windingcurrentwave forms forthesame mo -to as an UnR drive(Fig.18 ).Inthlstopology a tordrivenin thesethreetechniques.
Fig ure3:Chopper driveprovides bette r performance.
us
OSCILLATOR MOTO R
~INOI NG
SENSE RES ISTO R
Figure4:Motor c urrent using UR,USRand chopperconsta ntcurrent drive.
1.9
9.5
TIME
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APPLICATION NOTE
In general the best performance, in terms of torque, is achieved using the chopping current control technique[2].Thistechnique also allows easy implementation of multiple current level drivetechniques to improve the motor
perform-ance.
[1]bridgetransist~rs will be forward biased by the
~ransformeraction of the motor windings , provid-In9 an effective short circuit acrossthe supply.
Secondly the L298N,eventhoughithas split sup-ply voltages,may not be used withouta high volt-age supply on the chip since a portion of the drive current for the outputbridge is derived from this supply.
Selec ti ng Ena bl eor Phasechopping
Whe!1 implementingchoppingcontrol of the cur-rentIna stepper motor,there are severalways in which the current controlcan be implemented.A bridqe output,like theL6202 orL298N, may be drivenIn enablechoppinq, one phase choppingor two p~asechopping,as shown in Figure6.The L297I,mplements enable chopping or one phase choPPII:'!g, selected by the control input. The L6506 Implements onephase chopping, withthe recirculation path around the lower half of the bridge,if the fouroutputs areconnected to the 4 inputs of the bridge or enablechoppingif the odd numberedoutputs are connectedto the enable inputsof the bridge.Selecting the correct chop-ping mode is an importantconsideration that af-fectsthe stabilityof the system as well as the dis-sipation.Table 1 shows a relativecomparison of the different chopping modes,for a fixed cho p-ping frequency,motor current and motorinduc -tance.
Driving a UnipolarMotorwith the l298Nor L6202
Althoughitis not the optimal solution,design con-straints sometimes limitthe motor selection, In t~e ca~e where the designer is looking for a highly Integrateddriv~stage ~ith improved per-formance over previous designs but is con-strainedtod~iveaunipolar wound (6 leaded) mo-to~ItIS possibleto drive the motorwithH-Briclge drivers like thel298N orL6202. To drive such a motor the center tap of the motor shouldbe left unconnected and the two ends of the common winding,sar~connectedtothe ~ridgeoutputs,as shownIn Figure 5.Inthisconfiguration theuser shouldnotice amarked improvementintorquefor thesame coil current, or put another way, the same torqueoutputwillbe achieved witha lower coilcurrent.
A solution where theL298N or L6202 isusedto drive a unipolarmotorwhile keeping the center c,?nnectionof each coil connected to the supply Willnot work. First,the protectiondiodes needed from collector to emitter (drainto source) of the
Figu re5: Driving a unipolar woundmotor with a bipolar drive
C.T.
-Chapter 2 / Electronics
APPLI CATIONNOTE
Tabl e1:Comparativeadvantagesof choppingmodes
ChoppingMode RippleCurrent MotorDissipation BridgeDissipation• MinimumCurr ent
ENA BLE HIGH HIGH HIGH LOWER
ONEPHASE lOW lOW lOVV EST lOW
TWOPHASE HIGH lOW lOW Ippl2
n
Asrelatedto L2:l8N,l6203or L6202.Fig ure6a:TwoPhaseChopping.
1 Ilto l l-- - - - -==JF =-- - - ...J
Figure6b :OnePhaseChopping.
llt'loM !-- - --'=F=--- -- --'
'1r-<j.-{i ,:,:::r '"' • ..1tJ...Jf---D---1I~ '"'
Ii'll .l:! Ii'll
'1r-<j.--{i,:,:::r '" • .LjJ..Jf---D~Ii1 '"
JI't1 ~ lil t
!---..E.:::f=---l
4/12
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CNC Robotics
APPLICATIONNOTE
V=L .'!!
dt RIPPLECURRENT
Since the rate of current changeis related direct ly to the voltage appliedacrossthe coilbythe equa-lion:
theripplecurrent willbe determ inedprimarilyby thechopping frequencyand the voltage across the coil. Whenthe coil is driven on,the voltage acro ss the coilis fixedbythe powersupply minus the saturationvoltagesof thedriver. On theother hand thevoltage across the coil during the reci r-culation time depends on the chopping mode chosen .
Whenenab lechoppingortwophas e choppingis selected, the voltageacrossthecoil during recir-culatio n is thesupplyvoltage pluseither theVF of thediodes or the AIvoltage oftheDMOSdevices (when usingtheL6202intwophase choppin g).In this case the slopeof the current rise and decay are nearly the same and the ripple current canbe large.
When one phase chopping is used, the voltage across thecoilduring recirculationisVoo(Vsatfor Bipolardevices orI .ROSon forDMOS) ofthe tran-sistor that remainson plusVFof onediodeplus the voltagedropacross thesense resistor,ifitis intherecirculation path.In this case the current decays much slower thanit rises and the ripple currentismuchsmallerthan in the previouscase. The effectwill be much morenoticeable at higher supplyvoltages.
MOTOR LOSSES
The losses in the motor include the resistive losses (12R) in the moto r windingand parasitic losseslikeeddiecurrentlosses.The latter group of parasitic losses generall yincreases with in-creasedripplecurrentsandfrequency_Chopping techniquesthat have a high ripple current wi.
have higher losses in the motor.Enable ortwo phase choppingwill cause higher losses inthe motor with the effect of raisn g motor temper a-ture.Generallylowermotorlosses are achieved using phase chopping.
POWE RDISSIPATION INTHE BRIDGE IC. In thel298N , theinternaldrive circuitry provides active turn off forthe output devices whenthe outputsare switched in responseto the 4 phase inputs. However when the outputsareswitched offinrespon setothe enableinputs allbase drive isremovedfrom output devices but noactive ele-ment ispresent to removethe stored chargein thebase.When enablechoppingisusedthefall time of thecurrentin the powerdevices will be longerand the devicewillhave higherswitching lossesthan if phasechopping isused.
In the L6202 andL6203,theinternal gatedrive circuit works the samein response to eitherthe input orthe enable so the switchinglossesare the same using enableor
two
phase chopping, but would be lower using one phase chopping.However, the losses due to the voltage drops across the device arenot the same. During en-ablechoppingallfourof the output DMOS de-vicesare turned off and the currentrecirculates through the body to draindiodes of the DMOS output transistors. When phase chopping the DMOSdevices inthe recirculat ionpathare driven on andconduct currentin the reversedirection.
Sincethe voltage dropacrossthe DMOS device islessthanthe forwardvoltagedropofthediode for currents lessthan2A, the DMOStake a sig-nificant amountof the currentand the power dis-sipationismuch lower using phase chopping than enable chopping,ascanbe seen in the power
cissoa ton
graphs inthe data sheet.With these two devices,phase chopping will al-ways provide lower dissipation in the device. For discrete bridges theswitching loss and saturation lossesshould be evaluatedtodetermine which is lower.
MINIMUMCURRENT
Themlnlmmcurrent that canberegulated is im-portantwhenimplementingmicrostepping,when implementingmultilevelcurrentcontrols,or any-time whenatt emptingtoregulate a currentthat is very small compared to the peak current that wouldflow if the motorwereconne cteddirectlyto the supplyvoltage used.
With enablechoppingorone phasechoppingthe only problemisloss ofregulat ion forcurrents be-Iowaminimum value. Figure7shows atypical re-sponsecurve foroutput currentas a functionof theset reference.Thismlnlmnn value is set by the motorcharacteri stics,primarilythe motor re-sistance, the supply voltage and the minirmm duty cycle achievable bythe controlcircuit. The minimim currentthat can be supplied is the cur-rent that flows through the windingwhendriven by theminirmrn duty cyde.Below thisvalue cur-rentregulation is not possible.With enable chop-ping the current through the coil in responseto the minimumduty cycle can return completelyto zero during each cycle,as shown in figure 8.
When using one phase choppin g the current may or maynotreturn completelyto zero and there may be someresidual DC component.
When usingaconstantfrequencycontrollike the l297orL6506,the minimum duty cycle is basi-cally the duty cycle ofthe oscillator(sync) since the set dominance ofthe flip-flop maintains the output on during the time thesync is active. In constantoff timeregulators,likethe PBL3717A, theminimum output timeis setbythe propaga-tion delay through the circuitand it'sratio tothe selectedofftime.
--- l.Ti ~
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Chapter 2 / Electronics
APPLICATION NOTE
Figure7:The transfer function of peakdetect current control is nonlinear forlow current values . CURRENT OUT
Fig u re 8: A MinilTl.lrncurrent flows throughthe motor whenthe driver outputstheminimumduty cycle thatis achievable.
CURRENT
EHABLE C"'OPPIHG
tSI MINIMUM DUTY CYCLE
8.
A. 68
L..---.,f---l-_-+_+_+----.,f--+_-+_+_+.!..2T~Cho
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-Fortwophase chop ping the situationis quite di f-ferent. Although none of the available control chips implement this modeit isdiscussed here sinceit is easyto generatecurrentsthatcanbe catastrophiciftwophase chopping is usedwith peak detecting control techniques. When the peakcurrent islesstha n1/2 of theripple(Ipp) cur-re nttwophase chop p ingcan be especiall y dan -6112
gerous. In thiscase the reversedriveabilityof the twophas echopping tec hniqu e can causethe cur-rent in the motor windingto reverseand the con-trolcircuit to lose control. Figure 9shows thecu r-rent wa ve form in this case.When the current reaches the peakset by the reference bothsides oft,h~bridgeare switchedand thecurrentdecays untilitreache s zero.SlOcethe power tran s istors
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CNC Robot ics
APPLICATIO N NOTE Figu re9:Two phase choppingcanloo se controlofthewinding current..
lJ u
SYMCSENSE \,IOLTAGE
REFEREHCE UOl lA GE
\lIHOIHG CUAQEHT
are now on,the current will begin to increase in a negative direction.When the oscillator againsets the flip-flopthe inputs will then switch again and the currentwillbegin to become more positive.
However. the effect of a si~9le sense resistor usedwith a bridge is to rectify currentand the comparatorseesonly the magnitudeandnot the sign of the current.Ifthe absolutevalue of the currentinthe negative direction is above the set valuethe compa rat orwillbe fooledand reset the flip-flop . The current willcontin ueto becomemore negative and willnot becontro lledbythe regula-tioncirc uit.
Forthisreason two pha se choppingis not recom-mended with bridge circuits like the l298N or L6203 and isnot implemented inanyof the cur-rently available driverIC's. The problemcan be avoided by more complexcurrent sense tech-niquesthat donotrectifythecurrent feedback.
Chopper Stability and Aud ioNoise.
One problem commonlyencounteredwhenusing choppingcurrentcontrolis audio noisefromthe motorwhichis typically a high pitch squeal.In constant frequencyPWMcircuits this occurrence isusuallytracedto a stability problem inthe cur-rent controlcircuitwherethe effectivechopping frequencyhas shifted to a sub-harmonicofthe desired frequencyset by the oscillator. In con-stantofftime circuits the offtime is shiftedto a multiple of the offtime set by themonoslable.
There are two common causes forthis occur-rence.
The first cause isrelated to the electrical noise and current spikes inthe application that can fool the currentcontrolcircuit.In peak detect PWM ctrcnts, likethe l297 and L6506. the motor cur-rentis sensed by monitoring the voltage across the senseresistor connected to ground.When the oscillator sets the internal flip flop causing the bridge output totum on,there is typically a volt-age spike developed across this resistor. This spikeIScaused by noise in the system plusthe reverserecovery currentof the recirculatingdiode that flowsthrough the sense resistor.as shown in
Figure10.If the magnitude of this spikeishigh enough to exceed the reference voltage,the com-paratorcan be fooled intoresetting the flip-flop prematurely as shownin Figure 11.When this oc-curs the outputisturnedoff and the current con-tinuesto decay.Theresult is that the fundamental frequency of thecurrentwave formdeliveredto the motor is reducedtoasub-harmonicofthe os-cillator frequency, which is usuallyin theaudio range.Inpracticeit is notuncommonto encoun-ter instances where the period of the current wave formis two,three oreven four times the pe-riod of the oscillator. This problem is more pro-nounced in breadboard implementations where the ground isnotwell laidoutandgroundnoise contributesmakesthespikelarger.
Whenusing theL6506 andL298N,themagnitude of the spike shouldbe,intheory,smallersince the diode reverse recovery current flows to groundandnotthrough the senseresistor. How-Fig ure 10: Reverserecoverycurrentof the
recirculationdiode flows through the senseresistorcausingaspikeonthe sense resistor.
ReverseRecovery Current ....
Recirculation Current
c::::;>
- - - - - - - - - - - Iifi
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Chapter 2 / Electronics
APPLICATI ONNOTE
Figure 11:Spikeson the sense resistor causedbyreverserecoverycurrentsandnoise cantrick the current sensingcomparator.
ever, in applicationsusingmonolithicbridge driv-ers, liketheL29SN, internalparasitic structures often produce recovery current spikessimilar in naturetothe diodereverse recovery current and thesemay flowthrough theemitter leadofthe de-vice and hencethe senseresistor.When using
DMOS drivers,like theL6202,thereverse
recov-ery currentalways flowsthrough the sense resis-tor since the internaldiode in parallel withthe lowertransistoris connectedto the sourceof the DMOSdeviceand nottoground.InconstantofftimeFM control circuits,likethe