MC3423 Overvoltage Crowbar Sensing Circuit

1 Semiconductor Components Industries, LLC, 2005 March, 2005 Rev. 51 Publication Order Number: MC3423 /DMC3423 Overvoltage CrowbarSensing CircuitThis Overvoltage protection Circuit (OVP) protects sensitiveelectronic circuitry from Overvoltage transients or regulator failureswhen used in conjunction with an external Crowbar SCR. Thedevice senses the Overvoltage condition and quickly crowbars orshort circuits the supply, forcing the supply into current limiting oropening the fuse or Circuit protection voltage threshold is adjustable and the MC3423 canbe programmed for minimum duration of Overvoltage conditionbefore tripping, thus supplying noise MC3423 is essentially a two terminal system, therefore itcan be used with either positive or negative Pb Free Package is AvailableMAXIMUM RATINGSR atingSymbolValueUnitDifferential Power Supply VoltageVCC VEE40 VdcSense Voltage (1) Voltage (2) Activation Input CurrentIO300mAOperating Ambient Temperature RangeTA0 to +70 COperating Junction TemperatureTJ125 CStorage Temperature RangeTstg 65 to +150 CMaximum ratings are those values beyond which device damage can ratings applied to the device are individual stress limit values (notnormal operating conditions)

2 And are not valid simultaneously. If these limitsare exceeded, device functional operation is not implied, damage may occurand reliability may be 1. Simplified ApplicationVoutVinCoutCurrentLimitedDCPo werSupplyO. V. +MARKINGDIAGRAMSMC3423P1 = Device CodeA= Assembly LocationWL= Wafer LotYY= YearWW= Work WeekSOIC 8D SUFFIXPLASTIC PACKAGECASE 751 PIN CONNECTIONS12345678 VCCS ense 1 Sense 2 CurrentSourceDriveOutputVEEI ndicatorOutputRemoteActivation(Top View)See detailed ordering and shipping information in the packagedimensions section on page 7 of this data INFORMATION3423 ALYW1818MC3423P1 AWL YYWW3423= Device CodeA= Assembly LocationL= Wafer LotY= YearW= Work Week18 PDIP 8P1 SUFFIXPLASTIC PACKAGECASE 626 CHARACTERISTICS ( V VCC VEE 36 V, Tlow < TA , Thigh, unless otherwise noted.)CharacteristicsSymbolMinTypMaxUni tSupply Voltage RangeVCC 40 VdcOutput Voltage (IO = 100 mA)VOVCC VdcIndicator Output Voltage (IO(Ind) = mA)VOL(Ind) Trip Voltage (TA = 25 C)VSense1, Coefficient of VSense1 (Figure 2)TCVS1 %/ CRemote Activation Input Current(VIH = V, VCC VEE = V)(VIL = V, VCC VEE = V)IIHIIL 12040 180 ASource Current Risetime (TA = 25 C)tr 400 mA/ sPropagation Delay Time (TA = 25 C)tpd sSupply CurrentID : Tlow to Thigh = 0 to +70 C12486537+-+-+-ISourceVCCS ense 1 VEES ense 2.

3 Representative Block Diagram(B)Switch 1 Switch 2 VSense 1 VSense 2 Position APosition BClosedOpenRamp VI until output goes high; this isthe VSense (A)Switch 1 Switch 2 VCCVIF igure 3. Sense Voltage Test CircuitMC3423MC3423 4. Basic Circuit ConfigurationFigure 5. Circuit Configuration for Supply Voltage Above 36 VFigure 6. Basic Configuration for Programmable Durationof Overvoltage Condition Before TripFor minimum value of RG, see Figure 9.*See text for 10 k for minimum drift+-ToLoadPowerSupplyR1R2MC3423F11234 758RG(+ SenseLead)(- Sense Lead)S1**R2 10 k Q1: VS 50 V; 2N6504 or equivalentQ1: VS 100 V; 2N6505 or equivalentQ1: VS 200 V; 2N6506 or equivalentQ1: VS 400 V; 2N6507 or equivalentQ1: VS 600 V; 2N6508 or equivalentQ1: VS 800 V; 2N6509 or equivalentRSPowerSupplyMC3423(- SenseLead)(+ SenseLead)1823475Q110 F15V+1N474010V+ToLoad*R2R1 IndicationOutR3 Vtriptd =Vref C [12 103] C (See Figure 10)PowerSupplyR1R2MC3423R3C12475386V10 VCVO+VCCVCCV tripVrefVIOVOVC000tdRG-C1 >RS(R1 + R2) 10 FC1R1R210 mAIsourceVSVtrip Vref 1 R1R2 V 1 R1R2 RS VS 1025 k Vtrip Vref 1 R1R2 V 1 RRMC3423 INFORMATIONB asic Circuit ConfigurationThe basic Circuit configuration of the MC3423 OVP isshown in Figure 3 for supply voltages from V to 36 V,and in Figure 4 for trip voltages above 36 V.

4 The thresholdor trip voltage at which the MC3423 will trigger and supplygate drive to the Crowbar SCR, Q1, is determined by theselection of R1 and R2. Their values can be determined bythe equation given in Figures 3 and 4, or by the graph shownin Figure 8. The minimum value of the gate current limitingresistor, RG, is given in Figure 9. Using this value of RG, theSCR, Q1, will receive the greatest gate current possiblewithout damaging the MC3423 . If lower output currentsare required, RG can be increased in value. The switch, S1,shown in Figure 3 may be used to reset the , the power supply, across which the SCR isconnected, must be shut down to reset the Crowbar . If a noncurrent limited supply is used, a fuse or Circuit breaker, F1,should be used to protect the SCR and/or the Circuit configurations shown in Figures 3 and 4 willhave a typical propagating delay of s. If fasteroperation is desired, Pin 3 may be connected to Pin 2 withPin 4 left floating.

5 This will result in decreasing thepropagating delay to approximately s at the expenseof a slightly increased TC for the trip voltage for Programmable Minimum Durationof Overvoltage Condition Before TrippingIn many instances, the MC3423 OVP will be used in anoise environment. To prevent false tripping of the OVPcircuit by noise which would not normally harm the load, MC3423 has a programmable delay feature. To implementthis feature, the Circuit configuration of Figure 5 is used. Inthis configuration, a capacitor is connected from Pin 3 toVEE. The value of this capacitor determines the minimumduration of the Overvoltage condition which is necessary totrip the OVP. The value of C can be found from Figure Circuit operates in the following manner: When VCCrises above the trip point set by R1 and R2, an internalcurrent source (Pin 4) begins charging the capacitor, C,connected to Pin 3.

6 If the Overvoltage condition disappearsbefore this occurs, the capacitor is discharged at a rate 10times faster than the charging rate, resetting the timingfeature until the next Overvoltage condition , it is desired that immediate crowbarring ofthe supply occur when a high Overvoltage condition occurs,while retaining the false tripping immunity of Figure 5. Inthis case, the Circuit of Figure 6 can be used. The Circuit willoperate as previously described for small overvoltages, butwill immediately trip if the power supply voltage exceedsVZ1 + (+ SenseLead)(- Sense Lead)-+1kZ17345218 CFigure 7. Configuration for ProgrammableDuration of Overvoltage Condition BeforeTrip/With Immediate Trip atHigh OvervoltagesAdditional Features1. Activation Indication OutputAn additional output for use as an indicator of OVPactivation is provided by the MC3423 . This output is anopen collector transistor which saturates when the OVPis activated.

7 In addition, it can be used to clock an edgetriggered flip flop whose output inhibits or shuts downthe power supply when the OVP trips. This reduces oreliminates the heatsinking requirements for the Remote Activation InputAnother feature of the MC3423 is its remoteactivation input, Pin 5. If the voltage on this CMOS/TTLcompatible input is held below V, the MC3423operates normally. However, if it is raised to a voltageabove V, the OVP output is activated independent ofwhether or not an Overvoltage condition is present. Itshould be noted that Pin 5 has an internal pullup currentsource. This feature can be used to accomplish anorderly and sequenced shutdown of system powersupplies during a system fault condition. In addition, theactivation indication output of one MC3423 can be usedto activate another MC3423 if a single transistor inverteris used to interface the former s indication output to thelatter s remote activation input, as shown in Figure 7.

8 Inthis Circuit , the indication output (Pin 6) of the MC3423on power supply 1 is used to activate the MC3423associated with power supply 2. Q1 is any small PNPwith adequate voltage #1R1 PowerSupply#2+-+ 8. Circuit Configuration forActivating One MC3423 from AnotherNote that both supplies have their negative outputleads tied together ( , both are positive supplies). Iftheir positive leads are common (two negative supplies)the emitter of Q1 would be moved to the positive lead ofsupply 1 and R1 would therefore have to be resized todeliver the appropriate drive to SCR ConsiderationsReferring to Figure 11, it can be seen that the crowbarSCR, when activated, is subject to a large current surgefrom the output capacitance, Cout. This capacitanceconsists of the power supply output caps, the load sdecoupling caps, and in the case of Figure 11A, the supply sinput filter caps.

9 This surge current is illustrated in Figure12, and can cause SCR failure or degradation by any oneof three mechanisms: di/dt, absolute peak surge, or I2t. Theinterrelationship of these failure methods and the breadthof the applications make specification of the SCR by thesemiconductor manufacturer difficult and , the designer must empirically determine theSCR and Circuit elements which result in reliable andeffective OVP operation. However, an understanding of thefactors which influence the SCR s di/dt and surgecapabilities simplifies this the gate region of the SCR is driven on, its area ofconduction takes a finite amount of time to grow, startingas a very small region and gradually spreading. Since theanode current flows through this turned on gate region,very high current densities can occur in the gate region ifhigh anode currents appear quickly (di/dt).

10 This can resultin immediate destruction of the SCR or gradualdegradation of its forward blocking voltage capabilities depending on the severity of the , TRIP VOLTAGE (V) , RESISTANCE (k ) TypR2 = kMaxFigure 9. R1 versus Trip VoltageVCC, SUPPLY VOLTAGE (V)3530252015100 1020304050 607080RG, GATE CURRENT LIMITING RESISTOR ( )RG(min) = 0if VCC < 11 VFigure 10. Minimum RG versus Supply Voltagetd, DELAY TIME (ms)C, CAPACITANCE ( F) 11. Capacitance versusMinimum Overvoltage DurationMC3423 *Needed if supply not current limitedVinVinVoutVoutDCPowerSupplyDCPowe rSupplyCoutCout++*(11A)(11B)Figure 12. Typical Crowbar OVP CircuitConfigurationslpkSurge Due toOutput CapacitorCurrent LimitedSupply OutputdidtltFigure 13. Crowbar SCR Surge CurrentWaveformR & L EMPIRICALLY DETERMINED!ESRESLRLeadLLeadOutputCapToMC 3423 RLFigure 14. Circuit Elements AffectingSCR Surge and di/dtThe usual design compromise then is to use a gardenvariety fuse (3AG or 3AB style) which cannot be relied onto blow before the thyristor does, and trust that if the SCRdoes fail, it will fail short Circuit .