Thyristors & Triacs - Ten Golden Rules for Success In Your ...

1 APPLICATION NOTET hyristors & Triacs - Ten Golden Rules forSuccess In your Technical Publication aims to provide aninteresting, descriptive and practical introduction to thegolden Rules that should be followed in the successfuluse of Thyristors and Triacs in power control thyristor is a controlled rectifier where theunidirectional current flow from anode to cathode isinitiated by a small signal current from gate to 1. thyristor s operating characteristic is shown inFig. 2. thyristor V/I thyristor is turned on by making its gate positive withrespect to its cathode, thereby causing current flow intothe gate. When the gate voltage reaches the thresholdvoltage VGTand the resulting current reaches thethreshold current IGT, within a very short time known asthe gate-controlled turn-on time, tgt, the load current canflow from a to k . If the gate current consists of a verynarrow pulse, say less than 1 s, its peak level will haveto increase for progressively narrower pulse widths toguarantee the load current reaches the thyristor s latchingcurrent IL, load current flow will be maintained even afterremoval of the gate current.

2 As long as adequate loadcurrent continues to flow, the thyristor will continue toconduct without the gate current. It is said to be that the VGT,IGTand ILspecifications given in dataare at 25 C. These parameters will increase at lowertemperatures, so the drive circuit must provide adequatevoltage and current amplitude and duration for thelowest expected operating 1. To turn a thyristor (or triac) ON, a gate current IGTmust be applied until the load current is IL. This condition must be met at the lowestexpected operating gate Thyristors such as the BT150 can beprone to turn-on by anode to cathode leakage currentat high temperatures. If the junction temperature Tjisincreased above Tjmax, a point will be reached wherethe leakage current will be high enough to trigger thethyristor s sensitive gate. It will then have lost its abilityto remain in the blocking state and conduction willcommence without the application of an external method of spurious turn-on can be avoided by usingone or more of the following solutions:1.

3 Ensure that the temperature does not exceed Use a thyristor with a less sensitive gate such as theBT151, or reduce the existing thyristor s sensitivity byincluding a gate-to-cathode resistor of 1k or If it is not possible to use a less sensitive thyristor dueto circuit requirements, apply a small degree of reversebiasing to the gate during the off periods. This has theeffect of increasing IL. During negative gate current flow,particular attention should be paid to minimising the gatepower (commutation)In order to turn the thyristor off, the load current mustbe reduced below its holding current IHfor sufficient timeto allow all the mobile charge carriers to vacate thejunction. This is achieved by "forced commutation" inDC circuits or at the end of the conducting half cycle inAC circuits. (Forced commutation is when the loadcircuit causes the load current to reduce to zero to allowthe thyristor to turn off.) At this point, the thyristor willhave returned to its fully blocking the load current is not maintained below IHfor longenough, the thyristor will not have returned to the fullyblocking state by the time the anode-to-cathode voltagerises again.

4 It might then return to the conducting statewithout an externally-applied gate that IHis also specified at room temperature and,like IL, it reduces at higher temperatures. The circuit musttherefore allow sufficient time for the load current to fallbelow IHat the maximum expected operatingtemperature for successful 2. To turn off (commutate) a thyristor (or triac),the load current must be < IHfor sufficient timeto allow a return to the blocking state. Thiscondition must be met at the highest expectedoperating triac can be regarded as a "bidirectional thyristor "because it conducts in both directions. For standardtriacs, current flow in either direction between the mainterminals MT1 and MT2 is initiated by a small signalcurrent applied between MT1 and the gate 3. Thyristors , standard Triacs can be triggered bypositive or negative current flow between the gate andMT1. (The Rules for VGT,IGTand ILare the same as forthyristors.)

5 See Rule 1.) This permits triggering in four"quadrants" as summarised in Fig. 4. Triac triggering 5. Triac V/I the gate is to be triggered by DC or unipolarpulses at zero-crossing of the load current, negativegate current is to be preferred for the following internal construction of the triac means that the gateis more remote from the main current-carrying regionwhen operating in the 3+quadrant. This results in:1. Higher IGT-> higher peak IGrequired,2. Longer delay between IGand the commencement ofload current flow -> longer duration of IGrequired,3. Much lower dIT/dt capability -> progressive gatedegradation can occur when controlling loads with highinitial dI/dt ( cold incandescent lamp filaments),4. Higher IL(also true for 1-operation) -> longer IGduration might be needed for very small loads whenconducting from the beginning of a mains half cycle toallow the load current to reach the higher +--+MT1MT2+G+MT2+G+MT2-MT2-G-G-MT1MT1MT1 On-stateOff-stateReversecurrentForwardcu rrentReversevoltageForwardvoltageILIHLIH IT2-T2+QUADRANT1 QUADRANT3 QUADRANT2 QUADRANT4MT2MT1gIn standard AC phase control circuits such as lampdimmers and domestic motor speed controls, the gateand MT2 polarities are always the same.

6 This meansthat operation is always in the 1+and 3-quadrants wherethe triac s switching parameters are the same. Thisresults in symmetrical triac switching where the gate isat its most :- The 1+,1-,3-and 3+notation for the four triggeringquadrants is used for brevity instead of writing "MT2+,G+" for 1+, etc. It is derived from the graph of the triac sV/I characteristic. Positive MT2 corresponds withpositive current flow into MT2, and vice versa (seeFig. 5). Hence, operation is in quadrants 1 and 3 + and - superscripts refer to inward and outwardgate current 3. When designing a triac triggering circuit, avoidtriggering in the 3+quadrant (MT2-, G+) turn-on methodsThere are undesirable ways a triac can be turned are benign, while some are potentiallydestructive.(a) Noisy gate signalIn electrically noisy environments, spurious triggeringcan occur if the noise voltage on the gate exceeds VGTand enough gate current flows to initiate regenerativeaction within the triac.

7 The first line of defence is tominimise the occurrence of the noise in the first is best achieved by keeping the gate connectionsas short as possible and ensuring that the commonreturn from the gate drive circuit connects directly to theMT1 pin (or cathode in the case of a thyristor ). Insituations where the gate connections are hard wired,twisted pair wires or even shielded cable might benecessary to minimise noise immunity can be provided by adding aresistor of 1k or less between the gate and MT1 toreduce the gate sensitivity. If a high frequency bypasscapacitor is also used, it is advisable to include a seriesresistor between it and the gate to minimise peakcapacitor currents through the gate and minimise thepossibility of overcurrent damage to the triac s gate , use a series H triac ( BT139-600H).These are insensitive types with 10mA min IGTspecswhich are specifically designed to provide a highdegree of noise 4. To minimise noise pickup, keep gateconnection length to a minimum.

8 Take thereturn directly to MT1 (or cathode). If hardwired, use twisted pair or shielded cable. Fit aresistor of 1k or less between gate and a bypass capacitor in conjunction with aseries resistor to the , use an insensitive series H triac.(b) Exceeding the max rate of change ofcommutating voltage dVCOM/dtThis is most likely to occur when driving a highly reactiveload where there is substantial phase shift between theload voltage and current waveforms. When the triaccommutates as the load current passes through zero,the voltage will not be zero because of the phase shift(see Fig. 6). The triac is then suddenly required to blockthis voltage. The resulting rate of change ofcommutating voltage can force the triac back intoconduction if it exceeds the permitted dVCOM/dt. This isbecause the mobile charge carriers have not been giventime to clear the dVCOM/dt capability is affected by two conditions:-1. The rate of fall of load current at commutation,dICOM/dt.

9 Higher dICOM/dt lowers the dVCOM/dt The junction temperature Tj. Higher Tjlowers thedVCOM/dt the triac s dVCOM/dt is likely to be exceeded, falsetriggering can be avoided by use of an RC snubberacross MT1-MT2 to limit the rate of change of values are 100 carbon composition resistor,chosen for its surge current handling, and , use a Hi-Com that the resistor should never be omitted from thesnubber because there would then be nothing to preventthe capacitor from dumping its charge into the triac andcreating damaging dIT/dt during unfavourable 6. Triac dICOM/dt and (c) Exceeding the max rate of change ofcommutating current dICOM/dtHigher dICOM/dt is caused by higher load current, highermains frequency (assuming sinewave current) or nonsinewave load current. A well known cause of nonsinewave load current and high dICOM/dt is rectifier-fedinductive loads. These can often result in commutationfailure in standard Triacs as the supply voltage falls belowthe back EMF of the load and the triac current collapsessuddenly to zero.

10 The effect of this is illustrated in Fig. this condition of zero triac current, the loadcurrent will be "freewheeling" around the bridge rectifiercircuit. Loads of this nature can generate such highdICOM/dt that the triac cannot support even the gentlereapplied dV/dt of a 50Hz waveform rising from zerovolts. There will then be no benefit in adding a snubberacross the triac because dVCOM/dt is not the dICOM/dt will have to be limited by adding an inductorof a few mH in series with the , usea Hi-Com 7. Effects of rectifier-fed inductive load onphase control circuit.(d) Exceeding the max rate of change of off-statevoltage dVD/dtIf a very high rate of change of voltage is applied acrossa non-conducting triac (or sensitive gate thyristor inparticular) without exceeding its VDRM(see Fig. 8),internal capacitive current can generate enough gatecurrent to trigger the device into is increased at high this is a problem, the dVD/dt must be limited byan RC snubber across MT1 and MT2 (or anode andcathode).