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april 2009 doc id 14748 rev 1 1/13 AN2777 application note new high-temperature, high-performance triacs for optimized vacuum cleaner designs introduction a new high-temperature triac family, able to work up to a 150 c junction temperature in steady-state, has been introduced. this family helps to reduce the bulk of the required heatsink. these triacs are particularly suitable for hot or limited environments found in home appliances, such as vacuum cleaners. one key parameter in the design of triacs operating at high temperature is the turn-off capability. we explain here briefly how to optimize this parameter and present the performances of a new 12 a, 600 v device. test results are also presented to compare these performances to other high-temperature triacs available on the market today. these tests are performed in extremely severe temperature conditions as can appear in vacuum cleaners. www.st.com
contents AN2777 2/13 doc id 14748 rev 1 contents 1 triac turn-off behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 improvement of turn-off capability fo r new high-temperature triacs 6 3 vacuum cleaner requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1 steady state thermal design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.2 inrush current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.3 turn-off requirement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.4 jammed nozzle operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4 conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5 references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 6 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
AN2777 triac turn-off behavior doc id 14748 rev 1 3/13 1 triac turn-off behavior when a triac switches from on-state to off-state, the current passes through zero, and the line voltage is rapidly reapplied across the structur e. this voltage level is higher for inductive loads with low power factor, such as pumps or motors. indeed, for such loads, the phase shift between current and voltage is high, and a voltage in the range of 50 to 200 v can be applied for applications running on a 230 v rms line. under certain conditions, the component is not able to block this voltage, and so turns on spontaneously [see references 1. ] indeed, a triac can be compared to two thyristors mounted in back-to-back association and coupled with a single control area. to trigger the two thyristors, the control area overlaps the two conduction areas (see figure 1 ). during conduction, a certain quantity of charges is injected into the structure. these charges disappear by recombination during current decrease, and by extraction with the reverse recovery current after the turn-off. figure 3 shows this recombination current with a 230 v, 50 hz, 25 w pump (see figure 2 for test schematics). figure 1. simplified triac silicon structure figure 2. simplified test schematic the recombination of the charges takes place particularly in the neighboring regions of the gate. these charges can induce the triggering of the other conduction area when the mains voltage is reapplied across the triac. figure 4 shows this kind of behavior with the same load as given in figure 3 , but with a triac with a lower turn-off capability. n3 p2 n2 p1 n1 p2 n2 p1 n4 a2 a1 g n4 gates ctrl i - i + v t i t load v l r c
triac turn-off behavior AN2777 4/13 doc id 14748 rev 1 figure 3. triac turn-off with pump figure 4. charge recombination induces wrong triac turn-on during turn-off to characterize the triac turn-off capability, semiconductor manufacturers use a circuit where the rate of current decrease can be adjusted. in addition, the slope of the reapplied voltage can be controlled by using a circuit of resistors and capacitors connected across the triac [see references 1. ] for a given dv/dt off (a) (see figure 4 ), we progressively increase the di/dt off (a) to reach triac spontaneous re-triggering. this is the critical point that the triac is able to withstand. the rate levels of this point are called (di/dt)c and (dv/dt)c in triac datasheets. a. the expressions dv/dt off and di/dt off refer to the slopes induced by the ?natural? current and voltage across the load. v t (100 v/div) i t (10 ma/div) v mains (100 v/div) recovery current v t (50 v/div) i t (10 ma/div) dv/dt off di/dt off v t (50 v/div) i t (10 ma/div) dv/dt off di/dt off
AN2777 triac turn-off behavior doc id 14748 rev 1 5/13 the value of (di/dt)c decreases if the reapplied (dv/dt)c is increasing. the value of (di/dt)c also strongly decreases if the junction temperature is increasing. figure 5 gives the (di/dt)c relative variation according to the junction temperature for a snubberless triac from stmicroelectronics. this device is the btb12-600cw (12 a, 600 v, 35 ma i gt ). snubberless means that the specified (di/dt)c has been chosen so that it is guaranteed whatever the reapplied (dv/dt)c [see references 1. ] thus there is no need to add an r-c snubber circuit across the triac to help it to turn-off [see references 2. ] figure 5. btb12-600cw (di/dt)c vari ation with junction temperature 0 1 2 3 4 5 25 50 75 100 125 150 (di/dt) c [t j ]/(di/dt) c [t j =125c] t j (c)
improvement of turn-off capability for new high-temperature triacs AN2777 6/13 doc id 14748 rev 1 2 improvement of turn-off capability for new high- temperature triacs as shown previously (for example in figure 5 ), (di/dt)c drastically decreases with the die temperature. for example, the btb12-600cw capability would be 35% lower at 150 c compared to 125 c. this 35% decrease is quite usual for all triacs. as di/dt off only depends on the rms load current (as given in the equation below), the triac at 150 c would be able to drive loads with 35% lower power. equation 1 of course, such a load power derating cannot be accepted. compensating for this derating would lead appliance designers to use a higher current triac, if they wanted to increase the working temperature. stmicroelectronics has improved the design of the device to improve the triac tu rn-off capability. the following simulation indicates the results that can be obtained with the design improvements. figure 6 gives the simulation result of two different devices. one is able to turn-off, the other one not. figure 6. simulation results for two different triacs this breakthrough design has strongly improved device performance for high temperature applications. the new high-temperature triac family featur es very high commutation capabilities. for example, figure 7 compares the btb12-600cw with a new high-temperature t1235h-6 device. this figure shows that the turn-off ca pability is approximately four times higher with the new device over the whole temperature range. 3 ) hz ( ) a ( rms ) ms / a ( off 10 f 2 2 i dt / di - = 1 2 3 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 9.970e-04 9.990e-04 1.001e-03 1.003e-03 1.005e-03 1.007e-03 it (a) time (s) improved structure reference structure
AN2777 improvement of turn-off capability for new high-temperature triacs doc id 14748 rev 1 7/13 figure 7. (di/dt)c variation versus temperature for new and old device 0 20 40 60 80 100 120 140 25 50 75 100 125 150 t j (c) (di/dt) c [t j ](a/ms) btb12-600cw t1235-6h
vacuum cleaner requirements AN2777 8/13 doc id 14748 rev 1 3 vacuum cleaner requirements 3.1 steady state thermal design as for all power semiconductor applications, on e main point to check is the thermal design. it has to be checked to ensure the working junction temperature is below the maximum allowed temperature (t j max ). for this purpose, the heatsink thermal resistance (r thhs ) has to be chosen according to dissipated power (p) and maximum ambient temperature (t a max ) (see equation 2 and references 3. ) equation 2 in vacuum cleaners an efficient way to decrease the heatsink size is to put it in the air flow. however, it is quite difficult to evaluate the required heat-sink size. a good way to check the thermal design is then to measure the case temperature and check if this value is lower than the specified value (see figure 8 , from t1235h-6 datasheet). this figure shows that for a 10 a rms current, the case temperature can reach 116 c. experimental tests have been performed on a 2000 w vacuum cleaner. the maximum dissipated power occurs for the maximum speed (delay between line zero voltage and triac turn-on is 0.75 ms, refer to table 1 and figure 12). the rms load current equals 10 a. with a 20 cm2 white aluminum plate (2 mm width), the case temperature reaches only 100 c. this means that there is almost a 16 c safety margin. figure 8. maximum allowed current versus case temperature for t1235h-6 r thhs t jmax -t amax p 0 2 4 6 8 10 12 14 0 25 50 75 100 125 150 i t(rms) (a) t c (c) to-220ab/d2pak - to 220ab insulated
AN2777 vacuum cleaner requirements doc id 14748 rev 1 9/13 3.2 inrush current inrush current also causes significant stress on triacs, especially if the motor is turned on in full wave mode. today, with the electromagnetic standard applied to limit light flickering due to appliance inrush currents (iec 61000-3-3 standard), most vacuum cleaners feature a microcontroller which implements a soft-start fu nction. a full-cycle start-up thus occurs only with wrong triggering by the microcontroller. figure 9 gives the measured inrush current of a 2000 w motor started in full-cycle mode, with a 264 v rms line voltage (worst case for a 220-240 v line). the inrush current can reach up to 70 a. this level is well below the ma ximum peak current allowed for the t1235h-6 device (i tsm = 120 a for a 20 ms pulse). figure 9 also gives the calculated junction temperature for this device in a to220ab insulated package. the initia l device temperature is 60 c, as it could occur in the application if the motor has already operated before a new start-up. dissipated power is calculated with max v to and r d parameters given in our datasheet [see references 4. ] the thermal impedance taken into account is given in our datasheet (r th(j-c) = 3.3 c/w). it can be seen that the junction temperature remains below 150 c during this start-up. the operation is then totally safe for the device. figure 9. junction temperature and current at start-up (2000 w, 230 v motor, t1235h-6i triac) 3.3 turn-off requirement as explained above, the di/dt off constraint is one of the main points to check, especially for triacs working at high temperatures. furthermore, universal motors impose high di/dt off rates due to the brush commutations. figure 10 , for example, shows that the di/dt off rate ( ? ) can be approximately 50% higher than the value due to the 50 hz wave shape (di/dt off ? , as defined in equation 1 ). -100 -50 0 50 100 150 200 -0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 current (a) and temperature (c) it tj time (s)
vacuum cleaner requirements AN2777 10/13 doc id 14748 rev 1 figure 10. turn-off constraint (2000 w - 230 v motor) as the motor speed in vacuum cleaners is set by changing the triac turn-on delay (t on ), the back emf varies also with this delay. the worst di/dt off can then occur for a different setting than the maximum speed. ta b l e 1 gives some measurements performed on the same 2000 w, 230 v motor. it shows that even if the load rms current increases when the turn-on delay decreases, di/dt off increases. the worst case occurs then for the minimum speed, with a 7.3 a/ms rate. such a rate is less than half the level that the t1235h can withstand at a 150 c junction temperature (16 a/ms). figure 11. turn-on delay definition table 1. measurements with 2000 w, 230 v motor t on (ms) 6 5 0.75 i rms (a) 46.710 di/dt off 7.3 5.9 4.5 di/dt off v t i t 1 2 di/dt off v t i t 1 2 v t v l i t t on v t v l i t t on
AN2777 vacuum cleaner requirements doc id 14748 rev 1 11/13 3.4 jammed nozzle operation for vacuum cleaners, the worst operating condition occurs when the tube is blocked. this operation does not lead to a higher current. on the contrary, as there is no air flow anymore, the motor torque is lower and the motor rms current can decrease down to 8 a. in fact, the stress comes from the fact that the heatsink thermal impedance drastically increases as there is no cooling air flow anymore. the case temperature can then reach up to 120 or 140 c. the tr iac (di/dt)c capability is then highly reduced. this can cause failed turn-off. the motor suddenly goes from low speed to high speed with a half-cycle full conduction mode. such operation causes nois e variation and vacuum cleaner vibration. appliance manufacturers try to reduce this kind of behavior as much as possible, since it may give a poor quality image of their equipment to the end-user. this is the reason why some closed-box tests are usually performed by vacuum cleaner designers to check the triac?s ability to withstand such a st ressful operation. we have performed such a test with the following conditions: triac enclosed in a 10.5 x 8 x 5 cm cardboard box plastic foam around the box to thermally insulate it 1000 w, 110 v motor to reach 17 a/ms di/dt off rate motor rms current: 5.5 a (medium speed) line voltage: 120 v, 60 hz no heatsink figure 12. triac performance comparison for closed-box test the case temperature is measured from motor start-up to spurious triac turn-off. figure 12 gives the results with the t1235h-6i and other 12 a, 600 v, 35 ma, 150 c triac devices in insulated packages. device a case temperature increases faster than all other devices. this means that its power losses are higher than the other devices. this could be certainly due to a smaller die size. device b heating time is the slowest. but this device is only able to withstand the di/dt off rate up to a 116 c case temperature, whereas the t1235h works up to 156 c. device c presents the same power losses as the t1235h but works well only up to 144 c. using a t1235h device thus helps to withstand the closed-box test by more than 1 mn beyond the other devices. 0 20 40 60 80 100 120 140 160 180 0 50 100 150 200 250 time (s) tcase (c) t1235h - st device a device b device c
conclusion AN2777 12/13 doc id 14748 rev 1 4 conclusion an innovative high-tem perature triac family has been pr esented. the turn-o ff capability of this family of devices is four times high er than previous snubberless devices. the performances of a 12 a device have been pr esented. the turn-off performances have been specially improved and are far higher than other devices available today. such technology can be used to optimize vacuum cleaner design. it has been shown that such a 12 a device can be used in 2000 w, 230 v vacuum cleaners, whereas 16 a or 25 a devices were commonly used in the past. this allows the power board price to be reduced. as these devices are working up to a 150 c j unction temperature, the heatsink size can also be reduced, leading to another cost reduction. and above all, the performances of these devices also allow the end-product quality to be increased. indeed, time before bad operation can be increased by 50% during jammed nozzle operation. 5 references 1. ?triac turn-off behavior, logic level and snubberless technologies?, application note an489, stmicroelectronics. 2. ?rc snubber circuit design?, application note an437, stmicroelectronics. 3. ?scrs, triacs and ac switches: thermal management precautions for handling and mounting?, application note an533, stmicroelectronics. 4. ?t1235h, t1250h, high temperature 12 a triacs?, datasheet, stmicroelectronics. 6 revision history table 2. document revision history date revision changes 24-apr-2009 1 initial release.
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