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. April 2009 Doc ID 14748 Rev 1 1/13 www.st.com Contents AN2777 Contents 1 TRIAC turn-off behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 Improvement of turn-off capability for 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 2/13 Doc ID 14748 Rev 1 AN2777 1 TRIAC turn-off behavior 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 structure. 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 A1 G IN4 N1 P1 P1 Gates Ctrl N2 N2 P2 P2 N3 I+ A2 Figure 2. Simplified test schematic Load IT C VL R VT 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. Doc ID 14748 Rev 1 3/13 TRIAC turn-off behavior Figure 3. AN2777 TRIAC turn-off with pump VMains (100 V/div) VT (100 V/div) Recovery current IT (10 mA/div) Figure 4. Charge recombination induces wrong TRIAC turn-on during turn-off dV/dtOFF VT (50 V/div) dI/dtOFF IT (10 mA/div) 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/dtOFF(a) (see Figure 4), we progressively increase the dI/dtOFF(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/dtOFF and dI/dtOFF refer to the slopes induced by the “natural” current and voltage across the load. 4/13 Doc ID 14748 Rev 1 AN2777 TRIAC turn-off behavior 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 Igt). 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 variation with junction temperature (dI/dt) c [T j] / (dI/dt) c [T j=125°C] 5 4 3 2 1 Tj(°C) 0 25 50 75 Doc ID 14748 Rev 1 100 125 150 5/13 Improvement of turn-off capability for new high-temperature TRIACs 2 AN2777 Improvement of turn-off capability for new hightemperature 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/dtOFF 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 dI/dt OFF (A / ms)= IRMS(A)· 2·2π ·F(Hz)·10 - 3 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 turn-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 IT (A) 0.3 0.2 0.1 0 9.970E-04 9.990E-04 1.001E-03 1.003E-03 1.005E-03 1.007E-03 -0.1 Improved structure Reference structure -0.2 -0.3 -0.4 -0.5 1 2 time (s) 3 -0.6 This breakthrough design has strongly improved device performance for high temperature applications. The new high-temperature TRIAC family features 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 capability is approximately four times higher with the new device over the whole temperature range. 6/13 Doc ID 14748 Rev 1 AN2777 Improvement of turn-off capability for new high-temperature TRIACs Figure 7. (dI/dt)c variation versus temperature for new and old device (dI/dt) c [T j] (A/ms) 140 120 100 BTB12-600CW T1235-6H 80 60 40 20 0 25 50 75 Tj(°C) 100 Doc ID 14748 Rev 1 125 150 7/13 Vacuum cleaner requirements AN2777 3 Vacuum cleaner requirements 3.1 Steady state thermal design As for all power semiconductor applications, one 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 (Tj max). For this purpose, the heatsink thermal resistance (RthHS) has to be chosen according to dissipated power (P) and maximum ambient temperature (Ta max) (see Equation 2 and References 3.) Equation 2 RthHS ≤ Tjmax - Tamax P 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 IT(RMS) (A) 14 TO-220AB/D²PAK 12 TO- 220AB Insulated 10 8 6 4 2 TC (°C) 0 0 8/13 25 50 75 Doc ID 14748 Rev 1 100 125 150 AN2777 3.2 Vacuum cleaner requirements 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 function. 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 maximum peak current allowed for the T1235H-6 device (ITSM = 120 A for a 20 ms pulse). Figure 9 also gives the calculated junction temperature for this device in a TO220AB insulated package. The initial 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 Vto and Rd parameters given in our datasheet [see References 4.] The thermal impedance taken into account is given in our datasheet (Rth(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) Current (A) and Temperature (°C) 200 IT Tj 150 100 50 0 -50 time (s) -100 -0.05 3.3 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Turn-off requirement As explained above, the dI/dtOFF constraint is one of the main points to check, especially for TRIACs working at high temperatures. Furthermore, universal motors impose high dI/dtOFF rates due to the brush commutations. Figure 10, for example, shows that the dI/dtOFF rate (➀) can be approximately 50% higher than the value due to the 50 Hz wave shape (dI/dtOFF ➁, as defined in Equation 1). Doc ID 14748 Rev 1 9/13 Vacuum cleaner requirements AN2777 Figure 10. Turn-off constraint (2000 W - 230 V motor) 1 VT 2 IT dI/dtOFF As the motor speed in vacuum cleaners is set by changing the TRIAC turn-on delay (tON), the back emf varies also with this delay. The worst dI/dtOFF can then occur for a different setting than the maximum speed. Table 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/dtOFF 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 VL IT VT tON Table 1. 10/13 Measurements with 2000 W, 230 V motor tON (ms) 6 5 0.75 IRMS (A) 4 6.7 10 dI/dtOFF 7.3 5.9 4.5 Doc ID 14748 Rev 1 AN2777 3.4 Vacuum cleaner requirements 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 TRIAC (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 noise 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 stressful 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/dtOFF 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 Tcase (°C) 180 160 140 120 100 T1235H - ST Device A Device B Device C 80 60 40 20 time (s) 0 0 50 100 150 200 250 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/dtOFF 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. Doc ID 14748 Rev 1 11/13 Conclusion 4 AN2777 Conclusion An innovative high-temperature TRIAC family has been presented. The turn-off capability of this family of devices is four times higher than previous Snubberless devices. The performances of a 12 A device have been presented. 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 junction 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 6 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. Revision history Table 2. 12/13 Document revision history Date Revision 24-Apr-2009 1 Changes Initial release. Doc ID 14748 Rev 1 AN2777 Please Read Carefully: Information in this document is provided solely in connection with ST products. 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