"Enhance Triac Reliability Through Thermal Design"
Enhance Triac Reliability Through Thermal Design By Nick Ham, Principal Applications Engineer, Bipolar Product Line, NXP Semiconductors, Hazel Grove, United Kingdom Appliance applications illustrate how to perform the necessary thermal calculations using data- sheet information supplied by the semiconduc- tor vendor. T riacs are used to control ac mains loads in home �� appliances, and commercial and industrial equip- ment. In the majority of applications, the triac ��� will dissipate sufﬁcient power to make thermal considerations necessary. The size of heatsinks must be calculated, and the maximum junction temperature must be predicted. These thermal design procedures must be followed to ensure long-term reliability of the application. � The thermal design requires several stages of calculation � �� involving power, thermal resistance and temperature rise, as Fig. 1. With half-wave conduction of an SCR, the average and RMS load illustrated by several triac (and one silicon-controlled recti- currents are a function of IPK and the half-cycle time. ﬁer; SCR) application examples. These include a vacuum cleaner, refrigerator compressor, washing machine and ���������������� power tool designs. � Calculating Triac Power ��������� Triac power dissipation is inﬂuenced by the load current. ���������� Full sine-wave current (full-wave conduction) is assumed, as ��� it presents the worst-case condition of maximum triac power dissipation. It also makes for the easiest calculations. P = VO ITRIACAVG + RS ITRIACRMS2 (Eq. 1) ��� ��������������� ���������� I TRIACRMS I TRIAC AVG = 2 × 2 × , (Eq. 2) π where P is the triac power (W), VO is the triac knee voltage ��� (V), ITRIACAVG is the average load current (A), RS is the triac slope resistance (Ω) and ITRIACRMS is the root-mean-square (RMS) load current (A). VO and RS are given in the NXP Semiconductors data- ��� sheets on the ITRIAC / VTRIAC curve. If the values are not avail- able, they can be obtained from the ITRIAC / VTRIAC curve as described under the heading “Calculating VO and RS.” ITRIACAVG � �� ���� ���� �� ���� ���� �� is calculated from the application’s RMS load current using ���������� Eq. 2. (This assumes full-wave conduction and sinusoidal load current, which will give worst-case power dissipation.) Fig. 2. The tangent method is used to calculate VO and RS when these The value for ITRIACRMS is measured in the application. parameters are not given in the datasheet. For worst-case conditions If half-wave conduction is necessary, as shown in Fig. 1 and a hot triac, the maximum VTRIAC curve at TJ should always be used. MAX Power Electronics Technology September 2006 36 www.powerelectronics.com for a SCR, here’s how to calculate ITRIACRMS and ITRIACAVG: (°C/W), RTHMB-HS is the mounting base-to-heatsink thermal resistance (°C/W) and RTHHS-A is the heatsink-to-ambient I TRIAC AVG = 2 × IPK × T / π × 2T = IPK / π. (Eq. 3) thermal resistance (°C/W). RTHJ-MB is ﬁxed and governed by the device as it is inﬂu- I TRIACRMS = (IPK 2 × T) /(2 × 2T) = IPK 2 / 4. )/( (Eq. 4) enced by die size (refer to the relevant datasheet for the exact value). RTHMB-HS is controlled by the equipment manufacturer I TRIACRMS = IPK / 2. (Eq. 5) because it is governed by the mounting method (for example, with or without thermal grease, screw or clip-mounted, in- Calculating VO and RS sulating pad material). RTHHS-A is governed by the application If values for VO and RS are not given in the datasheet, and is under the sole control of the equipment manufacturer. you will have to generate the data yourself. These can be Fig. 3 illustrates these thermal resistance components. derived from the device’s datasheet, as shown in Fig. 2. First, make an enlarged I nsulated G ate B ipolar Transistors photocopy of the ITRIAC / VTRIAC curve to increase accuracy. Second, in the graph of ITRIAC versus the maximum VTRIAC for TJMAX , draw a tangent through the point 10 to 125 kHz Hard Switching on the curve corresponding to the rated current of the triac. Third, the point where the tangent crosses the VTRIAC axis gives VO. In the fourth and ﬁnal step, the slope of the tangent VTRIAC / ITRIAC gives RS. Calculating TJMAX TJ MAX is influenced by ambient temperature, triac power dissipation and the thermal resistance between junction and ambient. For this article, only the steady-state condition will be considered. In the short-term transient condition, transient thermal impedance Power MOS 7® IGBTs (600, 900, 1200V) (ZTH) applies. This will always be lower PT Technology than the steady-state thermal resistance Ultralow Gate Resistance and Charge (RTH). The transient condition is more Ultralow Switching Losses Low Cost Alternative to MOSFETs complicated to analyze and beyond the Excellent Noise Immunity scope of this article. Combi with High Speed Diode Available TJ = TA + P RTHJ-A, (Eq. 6) where TJ is the junction temperature Field Stop IGBTs (600 & 1200V) (°C), TA is the ambient temperature Trench Technology (°C), P is the triac power (W) and RTHJ-A C), Short Circuit Rated is the junction-to-ambient thermal Very Low Conduction Losses resistance (°C/W). Easy Paralleling Combi with High Speed Diode Available Analysis of RTHJ-A Thunderbolt® IGBTs (600 & 1200V) Thermal resistance is similar to NPT Technology electrical resistance, in that the total Short Circuit Rated resistance can be broken down into Moderate to High Frequency several smaller resistances in series. For Easy Paralleling the most popular package (TO-220), Combi with High Speed Diode Available RTHJ-A is composed of the following resistances: ADVANCED POWER TECHNOLOGY IS NOW RTHJ-A = RTHJ-MB + RTHMB-HS + RTHHS-A TM www.microsemi.com (Eq. 7) Phone: (541)382-8028 where R THJ-MB is the junction-to- mounting base thermal resistance www.powerelectronics.com 37 Power Electronics Technology September 2006 TRIAC RELIABILITY Note that there are some important caveats in the way tor power equals 1.8 kW max, the ac mains supply equals the thermal resistance is speciﬁed because it depends on 230 VRMS and, therefore: the package type and the practicality of isolating a metallic Max ITRIACRMS = P / V = 1800 W / 230 VRMS = 7.83 A. thermal reference point. For example, for plastic packages The triac is ﬁxed to an air-cooled heatsink, without without a metal mounting base, the expression RTHJ-MB + thermal grease. Bleed air is allowed to ﬂow through the RTHMB-HS is replaced by a single parameter of RTHJ-HS , since J-H heatsink at all times, even if the main airﬂow is blocked. The the heatsink is the nearest metallic reference point. Also, heatsink is double insulated. Absolute maximum heatsink for low-power plastic packages where a heatsink would not temperature is 70°C. be used, only RTHJ-LEAD is speciﬁed, because the leads are the A 12-A Hi-Com triac is recommended to cope with the nearest metallic reference point. Most of the heat would be inductive load and high inrush current. We will take as our conducted through the leads to the pc board, with a little example the BTA212-600B. Its IGATE of 50 mA is well matched radiated directly from the package to ambient. Finally, for to the typical discrete gate trigger circuit. some surface-mount packages without a mounting base but I TRIACRMSS 8 7.83 Using Eq. 2, I TRIAC AVG = 2 × 2 × RM = 2× 2 × = 7.05 A. 0 with a solder point instead, RTHJ-MB is replaced by RTHJ-SP. π π The table lists the NXP triac packages and the means From the datasheet, VO = 1.175 V and RS = 0.0316 Ω. of specifying their thermal resistance. It shows thermal Using Eq. 1, P = VO I TRIACAVG + R S I TRIACRMS2 = resistance values where they are ﬁxed by the package type 1.175 V 7.05 A + 0.0316 Ω (7.83 A)2 = 10.22 W. or mounting method. If a thermal resistance is inﬂuenced Using Eq. 7, RTHJ-A = RTHJ-MB + RTHMB-HS + RTHHS-A. by the triac die, the speciﬁcation becomes speciﬁc to that From the datasheet, RTHJ-MB = 1.5°C/W. particular device, so it will be given in the datasheet. From the table, for the TO-220 package screw mounted without insulator and without heatsink compound, Vacuum Cleaner Example RTHMB-HS = 1.4°C/W. A triac is used in a discrete phase-control circuit to RTHHS-A can be regarded as zero, since the maximum heat- control the speed of a vacuum-cleaner motor. Conﬁrm by sink temperature is ﬁxed at 70°C under worst-case airﬂow calculating for worst-case conditions that the triac’s TJMAX of 125°C will not be exceeded. For this application, the mo- ��� � ��� � ��� � ��� ��� ���� ����� ���� ��� �������� ����������������� �������������������� Fig. 3. There are three major thermal resistance components in the complete thermal path for a TO-220 package mounted to a heatsink. Power Electronics Technology September 2006 38 www.powerelectronics.com TRIAC RELIABILITY conditions. It can be regarded as an inﬁnite heatsink with a An 8-A Hi-Com triac is recommended to cope with the C. temperature of 70°C. Therefore, RTHJ-A = 1.5°C/W + 1.4°C/W inductive load and startup current. A suitable triac is the + 0 = 2.9°C/W. BTA208S-600E, which uses the DPAK package. Its IGATE of Using Eq. 6, TJMAX = TA + P RTHJ-A MA 10 mA is well matched to the drive capability of the micro- = 70°C + 10.22 W 2.9°C/W controller. = 100°C. I TRIACRMSS 1.4 Using Eq. 2, I TRIAC AVG = 2 × 2 × RM = 2× 2 × = 1.26 A. 2 This is below TJMAX of 125°C and, therefore, acceptable. π π MA From the datasheet, VO = 1.264 V and RS = 0.0378 Ω. Refrigerator Compressor Example Using Eq. 1, P = V O I TRIACAVG + R S I TRIACRMS2 = A triac is used in an electronic thermostat that controls 1.264 V 1.26 A + 0.0378 Ω (1.4 A)2 = 1.67 W. the on-off switching of a refrigerator compressor. The triac Using Eq. 6, TJMAX = TA + P RTHJ-A. MA gate is triggered from a microcontroller with 20-mA current TJMAX = 125°C, TA = 40°C and P = 1.67 W. MA sink capability. What maximum heatsink thermal resistance Rearranging the equation gives: is allowed to keep the triac’s junction temperature within its RTHJ-A = (TJ – TA) / P = (125°C – 40°C) / 1.67 W = 51°C/W. TJMAX of 125°C? Steady-state motor current equals 1.4 ARMS. MA Using Eq. 7, RTHJ-A = RTHJ-MB + RTHMB-HS + RTHH-SA. Maximum inrush current equals 17 APK in the ﬁrst half cycle. From the datasheet, RTHJ-MB = 2°C/W. We need to ﬁnd Mains supply equals 230 VRMS. A surface-mounted triac is RTHMB-A. required for direct soldering to the controller pc board. Rearranging the equation gives: Maximum ambient temperature is 40°C. RTHMB-A = RTHJ-A – RTHJ-MB = 51°C/W – 2°C/W = 49°C/W. Package Type Thermal Resistance Speciﬁcation Value (°C/W) SOT54 RTH 60 J-LEAD (TO-92) RTH (free air) 150 J-A SOT78 RTH See datasheet J-MB (TO-220) RTH (clip, with grease, no insulator) 0.30 MB-HS RTH (screw, with grease, no insulator) 0.5 MB-HS RTH (clip, no grease, no insulator) 1.4 MB-HS RTH (screw, no grease, no insulator) 1.4 MB-HS RTH (clip, with grease, 0.1-mm mica insulator) 2.2 MB-HS RTH (clip, with grease, 0.25-mm alumina insulator) 0.8 MB-HS RTH (screw, with grease, 0.05-mm mica insulator) 1.6 MB-HS RTH (screw, no grease, 0.05-mm mica insulator) 4.5 MB-HS RTH (free air) 60 J-A SOT82 RTH See datasheet J-MB RTH (clip, with grease, no insulator) 0.4 MB-HS RTH (clip, no grease, no insulator) 2.0 MB-HS RTH (clip, with grease, 0.1-mm mica insulator) 2.0 MB-HS RTH (clip, no grease, 0.1-mm mica insulator) 5.0 MB-HS RTH (free air) 100 J-A SOT186A RTH (with grease) See datasheet J-HS (plastic TO-220) RTH (no grease) See data sheet J-HS RTH (free air) 55 J-A SOT223 RTH See datasheet J-SP RTH (free air, minimum pad area, FR4 pc board) 150 typical J-A SOT404 RTH See datasheet MB J-MB (D2PAK) RTH (free air, minimum pad area, FR4 pc board) 55 typical J-A SOT428 RTH See datasheet MB J-MB (DPAK) RTH (free air, minimum pad area, FR4 pc board) 75 typical J-A Table. NXP triac packages and their thermal resistance speciﬁcations. www.powerelectronics.com 39 Power Electronics Technology September 2006 TRIAC RELIABILITY This is effectively the heatsink thermal resistance, since It uses the SOT186A all-plastic package. the pc board is the heatsink in this case. As an approximate I TRIACRMSS 1.3 Using Eq. 2, I TRIAC AVG = 2 × 2 × RM = 2× 2 × = 1.17 A. 1 guide, this thermal resistance can be obtained with a cop- π π per pad area of 500 mm2 (refer to the NXP application note From the datasheet, VO = 1.216 V and RS = 0.0416 Ω. “Surface Mounted Triacs and Thyristors,” document order Using Eq. 1, P = VO I TRIACAVG + R S I TRIACRMS2 = number 9397 750 02622). 1.216 V 1.17 A + 0.0416 Ω (1.3 A)2 = 1.49 W. Please note that the actual thermal resistance will be re- Using Eq. 6, TJ = TA + P RTHJ-A. duced by other, nondissipating components in close proxim- We already know that TA = 40°C and P = 1.49 W. ity to the triac, while it will be increased by any components From the datasheet, RTHJ-A for the SOT186A package in that dissipate power. It is essential to measure the prototype free air is 55°C/W. to discover the true thermal performance. Therefore, TJ = 40°C + 1.49 W 55°C/W = 122°C. This is below the TJMAX of 125°C. Therefore, the triacs can be Vertical-Axis Washing Machine Example operated without heatsinks. The washing machine uses a reversing induction motor that’s controlled by two triacs. Will the triacs’ TJMAX of 125°C Power Tool Example be exceeded if they are operated without a heatsink? A heavy-duty electric drill uses a universal (brush) mo- Full load motor power equals 300 W. The ac mains supply tor whose speed is controlled by a half-wave phase-control equals 230 VRMS. Therefore: circuit. Calculate the maximum power dissipation in the Max ITRIACRMS = P / V = 300 W / 230 VRMS = 1.3 A. SCR and calculate the heatsink thermal resistance required An isolated triac package is required, and the maximum to maintain the junction temperature below TJMAX. ambient temperature is 40°C. Calculations are as follows: Peak motor current during normal running = 5 A. This application requires 1000-V triacs to withstand the A surface-mounted triac is required for mounting within the high ac mains voltage that the motor imposes across them. trigger switch. Maximum ambient temperature is 50°C. A three-quadrant design is mandatory for maximum im- The SCR is air-cooled by the motor cooling fan. The munity to spurious triggering. The BTA208X-1000C is rec- BTH151S-650R is chosen for its high repetitive surge ommended. It is an 8-A Hi-Com triac with IGATE of 35 mA. guarantee for the repetitive overload conditions it will have to face. It is rated at 12 ARMS and comes in the SOT428 (DPAK) package. Using Eq. 3, ITRIACAVG= IPK / π = 5 / π = 1.59 A. Using Eq. 5, ITRIACRMS= IPK/2 = 5/2 = 2.5 A. From the datasheet, VO = 1.06 V and RS = 0.0304 Ω. Using Eq. 1, P = VO ITRIACAVG + RS ITRIACRMS2 = 1.06 V 1.59 A + 0.0304 Ω (2.5 A)2 = 1.88 W. Using Eq. 6, TJ = TA + P RTHJ-A. We already know that TA = 50°C and P = 1.88 W and, in this case, TJ = TJMAX = 125°C. Rearranging the equation gives: RTHJ-A= (TJ – TA) / P = (125°C – 50°C) / 1.88 W = 39.9°C/W. Using Eq. 7, RTHJ-A = RTHJ-MB + RTHMB-HS + RTHHS-A. From the datasheet, RTHJ-MB = 1.8°C/W. We need to ﬁnd RTHMB-A. Rearranging the equation gives: RTHMB-A = RTHJ-A – RTHJ-MB = 39.9°C/W – 1.8°C/W = 38.1°C/W. A maximum heatsink thermal resistance of 38°C/W will keep TJ at or below 125°C. This heatsink thermal resistance covers the steady-state condition and is easily achiev- Visit www.bussco.com/busbar to download our design guide or brochure. Call toll free: 866-481-5137 able with a small degree of airﬂow through the switch module. PETech Power Electronics Technology September 2006 40 www.powerelectronics.com