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Applying Power MOSFETs in an Unclamped Inductive Switching Environment

Understanding the Importance of Unclamped Inductive Switching. The vast number of loads driven today are inductive in nature such as solenoids, transformers, inductors, etc.Power MOSFET failure due to Unclamped Inductive Switching conditions is one of the most prevalent failure modes encountered

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Applying Power MOSFETs in an Unclamped Inductive Switching Environment

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    1. Applying Power MOSFETs in an Unclamped Inductive Switching Environment

    2. Understanding the Importance of Unclamped Inductive Switching The vast number of loads driven today are inductive in nature such as solenoids, transformers, inductors, etc. Power MOSFET failure due to Unclamped Inductive Switching conditions is one of the most prevalent failure modes encountered Proper MOSFET specification and proper application of the MOSFET within the circuit is one of the easiest ways a designer can improve the reliability of their power MOSFET components This tutorial will discuss the UIS failure mechanism and explore how a designer can properly specify a MOSFET component to avoid UIS failures

    3. Terminology and Definitions

    4. Terminology and Definitions “IAS” – Current (I) Avalanche Single Pulse; The magnitude of IDS that a part can sustain in the avalanche mode for a single non-repetitive pulse “EAS” – Energy Avalanche Single Pulse; the level of energy that a part can dissipate in the avalanche mode for a single non-repetitive pulse “tAV” – Time in Avalanche; A term used in specifying UIS capability that signifies the amount of time that the device is in the avalanche mode of operation

    5. Terminology and Definitions “EAR” – Energy Avalanche Repetitive Pulse; same as a single pulse but for a repetitive pulse sequence “IAR” – Current (I) Avalanche Repetitive Pulse; same as a single pulse but for a repetitive pulse sequence

    6. Power MOSFET Structure

    7. The Parasitic Bipolar Transistor

    8. UIS Test Circuit and Idealized Waveforms

    9. UIS Testing – Actual Waveforms

    10. Actual Measured HUFA76645P3 UIS Capability

    11. UIS Capability vs Starting Junction Temperature for the HUFA76645P3 Across Varying Values of Inductance

    12. Observations on the UIS Capability Tests Results When the starting junction temperature exceeds the rated TJ(MAX) a significant avalanche current capability exists That is IAS a TJ(FAILURE) – TJ(START) The avalanche capability as a function of L shows the following relationship: IAS3.2 a 1/L This result does not agree with the concept of constant energy which would have the relationship - IAS2 a 1/L The results obtained on Power MOSFETs, once the parasitic bipolar turn-on mechanism is suppressed, are similar to those obtained on rectifiers That is the Power MOSFET capability is the capability of a single PN junction device, that is the drain body PN diode

    13. Fairchild Standard UIS Rating Curve

    14. The UIS Rating Graph The UIS Rating Graph shows: IAS a 1/tAV1/2 Two starting junction temperature ratings are given. Ratings at other TJ(START) levels can be calculated using a linear curve fit The rating curve as published is guard banded from the measured capability Criteria to Safe Use If the circuit’s peak load current and tAV is plotted on the graph and it is below and to the left of the appropriate TJ(START) line the part is being used within its rating The tAV equations are given to assist the circuit designer in determining the tAV from known circuit and device information. The equations use the effective device breakdown voltage during the avalanche condition and is listed as 1.3 x Rated BVDSS

    15. Calculating TJ(AVG) TJ(AVG) = TA + PD * RQJ-A RQJ-A = RQJ-C + RQC-S + RQS-A Where TA is the highest operating ambient temperature expected RQJ-A = The junction-to-ambient thermal resistance RQJ-C = The junction-to-case thermal resistance RQC-S = The case-to-sink thermal resistance RQS-A = The sink-to-ambient thermal resistance PD = PCOND + PAV Where PCOND= on-state conduction losses taking into account the worst case RDS(ON) and its value at TJ(AVG) PAV is the avalanche losses and is equal to EAS * frequency If the avalanche energy cannot be obtained from direct observation, the EAS can be estimated by the following equation: EAS = ˝ * 1.3 * Rated BVDSS * IAS * tAV

    16. Single Pulse UIS Design Example Problem: A circuit contains a HUFA75344P3 MOSFET which drives a solenoid (inductive load) load of 1.7mh with a dc resistance of 2.1W from a supply voltage of 24V. A gate signal of VGS = 10V is applied to the MOSFET gate and the solenoid is energized for the first time. After some considerable time (greater than 5 L/R time constants). The gate voltage is returned to zero volts, VGS = 0V The system uses as heat sink and interface material described as follows: Heat sink data and interface material from Aavid Thermalloy: Heat sink part number 6109PBG RQJ-A = 17.0oC / W Interface isolation material In-Sil-8: Isolation material part number 1898 RQC-S = 1.25oC / W Determine if the HUFA75344P3 avalanche energy rating is exceeded and determine if this device can be used reliably in the application. Calculate the energy absorbed during the avalanche pulse

    17. Single Pulse UIS Design Example (con’t) Step 1 Calculate TJ(START) TJ or TJ(START) = TA + (PD x RQJ-A) RQJ-A = RQJ-C + RQC-S + RQS-A Given in the problem: TA = 80oC RL = 2.1W L = 1.7mh VDD = 24V TJ(MAX) = 175oC Rated BVDSS = 55V (HUFA75344P3 data sheet)

    18. Single Pulse UIS Design Example (con’t) Calculating RQJ-A: RQJ-A = 0.52oC / W + 17.0oC / W + 1.25oC / W = 18.77oC / W Determining PD: PD = I AS2 * rDS(on) Determining RTotal: RTotal = RL + rDS(on) @ 80oC = 2.1W + 0.010W = 2.11W Calculating the peak avalanche current: Since the on-time of the MOSFET is > 5 * L/R (99.3+% of its peak value), we can approximate IAS(PEAK) to be 100% of V/R to simplify our calculations IAS(PEAK) = VDD / RTotal = 24V / 2.11W = 11.37A Applying the rDS(on) value and peak current value to calculate power: PD = I AS2 * rDS(on) = (11.37A)2 * 0.010W = 1.29W TJ or TJ(START) = TA + (PD x RQJ-A) = 80oC + (1.29W * 18.77oC / W) = 104.2oC

    19. Single Pulse UIS Design Example (con’t) Re-calculating the rDS(on) based upon the 104.2oC calculated junction temperature: rDS(on) @ 104.2oC = 0.008W * 1.375 [est HUFA75344P3 data sheet] = 0.011W (This results in a 3 mO change) Re-calculating the temperature based upon the new rDS(on) value results in a junction temperature that is @ 2.5oC higher Several iterations can be made to drive the solution closer to its final value Each successive iteration will result in a smaller delta to the previously attained value We will use the rDS(on) value calculated at 80oC to simplify our calculations

    20. Single Pulse UIS Design Example (con’t) Using the time in avalanche equation contained on our Single Pulse UIS curve: tAV = (L/RTotal) * ln[(IAS * RTotal)/(1.3 * Rated BVDSS – VDD) + 1] = (0.0017H / 2.11W) * ln[(11.37A * 2.11W) / (71.5V – 24V) + 1] = 0.00081 * ln[(24) / (47.5) + 1] = 0.00081 * ln[0.505 + 1] = 0.00081 * ln[1.505] = 0.00081 * 0.409 = 331ms tAV = 331ms Calculate the Energy Absorbed by the MOSFET during Avalanche: EAS = ˝ * 1.3 * Rated BVDSS * IAS * tAV = (1.3 * 55V * 11.37A * 0.000331) / 2 = 135mJ

    21. Single Pulse UIS Design Example (con’t)

    22. Repetitive Pulse UIS Scenario

    23. Multiple or Repetitive UIS Usage The Single Pulse UIS rating graph can be used for Repetitive Pulse with the following considerations: By using the technique of superposition in which each UIS pulse is considered a separate event and the resulting TJ is evaluated as if no other pulse existed Determine the IAS, tAV, & TJ(START) just as in the single pulse case Usually the last pulse in a series occurs at the highest junction temperature and is therefore the severest stress. If the stress for the last pulse is within the rating then any previous pulse is also since it occurred at a lower temperature Usually the junction temperature variation of a device over the full repetitive period is small. The device’s thermal capacitance does not permit an instantaneous change in the average junction temperature. Therefore using the average junction temperature for TJ(START) does not result in appreciable error In the majority of applications the tAV is typically < 5% of the repetition period

    24. Repetitive Pulse UIS Design Example Problem: A circuit contains a HUFA75344P3 MOSFET which drives a solenoid (inductive load) load of 1.7mh with a dc resistance of 2.1W from a supply voltage of 24V. A gate signal of VGS = 10V is applied to the MOSFET gate and the solenoid is energized at frequency of 50HZ with a 75% duty cycle The system uses as heat sink and interface material described as follows: Heat sink data and interface material from Aavidthermalloy: Heat sink part number 6109PBG RQJ-A = 17.0oC / W Interface isolation material In-Sil-8: Isolation material part number 1898 RQC-S = 1.25oC / W Determine if the HUFA75344P3 avalanche energy rating is exceeded and determine if this device can be used reliably in the application Calculate TJ(START) TJ or TJ(START) = TA + (PD x RQJ-A) RQJ-A = RQJ-C + RQC-S + RQS-A

    25. Repetitive Pulse UIS Design Example (con’t) Given in the problem: Frequency = 50Hz (new operating condition) Duty Cycle = 75% (new operating condition) TA = 80oC RL = 2.1W L = 1.7mh VDD = 24V TJ(MAX) = 175oC Rated BVDSS = 55V (HUFA75344P3 data sheet) R?J-C = 0.52oC / W (HUFA75344P3 data sheet) R?C-S = 1.25oC / W (Aavid Thermalloy data sheet) R?S-A = 17.0oC / W (Aavid Thermalloy data sheet) rDS(on) @ 25oC = 0.008W (HUFA75344P3 data sheet) rDS(on) @ 80oC = 0.008W * 1.25[est HUFA75344P3 data sheet] = 0.010W. Actually the rDS(on) temperature multiplier is a function of TJ, but we will use TA for our first iteration) R?J-A = 18.77oC / W (calculated in the single pulse example)

    26. Repetitive Pulse UIS Design Example (con’t) Determining PD: PD = PCOND + PAV PD = IAS2 * rDS(on) * Duty Cycle + EAS * Frequency PCOND PAV Determine IAS(PEAK): L/R = 0.0017H/2.11W = 0.000806s On time = (1/Frequency) * Duty Cycle = (1/50Hz) * 0. 75 = 0.015s 0.015s / 0.000806s > 18 Since the on-time of the MOSFET is > 18 * L/R (99.9+% of its peak value), we can approximate IAS(PEAK) to be 100% of V/R to simplify our calculations IAS(PEAK) = 11.37A (Calculated in the single pulse example) Calculating PCOND: PCOND = IAS2 * rDS(on) * Duty Cycle = (11.37A)2 * 0.010W * 0.75 = 0.97W

    27. Repetitive Pulse UIS Design Example (con’t) EAS = 135mj (Calculated in the single pulse example) Calculating PAV: PAV = EAS * Frequency = 135mj * 50Hz = 6.75W Calculating PTOTAL PTOTAL = PCOND + PAV = 0.97W + 6.73W =7.7W TJ or TJ(START) = TA + (PD x RQJ-A) = 80oC + (7.7*18.77oC/W) = 80oC+144.5oC = 224oC TJ(START) > 175oC This part cannot be used in this application since TJ(START) exceeds TJ(MAX) of 175oC Since PCOND is a small percentage, @ 12%, of PTOTAL, it is recommended to choose a heat sink with an R?S-A such that: TA + (PD x RQJ-A) =175oC

    28. Repetitive Pulse UIS Design Example (con’t) Recalculating TJ(START) with a more efficient heat sink: Recalculating the rDS(on) @ 175oC: rDS(on) @ 175oC = 0.008W *2.0 [est HUFA75344P3 data sheet ] = 0.016W This results in a 8 milliohm change PCOND = IAS2 * rDS(on) * Duty Cycle = (11.37A)2 * 0. 016W * 0.75 = 1.55W Recalculating PTOTAL: PTOTAL = PCOND + PAV = 1.55W + 6.75W =8.3W RQJ-A = RQJ-C + RQC-S + RQS-A = 0.52oC + 1.25oC + 8oC = 9.77oC / W TJ or TJ(START) = TA + (PD x R?J-A) = 80oC + (8.3 * 9.77oC / W) = 80oC + 81.1oC = 161.1oC The TJ of the MOSFET now resides below the 175oC maximum operating temperature

    29. Conclusions A Power MOSFET’s UIS capability has a IAS2 * tAV = constant relationship A Power MOSFET’s avalanche energy is not a constant, but varies as a function of the time in avalanche A simple single pulse UIS Rating system has been defined. By plotting the device’s operating point of IAS and tAV on the rating graph, one can easily determine if the device is being operated safely A simple rating system for repetitive pulses has been presented. Using the single pulse information and TJ(AVG) the repetitive pulse operation can be quickly analyzed Significant, usable avalanche energy capability exists in a Power MOSFET as long as TJ(START) = TJ(MAX)

    30. Articles on Unclamped Inductive Switching “Single Pulse Unclamped Inductive Switching: A Rating System”, Fairchild Application Note AN-7514 “A combined Single Pulse and Repetitive UIS Rating System”, Fairchild Application Note AN-7515 “Boundary of Power MOSFET Unclamped Inductive (UIS) Avalanche Current Capability”, Rodney R. Stoltenburg, Proc. 1989 Applied Power Electronics Conference, pp 359-364, March 1989 “Rating System Compares Single Pulse Unclamped Inductive Switching for MOSFETs”, Harold Ronan, PCIM magazine, pp 32-40, Sept. 1991 “Power Rectifier UIS Capability”, Harold Ronan, John Worman

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