Applying Power MOSFETs in an Unclamped Inductive Switching Environment

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

• Avalanche A condition when the drain-source
  voltage exceeds the bulk break down of the Power
  MOSFET

• Ruggedness A term that signifies that a Power
  MOSFET has the ability to withstand energy
  dissipation in the breakdown mode of operation
• UIS Unclamped Inductive Switching A context
  sensitive term used to describe a Power MOSFETs
  ability to sustain energy in the avalanche mode
  of operation or it can be used to describe a
  circuit which is driving an inductive load
  without a drain clamp

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
Power MOSFET Chip Structure
Power MOSFET Structure With Parasitic Transistor
7
The Parasitic Bipolar Transistor
Breakdown Characteristics of the Parasitic
Bipolar Transistor
Schematic of the MOSFET with the Parasitic
Bipolar Transistor
8
UIS Test Circuit and Idealized Waveforms
UIS Test Circuit
Idealized UIS Test Waveforms
9
UIS Testing Actual Waveforms

• A few points of observation
• The observed BVDSS is higher then the datasheet
  rated value
• At low avalanche currents the BVDSS is nearly
  flat indicating a low temperature rise
• At higher avalanche currents the BVDSS rises to
  higher levels due to the effects of the bulk
  resistance and the increase in the PN junction
  bulk breakdown caused by the substantial increase
  in the junction temperature
• Failure of the device occurs roughly at 1/3 to
  1/2 of the expected tAV

Typical IDS VDS UIS waveforms 10us/div.,
10V/div., 10A/div. Curve I1 IDS(PK)14A Curve I2
IDS(PK)42A Curve I3 IDS(PK)48A
10
Actual Measured HUFA76645P3 UIS Capability
This graph is meant to depict the device
capability and the failure due to IAS as a
consequence of different starting junction
temperatures. The user must adhere to design
principles that ensures the maximum operating
junction temperature is kept within the data
sheet limits.
11
UIS Capability vs Starting Junction Temperature
for the HUFA76645P3 Across Varying Values of
Inductance
Intrinsic Temperature TJ(FAILURE)
This graph is meant to depict the device
capability and the failure due to IAS as a
consequence of different starting junction
temperatures. The user must adhere to design
principles that ensures the maximum operating
junction temperature is kept within the data
sheet limits.
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 circuits 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 ½ 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 (cont)

• 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)

R?J-C 0.52oC / W (HUFA75344P3 data
sheet) R?C-S 1.25oC /W (Aavidthermalloy data
sheet) R?S-A 17.0oC / W (Aavidthermalloy data
sheet) rDS(on) _at_ 25oC 0.008O (HUFA75344P3 data
sheet) rDS(on) _at_ 80oC 0.008O 1.25 est
HUFA75344P3 data sheet 0.010O (Actually the
rDS(on) temperature multiplier is a function of
TJ, but we will use TA for our first iteration)
18
Single Pulse UIS Design Example (cont)

• 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) _at_ 80oC 2.1W 0.010W
  2.11W
• Calculating the peak avalanche current
• Since the on-time of the MOSFET is gt 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 (cont)

• Re-calculating the rDS(on) based upon the 104.2oC
  calculated junction temperature
• rDS(on) _at_ 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 _at_ 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 (cont)

• 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 ln0.505 1
• 0.00081 ln1.505 0.00081 0.409 331ms
• tAV 331ms
• Calculate the Energy Absorbed by the MOSFET
  during Avalanche
• EAS ½ 1.3 Rated BVDSS IAS tAV
• (1.3 55V 11.37A 0.000331) / 2 135mJ

21
Single Pulse UIS Design Example (cont)
The calculated data point resides below the IAS
curve, therefore this product is acceptable to
use in this application
22
Repetitive Pulse UIS Scenario

• rDS(on) Losses
• PAV Losses
• PSWITCH Losses
• Body Diode Losses

Start-up
Increase / Decrease in rDS(on) Due to Die
Temperature Resulting in an increase / decrease
in rDS(on) Losses in the Next Cycle

• Starting TA
• MOSFET RQJC
• Systems RQCA
• Airflow

MOSFET TJ reaches steady state TJ(START)
Losses Power Dissipation HEAT
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 devices 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 lt 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 (cont)

• 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) _at_ 25oC 0.008W (HUFA75344P3 data sheet)
• rDS(on) _at_ 80oC 0.008W 1.25est 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 (cont)

• 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 gt 18
• Since the on-time of the MOSFET is gt 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 (cont)

• 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
  (7.718.77oC/W) 80oC144.5oC 224oC
• TJ(START) gt 175oC
• This part cannot be used in this application
  since TJ(START) exceeds TJ(MAX) of 175oC
• Since PCOND is a small percentage, _at_ 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 (cont)

• Recalculating TJ(START) with a more efficient
  heat sink
• Recalculating the rDS(on) _at_ 175oC
• rDS(on) _at_ 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 MOSFETs UIS capability has a IAS2 tAV
  constant relationship
• A Power MOSFETs 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 devices 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

31
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