Transcript Circuit Protection

Module Flow
• 11.1 Circuit Protection Overview
• 11.2 Circuit Protection Device Features and Options
• 11.3 Common Design Errors
• 11.4 Common Test Errors
• 11.5 eFuses
– 11.5a eFuse Overview
– 11.5b eFuse vs. Fuse
– 11.5c eFuse vs. Polyfuse
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Circuit Protection Fundamentals
11.1 Circuit Protection Overview
Circuit Protection – What is it ?
• Many things with many names
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Inrush Control
Hotswap
Hotplug
Current Limiting
Electronic Circuit Breaker
Short Circuit Protection
Soft Start
Over Voltage Protection (OVP)
eFuse
Load Power Limiting
FET SOA Limiting ( Protecting the Protector ! )
Reverse Current Protection (ORing)
~ the same functions
• Often Required for Agency Rating
– UL, CSA – North America
– EN, IEC, (CENELEC) – Europe
– CCC Mark (CNCA) - China
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Circuit Protection – What is it ?
• Circuits designed specifically to….
– Prevent Fire ! --“Keep the smoke in!”
– Keep small problems from growing big
• Minimize damage by quickly isolating failures
– Prevent potentially disruptive power bus disturbances
• One small transient can take down/reset an entire system
• What Gets Protected ?
SUPPLY
CONNECTORS
POWER FET
LOAD
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Circuit Protection – Where Is It Used ?
• Telecom Equipment
• Datacenters / Servers
• Storage / HDD, SSD,
Midplanes
• Industrial Control
– 24 or 48 V typically
• Tower Mounted
Antennas
• Merchant Power
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Circuit Protection – The Basic Parts
• Most Common Elements
Element for sensing current
Element for modulating current
LOAD BOARD
BACKPLANE
Power
Supply
Element for
controlling the FET
Load
= RL + CL
Control IC
• Location..
– Sometimes on the Load Side of the Connector
– Sometimes on the Supply Side of the Connector
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Thank you!
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Circuit Protection Fundamentals
11.2 Device Features and Options
“End customers can’t make you
design in protection circuits but
they can make you wish you had.”
– Design review wisdom
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Device Features/Options
Some of the Choices
• FET
– Internal or External
• Inrush control
– dV/dT, or di/dT
• Current Limit
– Always, Never, or just at startup
• Fault response
– Latch off or Retry
• Short Circuit Response
– Latch Off or Retry
• Control
– I2C or Analog Control
• Outputs
• ILIMIT Accuracy
– 20% Standard, 10% Pretty Good,
5% Very Good
• FET SOA protection.. Or not
– Allows use of smaller FET and
provides very high survivability
• Current Indicator Output (IMON)
– Analog or Digital Output ?
– Digital Output requires internal
ADC and typically includes PIV
Monitoring
• ORing Control
– Linear or Hysteretic
– Power Good, Fault, FET Fault
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Device Features/Options
Internal FET vs. External FET
Internal FET
• Highly Integrated
-
Few External Parts
Internal sense FET
Built it Power Limiting
Extremely well protected
• Compromises are made
- FET process vs. Analog process
- FET package vs. Analog package
• Require careful thermal design
• Generally not found in app > 5 A
External FET
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•
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•
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Flexible RDSON (Designers Choice)
More feature options
No limit on upper current limit
Generally more accurate
More external parts
- RSENSE, FET
- RS, CS for configuration
• Larger foot print
Power
Supply
Load
= RL + CL
Control IC
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Device Features/Options
FET SOA Protection
• One of the least understood but most appreciated features
• Allows use of smaller, less expensive FETs
• Analog multiplier calculates PDIS_FET in real time and compares result to
PROG pin
• If PLOAD > PROG then gate drive reduced to lower ILOAD and PFET
• TI is now the ONLY manufacturer to offer true Power Limiting !
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Device Features/Options
FET SOA Protection
• Dynamically adjusts ILIMIT to be approximately proportional to 1/(VDS)2
• Limits PDIS of FET to programmed limit
Orange = Violet x (VIN – Blue)
PDIS = ILOAD x VDS
PDIS
ILOAD
VOUT
ILOAD
VOUT
TPS2420 Startup into 15 Ω, 700 μF
TPS2420 Limits FET PDIS < 5 Watts
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Device Features/Options
Power Limiting – Startup into overload response
• SOA protection keeps FET safe
even when starting up into a
severe overload
ILOAD
• Fault timer limits T(ime) factor of
SOA
• Some competitive devices will
reduce ILIMIT over a limited range
and with limited protection.
• ONLY TI has true FET SOA Power
Limiting built into the Hotswap
Controller !!!
VOUT
CT
PDIS
TPS2420 Startup into overload
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Positive Low Voltage Protection
TI Device Portfolio Sample
PART
Input
Package
Range
TPS2420
TPS2590
3 to 20
TPS2421-1/2
ILIMIT
Lo
Internal FET
RDSON = 30 mW
Always
ILIMIT = ±10% @ 2
A
QFN16 (3x3)
Yes
No
No
2.5 to 18
MSOP10
25 ± 10%
LM25066/A
LLP24
(4x5mm)
LLP24
(4x5mm)
25 ± 10%
46.5 ± 11.8%
MSOP10
50 ± 10%
LM25066I/AI
2.9 to 17
LM25069-1/2
Startup
Only
MSOP10
50 ± 10%
PW20
50 ± 10%
9 to 26
9 to 36
No
Yes
Yes
Yes
No
Lo
No
Prog
l/l/h/h
N/A
No
No
Lo
2.40%
Yes
Yes
Always
LM25061-1/2
N/A
Prog
MSOP8
TPS2482/3
Yes
17%@2A
Lo
TPS24700/1
TPS2480/1
Int.
Imon
SOA OV I2C PG
FET
Acc.
SOIC8
TPS24720
TPS24710/1/2/3
QFN16
(4x4mm)
QFN16
(4x4mm)
VTHRESH
(mV)
No
Yes
1.00%
No
No
Hi
N/A
0.5%
Yes
0.5%
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Typical Inrush/OCP Design Steps
1. Select RSENSE to set ILIMIT and IFASTTRIP
– ILIMIT = VTH/RSENSE - VTH typically 25 – 50 mV
• Simplest controllers have fixed VTH
• High VTH → Better Accuracy but Higher I2R Losses
– Fast trip – (Short Circuit) threshold usually 1.5x -3x ILIMIT Level
2. Select CFAULT to get desired TFAULT
– Set TFAULT long enough to allow all downstream caps to charge
(TCHARGE)before time out
• TCHARGE ~ CV/I (C = Bulk Cap, V = VOUT, I= ILIMIT )
– Set TFAULT as short as reasonable to minimize FET stress during overcurrent
events
– Ensure that TFAULT x VIN X ILIMIT is within SOA curve
3. Select FET that can withstand TFAULT x VIN x ILIMIT x ~1.5 …..SOA !!
4. Set FET SOA Power Limit on devices so equipped
– Design tools available for some devices - check webpage
– TPS24700/10/20, TPS2490/1/2/3, TPS2480/1, LM5064/6/7/9, LM25061/6/9
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Questions To Ask During Design
1. Will a load get plugged into a live socket?
2. Will a load get unplugged from a live socket?
3. Is it OK for supply to collapse if one load shorts?
4. Are multiple loads connected to a common supply?
– OK for all loads to shut off if one load shorts?
5. Do loads need ability to ride through transients?
6. Do loads needs protection from voltage surges?
7. Do loads have large capacitance on the inputs?
8. Are multiple supplies powering the load or bus?
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Thank you!
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Circuit Protection Fundamentals
11.3 Common Design Errors
Common Design Errors
• SOA of FET too Small
• Layout Issues
• Inadequate Transient Protection
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SOA of FET Too Small
• SOA = Safe Operating Area
SOA Chart – Every FET has one
Defines Bounds of FET Operation
VDS_MAX = Vertical Limit
ID_MAX = Horizontal Limit
RDSON limits ID at lower voltages
Theoretical PMAX = 3000 W
ID_MAX
• Fault Time Is Critical
– Longer Fault time means bigger FET
– Shorter Fault Time allows higher
peak power
• Most Stressful FET Events
– Startup into short
– Shorted load while under full load
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IDS Drain to Source Current
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103
1 ms
101
10 ms
100
100 ms
1s
10-1
DC
10-2
10-2
10-1
100
101
VDS Drain to Source Voltage
102
VDS_MAX
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SOA of FET Too Small
Example - 12 V, 50 A Server
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Without Power Limiting
– PMAX = ILIMIT x VSUPPLY = 600 W
– TSOA_MAX = ~800 us
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102
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With Power Limiting
– PMAX_LIMITED = 50 W
– As VDS increases ILIMIT is reduced
– TSOA_MAX = 10 ms
– Smaller FET can be used
– @ 50 A will start limiting when
VDS > 1V
IDS Drain to Source Current
50 A
1 ms
101
10 ms
100
100 ms
1s
10-1
DC
• Common Error to Pick FET Too Small
10-2
10-2
10-1
100
101
VDS Drain to Source Voltage
102
12 V
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Layout Issues - A Little Stray R Can
Make a Big Error
• Critical Kelvin Connections
– Sense Runs
• Critical Short Run
– Ground
– Gate
• High Current Runs
• Poor Kelvin Runs…
– Inaccurate/variable ILIMIT
• Poor High Current Runs
– Voltage droop
– Power loss
– Overheating
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Inadequate Transient Protection
• All wires are inductive
• Inductance stores energy
LI2
E
2
• When the FET turns off, voltage spikes occur
di
VL
dt
di

dt
Positive Spikes at Input to Switch/FET
LOAD CURRENT
Negative Spikes at Output of Switch/FET
LOAD VOLTAGE
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Inadequate Transient Protection
• To squelch inductive spikes from supply / load leads
– Caps and/or TVS at UUT Input to clamp positive spike
– Schottky Clamp across output to clamp negative spike
– Short, Wide Leads and Runs
Power
Supply
Load
= RL + CL
Control IC
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Thank you!
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Circuit Protection Fundamentals
11.4 Common Test Errors
Common Test Error Sources
• Current Probes
• Electronic Loads
• Transients From Long Supply Leads
• Supply ILIMIT Too Low
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Current Probes
Current Probe Behavior
• ↑ Great For Observing Waveform Shapes
• ↑ Don’t have to be “In The Loop”…Nice !!
– Simply Clamps around feed or RTN wire
• ↓ Need Frequent Degaussing/Cal
• ↓ Not So Great for Precise Measurements
– Limited Bandwidth
– 1% Accuracy at Best
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Current Probes
For precise DC current measurements
• If ILOAD < 10 Amps use Multimeter on Current Scale
• If ILOAD > 10 Amps Use Shunt and Multimeter
– Pick RSENSE so VRSENSE @ ILIMIT = 50-100 mV
– Note….Now VOUT_SUPPLY ≠ VIN_LOAD...so measure VLOAD at The Load!
Power
Supply
DMM
DMM
10 Amp Scale
~100 mV Scale
Load
ILOAD < 10 A
Power
Supply
RSENSE
Load
ILOAD > 10 A
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Electronic Loads
• Good for DC Loading and Automated Tests
• Proper Setup Very Important
– Ex. - Constant Current, Constant Power, Constant Resistance
• But…often Have Switch Transients When Stepping Load
– Transients Can Cause Premature Trip When Measuring ILIMIT
• So What Do We DO ??
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Electronic Loads
For Minimal Transients While Adjusting Load
Method 1: Use Power Resistors as
Loads
• A bit tedious and Old School… but accurate
• A collection of fixed and variable resistors is
best
• Apply “Last Half Amp” With Small Wire
Rheostat
• Can be effective with eLoads also
Method 2: Use Power
FET as Load
• Connect FET and Series
Resistor as Load
• Adjust Potentiometer to
vary Current
• Make Sure the FET can
Handle the power !!!!
To UUT
RADJUST
RSHUNT
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Long Supply Leads
• All wires are inductive
• Long Supply Leads can have significant L
• Lab Test Environment Usually Worse Than Final Application!
– Reason for TVS and diodes on most TI EVMs
• When the FET turns off, voltage spikes occur
• To counter inductive spikes from supply / load leads
– Caps and/or TVS at UUT Input to clamp positive spike
– Schottky Clamp across output to clamp negative spike
– Twisted Supply leads
Power
Supply
Load
= RL + CL
Control IC
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Supply ILIMIT Too Low
• Lab Supply Limit Sometimes Set Slightly Above ILIMIT_LOAD
• VSUPPLY can sag due to I limiting during overload / short circuit testing
• Sagging VSUPPLY can cause UV shutdown before ILOAD reaches IFASTTRIP
• UV Shutdown is typically much slower than Fast trip (SC) Shutdown
• Slow shut down can violate FET SOA, resulting in dead FETs
• Fix 1: Ensure PS set to supply currents ABOVE fast trip level
• Fix 2: Attach bulk caps at input of UUT before test is run
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Trends in Circuit Protection
• Accuracy
– Current limit, power limit, monitoring
• Efficiency
– Low RDSON, low IQ
• High levels of integration
– i.e. bring FET, RSENSE into the package
• I2C, PMBus for control and monitoring
– Especially PMBus with Intel Grantley processors
• eFuses replacing/augmenting fuses & polyfuses
• High Power POE Systems (25-100 Watts)
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Thank you!
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Circuit Protection Fundamentals
11.5a eFuse Overview
Integrated Circuit Protection Types
Power controlling element contained therein
Initial $
好运
Wishful Thinking
Fuse
Polyfuse (PTC)
eFuse
Level
of
Protection
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What is an eFuse?
An active circuit protection device that…
• Will:
– Limit current at inrush
– Prevent load or source damage due to OC events
– Have an internal FET to control the load current
• Might:
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Provide OVP (none, fixed, adjustable)
Have adjustable fault time and/or current limit
Have indicator outputs (Fault, PG, etc.)
Be able to control turn on slew rate
Have a load current indicator output
Be on source side of a connector or load side or..
Be nowhere near a connector
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Typical Applications for eFuse
m-SATA SSD
SAS HDD
Enterprise Class SSD
Storage Server
Chassis
Set-Top Box
DVD Player
Internet TV
Appliances
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Brand Damage – A Hidden Cost
It’s not fair and that won’t change
• Most end users don’t know or care how a product works
– Even fewer know or care about circuit protection
• A good fuse design in a bad system can still get the blame
– Dirty power, poor transient control, can cause a fuse to blow
– A load with a blown fuse is viewed as the problem…not the faulty source
– Blame should go to the source of “bad” power...but rarely does
• END CUSTOMER DOESN’T CARE about power specs !!!
– Your board died….now fix it!!!
– Replacement board likely to blow a fuse, too
– Customer not happy – switches to competitive brand
• Control your products’ destiny !!
– Don’t rely on other systems to “do the right thing”
– Protect the product, the brand, the profit, your career !
• Someone will pay….don’t let it be you!
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Backend costs of “fuse only” designs
• Tangible costs
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Replacement of nonfunctional product
RMA admin costs / time
Shipping broken/new devices from/to customer
Truck rolls, service personnel
• Intangible backend costs
– Unhappy retailers
– Brand damage
– Loss of customer(s)
• In the end, we all want happy customers
– It’s that simple, it’s that complicated
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Thank you!
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Circuit Protection Fundamentals
11.5b eFuse vs. Fuse
Why Not Use a Fuse?
• Slow
• Inaccurate
• Lossy
• Leave a load unpowered after event
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“Fast Blo Fuse” Trip Time vs.
Current eFuse vs. Fuse
eFuse trip range
eFuse Limit !
Time (sec.)
Time and trip limit inaccuracies mean bigger power supplies
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Fuses Are Slow…Even the Fast Ones
eFuse Performance
• ILIMIT is programmable, predictable, and stable over temp
• Bus droop and supply stress reduced by tight over current tolerance
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Fuses are Lossy
• Higher resistance -> more energy -> more heat -> higher OPEX
• 13x more power lost with fuse!
– 800 mV/2A = 400 mΩ vs. eFuse @ 30 m Ω
• Lifetime cost of 1 Watt = $2 to $18 ( customer supplied numbers)
– Includes energy cost, distribution infrastructure, HVAC, product life
Little Fuse 231Series
0.12
0.19
0.30
0.48
0.75
1.19
Lower Losses using TPS2590 ( 30 mW )
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Fuses are Inaccurate
• Fuse makers recommend the INOMINAL< 75% IFUSE_RATED
• Power supplies must be overspec’d
– Accommodate fuse derating, fuse tolerance, PFUSE
– Bigger supplies = more CAPEX, more OPEX
Seconds
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Fuse’s Behavior is Sloppy
and Stressful
During Overload
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Much slower than eFuse
No active current limiting
Uncontrolled turn off time
Bus droop likely
More stress on supplies & load
High I2R losses
10x+ nom. trip current for 3 ms
After Overload
• No auto reset
• Inoperative system
• Module, fuse, or system must be
replaced
• Repair costs
• Field returns
• Unhappy Customers
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Fuse Summary
Challenges
Benefits
• Slow
• Low Cost
• Lossy
• Can provide Safety Compliance
• Inaccurate
- UL, IEC, CSA
• Load unpowered after event
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Fuses DO Excel in Some Apps!
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Thank you!
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Circuit Protection Fundamentals
11.5c eFuse vs. Polyfuse
eFuse vs. Polyfuse
eFuse (USB Power Switch)
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•
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Current based ILIMIT
Stable, accurate (20% - 30%) ILIMIT
Fixed or Programmable ILIMIT
Repeatable ILIMIT0
Fast ( < 1.5 us typ)
Wide temp range
-40° to +125° C
Polyfuse
•
•
•
•
•
•
•
Temp based ILIMIT
Sloppy, variable ILIMIT
No Programmable ILIMIT
RON Increases with each event
Slow to trip (several ms)
Not usable above 85° C
Auto-resets after trip event
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Polyfuses (PTC Devices)
Require Derating
D
D
Curve D
TPS2420/21,
TPS2590/910
eFuse vs. Polyfuse
• Brand conscious Tier 1, 2, 3s use USB Power Switch
• Low cost “bottom end” apps may use Polyfuse
True story #1 – Low end desktop maker melted wireless datacard during a short
condition. Three times. Now using USB switches.
True story #2 – Major ODM experiencing Power supply resets during STB
short testing. TPS2066C with faster response got designed in
and no more resets.
Polyfuse Summary
Challenges
Benefits
• Slow
• Resets after OC event
• Lossy – 2x regular fuse
• Low Cost
• Inaccurate
• Can provide Safety Agency
compliance
• Each OC event increases
resistance
UL, IEC, CSA
• Not suitable for high temp.
• R increases with Temp.-
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Thank you!
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