112 Circuit Protection Device Features and Options 113 Common Design Errors 114 Common Test Errors 115 eFuses 115a eFuse Overview 115b eFuse vs Fuse 115c eFuse vs Polyfuse 1 Circuit Protection Fundamentals ID: 675652
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Slide1
Module Flow
11.1 Circuit Protection Overview 11.2 Circuit Protection Device Features and Options11.3 Common Design Errors11.4 Common Test Errors11.5 eFuses11.5a eFuse Overview11.5b eFuse vs. Fuse11.5c eFuse vs. Polyfuse
1Slide2
Circuit Protection Fundamentals
11.1 Circuit Protection OverviewSlide3
Circuit Protection – What is it ?
Many things with many namesInrush ControlHotswapHotplugCurrent LimitingElectronic Circuit BreakerShort Circuit ProtectionSoft StartOver Voltage Protection (OVP)
eFuse
Load Power Limiting
FET SOA Limiting ( Protecting the Protector ! )
Reverse Current Protection (ORing)
Often Required for Agency RatingUL, CSA – North AmericaEN, IEC, (CENELEC) – EuropeCCC Mark (CNCA) - China
~ the same functions
3Slide4
Circuit Protection – What is it ?
Circuits designed specifically to….Prevent Fire ! --“Keep the smoke in!”Keep small problems from growing bigMinimize damage by quickly isolating failuresPrevent potentially disruptive power bus disturbancesOne small transient can take down/reset an entire system
What Gets Protected ?
LOAD
POWER FET
CONNECTORS
SUPPLY
4Slide5
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
5Slide6
Circuit Protection – The Basic Parts
Most Common ElementsLocation..Sometimes on the Load Side of the Connector
Sometimes on the Supply Side of the Connector
LOAD BOARD
BACKPLANE
Element
for controlling the FET
Element
for sensing
current
Element for modulating
current
6Slide7
Thank you!
7Slide8
Circuit Protection Fundamentals
11.2 Device Features and OptionsSlide9
9
“End customers can’t make you design in protection circuits but they can make you wish you had.”
– Design review wisdom Slide10
Device Features/Options
Some of the ChoicesFETInternal or ExternalInrush controldV/dT, or di/dTCurrent LimitAlways, Never, or just at startupFault responseLatch off or RetryShort Circuit Response
Latch Off or Retry
Control
I2C or Analog Control
Outputs
Power Good, Fault, FET Fault
ILIMIT Accuracy20% Standard, 10% Pretty Good, 5% Very GoodFET SOA protection.. Or notAllows use of smaller FET and provides very high survivabilityCurrent Indicator Output (IMON)Analog or Digital Output ?Digital Output requires internal ADC and typically includes PIV MonitoringORing ControlLinear or Hysteretic10Slide11
Device Features/OptionsInternal FET vs. External
FET11Internal FET
External FET
Highly Integrated
Few External Parts
Internal sense FET
Built it Power Limiting
Extremely well protected Compromises are madeFET process vs. Analog processFET package vs. Analog packageRequire careful thermal design
Generally not found in app > 5 A
Flexible R
DSON
(Designers Choice)
More feature options
No limit on upper current limit
Generally more accurate
More external parts
R
SENSE, FETRS, CS for configurationLarger foot printSlide12
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 P
DIS_FET
in real time and compares result to PROG pin
If PLOAD > PROG then gate drive reduced to lower ILOAD and PFETTI is now the ONLY manufacturer to offer true Power Limiting !12Slide13
Device Features/Options
FET SOA ProtectionDynamically adjusts ILIMIT to be approximately proportional to 1/(VDS)2Limits PDIS of FET
to programmed
limit
TPS2420 Startup into 15 Ω, 700 μF
I
LOAD
V
OUT
TPS2420 Limits FET P
DIS
< 5 Watts
P
DIS
V
OUT
I
LOAD
Orange
=
Violet
x
(V
IN
– Blue)
P
DIS
=
I
LOAD
x
V
DS
13Slide14
Device Features/Options Power Limiting – Startup into overload
responseSOA protection keeps FET safe even when starting up into a severe overloadFault timer limits T(ime) factor of SOASome 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 !!
!
P
DIS
V
OUT
I
LOAD
C
T
TPS2420 Startup into overload
14Slide15
Positive Low Voltage Protection
TI Device Portfolio Sample
PART
Input Range
Package
V
THRESH
(mV)
I
LIMIT
Int. FET
SOA
OV
I2C
PG
Imon Acc.
TPS2420
3 to 20
QFN16 (4x4mm)
Internal FET
R
DSON
= 30 m
W
I
LIMIT
= ±10% @ 2 A
Always
Yes
Yes
No
No
Lo
17%@2A
TPS2590
QFN16 (4x4mm)
-
N/A
TPS2421-1/2
SOIC8
Lo
TPS24720
2.5 to 18
QFN16 (3x3)
Prog
Startup Only
No
Yes
Yes
No
Lo
Prog
TPS24710/1/2/3
MSOP10
25 ± 10%
Yes
No
l/l/h/h
N/A
TPS24700/1
MSOP8
No
No
Lo
LM25066/A
2.9 to 17
LLP24 (4x5mm)
25 ± 10%
46.5 ± 11.8%
Always
No
Yes
Yes
Yes
Hi
2.40%
LM25066I/AI
LLP24 (4x5mm)
1.00%
LM25069-1/2
MSOP10
50 ± 10%
No
N/A
LM25061-1/2
MSOP10
50 ± 10%
No
TPS2480/1
9 to 26
PW20
50 ± 10%
Yes
0.5%
TPS2482/3
9 to 36
0.5%
15Slide16
Typical Inrush/OCP Design Steps
Select RSENSE to set ILIMIT and IFASTTRIP
I
LIMIT
= V
TH
/RSENSE - VTH typically 25 – 50 mVSimplest controllers have fixed VTH
High VTH → Better Accuracy but Higher I2R LossesFast trip – (Short Circuit) threshold usually 1.5x -3x ILIMIT Level Select CFAULT to get desired TFAULT Set TFAULT long enough to allow all downstream caps to charge (TCHARGE)before time outTCHARGE ~ CV/I (C = Bulk Cap, V = VOUT, I= ILIMIT )Set TFAULT as short as reasonable to minimize FET stress during overcurrent eventsEnsure that TFAULT x VIN X I
LIMIT is within SOA curveSelect FET that can withstand TFAULT x V
IN
x I
LIMIT
x ~1.5 …..SOA !!
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
16Slide17
Questions To Ask During Design
Will a load get plugged into a live socket?Will a load get unplugged from a live socket?Is it OK for supply to collapse if one load shorts?Are multiple loads connected to a common supply?OK for all loads to shut off if one load shorts?
Do loads need ability to ride through transients?
Do loads needs protection from voltage surges?
Do loads have large capacitance on the inputs?
Are multiple supplies powering the load or bus?
17Slide18
Thank you!
18Slide19
Circuit Protection Fundamentals
11.3 Common Design ErrorsSlide20
Common Design Errors
SOA of FET too SmallLayout IssuesInadequate Transient Protection20Slide21
SOA of FET Too Small
I
DS
Drain to Source Current
10
3
10
2
10
1
10
0
10
-1
10
-2
10
-2
10-1 100 101 10
2V
DS
Drain to Source Voltage
V
DS_MAX
I
D_MAX
R
DSON
1 ms
10 ms
100 ms
1 s
DC
SOA = Safe Operating Area
SOA Chart – Every FET has one
Defines Bounds of FET Operation
V
DS_MAX
= Vertical Limit
I
D_MAX
= Horizontal Limit
R
DSON
limits I
D
at lower voltages
Theoretical
P
MAX
= 3000 W
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
Putting FETs in parallel does NOT improve dynamic SOA !!!
21Slide22
SOA of FET Too Small
Example - 12 V, 50 A Server
10
3
10
2
10
1
10010-1
10
-2
10
-2
10
-1
10
0
101 102
VDS Drain to Source Voltage
IDS Drain to Source Current
1 ms
10 ms
100 ms
1 s
DC
Without Power Limiting
P
MAX
= I
LIMIT
x V
SUPPLY
= 600 W
T
SOA_MAX
= ~800 us
With Power Limiting
P
MAX_LIMITED
= 50 W
As V
DS
increases I
LIMIT
is reduced
T
SOA_MAX
= 10 ms
Smaller FET can be used
@ 50 A will start limiting when
V
DS
> 1V
Common Error to Pick FET Too Small
12 V
50 A
22Slide23
Layout Issues - A Little Stray R Can
Make a Big ErrorCritical Kelvin ConnectionsSense Runs Critical Short RunGroundGateHigh Current RunsPoor Kelvin Runs…Inaccurate/variable ILIMIT
Poor High Current Runs
Voltage droop
Power loss
Overheating
23Slide24
Inadequate Transient Protection
All wires are inductiveInductance stores energyWhen the FET turns off, voltage spikes occur
LOAD CURRENT
LOAD VOLTAGE
Positive Spikes at Input to Switch/FET
Negative Spikes at Output of Switch/FET
24Slide25
Inadequate Transient Protection
To squelch inductive spikes from supply / load leadsCaps and/or TVS at UUT Input to clamp positive spikeSchottky Clamp across output to clamp negative spikeShort, Wide Leads and Runs
25Slide26
Thank you!
26Slide27
Circuit Protection Fundamentals
11.4 Common Test ErrorsSlide28
Common Test Error Sources
Current Probes
Electronic Loads
Transients From Long Supply Leads
Supply ILIMIT Too Low
28Slide29
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 MeasurementsLimited Bandwidth1% Accuracy at Best
29Slide30
Current Probes
For precise DC current measurements If ILOAD < 10 Amps use Multimeter on Current ScaleIf ILOAD > 10 Amps Use Shunt and MultimeterPick RSENSE so VRSENSE
@ I
LIMIT
= 50-100 mV
Note….Now V
OUT_SUPPLY ≠ VIN_LOAD...so measure VLOAD at The Load!
30Slide31
Electronic Loads
Good for DC Loading and Automated TestsProper Setup Very ImportantEx. - Constant Current, Constant Power, Constant ResistanceBut…often Have Switch Transients When Stepping LoadTransients Can Cause Premature Trip When Measuring ILIMITSo What Do We DO ??
31Slide32
Electronic Loads
For Minimal Transients While Adjusting Load32
Method 1: Use Power Resistors as Loads
Method 2: Use Power FET as Load
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 alsoConnect FET and Series Resistor as LoadAdjust Potentiometer to vary CurrentMake Sure the FET can Handle the power !!!!Slide33
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 leadsCaps and/or TVS at UUT Input to clamp positive spikeSchottky Clamp across output to clamp negative spike
Twisted Supply leads
33Slide34
Supply ILIMIT
Too Low Lab Supply Limit Sometimes Set Slightly Above ILIMIT_LOADVSUPPLY can sag due to I limiting during overload / short circuit testingSagging VSUPPLY can cause UV shutdown before ILOAD reaches I
FASTTRIP
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 levelFix 2
: Attach bulk caps at input of UUT before test is run34Slide35
Trends in Circuit Protection
AccuracyCurrent limit, power limit, monitoringEfficiencyLow RDSON, low IQHigh levels of integrationi.e. bring FET, RSENSE into the package
I
2
C, PMBus for control and monitoring
Especially PMBus with Intel Grantley processors
eFuses replacing/augmenting fuses & polyfusesHigh Power POE Systems (25-100 Watts)
35Slide36
Thank you!
36Slide37
Circuit Protection Fundamentals
11.5a eFuse OverviewSlide38
Integrated Circuit Protection Types
Power controlling element contained therein
好运
Initial $
Level
of
Protection
Wishful Thinking
Fuse
eFuse
Polyfuse (PTC)
38Slide39
What is an eFuse?
An active circuit protection device that…Will:Limit current at inrushPrevent load or source damage due to OC eventsHave an internal FET to control the load currentMight:Provide OVP (none, fixed, adjustable)
H
ave adjustable fault time and/or current limit
H
ave indicator outputs (Fault, PG, etc.)
Be able to control turn on slew rateHave a load current indicator outputBe on source side of a connector or load side or..
Be nowhere near a connector39Slide40
Typical Applications for eFuse
Enterprise Class SSD
SAS HDD
Storage Server
Chassis
Set-Top Box
DVD Player
Internet TV
m-SATA SSD
Appliances
40Slide41
Brand Damage – A
Hidden CostIt’
s not fair and that won
’
t change
Most end users don’
t know or care how a product worksEven fewer know or care about circuit protectionA good fuse design in a bad system can still get the blame
Dirty power, poor transient control, can cause a fuse to blowA load with a blown fuse is viewed as the problem…not the faulty sourceBlame should go to the source of “bad” power...but rarely doesEND CUSTOMER DOESN’T CARE about power specs !!! Your board died….now fix it!!!Replacement board likely to blow a fuse, tooCustomer not happy – switches to competitive brandControl 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!
41Slide42
Backend costs of
“
fuse only
”
designs
Tangible costs
Replacement of nonfunctional product
RMA admin costs / timeShipping broken/new devices from/to customerTruck rolls, service personnelIntangible backend costsUnhappy retailersBrand damageLoss of customer(s)In the end, we all want happy customersIt’s that simple, it’s that complicated
42Slide43
Thank you!
43Slide44
Circuit Protection Fundamentals
11.5b eFuse vs. FuseSlide45
Why Not U
se a Fuse?SlowInaccurateLossyLeave a load unpowered after event45Slide46
“Fast Blo Fuse” Trip T
ime vs. Current eFuse vs. FuseTime and trip limit inaccuracies mean bigger power supplies
eFuse Limit !
Fusible fuse trip range
eFuse trip range
Time (sec.)
46Slide47
Fuses Are Slow…E
ven the Fast Ones
eFuse Performance
I
LIMIT
is programmable, predictable, and stable over temp
Bus droop and supply stress reduced by tight over current tolerance
47Slide48
Fuses are Lossy
Higher resistance -> more energy -> more heat -> higher OPEX13x 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 m
W
)
48Slide49
Fuses are Inaccurate
Fuse makers recommend the INOMINAL< 75% IFUSE_RATEDPower supplies must be overspec’dAccommodate fuse derating, fuse tolerance, PFUSE Bigger supplies = more CAPEX, more OPEX
Seconds
49Slide50
Fuse’s Behavior
is Sloppy and Stressful50
During Overload
After Overload
Much slower than eFuse
No active current limiting
Uncontrolled turn off time
Bus droop likelyMore stress on supplies & loadHigh I2R losses10x+ nom. trip current for 3 ms
No auto reset
Inoperative system
Module, fuse, or system must be replaced
Repair costs
Field returns
Unhappy
CustomersSlide51
Fuse Summary
51Challenges
Benefits
Slow
Lossy
Inaccurate
Load unpowered after event
Low CostCan provide Safety Compliance
UL, IEC, CSASlide52
Fuses DO Excel in Some Apps!
52Slide53
Thank you!
53Slide54
Circuit Protection Fundamentals
11.5c eFuse vs. PolyfuseSlide55
eFuse vs. Polyfuse
55eFuse (USB Power Switch)
Polyfuse
Current based I
LIMIT
Stable, accurate (20% - 30%) I
LIMIT
Fixed or Programmable ILIMITRepeatable ILIMIT0Fast ( < 1.5 us typ)Wide temp range-40° to +
125° C
Temp based I
LIMIT
Sloppy, variable I
LIMIT
No Programmable I
LIMIT
R
ON Increases with each eventSlow to trip (several ms)Not usable above
85° CAuto-resets after trip eventSlide56
Polyfuses (PTC Devices)
Require Derating
Curve D
TPS2420/21, TPS2590/910
D
DSlide57
eFuse vs. Polyfuse
Brand conscious Tier 1, 2, 3s use USB Power SwitchLow cost “
bottom end
”
apps may use Polyfuse
True story #2
–
Major ODM experiencing Power supply resets during STB short testing. TPS2066C with faster response got designed in and no more resets.True story #1 – Low end desktop maker melted wireless datacard during a short condition. Three times. Now using USB switches.Slide58
Polyfuse Summary
58Challenges
Benefits
Slow
Lossy – 2x regular fuse
Inaccurate
Each OC event increases resistance
Not suitable for high temp.R increases with Temp.-Resets after OC eventLow Cost
Can provide Safety Agency compliance UL, IEC, CSASlide59
Thank you!
59