Littelfuse OC Selection Training Anthony Chang. 2 Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. Who We

Littelfuse OC Selection Training

Anthony Chang

2Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved.

Who We Are...

• Littelfuse Inc. 成立於 1927 年，為專業製造保險絲公司。自 1999 至 2005 年投入了過電壓保護元件製造而成為一家線路保護元件的 Circuit Protection Provider 的跨國公司。總部設在美國伊利諾州 Des Plaines 市，擁有十幾家工廠分佈在美、歐、亞三大洲七個國家，在世界各地設有區域性營銷中心。


1927— First instrument fuses1976— ATO® blade fuse1992— Surface mount (D-Case)1994— Square Nano fuse1994— A-Case (1206)1996— 0603 surface mount1997— PTC Resettable Device 1999— ESD Protectors/Harris Suppression2002 Lead Free 1206, 0603 & 0402 size Fuse2002 Semitron Over Voltage Products (TVS Diodes & GDT)2003 TECCOR (Power Thyristor & Sidactor)2004 Wickmann Integration


Fuse selection• Fuse review• Basics of fuse• Application of Aspects• Selecting a fuse

– Fuse rating

– I2t rating

• Example of in-rush current• Demonstration of trip times• Downsides and solutions

Overview:


Fuse functionality - Limited current vs Potential current

Circuit response without fuse protection – current is eventually disrupted (in an unpredictable manner) due to the failure of a component or circuit board trace

Fused circuit - current is interrupted in a controlled manner (by the fuse) to avoid damage to circuit components - circuit is allowed to fail “gracefully”

I (A

)

t

Fuse

A Fuse is a Temperature Sensitive Device that serves as an intentional Weak Link in an electrical circuit. It interrupts overload and short-circuit conditions when its metallic element melts; thereby halting the overcurrent condition.


Fuse Review: Construction

Traditional Cartridge Type Vs. Slimline Thin Film Type

CAPCAP

SOLDERSOLDER

FILAMENTFILAMENT

BODYBODY

SOLDERSOLDER

CAPCAP

Cartridge fuseThin Film fuse

The metallic element (filament) melts and disconnects the power to the circuit.


SMD Fuse 1.- Ceramic Body. 2.- Fuse end caps Tin-Lead and RoHS (Silver or Gold) plated caps. 3.- Filament (Ag Alloy) for 451 & Ceramic Yarn and Tin plate wire for 452 451 series 452 series


Fuse

Solutions for:• AC mains overcurrent protection• Tip/Ring circuit protection

Cartridge and leaded fuse• Industry standard form factors (cartridge, axial leaded, radial leaded)

• Designed to IEC, UL, METI

• Fast-acting, medium-acting, and slo-blo versions available

• Low, enhanced and high-breaking capacity versions available

• Key feature is the wide array of form factors, amperage ratings, voltage ratings and breaking capacities that are available


Cartridge, leaded fuse• Power supply• Power adaptor• TVSS• Lighting ballast• Consumer electronics

– DVD, television, set top box

• Home appliances– Washer, dryer, refrigerator, air conditioner,

humidifier

• Tip/ring - Power Cross (2AG)

Target applications:

AC line protection for power supplies

AC line

Common/ground

Power modulePower plug

Po

we

r s

ys

tem

UltraMOV, CIII, LA series radial leaded MOV

Cartridge or leaded fuse

Fuse

Power supplyTip/ring circuit

Power supply


Fuse

Solutions for:• Battery pack and DC bus overcurrent protection

Surface Mount Chip Fuse• Space-efficient form factors (0402, 0603, 1206)

• Amperage ratings from 0.062A to 7.0A

• Low profile for height-sensitive applications

• Constructed on an epoxy-based substrate similar to the circuit board on which it will be installed. Avoids temperature coefficient of expansion (TCE) mismatch that could cause reliability problems in the field.

• Key feature of FR-4 substrate is higher melting i2t value than ceramic chip fuse, which minimizes nuisance tripping during in-rush current events. Lower resistance values also minimize power dissipation.


Solutions for:• AC mains/DC bus overcurrent protection• Tip/Ring circuit protection

Surface Mount Fuse• Includes Nano2, TeleLink® and UMF series

• Rated for AC and DC circuits


• Constructed using ceramic tube bodies to increase breaking capacity and working voltage

• Key features are higher melting i2t value than chip fuse, which minimizes nuisance tripping during in-rush current events; wire-in-air design minimizes thermal de-rating factor and extends usable temperature range.

Fuse

Nano2 UMF fuse

TeleLink® fuse

Nano2 fuse


Chip fuses• Battery pack• Hard disk drive• Cell phone• Digital camera• LCD monitor inverter (small panels)• MP3/PMP player• Bluetooth headset

Wire-in-Air SMD fuses• Motherboards (servers, notebooks)• LCD monitor inverter (large panels)• Telecom equipment (TeleLink)• Automotive Electronics modules

– Telematics, climate control, navigation, body controllers

Target applications:

Protection of inverter circuit (LCD)

DC input

Common/ground

Inverter circuitDC inputP

ow

er

sy

ste

m

MLV (MLA series)Diode (SMA series)

Chip fuseNano2 fuse

Protection of DC bus (HDD)

Data bus

VBUS

Signal Ground

HDD unitRibbon cable/ connector

Mo

the

rbo

ard

0402, 0603 MLV SMAJ diode

Shield Ground

0467 – Pb-free0434 – Pb/Sn0435 – Pb-free

D0

Dn

Inverter

Fuse

Inverter


What is a FUSE?

1. A fuse is a thermally responding device.

2. A fuse is designed and manufactured to protect by being the intentional weak link in an electrical circuit.

Purpose of the fuse is to prevent FIRE!

Fusing UltimateSystem End


基本保險絲學


保險絲基本工作原理 :

• 結構 : 金屬熔絲及兩端極• 功能 : 保險絲是串聯在電路中 , 當電路正常工作時 , 保險絲只相當予一根

導線能夠長時間穩定地使用 . 只有當電路中發生較大的過載電流和或發生短路時保險絲才會熔斷開電流來保護電路的安全 .

• 原理 : 保險絲通電時因電流轉換的熱量會使熔絲的溫度上升 . 當正常工作電流或允許的過載電流通過時 , 電流產生的熱量通過熔絲 , 殼體向周圍環境輻射 , 通過對流和傳導等方式散發的熱量能與產生的熱量逐漸達到平衡 . 當散熱速跟不上發熱時這些熱量就會在熔絲上逐漸累積 , 使溫度上升 , 一旦溫度達到和超過熔絲材料的熔點時會使熔絲熔化而斷開電流 , 達到保護的作用 .


保險絲允許的過載能力(Fuse Current Capacity)

Max. Carrying current:• UL STD 保險絲的最大不熔斷電流是 75% In ；• IEC STD 保險絲的最大不熔斷電流是 100%In 。

• 同時也要求保險絲在超過限量的過載電流時能及時燒斷，其中 :

Minimum Fusing Current:• UL STD 保險絲的最小熔斷電流在 200%In 左右；• IEC STD 保險絲的最小熔斷電流在 210% 左右。

Cold

HOT


選用保險絲的重要參數(Fuse selection factors)

• 選擇的保險絲是主要或次要的保護元件 ?(Fuse for Primary or Secondary Protection?)

• 1 。額定電流 (Rated current) (Minimum fuse rating)=In• 2 。額定電壓 (Rated voltage) AC/DC• 3 。應用環境温度 (Ambient temperature)• 4 。電壓降 (Voltage drop)• 5 。熔斷特性 (Fusing characteristics) - 時間 /電流特性 (TC curves)

• 6 。遮斷 / 分斷能力 (Breaking / Interrupting Capacity)• 7 。熔化熱能值 ( Melting I²t ) • 8 。結構特徵 (Structure)

---- 外形尺寸 ---- 安裝型式


额定電流 Rated Current ( In)

• 額定電流 In= Irmax / (Frf * Tdf)• 最大 / 重載工作電流 (max operating

current—Irmax*)• UL re-rating factor:

Frf---0.75• IEC re-rating factor:

Frf----1• Temperature de-rating factor,

Tdf – refer to temperature curve A, B or C

• UL 規格保險絲： In =Ir/(Frf*Tdf)• IEC 規格保險絲 : In = Ir/Tdf• 計算的 In 是 Absolute 最小的 In 額定

電流值可選或往上選用最佳適用的 In额定電流值。


應用環境温度(Ambient Temperature)

• UL / IEC 保險絲的各项要求都是指在室温條件下 ----250C 時

• 應用環境温度對保險絲的特性是有很大的影響 !!

• 温度折减率 ----Tdf• Ambient Temp: refers to the

air temperature immediately to the vicinity of fuse. (Note: At the worst Condition such as Burn In, high temp test, etc.)


實例 :應用電壓 : 12 V DC應用最大 / 重載電流 : 1A @ max 環境溫度緊鄰保險絲 : 80C 脈衝電流 : 20A @ 30uSec.*短路電流 : 35A max. at 12V保險絲種類 : 1206 size TFF應用最大電流是 1.0A 就將此值帶入下述公式 :

Tdf:0.75*(0.70)

Example:


Example :Steady state current


應用最大電流 (Irmax) 1.0ACat. Fuse Rating = -------------------------------------- = ------------------------

0.75 x 溫度折減率 (Tdf)@80C 0.75 x 0.70

Cat. Fuse Rating =1.91A [ 計算值 ]-Absolute minimum fuse rating can be selected. ( 可從 1206 size 的 466 or 468 系列 2A 以上安培數 , 請參考表 ( 一 ) 。 )

表 ( 一 )P/N Rating Voltage Melting I2t0466002 2.0A 63V 0.2326A^2Sec046602.5 2.5A 32 V 0.3516A^2Sec0466003 3.0A 32V 0.5760A^2Sec0468002 2.0A 63V 0.506A^2Sec0468003 3.0A 32V 1.270A^2Sec

Example: (Continue)


额定電壓 Rated Voltage( Vr ）

• 保險絲的额定電壓應大於或等於應用電路電壓 (Vr標示在保險絲目錄上 )

• 正常負載時额定電壓對保險絲影響很小 • 過載熔斷時额定電壓對保險絲有影響


FUSING

Basics: Hot spot 温

度T

emp

erat

ure

(°C

)

負載電流時的溫度

PCB Sn 锡球 PCB

In

I >In

Melting Point


t = 0

t =1 fusing to melting

fusing

Basic Fusing Process ( 保險絲熔斷過程 )


t =1

t =3 arcing continue depended on the I and V，

Arcing end

t =2, arcing starting

Basic Fusing Process

( 保險絲熔斷過程 )



保險絲的電壓降說明

保險絲的電壓降是保險絲在額定電流條件下，其兩端的電壓降。它反映了保險絲的內阻，其值不應過大。若將內阻（電壓降）過大的保險絲安裝在電路中，它將影響電路的系統參數，使得電路不能正常工作。標準對電壓降不僅有其值的上限規定，而且對其一致性也作了規定。


時間 /電流特性 Time/Current Characteristic

• 承受過電流時 , 電流與熔斷時間的關係• It is a characteristics determine how fast a fuse

responds to over-currents. Over-current is divided into overloads and short circuits.

• UL 和 IEC 的熔斷特性是完全不同的


UL/CSA248-14 規格 UL Vs IEC60127 規格（２）

額定電流値(%) UL & CSA STD 248-14

IEC Type F Sheet1

IEC Type F Sheet 2

IEC Type T Sheet 3

IEC Type T Sheet 5

100% 最低 4時間 --- --- --- --- 135% 最大 60分 --- --- --- --- 150% --- 最低 60分最低 60分最低 60分最低 60分 200% 最大 2分 --- --- --- --- 210% --- 最大 30分最大 30分最大 2分最大 30分

275% ---

32mA～3.15A 0.01～2秒 4～10A

0.01～3秒

32～100mA 0.01～0.05秒 125mA～10A 0.05～2秒

32～100mA 0.20～10秒 125mA～10A 0.60～10秒

1～80秒

400% --- 0.003～3秒

32～100mA 0.003～0.10秒 125mA～10A

0.01～0.03秒

32～100mA 0.04～3秒 125mA～10A 0.15～3秒

1～3.15A 0.095～5秒 4～10A

0.15～5秒

1000% --- 最大 0.02秒最大 0.02秒

32～100mA 0.01～0.3秒 125mA～10A 0.02～0.3秒

1～3.15A 0.01～0.10秒

4～10A 0.02～0.10秒


AGENCY APPROVALS

• UL / CSA 248-14

• Listed – Complied UL/CSA248-14

• Recognized - Manufacturer

• IEC 60127

• Certify agency: VDE, BSI, SEMKO, TUV, NemKo

• METI – PSE Marking

• CCC – China Safety per IEC

• K-mark – Korean Safety


時間 vs. 電流曲線 :(Time vs. Current curve)

• T/C Curve 是一個供設計參考用的工具而不是保險絲的規範• 是保險絲一個的平均熔斷時間曲線圖 . 實際熔斷時間必須考量實

際供應電壓 .


如何看讀時間 / 電流曲線(How to read the TC curve)

0.01

0.1

1

10

100

1000

0.1 1 10 100 1000CURRENT IN AMPS

TIM

E I

N S

EC

ON

DS

• 先選 3A 曲線 .• 找出 3 Ａ曲線與 6A 線交叉

點 .• 後再往 Y軸讀取相應時間 ---

0.1 seconds.

.180 1 2 3 5 7.5 15


遮斷 / 分斷能力 (Breaking/Interrupting Capacity)

• 在額定電壓條件下 ,保險絲能夠安全地分斷預期的電流且沒有對環境的損害 . (Interrupting rating also known as Breaking Capacity or short circuit rating, the interrupting rating is the maximum approved current which the fuse can SAFELY interrupt at rated voltage. )

• 保險絲最重要的安全指標 (The word “safely” here means that there should be no significant damage to the fuse housing nor to the surroundings as a result of arcing or molten metal emerging from the fuse.)

• 短路時保險絲能安全分斷的最大短路電流• IEC STD Fuse Type 有分高分斷 (High Breaking Capacity)，中分斷

(Enhance Breaking Capacity)和低分斷 (Low Breaking Capacity)保險絲 .


遮斷 / 分斷能力 (Breaking/Interrupting Capacity)

• UL 248-14 規定的分斷能力 : 125VAC 條件下 UL Listed 保險絲必須能切斷 10,000A for Subminiature

fuse(2AG) 的電流 ;250VAC 條件下保險絲必須能切斷的電流如下 : 保險絲的額定電流額定分斷能力

0 ~ 1 A 35A 1.1 ~ 3.5A 100A 3.6 ~ 9A 200A 10.1~ 15A 750A 15.1 ~ 30A 1500A

Remark: Recognized 認證的保險絲直接參考製造商的 Interrupting Rating 規格


遮斷 / 分斷能力 (Breaking/ Interrupting Capacity)

• IEC 127 規定的分斷能力 :

250VAC 條件下的分斷能力保險絲 -

低分斷 (LBC) 必須能安全切斷 35A MINIMUM 或它的 10In 中的最大的電流 ; 中分斷 (EBC) 必須能安全切斷 150A 的電流 ; 高分斷 (HBC) 必須能安全切斷 1500A 的電流


EXAMPLE: (continue)

• 0466 series Interrupting rating 50A @ 63V AC/DC for 0.5 ~ 2.0A

• 0466 series Interrupting rating 50A @ 32V AC/DC for 2.5 ~ 3A

• 0466 series Interrupting rating 50A @ 24V AC/DC for 4 ~5A

• 0468 series Interrupting rating 35A @ 63V AC/DC for 2A

• 0468 series Interrupting rating 50A @ 32V AC/DC for 3A


熔化熱能值 (Melting I2t )

• 切斷保險絲所需的熱能量值 ** (is a direct measure for the energy required to blow the fuse-element.) often, the fuse nominal melting I²t Method of fuse selection is applied to those applications in which the fuse MUST sustain large current pulses of a short duration.

• 選用保險絲规格的重要参数 (most important fuse selection factor) 防疲勞 , 多次的擴散 , 氧化 ( prevent de-grade, and oxidize, etc.)• 力特對每一種小型保險絲给出了 Fuse Nominal熔化 I2t值• 清除 I2t = 熔化 I2t + 飛弧 I2t [Clearing I2t =Melting I2t +Arcing I2t ] **Ratio < 22% =Pulse I2t / Fuse Melting I2t *Select either F-fast-acting or T-time lagged fuse* These high-energy currents are common in many applications and are described by

a variety of terms, such as “surge current”, “ start-up current”, “ Inrush current”, and other similar circuit “ transients” that can be classified in general category of “ PULSES.” Pulse current i.e. a flow of repetitive current pulses.


ť• Pulse I²t = ∫ i²p(t)dt =∑ i²p(t)

0• Safety factor is Less than 22%

= Pulse I²t / Fuse Nominal Melting I²t

OR

Fuse minimum Melting I²t = Pulse I²t / 22%


I2t of an Arbitrary Pulse - Conceptually

I2t = area under i2(t) curve

i(t)

Time (t)

Cur

rent

(A

)

tt11 tt22

i2(t)

Time (t)

Cur

rent

(A

)

tt11 tt22

i2(t)


Melting I2t

Melting I2t is an index that characterizes the energy required to melt a fuse element.

Melting I2t values are empirically measured as shown:

Cur

rent

(A

)

Timemelting time arcing time

Element melted,Arcing begins

Integrated to calculateMelting I2t


Example of Inrush Current

Ip2

Ip1Ip3

t1 t2 t3 t4 t5

Ir=4.5A


Example of Inrush

• Ip1= 25A• Ip2 = 20A• Ip3 = 25A• T1= 3mSec• T2 = 10Sec• T3 = 3mSec• T4 = 1hour• T5 = 5mSec• Pulse I2t1= Ip1 x Ip1 x t1 = 25 x 25 x 3mSec=1.875A2Sec• Pulse I2t2 = Ip2 x Ip2 x (t3-t2) = 20 x 20 x 3mSec = 1.2A2Sec.• Pulse I2t3 = Ip3 x Ip3 x ( t5 – t4) = 25A x 25A x 5mSec=3.125A2Sec• Total Pulse I2t = 6.2A2Sec (3.1A2Sec)• Fuse Melting I2t= Total Pulse I2t / 0.22 @ 100,000 cycles=28.2A2Sec(14.1A2Sec)• Fuse rating, In = Ir / (safety factor * Temperature factor)• UL factor= 0.75, IEC factor= 1, Temperature=100C• Fuse rating, In = 4.5 / ( 0.75 x0.9) = 6.67A for UL type fuse minimum.• In= 4.5 / ( 1 x 0.9) = 5A for IEC type minimum.


Example (continue) : Pulse I2t calculation

• T ： 30μsec • Ip ： Max 20A• Pulse I2t= （ Ip^2T ）＝（ 20 × 20 × 30μsec ） = 0.12A^2Sec (Assume worst inrush

condition of Square waveform**)• 100000Pulses Factor ： 0.22 –safety factor • Fuse Melting I2t= Pulse I2t / 0.22 = 0.12 / 0.22= 0.5455A^2Sec


Calculated Pulse I^2t( 計算單脈衝 I2t 值 ) = [(20A)^2 x 30uSec]=0.12A^2Sec / 2

= 0.06A^2Sec

( 請參照 LF’s Designer’s Guide waveform C, on page 6, chart I, 如下Note: 最佳以方形波計算單脈衝 Pulse I2t 值 )

C

Calculated Melting I^2t( 計算熱熔能量值 I2t) = 0.12 ÷ 0.22 = 0.5455A^2Sec** ( Per “A”)=0.06 / 0.22 = 0.273A^2Sec* (Per “C”) % ratio = 0.06 / 0.2326 =0.258 = 25.8% --NG (52%) % ratio = 0.06 / 0.3516 = 0.171 = 17.1% --OK(34.2%) % ratio = 0.06 / 5716 = 0.105 = 10.5% --OK(21%) % ratio = 0.06 / 506 = 0.119 = 11.9% --OK(23.8%) % ratio = 0.06 / 1.27 = 0.048 = 4.8% --OK(9.5%)

Example (Continue): Pulse I2T & Melting I2t


( 設計能耐 100,000 pulses per Chart II by 22% or less, on page 6 如下 )

22%

Example (Continue)


• Nuisance opening is most often caused by incomplete analysis of the circuit under consideration. Special attention must given to normal operating current, ambient temperature, and pulses. For trouble-free, long-life fuse protection, it is good design practice to select a fuse such that the I²t of the waveform is no more than 20% of the nominal melting I²t rating of the fuse.


Transient Currents

• One has to distinguish between transient currents as caused by energy reservoirs (capacitors, inductances) in the system to be protected, and transient currents imposed on the system and originating from outside. In general, internal transients are caused by switching operations in an electrical circuit containing energy reservoirs like capacitors and inductances. In such a circuit is closed, then during a short period of time after the instant of closing, a current will flow which is normally higher, sometimes considerably higher than the steady-state current flowing through the circuit when the transient current has disappeared.

• In real life internal transients or Inrush currents may have different shapes and magnitudes, some simple examples are in FIG 1 and FIG 2.


Fig 1, shows the Inrush current of an unloaded transformer, after connecting it to the AC mains.


Fig 2, shows the current after closing a circuit containing a capacitor


保險絲结構 Structure

• 管狀保險絲：玻璃管與陶瓷管熔絲形式與填充料體積與容量成正比

• 其他保險絲：• 電阻 /SMD型• 晶體管型• 微型與超微型• 插刀形與平板形• 带指示


安裝形式 Installation

• 不易更换的安裝：直接焊在印刷板上引線插焊表面貼焊

• 易更换的安裝：保險絲座


Soldering Recommendations

• Most fuse constructions incorporate soldered connections, caution should be used when installing those fuses intended to be soldering in place. The application of excessive heat can reflow the solder within the fuse and change its rating. Fuses are heat-sensitive components similar to semi-conductors, and the fuse of heat sinks during soldering is often recommended. For SMD fuses the amount of the metal in contact with fuse terminal (pad plus solder) will affect the fuse performance.


Soldering recommendation


Application Aspects

Application Aspects


• A fuse is a protective devices, this means that it is applied in an electrical system or subsystem to protect something: a load, a conductor etc., in general an object or objects. So, first of all we should know the characteristics of the object to be protected in order to know to which extent protection is required. More specifically, the proper selection of a fuse requires consideration of the following aspects –


• - Required electrical characteristics, especially rated current and time-current characteristics• - Environmental conditions such as temperatures, connecting wires/cooper trace etc.• - Required interrupting capacity at rated voltage (or system voltage)• - Voltage drop• - Combination of fuse link and fuse mounting means e.g. a fuse holder• - Over voltages• - Ageing aspects


• If a fuse opens, this operation is usually not caused by a fuse failure, BUT by an abnormal situation in the electrical system to be protected. This means that replacement of an open fuse does not normally correct the failure in the system. Opening of a fuse is caused by a current is too large. Such a current may be a continuous current or a transient current and it is likely that such a current is caused by a failure in the system to be protected. It should be normal practice to find and remedy this failure first before replacement of the fuse link is carried out


Trip time vs. Overload current(0467003, measured at 60C)

0.010

0.100

1.000

10.000

100.000

1000.000

3 3.5 4 4.5 5 5.5 6 6.5

Overload current (A)

Tri

p t

ime

(s)

Trip time (s)

Example – Effect of overload current on trip time0467003 (3A, 0603 fuse)

Data was taken at:

4A – 133% overload**

5A – 167% overload**

6A – 200% overload

This example shows that the trip time is inversely related to overload current. As current increases, the trip time decreases.

This shows that if a fuse rating is selected too high, the fuse may not clear before damage is done to the circuit

• In this case, a 4A overload typically cleared in 100 seconds


Cautions !

• Never design a fusing protection in the gray region, it means between 100 to 200%!!!!!

• No fuse supplier will guarantee !!!!! • Always refer to Littelfuse’s most update

products’ specification and application notes in the www.littelfuse.com


Summary of selection steps:

Understand the application and circuit parameters

Compare calculated and actual nominal melting I2t values to ensure fuse will not suffer nuisance opening. If there are multiple fuses qualified for the application, use secondary characteristics (size, voltage rating, etc.) to determine best solution

Determine Absolute minimum current rating of fuse (fuse re-rating, thermal de-rating)

Determine minimum Nominal melting I2t value of the application

Determine Pulse I2t value of the application

IMPORTANT!! Even though care may be used during the fuse selection process, it is recommended that application-level testing be performed to verify coordination of fuses to the circuit conditions


Note: 從以上計算所得的 Cat. Fuse Rating & Calculated Melting I^2t 值去選擇下列保險絲是否符合 Absolute minimum calculated I^2t rating of0.273A^sec or above (466 or 468 series的 2.A 或以上的安培值 ) 而後 , 向 LF經銷商或利特需求樣品實際地放入設計電路中以最嚴格狀態去做測試 , 以確認是否保險絲選擇正確 , 或如有需要可聯絡力特保險絲技術人員協助。

以上實例只供計算參考， Littelfuse技術人員隨時歡迎洽詢並給于協助。洽詢聯絡如下：

Littelfuse Far East Taiwan Rep. OfficeTel: 886-2-8751-1234, Ext. 105 ; 張廣兼Fax: 886-2-8751-1177E-mail: [email protected]:www.littelfuse.com


Design and Selection Tips

• Theoretical calculation is just a reference.• Fuse manufacturers’ fuse selection guide is the basic tool to

help design engineer to understand the important factors that need to take in to consideration.

• Fuse is a thermal sensitive device and so a proper thermo de-rating a MUST.

• Understand the type of application current waveforms when determine for the operating currents, pulse i2t and steady state currents with the worst application conditions to be considered.

• Design always need to take in to actual application end variations such as current, temperature and components, etc.

• Always verify the selection in the actual circuits or system end at the worst conditions to see if it is a good selection.


Fuse Selection Incorrectly

• Nuisance Opening.• Fuse degraded & oxidized.• Fuse ruptured, if over its breaking capacity or

interrupting rating.• Fuse performance reduced.• Poor pulse surges withstand due to improper

melting i2t value selected.• End products’ life expectance reduced.• Increase warranty and repair cost!• Lost business!!!!!


***Solution***

• Make Extra Safety Margins.• Design In with execution of TQRDC.• Consider Whole System Parameters during the Design In.• Always RE-Calculate the electrical parameters from scratch! When

modify your design!• ALWAYS EVALUATE THE SELECTION IN THE WORST CONDITION

(such as High Temp Burn In, Manufacturing Transients, etc) TO VERIFY THE SELECTION IS CORRECT!

• When needed, ALWAYS consult LF’s FAE in the region office for Technical support.

• Fulfill the GREEN Partner with 3R• Reuse; Recycle; Recovery by 2006!


Resettable PTCs


Resettable PTC

PTC operation – Resistance is low during normal circuit operation and increases during overcurrent events

Res

ista

nce

Time

Normalcurrent

OC event:currentincreases

Tripped statedue to heatfrom OC

OC clears:heat dissipates

Normal current condition:PTC returns to low resistance

D+D -

Vbus

Signal Ground

USB host

PTC

Bus powered device

IUSB


Solutions for:• Powered plug-and-play port overcurrent protection

Resettable PTC• Surface mount and radial leaded form factors


• Surface mount parts available in sizes of 1206 and 1812.

• Key feature is the ability to reset itself (return to low resistance state) and reconnect power after the overcurrent condition has been cleared

Resettable PTC


Resettable PTCs• Battery pack• Cell phone• Digital camera• Motherboards

– USB, FireWire (DC bus)

• Adapter cards• Powered Ethernet ports• Tip/ring circuits

Target applications:Protection of USB bus (Motherboard)

D+D -

VBUS

Signal Ground

USB ControllerUSB Port

Ou

tsid

e W

orl

d

V5.5MLA0603

(2) V0402MHS03

ShieldGround

1206L

USB port protection

Resettable PTC

Tip/ring protection

DC port protection


Under Normal Operation Under A Fault Condition

• At operating current– Many conductive paths– Very low resistance

• Excessive current causes device to heat

– Fewer conductive paths

– High resistance

– Cools down and resets when fault is removed

How a PTC Device Works


Resistance vs. Temperature


PPTC SELECTION CRITERIA

NORMAL OPERATING CURRENT

APPLICATION VOLTAGE ... AC/DC

AMBIENT TEMPERATURE

FAULT CURRENT AND DURATIONLENGTH OF PULSE

CIRCUIT TIME TO DAMAGE

MAXIMUM AVAILABLE FAULT CURRENT


Resettable PTC Applications

COMPUTER Parallel Port USB, FireWire Serial Ports Video Ports Mouse/Keyboard Ports

ELECTRONICS Control/Signal Lines I/O Interfaces

TELECOMMUNICATIONS I/O Interfaces on Controller Cards Sensitive Components within Telecom Equipment


What are the Downsides of PTC’s

Devices are current limiting not current interrupting.

Device can exhibit a soft failure Catastrophic Failure

Permanent damage Burnout


• Limits Current to Safe Level• User Transparent, No Replacement• Reduces Warranty and Service Costs• Ease of Use, Reset Simple• Ideal for Remote Applications

What are the Advantages of Resettable PTCs?

Documents

Littelfuse OC Selection Training Anthony Chang. 2 Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. Who We