Fuse Fundamentals

8/6/2019 Fuse Fundamentals

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Technical Paper

Advantages of Multi-layer Fuse Design:Raychem Circuit Protection FastActing Surface-mount Fuses

IntroductionRaychem Fast Acting Surface-Mount (SFF) fuses use a multilayer

design which sandwiches up to three fuse elements betweenlayers of a matrixed glass ceramic material. This configurationhas the benefit of supporting higher current ratings in a smallerpackage size, and providing better arc suppression characteristicswith less chance of a package breach exposing the surroundingenvironment to arcing fuse material when the fuse opens.

Multi-layer Design OverviewThe multi-layer design of Raychem's SFF fuse family allows fuseelements to be stacked in parallel within a single device. Theelements are embedded within the fuse body and surroundedon all sides by absorption material. Figure 1 compares the multi-layerdesign of Raychem's SFF fuses with typical single layer, glasscoated designs.

Figure 1. Multi-layer chip fuse vs single layer glass coated fuse

--

Glass/CeramicSubstrate

Multiple FuseElements

SubstrateMaterial

Single FuseElement

GlassCoating

Multi -layer Design Single-layer Glass Coated Design

The paralleled fuse elements provide very similar openingcharacteristics to single layer devices. Resistance imbalancesbetween the elements are very small. Any difference in resistancewill automatically be compensated by higher or lower currentflow in the elements. Because the fuse elements are in parallel,their voltage drop must be the same. As a result, the higherresistance fuse element will carry proportionally less current. This,and the close physical proximity of the fuse elements within theconstruction, maintains a very consistent temperature on both

elements. Therefore, premature opening of the fuse due to currentimbalance between the fuse elements does not occur. When the

fuse does open, one element will always open before the other.The small difference in time between the opening of each fuseelement has no impact on the effectiveness of the fuse.

Additionally, the absorption characteristics of the matrixed glass ceramicsurrounding each element results in a very clean opening characteristicas presented below. The matrixed glass ceramic material of thefuse body contributes significantly to the fuse performance. Thismaterial very efficiently absorbs the vaporizing fuse materialwhen the fuse opens. The multi-layer design surrounds eachfuse element with the matrixed glass ceramic material providinga larger volume of absorption material to the vaporizing fuseelement. The benefit of this combination of material and multiplelayers is excellent absorption and arc suppression characteristics.


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Arc ContainmentAnother advantage of the multi-layer design is that the fuseelements are deeply buried within the fuse body. This reduces thechance that a high energy open will rupture the fuse surface andexpose a high energy arc to the surrounding area, possibly evenemitting molten metal. Figure 4 displays the exterior of Raychem’s multi-layer fuse compared to glass coated designs after the fusehas blown.

Figure 4. Post blow views of Raychem’s multi-layer fuse vs

glass coated designs

No Visible Failure

or Breach

Fuse Element Failure Zones

Multi-layer Design Glass Coated Designs

Higher Current RatingsIn 0402(1005mm), 0603(1608mm) and 1206(3216mm) packagesizes Raychem’s SFF fuses have some of the highest currentlevels commercially available today. The multi-layer design makesthis possible. The ability to stack fuse elements in parallel allowsgreater current handling capacity within a given package size.

Enhanced Arc Suppression CharacteristicsThe multi-layer design has the benefit of exposing more fuseelement surface area to the glass-ceramic absorption material.

When the fuse elements open, there is more material for thevaporizing fuse metals to absorb into, resulting in a very efficientand effective quenching of the fuse arc.

Figure 1 compared the multi-layer design of Raychem’s SFF fuseswith standard glass coated designs. The glass coated designs relyon the coating on only one side of the fuse element to absorb thevaporizing fuse material when it opens. Therefore, there is muchless absorption material available to absorb the fuse metals. Theresult can be prolonged arcing and possible coating breach.

Figure 2 shows how the absorption characteristics of the twodesigns differ. The multi-layer design indicates a clean separationwith the fuse element evenly diffusing into the surroundingceramic substrate. In the glass coated design, theelement diffusiontakes place in a small portion of the device and is only absorbed

by the glass material directly above the area of failure.

Figure 2. Blown element comparison between multi-layer

and glass coated devices

Single-layer Glass Coated Design

Fault Zones

Multi-layer Design

The difference in the arc quenching performance can also beobserved in the electrical signal characteristic at the time the fuseopens. Figure 3 presents opening characteristics of both designsat close to maximum ratings. The noisy tail displayed on the single-layer glass coated device indicates a sustained arc that slows thecircuit disconnect in the presence of a fault. In this example, thearc is sustained for approximately 12ms, longer than the time ittakes the fuse to begin opening under the fault current. The multi-

layer design shows a very clean open characteristic indicating veryefficient arc suppression.

Figure 3. Electrical characteristic of multi-layer and glasscoated designs at failure

Single-layer Glass Coated DesignMulti-layer Design

Selecting Raychem Surface Mount Chip FusesFuse selection seems straightforward in that you pick one whichhas a current rating just a bit higher than your worstcase systemoperating current. Unfortunately, it’s not that simple. There arederating considerations for operating current and applicationtemperature. Turn-on and other system operations (likeprocessor speed changes or motor start up) cause current surgesor spikes that also require consideration when selecting a fuseSo selecting the right fuse for your application is not as simpleas knowing the nominal current drawn by the system. However, byfollowing the guideline presented in this paper, selecting the properfuse does become a straightforward process.



Fuse Selection FlowchartThe basic considerations for fuse selection are shown in theflowchart presented in Figure 1. Following this flow chart will helpyou select a fuse best suited for your application conditions.

Figure 1. Fuse Selection Flowchart

Temperature DeratingA fuse is a temperature sensitive device. Therefore, operatingtemperature will have an effect on fuse performance and lifetimeOperating temperature should be taken into consideration whenselecting the fuse current rating. The Thermal Derating Curvefor Raychem surface mount chip fuses is presented in Figure 2.Use it to determine the derating percentage based on operatingtemperature and apply it to the derated system current.

Apply Temperature Derating

The minimum fuse current rating is determined by the following

formula:

Ifuse (Isys /0.75)/Ktemp

Where:Ktemp is the % derating determined from the Thermal DeratingCurve in Figure 2

Figure 2. Thermal Derating Curve

If there are no pulse currents present in the application, thenthe fuse current rating may be selected. Finally, only the voltagerating must be reviewed for final fuse part number selection.

Example 1—Select a fuse for a system that draws 1.0 ampsteady state during normal operation and operates at ambienttemperatures up to 70°C. Ignoring for now the voltage

requirement, what is the minimum required current rating for thefuse in this application?

Ifuse ≥ (Isys /0.75)/Ktemp

Where:Isys = 1.0AKtemp = 88% (determined from Figure 2)Ifuse ≥ (1.0A/0.75)/0.88

Ifuse 1.52A

So a fuse with a current rating above 1.52A should be selected.

Application CurrentFuses require derating for both steady state and transient stateconditions. The final current rating for the fuse used in anyapplication will be a combination of both of these effects. It iseasiest to analyze the derating requirements separately for thetwo conditions. Some applications may not require considerationof transient currents.

Steady State DeratingIt is recommended that Raychem surface mount chip fuses notbe operated at steady state currents greater than 75% of thenominal 25°C current rating.

Apply Standard Derating (75%)Ifuse Isys /0.75

Where:Ifuse is the rated data sheet current at 25°CIsys is the nominal steady state system current

Transient State or Pulse DeratingThe term “Pulse” is used to describe any type of transient currentthat may be applied to the fuse. Common examples of pulses areinrush currents observed at system turn-on, motor start-up currentsand more extended duration peak currents observed during highspeed processing activity in computing systems.

These pulses have an effect on the fuse element because thetransient heating they induce causes thermal cycling within thedevice. Over a large number of pulses, this can affect the life ofthe fuse. Therefore, the pulse energy and the number of timesthe fuse may be subjected to the pulse must be considered whenselecting the proper fuse for the application.



The I2t parameter provides a measure of the fuse’s ability to

withstand the energy of a pulse. By determining the I2t energy of

the pulse, it can be compared to the fuse’s I2t curve to determine

what the rated current of the fuse must be to help ensure reliableoperation of the fuse.

Determine Pulse Waveform

Calculating Pulse I2t—The energy contained in a current pulse

depends on the pulse waveform’s shape, peak current andduration. The basic formula for I

2t is:

I2t = o≥

t[A (t)]

2dt

Figure 3 presents the most common current waveforms andsimplified equations for their I

2t. In most systems, current

waveforms could be approximated as either one of those onFigure 3 or their sequence. In case of such a sequence theresulting I

2t is the sum of I

2t of those individual waveforms.

Figure 3. Waveform I2t Formulas

Determine Pulse WaveformOnce the I

2t value for the application waveform has been

determined, it must be derated based on the number of cyclesexpected over the system lifetime. Since the stress inducedby the current pulse is mechanical in nature, the number oftimes the stress is applied has significant bearing on how muchderating must be applied to the fuse rating. Figure 4 presentsthe current pulse derating curve for Raychem surface mount chipfuses up to 100,000 cycles.

Pulse Temperature Derating

Once the pulse cycle derating has been applied to the

calculated I2t value, derating for operating temperature should

be applied according to the thermal derating curve shown inFigure 2.

Allowance for Circuit Variation

Apply an additional 30% safety margin to tolerate the variancesintroduced by other components in the circuit.

The derated I2t value can then be used to determine the rated

fuse current. This is done by referring to the minimal fuse I2t vs

clear time curves. The best way to explain how this is done isthrough an example.

Example 2—Select a 0603 size fuse that can handle a 1.0Asteady state system current, is subjected to a capacitor chargingturn-on pulse, peaks at 6 amps and lasts .005sec. The systemruns at 24VDC, expects to see 100,000 cycles over its lifetime andnormally operates at 70°C.

Step 1—Apply the 75% steady state derating and the 88%thermal derating at 70°C as determined in Example 1.

Ifuse ≥ (Isys /0.75)/Ktemp

Where:Isys = 1.0AKtemp = 88% (determined from Figure 2)

Ifuse (1.0A/0.75)/0.88

Ifuse 1.52A

This is the minimum current rating the fuse must have toproperly protect the system under steady state conditions only.Reviewing the datasheet indicates the 0603SFF200F/32 is therecommended device. (See tables in Appendix E.)

Step 2—Determine which pulse waveform in Figure 3 mostclosely represents the application and calculate the

waveform’s I2t value.

Since a capacitor must be charged every time the system ispowered, the fuse will be subjected to a current pulse most likethat of waveform #4 in Figure 3. Using the associated formulathe I

2t value is calculated as:

I2tpulse = (1/2)(6A)

2x 0.005sec

I2tpulse = 0.09 A

2sec

Figure 4. Raychem Surface Mount Chip Fuse Pulse DeratingCurve



Step 3—Apply Pulse Cycle Derating—Derate the I2t value basedon the number of cycles required by the application.

At 100,000 pulses, Figure 4 indicates a 40% derating of the I2t

value is required.

I2tdpulse = 0.09/0.4 A

2sec

I2tdpulse = 0.225 A

2sec

Where: I2tdpulse is the derated value for pulse waveforms.

Step 4—Apply Pulse Temperature Derating—Derate the I2t value

based on system operating temperature. In this example apply

88% thermal derating at 70°C from Figure 2.

I2tdpulse = 0.225/0.88 A

2sec

I2tdpulse = 0.256 A

2sec

Step 5—Apply Derating for Variance in the Circuit—Apply 30%safety margin (divide by 0.7) for variance introduced by othercomponents in the application.

I2tdpulse = 0.256/0.7 A

2sec

I2tdpulse = 0.365 A

2sec

Step 6—Select Fuse Current Rating for Pulse Environment—Determine which fuse has an I

2t rating greater than I

2tdpulse at

.005sec. The minimum I2t versus clear time can be found in

Figure 5 for Fast Acting Fuses and Figure 6 for Slow Blow Fuses.

(Larger charts are available in Appendices A and C).

Using Figure 5, the Fast Acting 0603SFF Family Minimum I2t vs

Clear Time curves, it can be determined that Curve H has an I2t

value greater than 0.365 A2sec at .005sec. This means that a 3.5A

fuse, part number 0603SFF350F/32, would meet the pulse andcycle derating requirements for this application example. Refer toAppendix E for part number selection.

Using the Slow Blow 0603SFS Family Minimum I2t vs Clear Time

Curves in Figure 6, it can be determined that Curve C has an I2t

value greater than 0.365 A2sec at .005sec. This means that a

2.0A fuse, 0603SFS200F/32, will meet this application examplerequirement. Refer to Appendix E for part number selection.

Step 7—Select Fuse Current Rating—Compare the steady statecurrent requirement from Step 1 to the current requirement forthe pulse environment in Step 6 and determine which value ishigher. The higher value will define the final fuse current rating.

In this case for the steady state condition, a 2.0A Fast Acting Fusefrom the 0603SFF Family is recommended. However, applyingthe pulse condition, a 3.5A Fast Acting Fuse from the 0603SFFFamily or a 2.0A Slow Blow Fuse from the 0603SFS Family couldbe used. The 3.5A Fast Acting Fuse has a higher current ratingthan the 2.0A Fast Acting Fuse; therefore, the 2.0A Fast Acting

Fuse can be eliminated as an option for this application example.Now, the choice is between the 3.5A Fast Acting Fuse and the 2.0ASlow Blow Fuse. Fast Acting Fuses open very quickly in responseto an overcurrent condition and transient current spikes not part oftheir normal operation. Slow Blow Fuses have a delayed openingsustained in overcurrent conditions or expected pulsed currents.Since this is an application where the fuse will be subjected to acurrent pulse, a Slow Blow Fuse is a better choice.

Step 8—Check Voltage Rating—The system voltage is 24V.The 0603SFF350F/32 and 0603SFS200F/32 are rated foruse at 32VDC. They both meet the voltage requirements.

NOTE: The fuse selection method presented in this paper is intended as a guideline todetermine the fuse most suitable for an application. We recommend testing the fusein the actual application with the worst case scenario to confirm suitable performance.

We also recommend verifying fuse clearing time to ensure that the right fuse isselected to provide the protection needed for the circuit and complies with requiredregulatory and safety standards. Visit our website at www.circuitprotection.com for themost up to date information.

Figure 5. 0603SFF (Fast Actiing) Family Minimum I2t vs ClearTime Curves

Figure 6. 0603SFS (Slow Blow) Family Minimum I2t vs ClearTime Curves

* Please note clear time curves are minimum threshold, not typical.



Appendix A



Appendix B



Appendix C



Appendix D


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Appendix E

Typical ElectricalCharacteristics

MaxInterrupt Ratings

0603SFF (1608mm) Fast Acting

Surface Mount Chip Fuses

0603SFF050F/32

0603SFF075F/32

0603SFF100F/32

0603SFF150F/32

0603SFF200F/32

0603SFF250F/32

0603SFF300F/32

0603SFF350F/32

0603SFF400F/32

0603SFF500F/32

0603SFF600F/24

0.50

0.75

1.00

1.50

2.00

2.50

3.00

3.50

4.00

5.00

6.00

0.485

0.254

0.131

0.059

0.044

0.032

0.025

0.024

0.018

0.013

0.010

32

32

32

32

32

32

32

32

32

32

24

50

50

50

35

35

35

35

35

35

35

35

* Measured at 10% of rated current and 25°C

Shape and Dimensions

Inch (mm)

Recommended Pad Layout

Inch (mm)

Typical Electrical

Characteristics

Max

Interrupt Ratings

0603SFS (1608mm) Slow BlowSurface Mount Chip Fuses

0603SFS100F/32

0603SFS150F/32

0603SFS200F/32

0603SFS250F/32

0603SFS300F/32

0603SFS350F/32

0603SFS400F/32

0603SFS450F/32

0603SFS500F/32

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0.200

0.100

0.052

0.041

0.031

0.021

0.017

0.015

0.013

0.09

0.18

0.82

0.63

0.87

1.20

2.30

2.70

3.20

32

32

32

32

32

32

32

32

32

* Measured at 10% of rated current and 25°C

† Melting I2t at 0.001 sec clear time

Shape and Dimensions

Inch (mm)

Recommended Pad Layout

Inch (mm)

Part

Number

Nominal

I2t

(A2sec)†

Voltage

(VDC)

Current

(A)

NOTE: Additional sizes and current ratings are available. Go to www.circuitprotection.com or refer to the latest catalog.

40

40

40

40

40

40

40

40

40

Rated

Current

(A)

Nominal

Cold DCR

(Ω)*

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Part

Number

Rated

Current

(A)

Nominal

Cold DCR

(Ω)*

Voltage

(VDC)

Current

(A)

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Fuse Fundamentals