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Advantages of eFuses
Versus PTC ResettableFuses
Introduction

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Overcurrent protection is a basic necessity for electricaldevices. While many people are familiar with fuses andhousehold circuit breakers, few are intimately familiar withthe kinds of overcurrent protection devices that are found inelectronics.
Though not a comprehensive list, there are basically threetypes of overcurrent protection devices in electronics. In
order of increasing sophistication they are:
• One-shot fuses• Positive Temperature Coefficient (PTC) resettable fuses• Electronic fuses (eFuses)

APPLICATION NOTE

• Programmable current limit• Enable pin, to turn on or off the device• Fault pin, to signal that something has gone wrong tocontrol logic or other power rails• Soft-start, to limit inrush current• Voltage clamp, to prevent voltage spikes from reachingComparison of technologies

One shot fuses, which are based on melting of a metal link,must be replaced after a single high current event. They arecommonly found in applications like LED bulbs wherea simple device makes sense. For LED bulbs the solution toa blown fuse is just to purchase another bulb. It is a small
expense and the fault leading to the open fuse likely requires
replacement of the bulb anyway.
PTC resettable fuses are a step-up from one shot fuses.
When a short circuit occurs, they heat up and transition froma low resistance state to a high resistance state. Allowing
them to cool down (typically by removing the power) resets
them to the low resistance state.
PTCs come in both ceramic (CPTC) and polymer (PPTC)
types. Ceramic types are used in sensitive application spaces
such as telecom where the resistance must not change muchafter tripping. The polymer type is used in many generalelectronics applications and is sometimes called
alternatively a resettable fuse, or polyswitch. In this article,
a polymer type is compared against an eFuse.
eFuses utilize a completely different operating principle
than one-shot or PTC fuses. Instead of limiting current based
entirely on heating, eFuses actually measure the current andturn off an internal switch if the current exceeds a specified
limit.
Also, since eFuses are semiconductor integrated circuit
devices they have a rapid (typically less than 10 ms) response
to short circuits as well as a plethora of features that may be
included:• Ability to operate over temperature with minimal shift
in parameters• No degradation after a fault; the on resistance does notdepend on how many faults have occurred© Semiconductor Components Industries, LLC, 2017

October, 2017 - Rev. 2

the load• Choice of latch-off or auto-retry, so that everything will
reset if the load recovers, but without needing to turnoff the power• Reverse current blocking
However, the eFuse arguably also has a few disadvantages
such as having more terminals and requiring bias current tooperate.
At the most basic level, an eFuse requires at least threeterminals due to its architecture. These are input, output, andground. In some cases it would be better to havea two-terminal device to make layout routing easier. For
example, there is no need to connect a PTC to the groundplane.
Also, since the PTC does not require a ground pin, current
only flows from input to output. This means that no bias
current is required at all. This is an obvious advantage forbattery-powered applications. However, the latest eFuses
have reduced bias current considerably. For example, when
the NIS5452 is turned off, its bias current is less than
100 mA. Its predecessor, the NIS5135 has a bias current
about 10 times higher.
While it is true that the eFuse is more complicated due toits semiconductor architecture, advances in semiconductor
technology have significantly reduced the size, andtherefore expense, of eFuses. For example, one of the first
eFuses from ON Semiconductor is the NIS5112 with 30 mW
RDS(on) in the SOIC−8 (5 × 4 × 1.75 mm) package. By
contrast the upcoming NIS5020 is in a much smaller (3 × 3 ×1.0 mm) DFN10 package and has half the RDS(on).

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eFuse vs. PTC Test Setup

A 12 ohm load is connected from output to ground. Sinceboth the eFuse and PTC have their current limits above 1 A,
in normal operation 1 A is allowed to flow.
The behavior of the PTC depends on the current capabilityof the power supply. To test this, three different supplies
were used:• Small wall adapter, CUI INC model SMI24−12, whichsupplies 12 V and up to 2 A DC. It has a cut off andauto-retry feature in the event of a fault• Benchtop DC power supply, GW Instek SPS−606,
capable of 60V and 6 A• Desktop computer power supply, Dell model
NPS−250KB F, capable of 16 A on the 12 V rail

To compare eFuse and PTC fault response an eFuse(ON Semiconductor NIS5132MN2) and a polymer PTC
with similar characteristics were chosen to provide fault
protection on a 12 V power rail. The eFuse Rlimit resistor waschosen as 37.4 W to match the current limit specifications.
Table 1 provides a brief comparison of the devices.
Table 1. PTC AND EFUSE SPECIFICATIONCOMPARISON
Specification

Polymer PTC

eFuse

Max Voltage

1625

Hold Current

1.5 A

1.7 A

Trip Current

3.0 A

3.0 A

On Resistance

40 to 110 mW

44 mW

Package1210

3 × 3 DFN

Length (mm)3.2

3Width (mm)2.5

3Height (mm)0.551The test scenario was the same for all cases: a 1 A load isrunning and then suddenly the output is short circuited toground. The current limit device is tasked with limiting thecurrent as soon as possible in order to maintain a stablesupply rail voltage. In a real-world application, the supply
rail staying high is important in case other parallel loads
running off the same supply need power.
Small Wall Adapter Power Supply

The test circuit board is shown in Figure 1. It has coaxial
connectors to measure input voltage, output voltage, current(voltage over a 20 mW resistor), and eFuse enable pin
voltage. It also has a Zener diode and 10 mF input capacitor
to suppress inductive spikes due to V = L(di/dt) transients on
the power cable.
There are also switches to change between the eFuse andPTC and to short the output to ground. Furthermore, it has
an onboard multimeter to read input voltage and output
current. Standard eFuse peripheral components such as theRlimit and the output capacitor were included too. Generally
eFuses do not require output capacitors for functionality, but
it is a good idea to have them if large inductive voltage spikes
are possible in the application.

Figures 2 and 3 show what happens while using the small
wall adapter supply. Referring to Figure 2, when a shortcircuit occurs with the PTC, over 4 A passes through for
50 ms, and then the power supply itself shuts off the current.
The PTC never actually trips, resulting in the power supply
shutting down.
In Figure 3, the eFuse responds to the fault. The input
voltage is able to stay up because the power supply current
limit does not activate. Subsequently the eFuse attempts toturn on again but recognizes that the output is shorted. TheeFuse limits the current so quickly that other loads could
continue to operate on the same power line if needed.

Figure 1. The Circuit Board Used for Comparing
the eFuse to the PTC

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Benchtop DC Power Supply

The benchtop supply has a 6 A limit but does not havea shut off. It supplied 6 A until the PTC tripped, but it took
about 30 ms (see Figure 4). In that time period the power
supply voltage was quite low. As shown in Figure 5, theeFuse responded just like in the wall adapter supply case:very quickly and allowing the input voltage to stay high.

Figure 2. At the beginning the PTC is conducting 1 A.
The input and output voltage are both about 12 V.
The power supply is the small wall adapter unit madeby CUI Inc. When a short on the output occurs, thepower supply provides about 4 A until its protection
circuit shuts off the current and the input voltage
drops to zero

Figure 4. In this case the PTC trips after the shortcircuit has been present for about 30 ms. Note thelarge drop in the input voltage until the PTC trips

Figure 3. Here the eFuse responds to the same
scenario as in Figure 2, and the power supply does
not need to shut down. The eFuse’s auto−retry
feature attempts a restart but the output is stillshorted so it does not return to 12 V. The input
voltage stays up. The ripple on the input voltage line
is caused by inductive voltage spikes and may be
dampened by using ceramic decoupling capacitors

Figure 5. The eFuse responds to the fault quickly,
keeping the input voltage up

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Desktop Computer Power Supply

Test Summary

The high current capability of the desktop computer
supply allowed the PTC to trip quickly (see Figure 6).
Clearly, the PTC performs much better with this supply thanwith the others; the input voltage trace is much more stable.
As Figure 7 shows, the eFuse responds in a similar manner
as with the other supplies, but even in this case the eFuse
results in less disruption of the input voltage than with thePTC.

It was observed during the tests that the eFuse respondedto faults more quickly than the PTC, allowing the power
supply rail to stay up better. The results are summarized inTable 2.
Table 2. FAULT RESPONSE TEST SUMMARY
SupplyPTC Response

eFuse Response

Fig 2: Did not trip

Fig 3: ~10 ms

Benchtop

Fig 4: Slow (~30 ms)

Fig 5: ~10 ms

Desktop
ComputerFig 6: Fast (~3 ms)

Fig 7: ~10 ms

Wall Adapter

Conclusion

In many cases, a PTC may provide adequate protection.
Its response should be fast enough to prevent burn damage
to wiring. In addition, its ability to maintain voltage levels
in response to a fault can be enhanced by choosing a higher
current supply, or perhaps even adding decoupling
capacitance on the power supply (input) side.
However, in other cases an eFuse is not only a better
choice but its superior performance and features are required
to satisfy application requirements. The use of an eFuse may
also save cost and board space if additional circuitry, needed
to prevent inrush currents or to provide overvoltage
protection, can be eliminated by using an eFuse. For more
information about eFuses, please visit www.onsemi.com orcontact your local ON Semiconductor sales representativefor evaluation boards and samples.

Figure 6. With the desktop computer power supply,
there is a dip in the input voltage but the PTC trips inabout 3 ms

Figure 7. The eFuse responds as it did with the otherpower supplies. The response is quick and the input
voltage stays highwww.onsemi.com
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ON Semiconductor andare trademarks of Semiconductor Components Industries, LLC dba ON Semiconductor or its subsidiaries in the United States and/or other countries.
ON Semiconductor owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of ON Semiconductor’s product/patent
coverage may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. ON Semiconductor reserves the right to make changes without further notice to any products herein.
ON Semiconductor makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does ON Semiconductor assume any liability
arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages.
Buyer is responsible for its products and applications using ON Semiconductor products, including compliance with all laws, regulations and safety requirements or standards,
regardless of any support or applications information provided by ON Semiconductor. “Typical” parameters which may be provided in ON Semiconductor data sheets and/orspecifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer
application by customer’s technical experts. ON Semiconductor does not convey any license under its patent rights nor the rights of others. ON Semiconductor products are notdesigned, intended, or authorized for use as a critical component in life support systems or any FDA Class 3 medical devices or medical devices with a same or similar classification
in a foreign jurisdiction or any devices intended for implantation in the human body. Should Buyer purchase or use ON Semiconductor products for any such unintended or unauthorized
application, Buyer shall indemnify and hold ON Semiconductor and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, andexpenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if suchclaim alleges that ON Semiconductor was negligent regarding the design or manufacture of the part. ON Semiconductor is an Equal Opportunity/Affirmative Action Employer. Thisliterature is subject to all applicable copyright laws and is not for resale in any manner.

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