Typical Causes of Fuse Failure in Linearmode Power Supplies
Dave Love, Engineering Manager, Jerome Industries, May 6, 2010
Abstract:
Fuse failure causes and user precautionary measures, for linearmode power adapters, which use small 50/60Hz transformers, to
power electronic apparatus.
Simple power adapter designs employing 50/60 Hz transformers for stepping down AC line voltage to lower voltage levels
typically used by electronic circuits are vulnerable to the failure of fuses, which are used to protect the power adapter from
excessive currents and temperatures.
These fuses are required by regulatory compliance agencies such as UL and TUV, and allow the linearmode power adapter to
remain safe under any common abnormal situation. Fuses can protect any of the following common abnormal situations:
a) output short circuit,
b) output overload,
c) transformer turn to turn insulation failure.
Two fundamental types of fuses are utilized to provide safety protection.
1) Current Fuse: which uses a high temperature metallic element which melts, and clears the circuit, when a high current is
applied.
2) Thermal Fuse: (AKA thermal cutout) which has a low temperature thermally sensitive element, which melts and clears the
circuit, when a high temperature is applied.
Note, the current fuse is somewhat temperature dependent, but fails primarily as a result of high current. Similarly, the thermal
fuse is somewhat current dependent, but fails primarily as a result of high temperature.
Fusing may be applied in either the primary or secondary circuit, based upon the specific protection objective. Many designs
utilize a combination of multiple fuses to meet all safety criteria stipulated by the specific safety agency. Power adapters
designed for usage powering medical apparatus, typically will need to meet a more stringent safety objective, due to the higher
risk associated with severely ill patients, or patients undergoing surgical procedures. Often, this will require more fuses to be
used than power adapters designed for general purpose use.
Linearmode power adapters, due to their inherently simpler design, allow more stress to occur when the output is short circuited
or overloaded, in comparison to a more complex switchmode designed power adapter. Thus, fusing margins tend to be be
thinner, for linearmode power adapter products.
The following issues can occur which can damage the protective fusing, during field deployment, or during internal incoming
inspection procedures of companies purchasing linearmode power adapters.
a) Input line transients due to lightning striking power lines or high power utility load induced transients,
b) Abnormal continuous AC line voltage or frequency deviation,
c) Incorrect AC voltage selection, when the optional 115/230 switch is used on a linearmode power adapter,
d) Output short circuit,
e) Output overload,
f) Output reversed battery.
The six above listed typical problems are elaborated upon below.
Input Line Transients:
Primary side current fuses may be exposed to very high levels of current during an AC line transient phenomenon. The
probability of fuse failure, is related to two fundamental issues, which are fuse susceptibility to failure, and magnitude of the
event. For short duration current transients, as is the case with this type of event, it is most common to analyze the fuse durability
by using the fuse I squared T characteristic.
I2T Defined
T
I
For instance, if a transient induced current pulse is 20 amps, and has a duration of 1 millisecond, the I squared T characteristic
would be: 20 x 20 x 0.001 = 0.4
If the I squared T rating of the fuse is greater than 0.4, the fuse would be expected to survive, if the I squared T rating of the fuse
is less than 0.4, the fuse would be expected to blow open.
Of course, the actual I squared T rating of a specific AC line transient event, is more or less impossible to to predict.
Measurement apparatus such as a digital storage oscilloscope with a current probe, can be used to measure the event, if the
event can be produced when needed.
The I squared T ratings for some common axial leaded current fuse types are shown below:
Typical
Fuse
Value
Littelfuse
Picofuse
Fast Blo
UL248-14 curve
Bel
3x7mm
Slo Blo
UL248-14 curve
Littelfuse
5x15mm
Slo Blo
UL248-14 curve
Littelfuse
5x20mm
Time Lag
IEC60127-3/IV curve
250 mA
0.0106
(type 251)
0.047
(type MS)
0.216
(type 230)
0.5405
(type 218)
500 mA
0.0546
(type 251)
0.20
(type MS)
1.16
(type 230)
2.825
(type 218)
1000 mA
0.281
(type 251)
0.81
(type MS)
5.64
(type 230)
6.730
(type 218)
The impedance of the transformer primary, will have a direct effect on the current transient level, when a specified voltage
transient level is applied to the transformer primary circuit. For instance, on a small transformer, the combined primary and
secondary winding resistances reflected to the primary winding may be roughly 250 ohms, whereas a larger power adapter
transformer may have a total reflected impedance level of perhaps 25 ohms. Therefore, should a 1000 V transient be applied to
the primary winding for 300 microseconds, the calculated I squared T value for the pulse on a small transformer would be
approximately:
(1000V / 250 Ohms) ^ 2 * 300/1000000 = 4 ^ 2 * 0.0003 = 0.0048
However, in the case of the larger transformer, the same pulse would generate an I squared T level on the fuse which would be
10 squared (100) times higher, and so would be equal to:
10 ^ 2 * 0.0048 = 0.48
Note 1, I squared T is a dimensionless parameter, in other words, there are no units of measurement.
Note 2, assumption of the output circuit modeled as a short circuit during the transient is a necessary simplification for the
analysis, in actuality, the I squared T would be expected to be slightly lower for a real world circuit (providing transformer core
saturation does not take place.)
Note 3a, a standardized (IEC61000-4-5) lightning strike may be approximated by a rectangular voltage pulse, 1600V x 40
microseconds. For a transformer with a total reflected primary side Z of 250 ohms, this equates to a I squared T level of 0.0016.
Note 3b, similarly, for a transformer with a total reflected primary side Z of 25 ohms, this equates to a I squared T level of 0.16.
Abnormal continuous AC line voltage or frequency deviation:
When transformers are designed for 60 Hz, and are operated at 50 Hz, abnormally high magnetic flux density will occur in the
core, this then typically causes core saturation, this then causes abnormally high line currents. The abnormally high line currents
can give rise to either current fuse failure or thermal fuse failure.
Thermal fuses may only respond to the usage environment in a slow manner. However, when a transformer winding is partially
overloaded, the resultant high currents will cause heating in the resistance of the copper winding, and this heating is a square
law effect, so if the current is doubled, the heating will be increased by a factor of 2 ^ 2, or 4 times. So in essence it is probable
that if both a thermal fuse and a current fuse are protecting a primary winding, that the thermal fuse will fail under overload
conditions first.
Both the input frequency and the input voltage effect the core magnetic flux density in a linear way, for instance, if the input
voltage goes up 20%, the transformer core flux density goes up 20%, if the input frequency goes down 20%, the transformer core
flux density goes up 20%. However there is a limit to how high the flux density can go before core saturation occurs, this limit is
at approximately 10% greater than the nominal AC input condition.
This would then equate the 132Vac, 60Hz if a transformer has a nominal input rating of 120Vac, 60Hz (standard
voltage/frequency rating across North America). If however, a voltage of 132Vac, 50Hz is applied to a transformer rated for
120Vac, 60Hz the core flux density would increase by (132/120) * (60/50) = 1.32 times. Since this value is greater than 1.1, core
saturation is expected.
The first order response to core saturation is a reduction in input inductance, consequent decrease in input impedance,
consequent increase in input current, consequent increase in power loss in the primary coil. These resultant effects can cause
either the input current fuse to fail or may cause the input thermal fuse to fail. Generally, only one or the other will fail, but not
both.
Note 3, if an incorrect input voltage such as 208Vac is applied to a 120Vac input, the transformer input current will spike
exceedingly high, and blow out an input current fuse in under 0.1 seconds. Therefore, care must be exercised when testing
power adapters, to ensure that the applied input voltage/frequency is within boundaries, to prevent power adapter fuse damage
during OEM incoming inspection procedures.
Incorrect AC voltage selection, when the optional 115/230 switch is used:
The typical failure mode in this scenario is that 230/240Vac is applied to a 120Vac rated primary winding, due to the input
configuration switch being in the wrong position. Per the explanation in the preceding section, the transformer core will then go
into deep saturation, resulting in exceedingly high input current, which will cause rapid input fuse failure.
Note 4, general purpose power adapter designs to safety standard IEC60950-1 oftentimes do not require an input current fuse to
meet the regulatory requirements. In this scenario, the input is always protected by a thermal fuse. The amount of time to
overheat the primary winding given the switch abnormal scenario, is typically on the order of 30 to 60 seconds.
Output short circuit:
Simple linearmode power adapters do not include any means to disable the output circuit under short circuit conditions, which
quite logically is the least expensive way to design and manufacture the product. However, this can give result in damage to the
power adapter fuse(s) caused by short circuit.
The most common scenario for output short circuit is when a power adapter is supplied (per the OEM's request) with a
unterminated output cord. Internally, we typically refer to this configuration as "strip and tinned". This configuration is analyzed
slightly differently dependent upon if the output includes a rectifier/filter (to produce DC) or if it does not.
The worst case, is a linearmode power adapter which has the rectifier filter in it, so this will be the one to look at first. In this
scenario, the output capacitor can maintain a charge for several minutes after the AC power is removed, providing the output is
unloaded when disconnected form AC power. Oftentimes, NEC class 2 output compliance will be required, which requires an
output fuse to be compliant, if the output power level is moderately high, or the output voltage is moderately low.
The output fuse, which will typically be provided after the rectifier/filter will be susceptible to failure, should the unterminated
output leads be allowed to touch one another, even with no AC present on the input. Therefore, all Jerome Industries ATE
equipment will automatically (even without programmer guidance) discharge all output loads before allowing the ATE operator to
remove the power adapter from the ATE test equipment. During incoming inspection at the OEM's premises, this simple
precaution, may be easily overlooked, resulting in unwitting damage to the power adapter being tested.
In the case of a linearmode power adapter with a AC output, short circuiting of the output is quite easily done, if the power
adapter is provided with an unterminated output cordset. Of course, here the difference is that the AC input must be powered in
order to cause the current fuse to fail. In this scenario, the typical time required to blow a current fuse will range between 0.1
seconds and 1 second. However if a unit is constructed without need for the current fuse, and only utilizes a thermal fuse, the
time to clear the thermal fuse would typically measure 40 to 60 seconds.
Output overload:
This abnormal condition is similar to the output short circuit condition, however, whereas a short circuit may produce output (and
input) currents which are anywhere from 5X to 10X the nominal full load condition, an output overload would be generally defined
as an output loading which is from 110% to 300% of the nominal full load condition.
Due to the typical problem of current fuse sizing with regards to nuisance tripping, during an output overload it is rare for a
protective current fuse to fail, typically the thermal fuse is the first fuse to fail. The OEM needs to verify the power adapter output
load current of the system, before proceeding to order mass quantities of power adapters. If an estimate of current draw is used,
it is possible to end up with a load current draw which is 110% to 120% of the nominal full load value. In this scenario, the
thermal fuse will typically be running close to the failure temperature of the fusing element. It may take anywhere from 1 week of
operation to several months of operation for the fuse to eventually fail, and it may require operation on a hot day, where the
ambient temperature of the power adapter is at it's upper limit for the thermal fuse to fail.
Note, if an OEM asks our customer service department, about optimum output loading, for a long product service life, 75% to
90% of the full load output rating is recommended. Note, if there is a little margin, then abnormal loading from conditions such as
damaged IC's or stuck relays in the OEM's apparatus, would be unlikely to overload the power adapter, and cause an internal
fuse to fail.
Output reversed battery:
Up to this point, analysis has focused on simple linearmode power adapters which power a system load, however oftentimes an
OEM customer will additionally require a battery charging feature. In this scenario, there is the potential for reversed battery
installation in the OEM's apparatus.
When a battery is installed backwards a number of bad things may occur. Unless there is a polarity protection diode, the
customers apparatus may be damaged, or may simply overheat without damage. With respect to the power adapter, the
reversed battery will forward bias the output rectifiers (even without input AC present), causing permanent damage to the
rectifiers. Should there be a current fuse located between the output rectifiers and the output cord, the current fuse will fail before
the rectifiers.
Typically, at first glance, a series output diode inside the OEM's apparatus between the incoming low voltage DC connector and
the battery would prevent reverse current flow into the charger in the event of reversed battery installation. However, a careful
analysis of the circuit with the added series diode shows that the series diode can not provide any protection to the rectifiers (or
current fuse) in the power adapter.