Reference & Information

Guideline for Derating Electronic Components



Cable, Capacitor, Circuit Breaker, Connector Diode, EMI Filter, FET, Fuse,
IC, Inductor, OPAmp, Potentiometer, Power Supply, Relay, Resistor, Switch, Transistor
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This page provides a number of guide-lines for Voltage and Temperature derating [Thermal Derating].
Derating guidelines are provided in tables for each component type, and should be regarded as design recommendations.
A good design always uses components that operate within their Safe Operating Region.
However there are absolute rules of design that should be followed when designing circuits.
The derating curves provided for Transistors, FETs, and Diodes should be considered rules to hold the junction temperature of these devices within specified limits.
These component derating recommendations do not account for mounting issues required by high vibration environments.
A short description is provided as required to understand How-to derate components;

Design Note; In all cases a component is considered to be operating alone, with no other components near-by.
In addition that the components are using ambient air [no conditioning], using natural cooling [no forced air].
Ambient air is considered to be normal room temperature, unless indicated otherwise.
Natural cooling is the dissipation of heat to surroundings by conduction, convection, radiation, or any combination thereof without the benefit of external cooling devices.




Connector Voltage Derating:

Derate connector voltage to 25% of rated operating voltage
.... Operating voltage = Connector rating / 0.25
Derate connector operational temperature by 50oC
.... Connector rating = operating temperature + 50oC
Current is normally specified as some number of pins carrying some specific amount of current, at some defined temperature. Increase the current carried over any particular pin and the number of pins carrying current most decrease. Decreasing the amount of current over the defined pins may allow more pins to carry current. Main section on Connector Derating. Refer to this page for a manufacturers listing for Connectors


Cable/Wire Derating:

Refer to this page for a manufacturers listing of Wire and Cable. Cable manufacturers will provide different numbers based on the insulation used for the wire. The table below lists copper wire with a Teflon [TFE] insulation. Teflon insulation has a higher operation temperature range then other insulators, for example PVC. The table below is based on data derived from MIL-STD-975, using 70oC as the operating temperature. To derate based on number of wires in a bundle:
IBW = ISW x (29 - #wire) / 28 @ [1 to 15 Bundled wires]
IBW = ISW x (0.5) @ [more then 15 Bundled wires]
ISW = Single wire
IBW = Bundled wires
To derate by temperature use; derate by 80% at 1500C, 70% at 1350C, or 50% at 1050C {Per MIL-STD-975}
To derate for Commercial wire use the table below;


Copper Wire Correction
Temperature Derate Ampacity
40oC 0.88
45oC 0.82
50oC 0.75
55oC 0.67
60oC 0.58
70oC 0.33

Ampacity Tables for many conditions (Commercial Wiring):
IEEE Standard 835, IEEE Standard Power Cable Ampacity Tables
IEEE Standard 848, Procedure for the Determination of the Ampacity De-rating of Fire Protected Cables
ICEA P-54-440, NEMA Pub. No. WC 51 - Ampacities of Cables in Open-Top Trays
Use the National Electrical Code [NEC] system for premises wiring, and not the Military data on this page.

Voltage Drop per Foot:
V = I x L x (R/1000) x 1.004 x (T-20)
V = Voltage Drop, I = Current, R = Resistance per 1000 foot
L = Length of wire in feet, T = Estimated wire Temperature under load oC
Refer to the AWG Table for Resistance per length

Copper Wire TFE Insulated
AWG
Current Carrying
AWG
Current Carrying
00 169 0 147
2 108 4 81
6 60 8 44
10 33 12 4425
14 19 16 13
18 9.2 20 6.5
22 4.5 24 3.3
26 2.5 28 1.8
30 1.3 - -

{Component Derating Guidelines}

Potentiometer Derating:

Variable Resistor Material types vs. Operational Temperature Range
Potentiometer Derating Curve

A list of Potentiometer manufacturers
This page shows the derating curve for an Adjustable WireWound resistor

{Component Derating Guidelines}

Resistor Derating Example:

Power ratings are normally specified at +25oC and must be reduced as the resistors temperature increases. A derating chart is often used, with derating starting at 70oC. Since parameters are application dependent, charts should be considered general rather than absolute, and only used as a guideline. However power de-rating curves should be considered more of a recommendation than a guideline The safest designs use the largest physical size operating at conservative temperatures and power ratings. Combining the resistor application and a power de-rating curve should start to form a design rule, once the location of the resistor on the PWB is known and the components around the device.

Resistor Derating Chart
% Rated load
Wire Wound
Temperature
Metal Film
Temperature
100% 70oC 70oC
80% 110oC 85oC
60% 150oC 100oC
40% 190oC 120oC
20% 240oC 140oC
10% 260oC 145oC

Military Resistor Derating Curves, Derating Resistor Networks, Derating Thermistors. A list of Resistor Manufacturers manufacturers

Design guidelines;
For resistors mounted in series, consider the heat being conducted through the leads to the next resistor. At the same time axial lead-mounted parts also act as a heat dissipator and allow heat to be conducted away from the device and into the printed circuit card, or pads on the PCB. Of course heat is also removed from the component via convection and radiation.
Large power resistors should be mounted to the metal chassis for heat dissipation. Do not mount resistors with power dissipation 1 Watt directly on terminal or printed wiring boards without use of heat sinks. A resistor that dissipates over one watt can damage a terminal board. A damaged board will have a lower insulation resistance. For the most efficient operation and even heat distribution, power resistors should be mounted in a horizontal position.

{Component Derate Guideline}

Fuse Derating:

Fuses are specified to interrupt within a maximum of 5 seconds when driven at 200% of their rated current for nominal ratings up to and including 10 amperes. A fuse with a nominal rating of 15 amperes is specified to interrupt within a maximum of 10 seconds when driven at 200% of its rated current. The power supply shall be capable of delivering appropriate levels of current to achieve short fusing times. There is an additional derating of 0.2%/°C for an increase in the temperature of fuse body. Main Fuse Derating Guidelines page. AC or DC Fuse operation. Fuse holders may also require derating.
A manufacturers listing for Fuses

{Component Derating Guidelines}

Switch Derating:

Switch Temperature Correction
  Current Derating Factor @ Application Ambient Temperature
  Military Commercial
Load Type 0oC to 85oC Above 85oC 0oC to 85oC Above 85oC
Resistive 75% of rated
Resistive load
60% of rated
Resistive load
75% of rated
Resistive load
60% of rated
Resistive load
Inductive /
Motor
75% of rated
Inductive load
60% of rated
Inductive load
40% of rated
Resistive load
30% of rated
Resistive load
Capacitive /
Lamp
75% of rated
Capacitive load
60% of rated
Capacitive load
25% of rated
Resistive load
20% of rated
Resistive load

Note the derating factor changes with the type of load.
A manufacturers listing of Mechanical Switches

{Component Derating Guidelines}

Inductor Derating:

Inductor Temperature Correction
  Current Derating Factor @ Application Ambient Temperature
  Military Commercial
Load Type 0oC to 85oC Above 85oC 0oC to 85oC Above 85oC
Resistive 75% of rated
Resistive load
60% of rated
Resistive load
75% of rated
Resistive load
60% of rated
Resistive load
Inductive /
Motor
75% of rated
Inductive load
60% of rated
Inductive load
40% of rated
Resistive load
30% of rated
Resistive load
Capacitive /
Lamp
75% of rated
Capacitive load
60% of rated
Capacitive load
25% of rated
Resistive load
20% of rated
Resistive load




Inductors are also derated by reducing the maximum operating temperature based on the insulation class and reducing the operating voltage.
Transformer Stress Ratio = Operating VA Load / Rated VA Load = 80% [Rule-of-Thumb Operating Recommendation]
A manufacturers listing of Inductor Vendors, Transformer Vendors, Choke Vendors

{Component Derating Guidelines}

EMI Filter Derating:

The general recommendation is to derate an EMI Filter to 50% of rated voltage and 50% of rated current, using a maximum temperature of 850C.
However that could be to strict for some applications, in which cause the next best option would be the dearting curve provided in the data sheet.
This example shows 50% operation doesn't apply until 125C and maximum operation out to 150C [Case Temperature].

A manufacturers listing of EMI/RFI Components

Note that many temperature-power derating curves are just a linear curve or set of curves, others are a bit more complicated.
Some curves show maximum power dissipation, others show percent rated load [as current or voltage].

EMI Surface Mount Chip Operational Temperature Range vs Maximum Current
EMI Chip Derating Curve

{Component Derating Guidelines}

Diode Derating:

Diode Temperature Correction
Diode Type Critical Stress Parameter Derating Maximum Junction Temp
General Purpose,
Rectifier, Switching,
PIN/Schottky, Thyristors
PIV 0.70 125oC
Surge Current 0.50
Forward Current 0.50
Varactor Power 0.50 125oC
Reverse Voltage 0.75
Forward Current 0.75
Voltage Regulator Power 0.50 125oC
Zener Current 0.50 (Imax+ Inom)
Voltage Reference Zener Current N/A
125oC
Zener Voltage Suppressor Power Dissipation 0.05 125oC
Bidirectional Voltage Suppressor Power Dissipation 0.5 125oC
FET Current Regulator Peak Operating Voltage 0.80 125oC

More data may reside on the Diode Derating page. Current-Power Diode Derating Curves for a number of diode types.

A manufacturers listing of Diodes, Zener Diodes and Varactor Diodes

{Back to Component Derating Guidelines}

Power Supply Derating Guidelines:

Operating Temperature range is normally 0 to 50oC, derate 20% for each additional 10oC increase above 50oC.
Operating Temperature range; 0oC to +70oC. Full power to +50oC then derate 2.5%/oC to +70oC. Or, derate linearly from 100% load @ 50oC to 75% @ 60, derated at 10% per oC to a Maximum of +65oC
Forced air cooling [20CFM nominal]: a power supply may require a forced air flow to operate at its rated maximum temperature rating. Derate 20% with out cooling
Open Frame supplies: may require an additional 15% power de-rating when covered by a frame.
About: The power supply specification should detail its operation, use the above as a rule of thumb. Normal operational temperature range is defined as ambient [Maximum]





Question; why derate electronic components. Answer; to reduce the stress they see and to keep them from over-heating.

Ambient temperature: The temperature of air or other media in a designated area, particularly the area surrounding equipment.
IEC 60134 Absolute maximum and design ratings of tube and semiconductor devices.
More data may reside on the Power Supply Derating page.
A list of Power Supply manufacturers.

{Back to Component Derating Guidelines}


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Modified 2/26/12
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