Understanding Derating Curves

Derating curves are essential tools in understanding an electrical product’s heat tolerance and maximum output power capabilities. They're also a necessary safety measure to secure the component’s functionality and your protection. 

What Is a Derating Curve?

A derating curve is a graph that displays how an electrical component's maximum current rating decreases as the ambient temperature increases. Engineers and researchers use it to reveal the possible combinations of voltage and current to reach the maximum output. Also known as a current carrying capacity or power rating curve, it graphically represents all the combinations of voltage and current that a particular power supply supports. 

When you find a derating curve that works for a mated pair of connectors, you’ll know the maximum current for any given temperature. The derated curve represents this maximum current. The supply will support any voltage or current below the curve. Derating curves ensure that your desired current, voltage, and power combination is supported by the component where the connection operates. 

Derating curves are typically found in the power supply specifications or the manual. It’s crucial to look at the graph before connecting any components to ensure they work with a suitable power supply. 

The Purpose of Derating Curves

Mathematically speaking, if both voltage and current can be changed, there are endless ways to produce the wattage to run an item. For example, if a product’s power demand is 40 watts, we could deliver it using 40 volts and 1 amp of current. We could also reach the wattage by using 100 volts and 0.4 amps. However, power supplies usually have limits on the output voltage and current they can produce.

In most cases, maximum current is the limiting factor. For example, if the power demand is 40 watts, it would be easy to design a 40-watt power supply using 40 volts and 1 amp. However, producing a 40-watt power supply that uses 1 volt and 40 amps is not as simple. 

Even considering standard voltages and currents, a power supply design might not accommodate all combinations leading to the maximum wattage. For example, a power supply can have a maximum current of 4 amps and a maximum of 100 volts but only support a 40-watt output. 

The item might only support certain combinations to attain the maximum output power. Since voltage and current may be suitable in a wide range of user-configured values, expressing all possible combinations in a table or list would be challenging. That’s where derating curves come in. These curves graphically represent all the possible combinations of voltage and current that the component allows. 

How to Read a Derating Curve

When reading a derating curve, note these important details: 

• The maximum power or temperature, depending on the item
• The maximum current
• The maximum voltage 

Remember that for most power supplies, the maximum output power is not equal to the maximum voltage multiplied by the maximum current. You’ll need to read the curve, depending on the type. 

How to Read Classical Derating Curves

A classical derating curve has a curved region called the maximum power output curve. All voltage and current points multiplied along this point will yield the maximum output power. 

To find a combination that gives you the maximum output on a classical derating curve, follow these steps:

1. Locate a voltage point under the maximum power output curve. 
2. Pinpoint a current point that’s also within the maximum power output curve.
3. Multiply them to find the point of intersection on the current axis. The result should equal the maximum output power. 

How to Read Alternate Derating Curves

When it comes to power supply and derating curves, many of them fall into the classic shape. However, other shapes are possible, and they’re known as alternate derating curves. Instead of a curve, they have a maximum power output line. Here’s how to read them: 

1. Find a voltage point under the maximum power output line. 
2. Locate a current point that’s within the maximum power output line.
3. Multiply them to get the maximum voltage or the maximum output power. 

How to Read Thermal Derating Graphs

Manufacturers typically include derating graphs because calculating thermal impedances and junction temperatures is complex. You wouldn’t want to expose most electrical products to unreasonable heat unless you're working with a heating item. Excessive heat could lead to increased failure rates and shorter service life.

Thermal derating graphs are generated to reveal the temperatures the product can run on without risking thermal failure. This derating curve shows the allowable load and ambient operating temperature. Follow these steps to generate these graphs: 

1. Use the Thermocouple Attachment

Wire a thermocouple as close to the contacts or connectors as possible. This step allows you to find the worst temperature rises possible close to the section that causes the heat. 

2. Capture the Room’s Ambient Temperatures

Measure the ambient temperatures in the room every time you capture a temperature rise. This step considers the temperature changes within the room that may otherwise affect the results. It ensures you’re only measuring the effect of the current, and it isn’t disturbed by other ambient temperatures. 

3. Test the Current

In this step, you’ll start ramping up the current in stages. Ensure that whichever level you’re on, it is set to DC continuous current and not pulsed. 

Waiting for the metal to heat up is important at each level. Allow the raised temperature to stabilize and note it down. Higher currents could take longer to stabilize, so be patient and only take the readings once they’re ready. 

Repeat this step for each level until you pass the upper limit of the item’s operating temperature. While you may be hesitant to do this, it’s crucial because it shows the product’s true operating capabilities. 

4. Calculate the Thermal Derating Curve 

When you’ve finished testing, you’ll have two sets of temperature readings — the ambient and contact temperatures. Subtract ambient temperature from the contact temperature, which rises for each level. 

To achieve a reliable dataset, perform this test on multiple mated pairs. 

Precautions to Take When Applying Derating Curves 

Understanding power supply and derating curves is crucial to real-world power supply and electronics applications. However, outside of a laboratory’s ideal conditions, safety margins should be added to prevent damage to products, workers, and the environment. Here are three safety measures you can take: 

• Using the cooling functions to lower heat on the contacts 
• Building prototypes that represent the end usage methods
• Performing climate testing solely in experimental chambers 

When testing heating limits, go up in small intervals. Pushing a component way past its operating temperature limits can have dire consequences. It can degrade the performance specs and cause electromagnetic interference (EMI). 

Contact Astrodyne TDI for Your Unique Electrical Requirements

Testing derating curves for power supplies can affect the application and functionality of your products by causing EMI interference, which impacts electrical circuits. At Astrodyne TDI, we manufacture EMI filters that protect against EMI interference. We can customize these filters upon request and provide exceptional quality at cost-effective prices. 

We are industry leaders in unique electrical requirements, from programmable outputs to utility power sources. Our team of experts is only a click away. Request a quote or find a distributor to simplify your electrical needs.