How Hot is Too Hot? Ask Sherlock

Jan 27, 2012 1:58 PM
Craig Hillman

The power supply is the core of all electronic equipment. It often has to inexpensively manage high levels of power and voltage in relatively tight spaces. To meet these requirements, power supply designers need to call upon an impressive level of creativity and skill.

But, creativity relies on knowledge. This is especially true in power supply design, where solutions to electrical noise, timing, and efficiency are born from expertise and experience. Unfortunately, the feedback loop on thermal solutions is not always so direct. While amazingly powerful thermal tools can predict the distribution of junction, case, and ambient temperatures in minute detail, it is often difficult to know what the temperatures should be, as opposed to knowing what the temperatures will be.

The classic approach for determining appropriate component temperatures is based on a combination of datasheet information and derating strategies. As an example, if a power MOSFET is rated to 125°C and internal derating guidelines specify a 20°C margin, the selected thermal solution must ensure a temperature no higher than 105°C.

Beyond the typical confusion over which 125°C (Junction temperature? Case temperature? Lead temperature?) and if thermal derating should be absolute (e.g., 20C margin) or relative (e.g., 80% of the rated temperature), this methodology is popular because it is so straightforward.

Increasingly, the electronics industry is realizing that classic derating is no longer satisfactory. Its broad assumptions, which are not based on actual failure modes and degradation mechanisms, can result in overly conservative and expensive designs or products with insufficient reliability. Either case loses customers and shrinks market share. A more effective approach is one that takes the results of thermal modeling or measurement and inserts these results into design rules or predictive tools that are based on reliability physics.

We will identify the most temperature sensitive components in power supply design, discuss how temperature induces degradation in these components, and examine what existing knowledge is available to allow the designer to make physics-based decisions on how hot is too hot.

Components At Risk

The derating approach was always a questionable practice, but had some legitimacy in older electronics because solid state mechanisms typically took decades, if not hundreds of years, to evolve and induce any significant number of failures. So, derating was more about function (parametric drift, etc.) then reliability. Reliability has become a greater concern as the need for more functionality in less space has resulted in such fine feature sizes that degradation mechanisms are now evolving in a matter of years, or even months, even when designers adhere to classic derating guidelines.

The component technology of greatest concern in regards to temperature and reliability in power supply design are:

• Magnetics (Transformers / Chokes)
• Optocouplers / Light Emitting Diodes (LEDs)
• Capacitors (Electrolytic / Ceramic / Film)
• Integrated Circuits
• Solder Joints
• Magnetics

Magnetics, such as transformers and chokes, get top billing as they are the component technology often least considered when concerns about temperature arise during design reviews and component stress analysis. Since transformers are also typically custom, many do not even come with a temperature rating. So, how to determine how hot is too hot for magnetics? There are three key issues of concern.

First, the saturation current in ferrite material has a soft saturation curve which can tend to obscure when a material starts to saturate. Nevertheless, this will vary with temperature. For example, BSAT could be 3000 gauss at room temperature, but at 100°C it can drop down to 2000 gauss. Saturating the magnetic material will not damage a magnetic, but it will appear to be shorted to the electronic circuit and can cause the circuitry to fail. Debugging this scenario is quite difficult, because the transformer or inductor can seem ok at room temperature.

Second, designers are sometimes under the mistaken impression that maximum temperature rating is equal to the Curie temperature (this can be between 100°C to 300°C). However, core loss usually reaches a minimum at temperatures between 50°C to 100°C, which is typically below the Curie temperature. Depending on the ferrite design, structure and cooling, the magnetic can go into a thermal runaway if the core temperature is on the right side of this minimum. This is because for some magnetic designs, the core loss will increase over time, resulting in a higher temperature, which causes an even higher core loss, and so on and so on. This increase in core loss is due to thermal aging. Fig. 1 shows a relative relationship between core loss and temperature for low and high frequency materials.

Thermal aging is primarily a concern for powder iron cores, which are lower cost and sometimes more appropriate than ferrite cores. Powder iron cores are mixture of extremely small particles of iron oxide or other magnetic material mixed with an organic binding agent. It is well known that long-term exposure at elevated temperatures can induce “thermal aging” of the binding agents. There are several variables that affect the rate of thermal aging of magnetic components, including the core material, the peak AC flux density, the operating frequency, the core geometry (larger cores age faster), copper loss and core temperature. As thermal aging progresses, the eddy current loss, which is a critical component of the core loss, becomes significantly higher. Increasing core loss eventually results in higher core temperatures and failure of the magnetic component. Fig. 2 describes the relative changes in core temperature as a function of operating time, operating temperature, and core volume.

There are several things that a designer can do to mitigate the higher temperature effects on magnetics. For example, the device could utilize a low loss core material. Also, higher frequency with total turns reduction while maintaining turns ratio or using Litz wire for reduced coil heating can be done. Or, varnish applied with vacuum evacuation (also allows for washing) can also facilitate improved thermal performance.

Light Emitting Diodes / Optocouplers

Light emitting diodes (LEDs) are typically incorporated into power supplies as indicator lights or as one half of optocoupler technology. LEDs have a natural lifespan that ends in a wear-out mechanism. Defects within the active region can spur nucleation and dislocation growth and are particularly affected by temperature and current. Lifetime of LEDs is typically described by the following functions

Where:
I = Forward current
n = Forward current exponent
EA = Activation energy
KB = Boltzmann constant
T = Kelvin temperature

For LED technology, the forward current exponent is typically 1.5 and the activation energy is 0.7 eV.

Indicator lights rarely experience “failure” because designers typically introduce a low duty cycle (refresh rate simply must be 60 Hz or faster) and a low forward current. Because of the high intensity of LEDs at rated forward current, use of forward currents 1/10th the rated forward current is typically sufficient for all but the brightest environments. With such a high exponent, the derating on the forward current is more than sufficient to extend lifetime out by several decades. In addition, failure definition in LEDs is a 50% reduction in brightness. Even with this reduction in brightness, most users can still perceive the indicator lights and can respond to their signaling.

The real issue with LEDs comes with their use in optocouplers. Low voltage optocouplers are typically rated to 40 to 60 mA. While derating guidelines typically allow for up to 80% of the rated forward current, most designers, to optimize current transfer ratio (CTR) (the ratio of the output current over the input current), typically drive the LED at 1 to 10 mA. And this approach is a good thing when the optocoupler sees anything close to elevated temperatures.

As an example case study, a solar power conversion company was operating a 50mA optocoupler in their design at 11mA, which was well within derating guidelines, at a 50% duty cycle (solar panels don’t produce much power at night). This particular manufacturer’s reliability goals were 95% reliability after 15 years.

The challenge was in where to locate the optocoupler. The three locations had various advantages and disadvantages, but they had distinct differences in temperature. Option 1 was at ambient, Option 2 was 5°C above ambient, and Option 3 was 10°C above ambient. Because of this application, ambient was driven by diurnal temperature changes (Table 1).

To use Equation (1), a rated lifetime is necessary. While a surprising number of optocoupler manufacturers do not provide this information, the standard LED in optocouplers is typically specified to have a mean time to failure (MTTF) of 50,000 hours at maximum rated forward current at 25°C. It is important that MTTF is not lifetime; MTTF is the time to 63% failure. The more important reliability metric of time to 1% failure is typically far shorter.

Using the relevant equations and inputs, the manufacturer quickly realized that the criticality of keeping the optocoupler as cool as possible (Fig. 3). The 20X difference in unreliability would not have been captured using the traditional approaches of derating to determine appropriate temperatures.

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