Don't Be Misled by Power Device Specs

May 1, 2008 12:00 PM
By Roger Stout, Senior Research Scientist, ON Semiconductor, Technology Development, Advanced Packaging, Phoenix

More than once, I've fielded this question from applications engineers on the telephone: “I just found a data sheet for the XYZ device, and it has two different maximum power values listed. Which one is correct?” Or, perhaps designers have seen one of those device sample boards that have a dozen or more package types mounted on the front, with a table on the back showing two columns with two different maximum device power values for each package style. How can this be?

A couple of years ago at a major industry conference, at the first break in a half-day seminar I was presenting (after I'd already covered the basics), someone came up to me and pleaded, “But for a given package, shouldn't there just be some intrinsic amount of power dissipation that results in that maximum junction temperature?”

Speaking as a thermal expert, the answer to the question is yes and no. Unfortunately, this answer probably doesn't help the designer much. However, a mental model rooted in electrical fundamentals will help clarify these concepts.

An Electrical Analogy

For starters, let's take a 10-Ω resistor and pass 1 A of current through it (Fig. 1). Now, we may ask, “What's the voltage of this resistor?” Actually, such a question doesn't make any sense. For the question to make sense, what needs to be asked is, “What's the voltage drop across the resistor?”

Here's the problem. Voltage, as such, implies a reference. With no reference specified, it might reasonably be assumed that the voltage at one terminal be compared with the voltage at the other. Then the answer is clear and simple: 10 V.

It should be intuitive that the actual voltage at one end or the other could really be anything; this resistor would act the same in any circuit and, as long as it was conducting 1 A, the voltage at the input end would always be 10 V relative to the output end. If the voltage at the output end were determined by other things in the circuit, it might be ground (that is, a systemwide 0-V reference), or it might be something else entirely. For example, there's no reason the input end of the resistor couldn't be ground, and then the output end would be -10 V.

Let's suppose we have three connected resistors with two current sources (Fig. 2). If I1 is 1 A, can we tell what the value of V_A is? How about this variation: What if we don't allow VA to be higher than 10 V, what would the maximum current of I1 be?

Obviously, we can't answer either of those questions without more information. Believe it or not, most semiconductor package thermal systems boil down to a thermal circuit analogous to Fig. 2; yet most designers who ask for maximum power dissipation think of Fig. 1 (with the output end grounded).

The Thermal Domain

Let's formally switch the electrical analogy back into the thermal domain. First, temperature is like voltage. We're mainly interested in temperature differences between points in a system. An important point to consider is thermal ground, otherwise known as ambient. But ambient is probably not zero in a given system. Another important point is the junction temperature. Although we need to get to it relative to ambient, we probably have a nonrelative maximum junction temperature to worry about.

Second, thermal heat is like electrical current. Be very careful here. Because we so often talk about electrical current flowing in a semiconductor device, it's all too easy to forget that when we're specifying amperes of electrical current, we basically haven't said anything at all about how much heat is being dissipated. We'll have to do a little electrical analysis to get from electrical current into power, but we have to make sure to use power as the current analog in the thermal model.

Third, thermal resistance must have units of °C/W. Then, when we multiply a thermal current (W) by a resistance, we get a temperature difference in °C. Note again that we're referring to a temperature difference, not a temperature.

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