Power Supply Parameter Estimator
This tool estimates the voltages in a linear regulated power supply, and then calculates the heat produced from that. This allows you to iteratively work out a suitable choice of power source and heat sink for a linear power supply.
It’s Only a Model
The estimator only provides a rough model of typical linear regulated power supply behavior. It does not attempt to model the low level physical behavior of individual components, or take all the known sources of variability into account. It’s fair to say that its results are just plain inaccurate.
More importantly, this is just a dumb computer program running in a web browser. It cannot do your thinking for you. It attempts to provide an analysis of its calculated values, but in the end, it’s up to you, the DIYer, to evaluate what it says and act intelligently on that information.
This is not to say that the estimator is useless. It’s my experience that when faced with the calculations required to pick the proper transformer, many will give up and simply choose something that’s way over-specified to be sure there’s no possibility that the output voltage will be too low. This sort of overspecification often results in a power supply that either fails to work correctly, or is in danger of overheating. You don’t need a detailed physical model or an impeccable analysis to avoid such problems.
Likely Sources of Inaccuracy
The rectifier type selector simply uses 0.5 V per diode for a Schottky bridge rectifier, and 1 V for a silicon rectifier. These values are representative of power diodes of this type, but the actual values will be somewhat different. The exact diode type and the load current play into the actual voltage drop; the estimator doesn’t even attempt to model this.
The calculation for ripple in AC-input supplies is very much an approximation. First, the formula used assumes that you’re using a simple capacitor type filter, and that the cap is large enough that the load current won’t cause it to let through too much ripple. Second, the estimator makes no attempt at modeling the nonlinear IR drops in the transformer’s output coil, or their interactions with the filter stage.
The thermal resistance calculation assumes that you’ve tightened the mounting bolt well and used the correct amount of heat sink compound. If you wanted to model these errors, you can do it by increasing any of the thermal resistance numbers. (They’re just added together in the calculation, so it doesn’t matter which one you change.)
The calculated temperature isn’t an absolute value. To get that, you need to add that temperature to the temperature of the air surrounding the heat sink, called the “ambient temperature.” You will almost certainly not know this accurately. Chances are that you’re going to box the regulator up, which allows the regulator to heat the air up inside the enclosure. The only way to figure this out accurately short of just building it and trying it requires high-end fluid dynamics software. This simple little estimator can only make an arbitrary guess about ambient temperature when deciding whether your configuration makes sense or not. It may not even come close to your actual situation.
References
The ripple voltage calculation comes from The Art of Electronics, second edition, page 46.
Section 7.2.8 in The Circuit Designer’s Companion (pp. 236-237) by Tim Williams was helpful in putting together the rest of the calculations. Those calculations work the problem from the other direction and have a different purpose, however, so I was unable to use any of them exactly. Thus, all errors in application are mine.
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