Real World Power Supply Design

Power supplies come in a wide variety of voltage outputs and packaging. Here are just a few examples of them.

The photo is one of the most common multi-voltage switching power supplies used in desktop computers. This power supply will fit nicely into almost every kind of PC enclosure. Its typical efficiency is 75% at full load with minimum ripple and noise. These power supplies also have auto voltage select (AVS) functions, which means that they can automatically detect either 110 VAC or 220 VAC inputs and adjust for either input voltage accordingly.

Figure 5-30

Computer Power Supply

Figure 5-30

Computer Power Supply

Pictured below is another common type of power supply called an "open frame" power supply. The "open frame" designation comes from the fact that the power supply components are not enclosed in a metal box like the computer power supply above. This not only saves cost but also provides for more air circulation around the individual components, especially the diode rectifiers, which are attached to the black metal heat sinks. Notice the cut out for a fan in the computer power supply above. Because it's enclosed, a fan is needed to circulate cool air around its components, which, by the way, look quite similar to the open frame model pictured here.

Figure 5-31

Open Frame Power Supply

Figure 5-31

Open Frame Power Supply

Linear Power Supply Design

At the end of Experiment 4 we mentioned that this chapter would have more information on power supply design, and specifically on how to convert the full wave rectified circuit you built into a classic linear power supply. True to our word, here are more details on linear power supply design beginning with Figure 5-32.

The first thing that's different is that the wind-powered three-phase AC alternator has been replaced by a conventional transformer, T1, which isolates the rest of the circuit from the 110 VAC, 60 Hz input and also steps the voltage down according to the power supply's DC output requirements. Every transformer has a "primary" and a "secondary" winding. The part of the transformer that is connected to the 110 VAC source is called the primary winding, or just the primary, and the secondary winding, or secondary, is connected to the 4-diode bridge rectifier. Some transformers also have multiple primary and secondary windings, as well.

In general, the higher the DC output, the higher the secondary voltage. In this simple power supply circuit the DC output is approximately equal to the secondary voltage multiplied by 1.414, however this is a rather simplistic calculation and does not take into considerations the many variables that can affect this assumption like transformer loss, output ripple and especially varying loads. At light loading this rule can be applied without much concern and it will be accurate enough for most applications. However when an appreciable amount of current is drawn, this simple approach may not yield the proper results. Nevertheless, we will not stress our power supply hard enough to violate this assumption in this example.

Since the applied 110 VAC spends so much of its time at a voltage lower than that of the capacitor, there is no diode conduction during this time. You learned this in Experiment 4 when you added a smoothing capacitor to the circuit. During the periods when the diodes do conduct, the transformer must replace all the energy that drained from the capacitor in the intervening 8 to 9 ms assuming a 60Hz AC input and full wave rectification as we have here. Our smoothing capacitor helps to do this but unfortunately it's not enough. As you saw in Figure 5-33 - Oscilloscope Plot of Smoothed Full Wave Rectifier Output (repeated here) the smoothing capacitor still leaves a bit of "ripple" on the DC output.

Figure 5-33: Oscilloscope Plot of Smoothed Full Wave Rectifier Output

So, what is "ripple" and why is it not desirable? Ripple is a slang, but accepted, term for the voltage variations that "ride on top" of the DC output. It's not desirable because when the power supply output is applied to power an audio amplifier, for example, this ripple will translate into an annoying hum on the speakers. There are other deleterious effects that ripple can cause, however this is most pressing of them all for the majority of commercial products. The idea is to add a "choke" inductor in series with the DC output to help eliminate the ripple.

In our schematic diagram in Figure 5-32 our choke inductor, L1, serves to smooth out the ripple by "choking off" the AC component of the DC output, which is why engineers and technicians long ago coined the word for it. While a capacitor will not pass DC and allow AC to charge it up, an inductor will not pass AC but will allow DC to pass without much loss. These are very simplistic definitions for capacitors and inductors, however they will suffice for this example. With its' "back EMF" capability inductor, L1, acts to cancel the AC component that rides on the DC output. The combination of capacitor, C1, and inductor, L1, form a low-pass filter that helps to smooth out the DC output.

Capacitor, C2, is generally called a filter capacitor since its purpose is to cancel the remaining AC ripple even more. So with a constant resistive load as represented by resistor, R1, we have a linear power supply that is effective in delivering a reasonably "clean" DC signal to whatever it happens to be connected to.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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