Ideal Versus Real: Understanding Some Fundamentals When Selecting Power Supplies

Introduction
In college, we learned about electronics using ideal components: pure resistors without series inductance, pure capacitors without ESR, op amps with infinite gain and zero offset voltage. For power supplies, the situation was no different: a constant voltage source with zero output impedance, unlimited current compliance, and infinite bandwidth. With components like these, how difficult could it be to design electronic circuits and systems?

Then, we got jobs as engineers in the real world and discovered things like temperature coefficients in resistors, dielectric absorption in capacitors, and phase shifts in the gain of amplifiers. Power supplies did not escape the omnipotent forces determined to destroy our idealistic view of electronics. Non-zero output impedance, output current limitations, and finite bandwidth have all conspired to make our lives a little more complicated when applying power supplies. The effects of these non-ideal power supply attributes and others are discussed in this post.

The Ideal Voltage Source
An ideal voltage source would maintain its output voltage constant irrespective of the loading conditions. For example, if the source were a 5 V DC source, the output would measure exactly 5.0 V with no current flowing, or with 1 A flowing, or 10 A, or 500 A, and so on. Additionally, when the load current changed from one value to another, such as from 5 A to 10 A, the output voltage would be maintained at exactly 5.0 V, unperturbed throughout the change. See Figure 1a.

The Real Voltage Source
Unfortunately, power supplies like the ideal one described above do not exist in real life. Real power supplies try to maintain a constant voltage on their outputs by employing a feedback loop that monitors the output voltage, compares that voltage to a reference, and continuously makes adjustments based on the difference. They also have to be designed to fit in a limited space, with limited input power, and limited ability to dissipate the inevitable heat generated internally. Consequently, real power supplies have limited current compliance, finite output impedance, and finite bandwidth. The effects of these attributes become apparent when drawing current from the power supply, whether that is a static current or dynamic current. For example, a 5.0 V output at no load with 10 milliohms of output impedance will drop to 4.9 V with a 10 A static load. The output voltage will continue to decrease as the current increases. See Figure 1b.

With dynamic loads, the non-ideal nature of the real power supply output becomes more evident. Consider the output voltage behavior shown in Figure 1b immediately following the load current changes. The voltage overshoots and undershoots of the real source are a result of its non-zero output impedance which is a function of frequency (Zo(f)) and is determined by the internal feedback loop used to maintain the output voltage.

Power Supply Output – Deviant Behavior?
When selecting a power supply to meet your needs, first make sure you know what output voltage deviations you can tolerate. Evaluate your needs with respect to both static and dynamic conditions. For example, some devices, such as cell phones, have a low voltage detection circuit built-in. Make sure you are aware of the voltage level at which this circuit takes effect and how long the voltage must be below that level for the circuit to trip. The power supply used for testing should be selected to maintain its output voltage to meet your needs under changing load current conditions, especially to avoid nuisance tripping of a low voltage detection circuit. The load regulation (or load effect) specification tells you how well the power supply will maintain its output voltage when subjected to static load changes. The transient response specification will tell you how long it will take for the output voltage to recover to within a voltage band around the output voltage following a current change. Power supplies with different performance levels have correspondingly different specifications as shown in the table.

Other Non-ideal Attributes to Consider
In addition to the output voltage response to static and dynamic load changes, real power supplies also exhibit many other non-ideal behaviors. Line regulation, output noise, and cross regulation for multiple output power supplies are some examples.

• Line regulation is a measure of how the output voltage responds statically to input line voltage changes. It is primarily caused by finite loop gain, with some secondary effects from internal bias supply line regulation effects.
• Output noise is usually specified in either peak-to-peak volts, or rms volts, or both, and within a specified bandwidth such as 20 Hz to 20 MHz. Output noise has many sources, including residual effects from rectification circuits, internal digital circuits, and even the op amps themselves that are used for output voltage regulation.
• On a multiple output power supply, cross regulation is a measure how one output voltage responds to load current changes on the other output(s).

Clearly, for all of these attributes, the lower the specified behavior, the more “ideal” the power supply. While it may be tempting to look for a power supply with the lowest specifications in all of these areas, it is always prudent to evaluate your true needs, and make your selection based on those needs. Since tradeoffs must often be made, knowing your requirements will always make the selection process easier by broadening the choices when compared to just looking for the best specifications in all areas.

Other, more subtle behavior of non-ideal power supply outputs can also be important, depending on your application:

• Overshoots at AC (or DC) input turn-on and turn-off should be considered.
• Output voltage behavior when the power supply enters or leaves a current limit condition (mode crossover overshoots) can sometimes cause problems.

These behaviors are often unspecified by the power supply manufacturer. However, relying on reputable power supply vendors helps avoid problems since these vendors frequently take steps during the design process to minimize these effects.

Wrap-up
Obviously, real power supplies don’t behave like ideal power supplies. Sometimes this non-ideal behavior makes a difference in your application, and sometimes it does not. When selecting a power supply, it is important for you to know your true requirements in order to make the selection process go as smoothly as possible and to avoid overspending. A power supply’s specifications outline its non-ideal behavior, so look for specifications that meet your needs. Also realize that there are unspecified performance issues that could be important in your application as well. If you don’t see the specification for which you have interest, ask your power supply vendor about parameters you feel are important in your application.

Use Remote Sense to Regulate Voltage at Your Load

Have you ever set your power supply output voltage to a particular value and found the voltage at your load was lower than you expected? If this was acceptable for your test, then you probably just left it alone. But if you wanted the voltage at your load to be equal to the voltage you set, then you should have used remote sensing.

Remote sensing is a feature on many power supplies that allows the power supply to regulate the voltage right at your load (“remotely”). This is accomplished by using a set of remote sense leads that are in addition to your load leads. The power supply uses the voltage on the remote sense lead terminals to sense the voltage right at the load terminals and regulate the voltage right at the load by adjusting the output terminal voltage.

Consider the example in Figure 1 showing a power supply set for 5 V, the desired voltage at the load. If the load is located six feet away from the output terminals, and you are using 14 AWG wire (about 2.5 mΩ/ft), each load lead will have about 0.015 Ω of resistance. If 10 A is flowing through the load leads, each load lead will drop about 0.15 V (10 A x 0.015 Ω) for a total drop of 0.3 V. When the power supply regulates its output voltage right at the output terminals, the result at the load is 4.7 V instead of the desired 5 V.



Figure 2 shows the same setup using remote sensing. The remote sense terminals are connected to the load at the points where you want the 5 V setting to be regulated. In this case, the power supply regulates 5 V at the load by adjusting its output voltage to 5.3 V to make up for the drops in the load leads. It does this by using the voltage across the sense leads as part of the feedback loop inside the power supply to adjust the voltage on the output terminals. The purpose of the power supply is to keep the sense lead voltage constant at the setting; the power supply changes the output terminal voltage based on the sense terminal voltage. The input impedance of the sense terminals is high enough to prevent any significant current flow into the sense terminals – this makes any voltage drop on the sense leads themselves negligible.

"Where's the CC button?"

I have worked at Agilent (was HP, of course) for more than 30 years now (since 1980) and I have spent the majority of that time involved with power products. One thing has been consistent over that period of time: we are asked about how to put a power supply in constant current (CC) mode. The question takes on many forms, one of which is "Where's the CC button?". Given that this is my first blog post, and given that this is an important fundmental concept about power supplies, I figured it was a good place to start this power blog. Well, the simple answer is: there is no CC button, but continue reading to find out how to "put" a power supply in CC mode....

There are two primary output operating modes for most power supplies: constant voltage (CV) mode and constant current (CC) mode. While you don't set the mode, you do set the output voltage setting and the output current setting. Then, the output operating mode is determined by what you connect to the output (the load).

The output operating mode is detemined by three things:

1. Output voltage setting (Vset)


2. Output current setting (Iset)


3. Load value (Rload)

If the load current is low enough such that the current that is drawn is LESS than the current setting, the power supply will operate in CV mode regulating the voltage at a constant value with the current determined by the load.

If the load current is high enough such that the load is trying to draw MORE current than the current setting, the power supply will limit the current at the current setting value and operate in CC mode regulating the current with the voltage determined by the load.

Consider a simple resistive load, Rload:
If Rload > Vset / Iset, the power supply will be operating in CV mode.
If Rload < Vset / Iset, the power supply will be operating in CC mode.

The two extreme examples of the above are with Rload open (near infinite ohms) and Rload shorted (near zero ohms). When a power supply output is open (Rload = infinite, a vertical line from the origin on the graph), it should be obvious that the output will be in CV mode with no current flowing. When a power supply output is shorted (Rload = zero, a horizontal line from the origin on the graph), it should be obvious that the output will be in CC mode with near zero voltage.

Note that Agilent power supplies typically show the dynamic operating mode on the front panel. If the power supply is unable to regulate either the voltage or the current, the indicator will show UNR (unregulated) since neither the voltage nor the current is being regulated. This condition is rare, but can happen sometimes if Rload = Vset/Iset, or if there is a problem with the internal circuitry.