On DC Source Voltage and Current Levels and (Compliance) Limits Part 2: When levels and limits are not the same

In part 1 my colleague made a good argument for current and voltage level and limit settings actually being one and the same thing and it was really just a case of semantics whether your power supply was operating in constant voltage or in constant current mode. I disagreed and I was not ready to admit defeat on this yet. Now is my chance to explain why I believe they’re not one and the same thing.

I have been doing quite a bit of work with source measure units (SMUs) that support multi quadrant output operation. They in fact feature (constant) voltage sourcing and current sourcing modes of operation. This tailors the operation of the SMU for operating as a voltage source with a set current compliance range or conversely as a current source with a set voltage compliance range. Right at the start one difference is the set up conditions. The output voltage or current level is set to zero while the corresponding current or voltage limit is set to some value, often maximum, so that the DC source accordingly starts out in either constant voltage or constant current for normal operating conditions.

Some products feature a programmable or fixed power limits. In one product I know of, the programmable power limit acts accordingly to override and cut back the either the voltage limit when set for current sourcing, or the current limit when set for voltage sourcing. It does not do this in true real-time however. It cuts back the limit based on the level setting, as a convenient means as to help prevent the user from accidently over-powering the DUT. Alternately many auto-ranging output DC power sources exist that provide an extended range of output and voltage for a given output power capacity. They incorporate a fixed power limit to protect the power supply itself from being inadvertently overloaded, as shown in Figure 1. Usually the idea is for the user to stay below the limit, not operate in power limit. The point here on these examples is that the power parameter is an example of being a limit but not really a level.

Figure 1: Auto-ranging DC power supply power limit

More to the point is some SMUs may incorporate two limits to provide a bounded compliance range with specified positive and negative limits. Not all DUTs are passive, non-reactive devices. As one illustrative example a DUT may be the output of 2-quadrant DC voltage source which you want to force up or down, within limits, or a battery you want to charge and discharge at a fixed rate, with your test system DC source. This set up is illustrated in Figure 2.

Figure 2: Test system DC source driving the output of a DUT source

Figure 3 shows the constant voltage or voltage priority output characteristic for one particular SMU having two programmable current limits. Clearly both limits cannot also be the current level setting as you can only have one level setting. For the case of the external voltage source load line #1 (not all load lines are resistances!), when SMU voltage is less than the DUT source voltage (VEXT1 load line), the current is –ILIM. Conversely when SMU voltage is greater than the DUT source voltage (VEXT2 load line), the current is then +ILIM. In the case of the battery as a DUT this can be used to charge and discharge the battery to specified voltage levels. This desired behavior is achieved using voltage priority operation. Current priority operation would yield very different results. Understanding the nuances of voltage priority, current priority, levels, and limits is useful for getting more utility from your DC sources for more unusual and challenging power test challenges.

Figure 3: Example of a current priority output characteristic driving a DUT voltage source

In closing I’ll concur with my colleague, in many test situations using most DC sources the voltage and current levels and limits may not have a meaningful difference. However, in many more complex cases, especially when dealing with active DUTs and using more capable DC sources and SMUs, there is a clear need for voltage and current level and limit controls that are clearly differentiated and not one and the same! What do you believe?

Six of seven new Agilent power supplies are autorangers, but what is an autoranger, anyway?

In this blog, I avoid writing posts that are heavily product focused since my intention is generally to provide education and interesting information about power products instead of simply promoting our products. However, when we (Agilent) come out with new power products, I think it is appropriate for me to announce them here. So I will tell you about the latest products announced last week, but I also can’t resist writing about some technical aspect related to these products, so I chose to write about autorangers. But first…..a word from our sponsor….

From last week’s press release, Agilent Technologies “introduced seven high-power modules for its popular N6700 modular power system. The new modules expand the ability of test-system integrators and R&D engineers to deliver multiple channels of high power (up to 500 watts) to devices under test.” Here is a link to the entire press release:

https://www.agilent.com/about/newsroom/presrel/2012/17jan-em12002.html

I honestly think these new power modules are really great additions to the family of N6700 power products we continue to build upon. We have several mainframes in which these power modules can be installed and now offer 34 different power modules that address applications in R&D and in integrated test systems. Oooooppps, I slipped into product promotion mode there for just a short time, but it was because I really believe in this family of products….I hope you will forgive me!

OK, now on to the more fun stuff! Since six of these seven new power modules are autorangers, let’s explore what an autoranger is. Agilent has been designing and selling autorangers since the 1970s (we were Hewlett-Packard back then) starting with the HP 6002A. To understand what an autoranger is, it will be useful to start with an understanding of what a power supply output characteristic is.

Power supply output characteristic
A power supply output characteristic shows the borders of an area containing all valid voltage and current combinations for that particular output. Any voltage-current combination that is inside the output characteristic is a valid operating point for that power supply.

There are three main types of power supply output characteristics: rectangular, multiple-range, and autoranging. The rectangular output characteristic is the most common.

Rectangular output characteristic
When shown on a voltage-current graph, it should be no surprise that a rectangular output characteristic is shaped like a rectangle. See Figure 1. Maximum power is produced at a single point coincident with the maximum voltage and maximum current values. For example, a 20 V, 5 A, 100 W power supply has a rectangular output characteristic. The voltage can be set to any value from 0 to 20 V, and the current can be set to any value from 0 to 5 A. Since 20 V x 5 A = 100 W, there is a singular maximum power point that occurs at the maximum voltage and current settings.

Multiple-range output characteristic
When shown on a voltage-current graph, a multiple-range output characteristic looks like several overlapping rectangular output characteristics. Consequently, its maximum power point occurs at multiple voltage-current combinations. Figure 2 shows an example of a multiple-range output characteristic with two ranges also known as a dual-range output characteristic. A power supply with this type of output characteristic has extended output range capabilities when compared to a power supply with a rectangular output characteristic; it can cover more voltage-current combinations without the additional expense, size, and weight of a power supply of higher power. So, even though you can set voltages up to Vmax and currents up to Imax, the combination Vmax/Imax is not a valid operating point. That point is beyond the power capability of the power supply and it is outside the operating characteristic.

Autoranging output characteristic
When shown on a voltage-current graph, an autoranging output characteristic looks like an infinite number of overlapping rectangular output characteristics. A constant power curve (V = P / I = K / I, a hyperbola) connects Pmax occurring at (I1, Vmax) with Pmax occurring at (Imax, V1). See Figure 3.

An autoranger is a power supply that has an autoranging output characteristic. While an autoranger can produce voltage Vmax and current Imax, it cannot produce them at the same time. For example, one of the new power supplies just released by Agilent is the N6755A with maximum ratings of 20 V, 50 A, 500 W. You can tell it does not have a rectangular output characteristic since Vmax x Imax (= 1000 W) is not equal to Pmax (500 W). So you can’t get 20 V and 50 A out at the same time. You can’t tell just from the ratings if the output characteristic is multiple-range or autoranging, but a quick look at the documentation reveals that the N6755A is an autoranger. Figure 4 shows its output characteristic.

Autoranger application advantages
For applications that require a large range of output voltages and currents without a corresponding increase in power, an autoranger is a great choice. Here are some example applications where using an autorangers provides an advantage:
• The device under test (DUT) requires a wide range of input voltages and currents, all at roughly the same power level. For example, at maximum power out, a DC/DC converter with a nominal input voltage of 24 V consumes a relatively constant power even though its input voltage can vary from 14 V to 40 V. During testing, this wide range of input voltages creates a correspondingly wide range of input currents even though the power is not changing much.
• There are a variety of different DUTs of similar power consumption, but different voltage and current requirements. Again, different DC/DC converters in the same power family can have nominal input voltages of 12 V, 24 V, or 48 V, resulting in input voltages as low as 9 V (requires a large current), and as high as 72 V (requires a small current). The large voltage and current are both needed, but not at the same time.
• A known change is coming for the DC input requirements without a corresponding change in input power. For example, the input voltage on automotive accessories could be changing from 12 V nominal to 42 V nominal, but the input power requirements will not necessarily change.
• Extra margin on input voltage and current is needed, especially if future test changes are anticipated, but the details are not presently known.