During the last couple of decades of technology evolution, we’ve seen how electrical and electronic design platforms have product cycles shortened to three to five years. Measurement instruments need to catch up to fast-moving innovation and technology advancements, or they will quickly become redundant.
Mobile device chargers have evolved from an output of 5 V to variable voltage outputs of 5 V, 9 V, or even 12 V for fast charging. For high-power application, electrification of vehicles no longer uses 12 V or 42 V. Voltages now range from tens of volts to power air-conditioning systems and other car electronics to a few hundred volts to drive the powertrain. These demands require a power supply that is equipped with multiple output ranges.
In this blog, we are going to discuss the types of power supply output ranges available in the market and why they are important. Let’s start the discussion by understanding a power supply characteristic.
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 auto ranging. The rectangular output characteristic is the most common.
Rectangular output characteristic
It’s not surprising to see a rectangle shape power supply output characteristic on a voltage-current graph (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.
Figure 1. Rectangular output characteristic.
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.
Autorange 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, Keysight N6755A has 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 autoranger 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.
We have learned that an auto ranging power supply has many great advantages over single range and dual range power supplies if you plan to use your power supply in a variety of DUT testing. Aside from saving space and the cost of using multiple units, it also provides future proof to your test system if your DUT design changes again. For more information on tips that help your power testing, download the 10 Practical Tips to Help Your Power Testing and Analysis application note.