What are voltage modes?

While all programmable AC power sources offer AC output mode, some also provide DC output mode. This may be useful for applications where only DC is required to power an EUT.  However, a growing number of power related products are coming on the market than can operate from both AC grid power and direct DC power.  This often eliminates a AC to DC conversion stage which reduces losses and improves energy efficiency. Such EUTs must not only be testing in both AC mode and DC mode, they also must be qualified for their ability to transfer between AC and DC power input without any undesirable effect on their output. For that purpose, the programmable power source used must offer AC+DC output mode in addition to Mode and DC Mode.  Not all power sources can support this but the AFX Series from Pacific Power does.

Why Does it Matter?

Most AC power sources only offer AC output and those that offer either AC or DC output cannot be used as there is a considerable amount time the output is dropped when switching between AC and DC modes or vice versa.  An alternative used by design and test engineers was to construct a power transfer switch between two lab supplies, one AC source and one DC supply. Such a method can obtain better transfer characteristics but transfer switches can be failure prone and still exhibit a certain amount of cross over delay, also known as dead-time as the two supplies cannot be shorted together under any circumstance. See Figure 1.

Figure 1:  Electronic Transfer Switch Setup

Figure 1: Electronic Transfer Switch Setup

Power Source with native AC+DC Mode

A far better solution is to use an AC power source capable of AC+DC and DC+AC output modes.  This allows transfers between AC and DC input power the be accomplished with zero cross over time as no switching between output modes or two power supplies is required at all. A scope capture of the input power to the unit test in AC+DC mode operation is shown in Figure 2 below.

Figure 2: Scope Capture of alternating AC and DC input

Figure 2: Scope Capture of alternating AC and DC input

Programming Fast Transition Times

To ensure the transition between AC input and DC input occurs instantaneously, we have to program a small transient program that alternates between 230Vac and 325Vdc for our example. This can be done using the TRANSIENT screen of the AFX power source or if more convenient, using the built in web server and a browser or the user’s smartphone, tablet or PC. Figure 3 shows the Web interface transient setup.

Figure 3: Two step transient program

Figure 3: Two step transient program

By programming the transition from AC to DC mode to occur at the 90° point of the sine wave, we can cause the AC to DC transition to occur at the positive peak of the AC waveform, causing a seamless transition as shown in Figure 4.

Figure 4: AC to DC Transfer at 90°

Figure 4: AC to DC Transfer at 90°

Conclusion

The new AFX Series is one of the most versatile and compact programmable AC & DC power source on the market that should be part of every power design and test engineer’s tools kit.  To learn more, check out the full application note on “AC+DC Mode Power Converter Testing”.