FAQ: Offline AC-DC and DC-DC Power Supplies

Feb. 5, 2020

This article originally appeared in ElectronicDesign. It is reprinted with permission.    

Designing power supplies that work from wall outlets has multiple challenges. This FAQ should answer many of your questions.

What are offline voltages?

Worldwide ac mains voltages can range from 100 to 240 V. To allow for sags (brownouts) and surges, offline ac supplies intended for universal input are rated at 85 to 165 V. The ac frequency ranges from 50 Hz in most of the world to 60 Hz in the U.S. There are some 400-Hz systems, but those are mostly in aircraft, not wall power. Common industrial offline voltages are 208, 240, and 440 in the U.S., and 380, 400, and 415 V ac in Europe. Residential power tends to be single-phase, whereas industrial offline power is usually three-phase, with perhaps a neutral return.

The need to power LED lighting and other dc loads has resulted in dc line voltages in commercial and residential buildings, usually 24 V dc. These require a dc-dc converter, which is a type of switching voltage regulator. Power plants and industrial installations often use high-voltage dc, typically 120 to 330 V dc, since they can’t always depend on outside ac power. This dc voltage is frequently backed up with batteries.

What are the fundamental specs of an offline supply?

The input voltage range and the output voltage or voltages for a multiple-output supply are a basic spec. There’s a line regulation spec, relating the change in output voltage to the change in input voltage. There’s a load regulation spec that relates the change in voltage as the load increases. Another fundamental spec is the efficiency of the supply—the amount of power input related to the output power. This changes over load and gets worse at low and high loads.

The transient response of the supply is another important spec. It relates how quickly the supply responds to a sudden load increase. It also might specify the level of overshoot with a sudden removal of load. There may also be a spec that requires the supply to work at zero load. Many switchers need some load in order to properly regulate.

The ripple spec of an offline supply characterizes the small changes in the output voltage that are periodic, such as 60 Hz or at the switching frequency. The noise specification relates to unintended voltage or current excursions on the output. It’s often stated as an RMS voltage, in microvolts or millivolts, but is more useful when shown in a graph of noise spectrum over frequency. Amplifiers and other analog chips are much more sensitive to high-frequency noise, so knowing the frequency band where there’s power-supply noise will help you make a better design. This output noise isn’t necessarily related to radiated noise of the supply.

What is power factor correction (PFC)?

Some standards require your offline ac supply have power factor correction. PFC makes sure the input current to the power supply is a sinusoid that’s in phase with the input voltage. Without special circuitry, offline supplies will draw a spike of current when the applied line voltage reaches its positive and negative peaks.

What protection features might I design in?

Input-current inrush protection lowers the turn-on surge off current when you connect the supply to the source. PFC circuits can do this as part of their function. Such is the case with soft-start circuits, too. Output-current limiting protects the supply from shorts and surges. You can design the supply to just limit the current at a maximum as the voltage drops, or you can do foldback protection, where the current drops to a low value until you remove the load.

You should consider temperature protection. Therefore, if the supply gets too hot, it will limit current, or shut down. You might consider input overvoltage protection as well as undervoltage lockout (UVLO), where the supply will not work if the input voltage is too low.

What communication should the supply have?

Some supplies are just expected to work, with no communication features. At the least, you might want a “power good” logic signal that asserts when the supply is working. You can also design in various fault signal outputs. For full digital communication, there’s the PMBus open standard, which lets you control the supply and read back information from it. A lab supply might have IEEE 488, RS-232, or even Ethernet communications. There’s no doubt a Wi-Fi-controlled offline power supply is coming soon.

How do I increase efficiency?

The biggest efficiency improvement is going from a linear supply to a switcher. There’s still a place for linear supplies, as laboratory power supplies and for high-end audio power. Most offline supplies are now a switching type. In general, a larger inductor or transformer can provide better efficiency, as will thicker copper conductors, and higher-cost transistors, diodes, and capacitors. More complex architectures like zero-voltage and zero-current switching, as well as series resonant topologies can improve efficiency.

Whereas 85% efficiency was typical years ago, now efficiencies range well into 90% and higher, albeit not at all loads. This is why sizing the supply output to the load is important. All supplies need a quiescent current just to run the control circuits and switch the transistors. As the output current drops, the quiescent current becomes a greater part of the input current, and by definition, reduces the power-supply efficiency. If you can eliminate cooling fans, this will improve efficiency as well—fan power is a form of quiescent current.

Can I do remote sensing?

Many times, you want to control the power-supply voltage at the load, after it has traveled across long wires to get to the load. The long sense lines can cause problems if they extend to lengths that make the supply go unstable. Long sense lines will also receive ambient noise and inject that to the sense circuitry in the power supply, affecting the output voltage.

What standards apply to offline power supplies?

The original concern with electrical equipment was fire. As a result, Underwriters Laboratories (UL), which was started for fire insurance companies, specifies fire—and now also safety—standards such as creepage and clearance distances. The Canadian Standards Association (CSA) has similar standards, and the two organizations can cross-certify your product for both sets of standards to get you a UL and CSA marking on your supply. The European Community (EC) directives are a similar, but distinct, set of standards that will allow you to display a CE marking.

The FCC has standards for radio-wave emissions from supplies—both emissions over the air (radiated) and via the cord from the supply back into your wall (conducted). The EC directives also include these electromagnetic-compatibly (EMC) regulations, as well as immunity regulations, that require your design to withstand electromagnetic radiation from external devices without malfunction. Different standards exist for information, medical, and telecommunication products.

There are also different regulations for Class I, where the plug has a ground pin, and Class II, often called “double insulated,” where the power supply isn’t connected to earth ground. There’s also a limited power source (LPS) class with relaxed safely specs due to the limited nature of its power availability. The body of regulations are so complex that many designers turn to an outside listing company, such UL or TUV or the dozens of testing laboratories familiar with all of the worldwide standards, for a product application.

How do I increase the reliability of my supply?

One way is to remove the fan. The other low-reliability components are electrolytic and tantalum capacitors, and potentiometers. The poor reliability of power-supply electrolytic capacitors will swamp out the reliability of all other components, even in a large system. If you can’t get rid of them, then over-design them, putting in much higher voltage ratings than needed. Also, temperature makes the poor reliability even worse. If you can use low-ESR capacitors and reduce the ripple currents they have to carry, it will lower the self-heating of the capacitors.

Mean time between failure (MTBF) is a calculation done by adding up the statistical reliability of all the parts in your system. You can do this under a military standard, MIL-HDBK-217F Notice 2, or the Telcordia standard SR/TR-332 (Bellcore) that evolved for the telecommunications industry.

There are tables of component reliability that you use for each standard. You’re allowed improved reliability if your electrolytic capacitors run at a lower-than-rated voltage and at a lower temperature. Alternatively, certain lifecycle calculations estimate the life of electrolytic capacitors based on their temperature rise at maximum operating temperature.

What is multiple-output cross regulation?

Many multiple-output supplies only regulate one output voltage, such as 5 V, and utilize circuit design to make reasonably constant secondary voltages like ±15 V. This will mean that if you draw lots of current from the 5-V output, it will change the ±15-V outputs. If you need multiple precise voltages, you may have to use separate supplies, or post-regulate the secondary outputs with another linear or switching regulator. For very precise voltages, you can post-regulate with a voltage-reference IC.


To join the conversation, and become an exclusive member of Electronic Design, create an account today!