AC to DC conversion is one of the fundamental tasks of electrical engineering. I suspect that the classic AC to DC circuit—transformer, bridge rectifier, capacitor, and maybe a Zener diode or linear regulator—was, for many of us, an important introduction to the world of electrical design and analysis. There is nothing particularly thrilling about converting AC voltage to DC voltage, but it is an inescapable necessity in a world that distributes almost all of its electrical power by means of alternating current.
It is possible to generate a DC supply voltage from an AC source without the use of a transformer. However, the absence of a transformer makes these circuits rather dangerous. My great desire to avoid electrical shock induces me to prefer circuits that step down the AC voltage as soon as it enters a board. It’s true that simple transformerless power supplies for low-current applications are both smaller and cheaper than transformer-based counterparts. But for me, safety takes precedence over size and cost.
Improving AC to DC Conversion
For those who wish to avoid transformerless power supplies, the topology described above still forms the basis of an AC to DC converter. However, various beneficial features can be incorporated, and we can make the circuit more efficient by delivering rectified and smoothed voltage to a switching regulator instead of a Zener diode or a linear regulator.
The following block diagram, originally published by National Semiconductor, gives you an idea of what you might find in a modern AC to DC converter. This circuit accepts a 24 VAC input (the initial step-down transformer is not shown), and it can provide 100 mA of current at 3.3 V. It is built around the LM3481, a controller IC for DC/DC converters.
For more information (including the complete schematic) and photos of an actual PCB implementation, take a look at this app note.
The next block diagram, from Microchip, shows that an AC to DC converter can be quite complex. This design actually incorporates two digital signal controllers (a digital signal controller is an advanced microcontroller that doesn’t quite qualify for the “digital signal processor” category). As you can see, the AC input is sent directly to a bridge rectifier—there’s no step-down transformer to reduce the voltage. I don’t know about you, but I don’t like to experiment with circuits that generate 420 VDC.
If you want to take a deep dive into the topic of switching-regulator-based AC to DC conversion, this document from Microchip may be helpful.
The PSK-S3 and PSK-S5B Modules
These new AC to DC converters from CUI’s Power Group have six output-voltage options. The PSK-S3 devices can deliver 3 W of power, and the PSK-S5 series delivers 5 W. As is so often the case these days, the performance and features offered by these off-the-shelf modules make it very difficult to justify the effort and risk involved in designing a custom power supply.
For one thing, I trust a well-known manufacturer more than myself when it comes to safety and reliability. These converters have an isolation rating of 4 kV, they operate down to –40°C, and they provide overvoltage and short-circuit protection.
The MIL‑HDBK‑217F reliability analysis indicates that the modules have a minimum mean time between failures (MTBF) of 300,000 hours at room temperature. That’s 34 years. Maybe some industrial applications would actually require 34 years of continuous use, but if we’re talking about consumer electronics, my guess is that the equipment will be in the landfill long before one of these converters experiences a failure.
I wouldn’t mind having a general idea about what’s actually inside these encapsulated converter modules. The datasheet doesn’t say anything about the internal architecture. Even a basic block diagram would be appreciated.
However, the datasheet does have helpful application circuits. The basic implementation, shown in Figure 1 on page 7, recommends that the module be complemented by a fuse, a metal oxide varistor, two output capacitors, and a transient voltage suppressor diode. This figure is even accompanied by a table that provides specific recommendations for the external components.
Taken from the PSK-S3 datasheet.
The application circuit is a bit more elaborate if you’re concerned about EMC. The component labeled “LCM” in the input filter is a common-mode inductor.
Taken from the PSK-S3 datasheet.
Do you have any thoughts on these new AC to DC converters? Have you ever designed a circuit that remained functional for 34 years? Let us know in the comments section below.