Which has less energy loss - AC step up/step down transformers or DC to DC step up/down converters?
I don't hear much of DC to DC converter modules, even though DC is widely used today, mostly in smaller electronics.
You might not have heard much about DC/DC converters because they are often built into the device. The vast majority of mains powered 'smaller electronics' today have switching AC/DC converters to produce low voltage DC from the AC mains. A switching AC/DC converter (also known as a switched-mode power supply or SMPS) consists of a mains voltage rectifier and filter, followed by a DC/DC converter. Many devices also have on-board DC/DC converters to further reduce voltage.
Mains frequency transformers are used less to power 'smaller electronics' today because they are more bulky and less efficient, especially once the output has been rectified, smoothed and regulated. Although the power loss in an individual device might be so low that the consumer doesn't consider the cost, there are so many of them that the total loss could be significant.
Transformers are used mostly to increase or decrease AC voltages for mains transmission and supply. Large transformers are more efficient than small ones, and cheaper and less complex than using a DC/DC intermediary when the the input and output are both AC. However at the other end of the scale, very long transmission lines and undersea cables have less loss using DC, even after DC/AC conversion at each end.
So in deciding which is more efficient you have to consider not just the component that changes the voltage, but what associated losses may occur in other parts of the system. In the case of 'smaller electronics' seemingly insignificant transformer losses could become a big problem when DC conversion losses and power factor are taken into account.
Efficiency Standards for External Power Supplies
In the early 90’s, it was estimated that there were more than one billion external power supplies active in the United States alone. The efficiency of these power supplies, mainly utilizing linear technology, could be as low as 50% and still draw power when the application was turned off or not even connected to the power supply (referred to as “no-load” condition). Experts calculated that without efforts to increase efficiencies and reduce “no-load” power consumption, external power supplies would account for around 30% of total energy consumption in less than 20 years.
It’s important to compare apples to apples.
If you’re talking about long-lines distribution, HVDC as a system is more efficient due to the absence of skin effect in the conductor: current flows in the entire cable making the I2R losses much lower. Also, dielectric losses are nullified since DC avoids the charging/discharging of AC, and the cable the can be sized without needing to allow for reactive power and RMS peak.
Taken together, these advantages make HVDC long lines about 30-40% more efficient than AC. This increased capacity and efficiency justifies the cost and losses of the HVDC inverters at each end.
More here: https://sinews.siam.org/Details-Page/direct-current-transmission-and-the-future-of-electricity
For medium and low voltage distribution, the simplicity and low cost of AC with transformers wins out. It’s hard to say if AC is actually more efficient than DC at this scale, given advances in conversion technologies since Edison and Tesla’s era. AC is however more convenient, especially for motors and some lighting systems. Even in this realm however, the push for 48V DC data center power would suggest that different approaches using DC-DC are more efficient.
More here: https://www.edn.com/design/power-management/4442710/2/48V-direct-conversion-dramatically-improves-data-center-energy-efficiency-
That said, even at smaller scale, for managing renewable power generation there is work going on with voltage-source conversion using local HVDC at tens to hundreds of kilovolts. This development, called ‘HVDC Light’, arose from using HVDC for offshore windmill plants where using undersea AC cabling was impractical.
More here: https://library.e.abb.com/public/b35718ff8f3fa4c0c1256fda004c8ca2/VSC%20TRANSMISSION%20TECHNOLOGIES.pdf
I suppose it mostly matters what you're coming from and what you're going to, i.e. AC/AC, AC/DC, DC/DC, or DC/AC. That said in a direct comparison between transformers vs. DC/DC converters, I'd wager transformers will have better efficiency. Obviously there's lots of use case considerations such as input and output voltage differential, but generally, it's difficult to get much better than 90% effeciency out of a DC/DC converter. Where according to this...
Most of the transformers have full load efficiency between 95% to 98.5%