Board Mountable Converters for AC/DC And DC/DC Applications

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Modern power distribution often requires a sophisticated combination of isolated and non-isolated AC/DC and DC/DC converters. Isolated converters are primarily required for system and user protection in the event of single or multiple faults.

Today, power supplies have to fulfill a wide range of requirements at the same time. Accordingly, both isolated and non-isolated AC/DC and DC/DC converters are usually used. While non-isolated solutions are primarily compact and low-loss, isolated converters perform safety-relevant tasks.

In principle, you can develop your own converter for low to medium power (up to around 1,000 W) using advanced ICs, with different architectures offering different performance benefits. However, the reality of developing and validating these converters is different. At a minimum, they must meet a number of basic functional and performance requirements, including output voltage and current, efficiency, transient response, size and protection from line, load and supply faults.

The development challenges are not limited to the basics. There are a number of regulations relating to safety, efficiency at different load levels, shutdown power, thermal performance, emissions and susceptibility to electromagnetic interference (EMI), among others. These properties must be verified by a certified test laboratory, which significantly extends the development schedule. The simple DIY option quickly becomes very risky, and you will soon find that the decision to buy or not to buy is heavily in favor of buying.

If you are not convinced, remember that the converter must also have galvanic isolation. While this is a common requirement for almost all AC/DC converters, it is also required for some DC/DC converters. This need leads to new regulations, safety and certification requirements that further shift the decision between manufacturing and buying in favor of buying, regardless of the size of the offering.

The good news is that board-mountable isolated and non-isolated current transformers are available in a wide range of voltage and current ratings. These simplify the development and deployment of products for defense, communications, test and measurement (isolated) and mobile robotics (non-isolated) applications and can be used in combination for power distribution.

As components that can be implemented immediately, they can be placed on the main board in a location optimized for the power distribution rails. Furthermore, they do not require discrete brackets. In short, they provide the power supply function as a closed, complete and ready-to-use solution.

Insulation Basics

Galvanic isolation is an electrical barrier that prevents a conductive ("resistive") path from forming between two sides of a signal or power path. However, this isolation must still allow the passage of energy and power using other transmission methods. Depending on the design, isolation may be required for signals, power or both. The techniques used to implement the isolation depend on the specifics of the current flow to be isolated.

Isolation may be required for a variety of reasons. For signals, it can improve sensor integrity, eliminate ground loops or protect users and circuits in the event of faults that allow current to flow into the signal paths.

For power supplies, it is primarily required to ensure user safety and to prevent electric shock from accidental contact with AC power lines or high voltage DC. It also supports the need for "floating" circuits (no connection to circuit ground) used for non-power supply signals.

In general, insulation is a method of directing the flow of current according to Kirchhoff's current law (KCL). For a current to flow, there must be a return path to the source, and the purpose of insulation is to interrupt this path. In a possible electric shock scenario, the entire fault current path through the user and back to earth (Figure 1, left) is interrupted by the isolating transformer in the power source (Figure 1, right).

A common scenario for a potential electric shock is when a live ("hot") cable comes into direct contact with the metal housing of the appliance through frayed insulation. Even if the appliance is still working as intended, the user may suffer an electric shock if the earth connection is broken (which is often the case) and the fault current flows through the user to earth instead of safely via the earth cable.

To counter this risk, the isolation function of the power supply unit interrupts the current path between the original voltage source and the appliance. This prevents a circuit from being created between them so that there is no risk of electric shock despite the wiring error.

It should be noted that hazardous voltages include both AC mains voltage and comparable DC voltages, such as those from multi-cell batteries. Most legal standards define dangerous voltages as those above around 60 V, depending on the situation and type of voltage.

Power isolation is almost always achieved by electromagnetic coupling via a transformer. Electromagnetic coupling is electrically efficient, technically effective, very flexible, extremely reliable and can be easily adapted to legal and circuit requirements.

The Reasons for So Many Good Decisions

There is no shortage of architectural options when planning power distribution systems, as modern systems use many busbars. However, choosing the right power converter can be difficult if parts of a circuit need to be insulated while other parts either do not need insulation or must not be insulated.

There are cases where an AC/DC or DC/DC rail for the high voltage supply does not need to be isolated, but isolation is required further down the power distribution chain. Designers must decide, among other things, whether to use a single larger isolated power supply or several smaller ones, and whether to use an isolated power supply only where necessary and a non-isolated power supply elsewhere (Figure 2).

To meet these requirements, TDK-Lambda offers a wide range of board-mountable isolated and non-isolated AC/DC and DC/DC power converters for numerous input/output voltages and currents (step-down and step-up converters). Examples include:

Isolated AC/DC conversion: The PFE500F-28/T is a 28 V/18 A converter with an output for 85 to 265 V AC (V_AC). It features 3,000 VAC input/output isolation in a 4.8 × 2.8 × 0.5 inches 1-brick module for use in environments where convection or forced air cooling is not possible.

Non-isolated AC/DC conversion: The PF1500B-360 enclosed module also has a 1-brick size and converts the AC input to a regulated 360 V DC output (V_DC) for use in distributed power systems that use isolated DC/DC high-voltage converters or for loads that require a high-voltage source. It has a rated power of 1,512 W at 170 to 265 V_AC input and 1,008 W at 85 to 265 V_AC. The module has a power factor of 0.98 and an efficiency of up to 96.5 %.

Isolated DC/DC converter: The isolated DC/DC converter GQA2W024A050V-007-R delivers 120 W in a compact and powerful 1/4 brick housing with an isolation of up to 3000 V_DC between input and output. It operates over an input range of 9 to 36 V and delivers 5 V at 24 A. Its mechanical housing is available in multiple baseplate, case and potted configurations (Figure 3) and supports convection and conduction cooling via an external cold plate or heat sink.

Non-isolated DC/DC converter: The non-isolated PoL DC/DC converter I6A24014A033V-003-R is well suited for generating high current output voltage rails from a 12 or 24 V_DC power supply. It has an input range of 9 to 40 V, delivers up to 14 A and offers a wide output setting range of 3.3 to 24 V in a compact 1/16 brick package.

Conclusion

TDK Lambda offers isolated and non-isolated board mountable converters for a variety of topologies and input and output voltage specifications. The result is an optimized line of high-performance, off-the-shelf converters suitable for a wide range of power configurations. (mr)

Rolf Horn is an application engineer at DigiKey.

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