DC/DC converters can come in isolated and non-isolated varieties, but isolated converters are preferred in certain situations. Issues with safety, EMC compliance, and the power levels involved can dictate the need for an isolated DC/DC converter. These can be available in module format, such as a brick form factor, or you can design your own from discrete components.
Whether you buy off the shelf or design your own DC/DC converter, how will you know whether to use an isolated or non-isolated regulator circuit? We'll run over some common applications that use an isolated system and the motivation for this. There are also some challenges with isolated DC/DC converters, some of which relate to the construction of magnetic components and their parasitics.
Reasons to Use Isolated DC/DC Converters
An isolated DC/DC converter takes advantage of magnetic induction to transfer power between two systems that are not connected with a conductor. This separation of conductors from an input side and output side (or primary and secondary sides of a transformer) provides some important benefits and is necessary to comply with multiple industry/regulatory standards. Briefly, the benefits of an isolated DC/DC converter include:
- Galvanic isolation, which shields a user from high voltage
- Large step-down voltages, such as line input voltages to low DC voltages
- High output current capability through the use of a step-down transformer
- Typically high efficiency over a broad input voltage range
- Excellent stability against transients or surges
- Easy to integrate circuit protection
These points reveal several areas where an isolated DC/DC converter is necessary. Some of the applications of these converters are specified in certain industry standards for safety and efficiency reasons. Hardware that draws power from an AC line is typically recommended to be built with an isolated DC/DC converter on the main power input to protect the user from the unregulated AC input voltage.
With these points in consideration, we see isolated DC/DC converters as the primary input power regulator type on telecom equipment, rack-mount or desktop computers, commercial or industrial equipment, power systems for lighting, and some embedded systems. Embedded systems are one area where multiple regulators may be needed, not all of them being isolated.
Custom Circuit or Off-the-Shelf Module?
Isolated DC/DC converter circuits come in two varieties: as off-the-shelf modules or they are built as custom circuits. Off-the-shelf regulators are helpful in many situations where a team wants to cut down their design time. Typically, these are in brick format with standard voltage outputs (12V, 24V, 48V, etc.). They tend to have high current output capability and they can come in isolated or non-isolated versions. Some of these regulators offer a wide input range, so they are in an isolated buck-boost topology.
Are there potentially problems with using an off-the-shelf module? Are they a drop-in solution or is additional design work needed? The answer varies and depends on how the DC/DC converter module was designed. For example, some brick format modules require additional filtering to remove EMI on the input and output. The EMI filter circuit design may not be documented, so the design engineer will have some additional work to do creating the filter circuit and testing it. Sometimes, these filter circuits are documented thoroughly and can be used as specified directly from the datasheet. An example is shown below.
In this example DC/DC converter module schematic, the input filter section uses standard LC filtering to address differential mode noise and a common-mode choke to suppress common-mode noise. The isolated ground regions are bridged with a Y-type capacitor to provide a return path for higher-order harmonics in the switching waveform and suppress radiated EMI at high frequencies. The manufacturer has specified the filter components in this example, as well as the output capacitance required for stability, but this will not always be the case with off-the-shelf isolated DC/DC converter modules.
Before starting your next project and contracting with a PCB design firm, make sure you closely consider these points as they will affect your BOM costs and your overall design approach. In some cases, the wrong isolated DC/DC converter module decision can blow up your form factor, and you will have no choice but to use a custom solution.
Whether you're designing high-speed PCBs for mil-aero embedded systems or a complex RF product, you should work with a design and development firm that can ensure your product will be reliable and manufacturable at scale. NWES helps aerospace OEMs, defense primes, and private companies in multiple industries design modern PCBs and create cutting-edge embedded technology, including power systems for high reliability applications and precision control systems. We've also partnered directly with EDA companies and advanced ITAR-compliant PCB manufacturers, and we'll make sure your design is fully manufacturable at scale. Contact NWES for a consultation.