DC-DC Converter Topologies: Which is Best for Your System?By ZM Peterson • Apr 16, 2020
DC-DC conversion is a critical part of any new system, and you’ll need to choose the right regulator topology for use in your next design. The simplest form of DC-DC conversion simply involves linear regulation with a voltage divider, but this is an inefficient and elementary way to step down voltage in your system.
Real DC-DC converters are more sophisticated, and there are a number of ways you can easily regulate the output from your power supply, whether it’s as a standalone unit or an integrated circuit. There are many common DC-DC converter topologies, and each provides certain advantages in terms of easy adjustment and redesigns. These topologies are used at the integrated circuit level and in larger power systems that are built from discrete components. If you’re unsure which type of power regulator you need to use for your new design, it will depend on the availability of different ICs that can supply the power you need and how you intend to control your power output. If you need access to larger power outputs, you’ll need to create a regulator/converter from discrete components using one of the standard DC-DC converter topologies.
Standard DC Regulation Strategy
AC-DC and DC-DC conversion follow nearly the same strategy, with the only difference being the use of a rectifier stage and optional PFC circuit in AC-DC conversion. The most common DC-DC converter topologies are buck, boost, and buck-boost (flyback); see the section below for more information on these and other topologies. Another common converter is an LDO regulator, which is often paired on the output from another DC-DC converter. This is the case in high power and low power designs. This combination of regulators creates the following typical workflow for power regulation:
- Rectification stage (AC only). To create a DC output, you need to convert the AC input to an unregulated DC input with a full-bridge rectifier. An output capacitor
- 1st regulator stage. This generates a stable DC output with specified current. The current output will be limited and depends on the supply current and the components used for regulation. DC current regulator ICs are also available, and high-current regulators can be easily found by searching for "LED driver" on major search engines.
- Output regulator stage and filtering. Later regulator stages are normally used to set the final voltage value to a desired level. This is typically where an LDO comes into play as it provides fixed DC output as long as the input voltage does not drop below the LDO’s headroom threshold. Any switching noise from the regulator stages can be removed with filtering.
This DC-DC converter layout for an SMPS is intended for higher power output.
Note that switching regulators have become the standard components for use in various DC-DC converter topologies as they provide high efficiency output with some control over voltage. If the input voltage is expected to vary, a programmable controller IC or an MCU can be used to adjust the output voltage by changing the duty cycle of the PWM signal in the switching regulator. EMI filter circuits might be placed between any of the stages shown above. On the output, the standard method is to use a shunt capacitor or an LC filter to remove switching noise, which can propagate to a downstream circuit as conducted EMI.
For switching regulators that use PWM signals with fast edge rate, it’s common to include a power factor correction (PFC) circuit after the rectifier stage but before the input capacitor. This circuit smooths out the input current into the DC-DC converter, thereby increasing the overall efficiency of the regulator system. The output from the PFC circuit is then connected to a large capacitor to provide semi-regulated DC power to the regulator stage. The remaining stages follow the same process as that shown above.
Common DC-DC Converter Topologies
All DC-DC converters will be isolated or non-isolated. The idea in using an isolated DC-DC converter is to create a new ground potential on the output side of the system (e.g., such as in 3-phase AC-to-DC conversion). An isolated converter is also desirable in high voltage systems as they provide safety during service; the operator will be less likely to come into contact with a high voltage source when servicing the output side of the system.
The common non-isolated DC-DC converter topologies and their DC transfer functions are shown below.
Common non-isolated DC-DC converter topologies and their DC transfer functions
The topologies shown above are common and can be found in various ICs. ICs are available that will output up to ~50 W at high voltage or high current. Getting more power out of your regulator requires a design from discrete components. The same applies to the common isolated DC-DC converter topologies. As there is a large number of isolated DC-DC converter topologies, I’ll direct interested readers to smps.us for a long list of different types of isolated switching converters.
There are other functions that might appear in an integrated circuit or discrete circuit for power regulation. As the use of a PWM signal provides regulation and control, you may find error amplification and PWM regulation integrated into a single IC. An example block diagram for a constant current limiting circuit is shown in the accompanying circuit diagram. This is a common feature in DC-DC converter topologies on integrated circuits or in larger power supplies as it provides constant current limitation once the load resistance drops too low. The error amplifier in this circuit is used to ensure stable voltage output if the input voltage changes. This is the same functionality you’ll find in a typical LDO.
Current sense limiter in a DC-DC converter
At NWES, we’ve created low power and high power systems with different DC-DC converter topologies. We’ll also create a high quality, fully manufacturable PCB layout for your system. We're here to help electronics companies design modern PCBs and create cutting-edge technology. We've also partnered directly with EDA companies and several advanced PCB manufacturers, and we'll make sure your next layout is fully manufacturable at scale. Contact NWES for a consultation.