DC/DC Converter Topologies – Switching Principles, Types, and Selection Criteria
What are a DC/DC converter topologies?
DC/DC converter topologies describes the circuit configuration of power components—transistors, inductors, capacitors, and, if applicable, transformers—that together enable a defined voltage conversion.
The topology determines how energy flows between the input and output circuits, the power range in which a converter operates at its ideal operating point, and whether and what type of galvanic isolation is implemented. However, the topology does not include the control architecture, housing, or cooling solution—these parameters are defined independently of the circuit configuration. Choosing the right topology is therefore the most fundamental decision in the design of any DC/DC converter.
What determines the topology of a DC/DC converter?
The topology of a DC/DC converter is determined by the following design parameters:
- Number and arrangement of switching elements (one to four transistors, synchronous or asymmetric)
- Energy storage: inductance alone (non-isolated) or a transformer with inductance (galvanically isolated)
- Galvanic isolation: circuits with or without a common ground
- Energy flow direction: unidirectional (input → output) or bidirectional (both directions)
Each of these parameters narrows down the range of possible topologies. The combination of voltage level, power range, isolation requirements, and energy flow direction typically leads unambiguously to a preferred topology.
How do DC/DC converters differ from linear voltage regulators?
Linear voltage regulators convert excess voltage directly into heat—their power dissipation is equal to the product of the voltage difference and the load current. DC/DC converters, on the other hand, switch power semiconductors at high frequencies and store energy in inductors or capacitors, enabling efficiencies typically ranging from 85 to 97%. When there are large voltage differences between the input and output, linear regulators are subjected to significant thermal stress. A switching regulator performing the same task dissipates only the unavoidable conversion loss. Depending on the power class, the difference in thermal load—and thus in system size—is a factor of 5 to 10.
What are the different DC/DC converter topologies?
DC/DC converters are divided into two basic topology groups: non-isolated and isolated topologies. The following overview shows all nine relevant circuit principles along with their key characteristics
| Topology | Isolation | Bidirectional | Typical power range |
|---|---|---|---|
| Buck Converter | No | No | 1W – 10kW |
| Boost Converter | No | No | 1W – 10kW |
| Buck-Boost Converter | No | No | 1W – 150W |
| Bidirect. Buck/Boost | No | Yes | 100W – 10kW |
| Flyback Converter | Yes | No | 1W – 150W |
| Forward Converter | Yes | No | 50W – 500W |
| Full-Bridge Converter | Yes | possible | >1kW |
| Dual Active Bridge (DAB) | Yes | Yes | >1kW |
| LLC / CLLC Resonance transducer | Yes | CLLC: Yes | >100W |
Non-galvanically Isolated DC/DC Converter Topologies
Non-isolated DC/DC converters connect the input and output circuits to a common electrical potential—the input and output grounds are directly connected. This switching principle enables compact designs with few components and high efficiency in low-voltage applications. The most important topologies in this group include buck converters, boost converters, bidirectional buck/boost converters, and buck-boost converters.
What is a Buck Converter (Step Down Converter)?
A buck converter, also known as a step-down converter, is a non-isolated DC/DC converter that reduces the input voltage to a lower output voltage. The switching principle is based on a power switch connected in series with the load path, a storage inductor, and a free-wheeling diode: During the on-phase of the transistor, current flows through the inductor to the load; during the off-phase, the inductor releases stored energy via the free-wheeling diode.
Typical efficiencies range between 85% and 96%. The buck converter is suitable for applications in which the output voltage is always lower than the input voltage—such as when powering control units or sensors from a DC bus. In the DDL series, Querom implements this topology in the form of the DDL3050-48. Learn more about the technical parameters in the DDL3050-48 datasheet.
What is a Boost Converter (Step-Up Converter)?
A boost converter, also known as a step-up converter, is a non-isolated DC/DC converter that converts a lower input voltage into a higher output voltage. The switching principle uses a storage inductor connected in parallel with the input: During the transistor’s on phase, the inductor charges with input current; when the transistor turns off, the inductor’s back-EMF superimposes on the input voltage and drives current through a diode to the higher output voltage. A typical application for a boost converter is stepping up the voltage of a supercapacitor module to a higher DC bus.
What is a Buck-Boost Converter?
A buck-boost converter generates an output voltage that can be either higher or lower than the input voltage. One possible application is connecting a fuel cell—which typically has a wide voltage range—to a battery. While the high open-circuit voltage is stepped down, the lower full-load voltage is stepped up.
What is a bidirectional DC/DC converter?
A bidirectional DC/DC converter transfers electrical energy in both directions between the input and output circuits: In charging mode, it operates as a buck converter; in discharging mode, as a boost converter. This characteristic is the fundamental prerequisite for energy recovery—the process of feeding kinetic energy back into an energy storage device during braking.
With the DDL series, we develop and manufacture a product family of bidirectional, non-isolated buck/boost converters: the bidirectional DDL2148-24, DDL4848-48, and DDL5096-96, each with a power rating of 5 kW, as well as the unidirectional DDL3050-48 with a power rating of 3 kW. Typical applications include mobile systems such as AGVs or the coupling of different vehicle electrical system voltages.
How does bidirectional differ from unidirectional?
Unidirectional DC/DC converters transfer energy exclusively from the input to the output—the direction of energy flow is fixed. Bidirectional converters also allow energy to be transferred in the opposite direction, which is what makes energy recovery and active battery management possible in the first place. In AGV systems and BESS applications, bidirectional topology is therefore not an option but a system requirement for maximum energy efficiency.
Galvanically Isolated DC/DC Converter Topologies
Galvanically isolated DC/DC converters electrically isolate the input and output circuits from each other via a transformer—the input and output grounds are electrically isolated. According to IEC 62368-1, this isolation is required for all systems without protection against accidental contact [5.1] [5.2] with voltages above safety extra-low voltage (SELV: >60 V DC), as well as when coupling high-voltage and low-voltage networks. The most important galvanically isolated topologies are flyback, forward, full-bridge, DAB, and LLC resonant converters.
What is a Flyback Converter?
A flyback converter is an electrically isolated topology in which the transformer serves both as an energy storage device and for electrical isolation. During the transistor’s on phase, the transformer core stores magnetic energy; during the off phase, the core releases the energy to the load via the secondary side. The flyback converter is particularly suitable for power ratings up to approximately 150 W and, thanks to multiple secondary windings, easily provides several galvanically isolated output voltages.
What is a Forward Converter?
A forward converter is a galvanically isolated topology that transfers energy directly from the primary side to the secondary side during the transistor’s on-phase. Unlike a flyback converter, the transformer serves solely for galvanic isolation and voltage conversion, not as an energy storage device—the output inductor handles the energy smoothing. The forward converter thus operates with lower core losses and is suitable for power ranges between 100 W and 500 W, particularly at low output voltage-to-current ratios.
What is a Full-Bridge / Push-Pull Converter?
A full-bridge converter uses four power switches in an H-bridge configuration to drive the transformer with an AC voltage, thereby transmitting high power with galvanic isolation. The four transistors switch in pairs, which ensures that the transformer core is driven symmetrically and no DC component is generated. Full-bridge converters are the preferred topology for power levels above 1 kW in industrial systems. Push-pull converters follow a similar principle but use a center-tapped primary winding with two switches. The full-bridge topology also forms the basis for the Dual Active Bridge (DAB) converter.
What is a Dual Active Bridge (DAB) Converter?
A Dual Active Bridge (DAB) converter consists of two active full bridges—one on the primary side and one on the secondary side of the transformer—and enables galvanically isolated, bidirectional power flow. Power transfer and direction are controlled by the phase shift between the primary- and secondary-side bridges: A positive phase angle drives power from the primary to the secondary side, while a negative phase angle drives it in the opposite direction. In BESS applications with high voltages ranging from 800 to 1,500 V, the DAB converter handles the bidirectional coupling between the DC bus and the battery module, enabling black start capability, inrush current limiting, and droop control.
What are LLC und CLLC Resonance transducer?
LLC resonant converters are galvanically isolated DC/DC topologies that use a resonant circuit consisting of series inductance (L), magnetizing inductance (L), and series capacitance (C) to minimize switching losses through zero-voltage switching (ZVS) and zero-current switching (ZCS). Unlike hard-switching topologies, the transistors in an LLC converter switch under nearly lossless conditions, enabling efficiencies of over 97% in the rated load range. CLLC topologies extend the resonant principle to bidirectional operation by incorporating a mirror-symmetric secondary side.
What topologies do Querom DC/DC converters use?
Querom DC/DC converters are available in two topology classes: the DDL series with a non-isolated bidirectional buck/boost topology and the DDH series with electrically isolated high-voltage converters. Both classes are designed for industrial DC power networks and are developed and manufactured entirely in Germany.
DDL Series: Non-Isolated Bidirectional Buck/Boost Topology
The Querom DDL series uses a non-isolated bidirectional buck/boost topology: In charge mode, the converter operates as a buck converter (input → output); in discharge mode, it operates as a boost converter (output → input). The association of Charge = Buck Mode and Discharge = Boost Mode is documented in the DDL4848-48-C (V1.2) operating manual for this converter; it specifically describes the DDL4848-48 and is not a general characteristic of the topology—it may be implemented differently in other converters with the same topology.
The DDL series is available in four product variants:
- DDL3050-48: 40–60 VDC, 60 A, 3 kW (unidirectional)
- DDL2148-24: 30–58 VDC, 100 A, 5 kW (bidirectional)
- DDL4848-48: 30–58 VDC, 100 A, 5 kW (bidirectional)
- DDL5096-96: 30–120 VDC, 50 A, 5 kW (bidirectional)
All DDL models communicate via CAN bus, except for the DDL3050-48, which uses Modbus. Parallel operation is possible and, with symmetrical load distribution, requires all converters to communicate with a higher-leveld, in the case of symmetrical load distribution, requires all converters to communicate with a higher-level control unit. All converters achieve an efficiency of approximately 97%.
DDH Series: Galvanically Isolated High-Voltage Topologies
The Querom DDH series comprises galvanically isolated DC/DC converters for high-voltage applications with input voltages up to 900 VDC. The DDH3009-24 is the current model in the series: It converts input voltages up to 900 VDC to a 24 V output voltage with galvanic isolation. The DDH topology is designed for applications in which high-voltage DC buses must be safely isolated from low-voltage control levels. Technical data sheets and inquiry forms are available directly from our product team.
In which applications are which topologies used?
The choice of DC/DC converter topology depends largely on the application. Different application areas have specific requirements regarding galvanic isolation, energy flow direction, and power range, which directly influence the choice of topology. The following sections describe which topologies are used in the areas of BESS, AGVs, and DC grids.
DC/DC Converter Topologies in AGVs and Mobile Robotics
Automated guided vehicles (AGVs) are mobile robotic platforms for automated intralogistics. These systems use non-isolated bidirectional buck/boost converters for traction and benefit directly from the energy recovery capabilities of this topology. In AGV applications, energy recovery during braking extends operating time by up to 30%. In addition to providing traction power, the bidirectional converter supplies peripheral voltages for control, sensors, and communication (24V, 12V, 5V, and 3.3V). In standby mode, Sleep Mode reduces quiescent current and conserves battery power. Learn more about AGV applications on our AGV applications page.
DC/DC Converter Topologies for DC Grids and producers
DC grids are direct current distribution networks in which multiple generators, storage systems, and consumers are connected via a common DC bus. The voltage of 650 VDC defined by the ODCA (Open DC Alliance) serves as a reference here, offering measurable efficiency advantages over AC grids.
Modern photovoltaic modules integrate MPPT tracking directly into the module and thus operate at significantly lower voltages. In fuel cells, the voltage is typically reduced because the high open-circuit voltage delivers little power at the operating point, and a converter’s efficiency is optimal when the voltage difference between the input and output remains small. A typical boost application in a DC grid, on the other hand, is stepping up the voltage of a supercapacitor module to the DC bus. The maximum efficiency of a DC/DC converter is typically achieved at 60–80% of the rated load—a factor that system designs must take into account during sizing.
DC/DC Converter Topologies in BESS
Battery Energy Storage Systems (BESS) store electrical energy in batteries and feed it back into the DC grid as needed. BESS systems require bidirectional DC/DC converters with galvanic isolation, since battery voltages and the DC bus are in the range of 800–1,500 V, and potential leakage must be prevented for safety reasons. In high-voltage BESS configurations, the DAB topology is becoming the standard: it combines high power density, galvanic isolation, and bidirectional operation with phase angle control. For reliable grid operation, black start capability, inrush current limiting, and droop control are essential functions of the converter.
FAQ – DC/DC Converter
Efficiency, power range, electrical isolation, bidirectional capability, and complexity are the key factors in selecting the right DC/DC converter topology. The following comparison matrix summarizes all nine topologies based on these criteria:
| Topology | η-range | Power | Isolation | Bidirectional | Complexity |
| Buck Converter | 85 – 96% | 1W – 10kW | No | No | Low |
| Boost Converter | 85 – 96% | 1W – 10kW | No | No | Low |
| Bidi. Buck/Boost | 90 – 97% | 100W – 10kW | No | Yes | Medium |
| Buck-Boost (Invers) | 80 – 92% | 1W – 150W | No | No | Low |
| Flyback / Sperrwandler | 80 – 90% | 1W – 150W | Yes | No | Medium |
| Forward / Flusswandler | 85 – 93% | 50W – 500W | Yes | No | Medium |
| Full-Bridge | 90 – 96% | >1kW | Yes | Possible | High |
| DAB | 92 – 97% | >1kW | Yes | Yes | High |
| LLC / CLLC | 93 – 97%+ | >100W | Yes | CLLC: Yes | High |
Galvanic isolation is required in DC/DC converters in the following situations:
- Touch voltages above the SELV limit (60 V DC according to IEC 62368-1): If people may have direct access to circuit components, isolation is required.
- Coupling of different voltage levels: In high-voltage/low-voltage interfaces (e.g., 400 V DC bus to 24 V sensors), galvanic isolation prevents potential leakage.
- System protection during fault conditions: In redundant systems and in ground fault scenarios, isolation protects against uncontrolled fault currents.
- Normative requirements: IEC 62368-1 (successor to IEC 60950-1) defines mandatory isolation voltage levels; for systems with hazardous energy levels, basic insulation is the minimum requirement.
The choice of topology follows a systematic decision-making process:
- Specify the input voltage range (Vmin / Vtyp / Vmax)
- Define output voltage requirements (nominal voltage, current, ripple tolerance)
- Check for galvanic isolation: Are there potential differences or normative requirements according to IEC 62368-1? Determine the direction of energy flow: Is unidirectional (input → output) or bidirectional required? Classify the power class: <150W (flyback, buck), 150W – 1kW (forward, boost, buck), >1kW (full-bridge, DAB, LLC)
- Efficiency priority: Resonant topologies (LLC/CLLC) for maximum efficiency; non-isolated topologies for compact low-voltage designs
Do you have specific requirements for your DC/DC converter? Tell us about your project—we’ll analyze the best topology for your application. You can find an overview of our available converters on the DC/DC Converter overview page.
LLC resonant converters achieve the highest efficiency among galvanically isolated topologies—over 97% can be achieved in the rated load range through zero-voltage switching (ZVS). Non-isolated bidirectional buck/boost converters achieve comparable values: The DDL5096-96 from Querom achieves 97% efficiency. The maximum efficiency of DC/DC converters generally ranges from 60% to 80% at rated load—it drops slightly at partial and full load.
Yes, DC/DC converters with different topologies can be operated in parallel on a common DC bus, provided that equal output voltages and coordinated load sharing are ensured. Without load-sharing control, the converter with the lowest output voltage would not supply any current, while the other would be overloaded. For regulated parallel operation in a DC grid, droop control has become the standard: Each converter reduces its output voltage in proportion to the current it is supplying, which supports load-proportional distribution; however, precisely symmetrical load sharing requires all converters to communicate with a higher-level control unit.
Yes, non-isolated and galvanically isolated DC/DC converters are commonly combined in industrial system architectures. A typical configuration: A galvanically isolated converter connects the high-voltage DC bus to a 48V low-voltage segment; a non-isolated bidirectional converter handles power management within this segment between the traction battery and the loads. This requires well-thought-out potential planning and a coordinated protection concept for both topology stages.