By zhanqun1 
The main difference between a bidirectional AC/DC converter and a bidirectional DC/DC converter lies in their working principle and application scenarios.
1. Two-way AC/DC converter
Function: It uses two-way AC/DC converters for energy conversion between AC and DC, and the flow of power is bidirectional between AC and DC. It can convert alternating current into direct current and direct current into alternating current.
Applications: These converters are commonly used in solar energy systems, battery energy storage systems, electric vehicle (EV) charging/discharging systems, and grid connections. For example, the charging piles of electric vehicles use two-way AC/DC converters, which can convert the alternating current of the grid into the direct current required by the battery, and can also provide power to the grid when the battery is discharged (V2G, Vehicle-to-Grid).
Operating principle: In the bidirectional AC/DC converter, there is an AC/DC conversion part, like a rectifier, and a DC/AC conversion part, like an inverter. The converter can change the direction under different working modes to realize the energy flow in both ways.
2. Two-way DC/DC Converter:
Functions: The two-way DC/DC converters are used for energy conversion between DC and DC, which can be converted bidirectional between different voltage levels. It can not only achieve voltage rise and fall but also support the forward and reverse flow of energy.
Applications: These converters are often used in BMSs, electric vehicles, and energy storage systems to adjust either the match between battery voltages and device voltages or to achieve battery charging or discharging. These converters are used in charging and discharging processes for electric vehicle batteries.
Working principle: The bidirectional DC/DC converter will realize the energy transfer of two different DC voltages under the control of switching component cooperation, inductors, capacitors, and other devices. It can adjust voltage and power in a bidirectional manner between the DC supply and the load.
3.Main Difference Summary
Energy Conversion Direction
Two-way AC/DC transformer: can be converted to and from alternating current and direct current.
Two-way DC/DC converter: Only can be converted between direct current (DC), mainly regulating voltage and current.
Application Scenario:
Two-way AC/DC converter: used for energy exchange between AC and DC, such as an electric vehicle charging pile and power grid interface.
The two-way DC/DC converters are used in a DC system: such as in battery management, energy storage systems, and electric vehicle battery charging and discharging.
Functional Difference:
Two-way AC/DC converters can handle the conversion between alternating current and direct current, so more complicated circuit design is needed, such as rectification and inverter functions. In a two-way DC/DC converter, there is primarily a concern with the regulation of DC voltage, so the design is comparatively simple and mainly used for rise and fall of DC voltage, two-way flow of energy.
Two-way AC/DC converter practical application case
Energy storage system and grid interaction
In large-scale grid-level energy storage power stations, two-way AC/DC converters are very important. In the night, when consumption of power is low, the alternating current of the grid is rectified into direct current through the two-way AC/DC converter for storage in the battery energy storage system. In peak power consumption in the daytime, the DC in the system is inverted back into AC through its inverter and transmitted back to the power grid. In this respect, it plays the role of “peak cutting and valley filling” in effectively balancing the supply and demand of electric power.
Uninterruptible Power Supply System
UPS is an essential device for places such as data centers where continuity of power supply is highly required. One of the core components of UPS is the bidirectional AC/DC converter. When the mains is normal, it converts the mains alternating current to direct current to charge the battery inside the UPS. The DC power in the battery quickly transforms into AC by the converter when the mains happens to be faulty. Power will be supplied for these key loads, which are very important for ensuring the continuity of devices like servers and storage devices running within a data center, hence avoiding loss of data or even service interruption.
Electrical Vehicle Charging Pile and Grid Interaction
In the two-way flow of energy in some EV charging stations supporting V2G, a two-way AC/DC converter is used. When there is enough electric vehicle battery, the bidirectional AC/DC converter will convert the direct current in the vehicle battery into alternating current and feed it back to the power grid. While recharging is required in a vehicle, it has been designed to convert grid alternate current to direct current to charge the vehicle’s battery, allowing the participation of the electric vehicle even in auxiliary services like frequency modulation and peak regulation due to its capacity to behave as a mobile energy storage unit in the grid.
Microgrid systems are connected into a main grid.
In the microgrid systems containing distributed power sources like solar, wind, etc., bidirectional AC/DC converters are used to connect the microgrid to the main grid. When the power generated by the distributed power supply in the microgrid is excess, the excess AC is converted to DC by the bidirectional AC/DC converter and stored in the energy storage system, or the AC is directly transmitted to the main grid. When the microgrid cannot support enough power, some alternative current can also be accessed from the main grid to then be reconverted with the converter into the energy that will be supplied in the microgrid. So it allows exchanging energy in a flexible mode and can perform in stable operation among microgrids and a main grid.
Two-way DC/DC practical application case
Electric drive vehicle battery management system
In electric vehicles, two-way DC/DC converters are used to connect battery packs and motor drive systems with different voltage levels. When the vehicle is in an accelerating state, the high voltage direct current of the battery pack, after passing through the two-way DC/DC converter for voltage reduction, supplies the motor to drive the vehicle forward. When the vehicle is braking, the feedback current generated by the motor is stored back to the battery pack after boosting by the bidirectional DC/DC converter, so as to realize energy recovery and improve the energy utilization efficiency and driving range of the vehicle. Meanwhile, it can also be used to balance the voltage of different individual cells in the battery pack to extend the battery life.
DC voltage regulation in DC microgrids: Various DC power sources of different voltage values – like photovoltaic cells and fuel cells – might exist in a DC microgrid together with various DC loads. The two-way DC/DC converters can transform either the direct current of the DC power supplies of different levels into a voltage that may be fed to the DC load or transform the feed-backed energy generated by DC loads into voltage acceptable for accumulation in the DC power supplies. For instance, boosting low-voltage DC output from the photovoltaic cell and connecting it to a high-voltage DC microgrid bus, or bucking DC on the high voltage bus supplying to low voltage DC load.
New energy power generation system power optimization
In large-scale solar photovoltaic power generation systems, the output voltage and power of several photovoltaic strings will be different owing to factors such as illumination and temperature. Bidirectional DC/DC converters can connect different PV strings, adjust and match their output voltage and power, and achieve MPPT. By adjusting the operating mode of the converter, the output voltage of each series of PV is adjusted to the voltage corresponding to the maximum power point, and the power generation efficiency of the whole PV power generation system is improved. Meanwhile, in the context of integration between the energy storage system and the photovoltaic power generation system, the bidirectional DC/DC converter can also implement the energy bidirectional conversion, balance the power between the energy storage system and the photovoltaic power generation system.