Bidirectional DC conversion: The quest for the elusive “DC-DC power transformer.”

The physical phenomenon of induction — the presence of a dynamic magnetic field generating an electromotive force in an electrical conductor — is what allows a transformer to change the level of an ac voltage. It’s a simple device that can work in either direction; depending on which winding is used as the primary, the voltage can be stepped up or down.

It’s not so simple when it comes to changing DC voltage levels. Since direct currents create static magnetic fields, they can’t induce voltages in secondary windings. Thus, DC-to-DC converters require the input voltage to the primary of a transformer to be switched on and off to generate a dynamic magnetic field. Then, the induced secondary voltage is rectified. But, in this sequence power flows in one direction only — from a dedicated source to a load.

Some types of modern systems, however, require a device that allows bidirectional power flow, the equivalent of a “DC-DC power transformer:” In hybrid and electric vehicles bidirectional converters can charge a low voltage (12 V) battery during normal operation and reverse the current flow to charge the high-voltage (400 V/600 V) battery, if it needs an urgent boost. Bidirectional power processing is also important in energy storage systems. In such systems the “DC-DC power transformer” would provide current to charge a battery bank in the forward direction and then provide energy from the battery bank to hold up a bus voltage in the backward direction.

In fact, any application where there is energy storage and energy recovery — from an electronic load that can also function as power supply — can benefit from using bi-directional components.

One way to create bidirectional current flow is by using two “anti-parallel” converters. A more elegant, compact and energy efficient solution is to use “synchronous rectification;” turns out that most switching regulator topologies can be made to move power in either direction by replacing all diodes with low on-resistance MOSFET switches that are appropriately turned on and off under the control of a microcontroller.

In the pursuit of efficient energy management, engineers are well advised to learn how to use bidirectional products. This reference design for a bidirectional 400V-12V, DC-DC converter could be a good place to start. A phase shifted full-bridge (PSFB) with synchronous rectification controls power flow from a 400V bus/battery to the 12V battery in step-down mode, while a push-pull stage controls the reverse power flow from the low voltage battery to the high voltage bus/battery in boost mode. More story..