Transitioning Voltage Regulator Design From Unidirectional To Bidirectional

An interesting article by Nazzareno Rossetti, published in How2Power Today, explores the transition from designing traditional unidirectional voltage regulators to bidirectional converters essential for modern energy management systems (EMSs). These systems optimize energy flow and storage among electric vehicles, photovoltaic arrays, home batteries, and the grid, requiring components like regulators and chargers to handle power in both directions seamlessly.

Most power electronics experience stems from unidirectional applications, such as buck converters supplying CPUs or mobile devices. Here, energy flows one way from a variable input to a regulated output powering passive loads. Transitioning to bidirectional operation, exemplified by the phase-shift dual active bridge (DAB) converter, represents a significant shift. Designers accustomed to unidirectional PWM buck control may feel unprepared for DAB complexities. Rossetti bridges this gap by demonstrating that bidirectionality is not entirely foreign—elements already exist in conventional designs.

Synchronous power stages reveal inherent bidirectionality. In a synchronously rectified buck converter (e.g., 12 V to 1.8 V), inductor current ripple at light load can dip negative, causing brief reverse energy flow from output to input. This “annoyance” disrupts regulation and efficiency, often requiring designers to disable synchronous rectification at low loads. Similarly, in PWM full-bridge motor drivers, current recirculates back to the input during off-time, risking capacitor overvoltage.

These examples show synchronous half-bridges naturally support bidirectional flow. Rossetti illustrates this by reconfiguring the same power train: placing feedback at the low-voltage side yields a buck converter (current from high to low), while placing it at the high-voltage side creates a boost (current from low to high). Thus, the topology can exchange energy between nodes bidirectionally for instance, allowing a depleted 1.8-V battery to draw from a 12-V battery or vice versa.

Conventional hard-switched converters suffer efficiency losses during switching transitions. Negative current in light-load scenarios enables zero-voltage switching (ZVS) by bootstrapping the switching node, but only with excessive ripple relative to DC load current, impractical for most uses. The solution lies in embracing reverse current fully via two-stage topologies that generate a zero-average, 50% duty-cycle square-wave current for soft switching, then rectify it to DC.

The phase-shift DAB exemplifies this approach. It features isolated full bridges on primary and secondary sides, connected via a high-frequency transformer. Phase-shift modulation between bridges controls power direction and magnitude at fixed frequency. Primary and secondary phase-shift chains, with proportional-integral compensation, regulate the designated output port. Auxiliary flyback converters and pulse transformers provide high-side gate drive and isolated feedback. Dead-time generation prevents shoot-through.

Simulations for grid-tied/off-grid storage applications (100 V to 5000 V DC link) show bipolar inductor current enabling consistent ZVS. Phase-shift control outperforms PWM by maintaining fixed frequency, simpler filtering, easier gate-drive design, and fewer diode conductions, yielding excellent full-load efficiency (though light-load performance degrades as ZVS weakens).

Rossetti concludes that emerging bidirectional markets challenge designers under tight deadlines. Yet, familiar tools like synchronous half-bridges form the foundation. A full bridge extends two half-bridges, and with imagination, unidirectional concepts evolve into mature bidirectional architectures like the DAB. This perspective eases the mindset shift from one-way to two-way energy control, helping engineers deliver innovative solutions for EMS applications.

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