Toshiba Demos New Switching Topology for Non-Isolated DC-DC Converters

The new topology eliminates the need for a transformer and significantly reduces the number of capacitors in DC-DC converter ICs.

Toshiba recently announced a new non-isolated DC-DC converter technology that operates from 48 V to 1 V. These devices address conduction losses associated with mounting server and data center demands, which increases load currents in DC-DC converters. The result of these higher load currents and conduction losses is heat and lower overall efficiency.

To mitigate these losses, industry standards have raised the input voltage from 12 V to 48 V. This reduces the current for a given power level, thus lowering conduction losses. However, this shift also introduces new challenges for DC-DC converter designs, particularly with the buck topology.

Toshiba develops a host of non-isolated buck DC-DC converter power supplies. Image (modified) used courtesy of Toshiba

Toshiba claims its new star-delta switching topology reaches the industry’s highest current densities while negating the need for a transformer for DC-DC converter ICs with 48-V input and 1-V output.

Toshiba Leverages Star-Delta Switching Topology

At the 2024 IEEE Symposium on VLSI Technology & Circuits, Toshiba demonstrated its new 48 V to 1 V non-isolated DC-DC converter technology. The test device achieved current densities of up to 790 mA/mm² and a high power conversion efficiency of up to 88%.

The proposed star-delta switching network. Image used courtesy of Toshiba

According to Toshiba, its star-delta switching topology eliminates the need for transformers, which are typically used in isolated topologies to manage pulse-width expansion. Instead, Toshiba uses a hybrid configuration of inductors and capacitors, judiciously controlled by FETs, to significantly reduce the volume and number of external components. Toshiba claims that its star-delta topology reduces the capacitor count per pulse-width expansion ratio from 0.5 to 0.6, as opposed to 0.8 to 1.0 in traditional non-isolated hybrid topologies. 

Toshiba demonstrated the effectiveness of this topology with a suite of test chips. The company developed a bootstrap circuit that cut layout area by up to 61% and a level shifter circuit that supports an active bias current scheme, reducing bias current by up to 92%.  

Eliminating the Bulk of Transformers and Capacitors

In a buck converter, the pulse width driving the power switch must be four times shorter than 12 V to increase the input voltage to 48 V. This reduced pulse width increases switching losses as the transitions between on and off states become more frequent and less efficient. These switching losses then directly degrade the overall power conversion efficiency of the system.

Designers often use transformers in isolated topologies to address these efficiency issues. While transformers expand pulse width and prevent switching losses, they also add significant bulk to a design, which is problematic in applications where space is constrained.

Non-isolated multiport converter. Image used courtesy of MDPI

Non-isolated hybrid topologies are a compact alternative. These designs use a combination of inductors and capacitors to manage pulse width expansion without bulky transformers. Compared to transformer-based solutions, this approach can reduce the overall volume of the converter by a factor of 10 to 100. Despite these space-saving advantages, hybrid topologies introduce their own challenges.

One significant drawback is the requirement for a large number of capacitors—typically 0.8 to 1.0 capacitors per pulse-width expansion ratio. This increased capacitor count leads to higher external component density and congestion in pin wiring, complicating the PCB layout and increasing mounting costs. The additional capacitors and complex wiring elevate manufacturing costs and pose challenges to system reliability and maintenance.