IP Library › Granted Patent US 12,633,816
Granted Patent B1
US 12,633,816 · App. 19/409,698 · Granted May 19, 2026

Cascaded DC modular solid-state transformer

Inventor: M A Awal (Poway, CA)
Assignee: EPC POWER CORPORATION
H02M1/007H02M1/0095H02M1/083H02M3/01H02M3/33584H02M3/3378H02M1/44H02M3/33523
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Quick Facts
Patent No.
US 12,633,816
App. No.
19/409,698
Granted
May 19, 2026
Kind
B1
Abstract

A solid-state transformer system comprises a plurality of isolated DC-DC converter modules, each module having a medium voltage port and a low voltage port. The medium voltage ports are series-connected in each phase of a single-phase or three-phase configuration to form a medium voltage AC interface, and each module comprises two quadrant switches with no active front end. The system includes a controller configured to generate voltage references for each module, the voltage references comprising a DC common-mode component and an AC differential-mode component, and optionally an AC common-mode component, wherein the DC common-mode component ensures unidirectional voltage operation across the medium voltage port of each module. The low voltage ports are interconnected to form a low voltage DC interface. The isolated DC-DC converter modules utilize conversion topologies including frequency modulated resonant converters, phase-shift modulated converters, and hybrid conversion topologies, operating at frequencies between 20 kHz and 100 kHz to achieve soft switching operation over wide operating ranges for improved efficiency and simplified manufacturability.

Claims (55)

1 . A solid-state transformer system, comprising:

a plurality of isolated DC-DC converter modules, each module having a medium voltage port and a low voltage port, wherein the medium voltage ports of the modules are series-connected in each phase of a three-phase Y-configuration to form a medium voltage AC interface; and

a controller configured to generate voltage references for medium voltage port of each module, the voltage references comprising a DC common-mode component and an AC differential-mode component, and optionally an AC common-mode component, wherein the DC common-mode component ensures unipolar voltage operation across the medium voltage port of each module; and

wherein each isolated DC-DC converter module comprises two quadrant switches on the low voltage and the medium voltage ports.

2 . The solid-state transformer system of claim 1 , wherein the low voltage ports of the modules are interconnected to form a low voltage DC interface.

3 . The solid-state transformer system of claim 1 , wherein the low voltage ports of the modules are series or parallel connected in each phase of a three phase Y-configuration to form a low voltage AC interface; and

a controller configured to generate voltage references for low voltage port of each module, the voltage references comprising a DC common-mode component and an AC differential-mode component, and optionally an AC common-mode component, wherein the DC common-mode component ensures unipolar voltage operation across the low voltage port of each module; and

wherein three-phase four-wire connection is used on either or both medium voltage and low voltage AC interfaces or a separate isolated or non-isolated DC-DC stage is used on either of the AC interfaces to maintain a common mode DC voltage on both medium voltage and low voltage sides.

4 . The solid-state transformer system of claim 1 , wherein the low voltage ports of the modules are series or parallel connected to form an upper arm and a lower arm in each of the three phases;

a low voltage DC interface formed by connecting positive terminals of uppermost modules of the upper arms and negative terminals of lowermost modules of the lower arms;

a low voltage AC interface formed at junctions between the upper arm modules and lower arm modules of each phase;

a controller configured to generate voltage references for a low voltage side of the modules, wherein the voltage references comprise a DC common-mode component and an AC differential-mode component, and optionally an AC common-mode component, wherein the DC common-mode component is determined by a desired voltage across the low voltage DC interface and ensures unipolar voltage operation across the low voltage port of each module; and

the controller generates AC differential-mode voltage references with opposite polarity for the upper arm modules and the lower arm modules.

5 . A solid-state transformer system, comprising:

a plurality of isolated DC-DC converter modules arranged as upper arm modules and lower arm modules in each phase of a three-phase configuration, wherein each module has a medium voltage port and a low voltage port;

a medium voltage DC interface formed by connecting positive terminals of uppermost modules and negative terminals of lowermost modules;

a medium voltage AC interface formed at junctions between the upper arm modules and lower arm modules of each phase;

a controller configured to generate voltage references for a medium voltage side of the modules, wherein the voltage references comprise a DC common-mode component and an AC differential-mode component, and optionally an AC common-mode component, wherein the DC common-mode component is determined by a desired voltage across the medium voltage DC interface and ensures unipolar voltage operation across the medium voltage port of each module; and

the controller generates AC differential-mode voltage references with opposite polarity for the upper arm modules and the lower arm modules.

6 . The solid-state transformer system of claim 5 , wherein a low voltage DC interface formed by interconnecting the low voltage ports of the modules.

7 . The solid-state transformer system of claim 5 , wherein the low voltage ports of the modules are series or parallel connected to form an upper arm and a lower arm in each of the three phases;

a low voltage DC interface formed by connecting positive terminals of uppermost modules of the upper arms and negative terminals of lowermost modules of the lower arms;

a low voltage AC interface formed at junctions between the upper arm modules and lower arm modules of each phase;

a controller configured to generate voltage references for the low voltage side of the modules, wherein the voltage references comprise a DC common-mode component and an AC differential-mode component, and optionally an AC common-mode component, wherein the DC common-mode component is determined by a desired voltage across the low voltage DC interface and ensures unipolar voltage operation across the low voltage port of each module; and

the controller generates AC differential-mode voltage references with opposite polarity for the upper arm modules and the lower arm modules.

8 . The solid-state transformer system of claim 6 , using integrated three-phase DC-DC modules, wherein an integrated three-phase DC-DC module uses three separate transformer cores and low voltage side windings are delta-connected to provide one or more low voltage ports.

9 . The solid-state transformer system of claim 6 , using magnetically coupled three-phase DC-DC modules, wherein a magnetically coupled three-phase DC-DC module uses a single transformer core with at least three windings on the medium voltage side and one or more windings on the low voltage side.

10 . The solid-state transformer system of claim 6 , wherein each isolated DC-DC converter module utilizes a conversion topology selected from the group consisting of:

pulse-width modulated push-pull converters;

frequency modulated resonant converters;

phase-shift modulated converters comprising dual active bridge converters and multi-active bridge converters; and

hybrid conversion topologies comprising quasi-resonant dual active bridge or multi-active bridge converters.

11 . The solid-state transformer system of claim 10 , wherein the isolated DC-DC converter modules achieve soft switching such as zero voltage switching (ZVS) or zero current switching (ZCS) or both for medium voltage side switches or low voltage switches or both based on power flow direction.

12 . The solid-state transformer system of claim 10 , wherein the controller generates an AC common-mode component using third-harmonic injection to maximize bus voltage utilization.

13 . The solid-state transformer system of claim 10 , wherein the AC common-mode component does not contribute to power flow at the medium voltage AC interface.

14 . The solid-state transformer system of claim 1 , wherein the modules comprise magnetically coupled three-phase modules having a core configuration with three legs arranged in a common magnetic structure fabricated from ferrite material or nanocrystalline magnetic material.

15 . The solid-state transformer system of claim 13 , wherein magnetically coupled three-phase modules provide constant power flow over a transformer core by combining double-line frequency oscillating single-phase power from three grid phases.

16 . The solid-state transformer system of claim 1 , wherein each module includes two low voltage ports with independent transformer secondary windings.

17 . The solid-state transformer system of claim 1 , wherein the low voltage ports are connected in series to increase total output voltage.

18 . The solid-state transformer system of claim 1 , wherein the low voltage ports are connected in parallel to increase total output current capacity.

19 . A method of controlling a cascaded modular solid-state transformer, the method comprising:

generating voltage references for medium voltage buses by combining a DC common-mode voltage component, an AC common-mode voltage component, and an AC differential-mode voltage component;

regulating the DC common-mode voltage component to ensure unidirectional voltage operation across medium voltage ports while connected to a three-phase AC source; and

controlling power flow between a medium voltage AC interface and a low voltage DC interface.

20 . The method of claim 19 , wherein the AC common-mode voltage component is generated using third-harmonic injection to maximize bus voltage utilization.

21 . The method of claim 20 , wherein the AC common-mode voltage component does not affect power flow at the medium voltage AC interface.

22 . The method of claim 19 , wherein controlling power flow comprises regulating only one module in each phase to produce time-varying voltage while other modules produce constant medium voltage port voltage to optimize component losses.

23 . The method of claim 19 , further comprising:

pre-charging medium voltage DC bus capacitors in each module through controlled current injection from the low voltage DC interface;

limiting pre-charging current to 10% to 20% of nominal module current rating; and

proceeding sequentially from lowest voltage module to highest voltage module.

24 . The method of claim 19 , further comprising:

monitoring overcurrent conditions with thresholds set at 110% to 150% of nominal current ratings;

monitoring overvoltage conditions with thresholds set at 110% to 120% of nominal DC bus voltage; and

monitoring overtemperature conditions with thresholds set at 125° C. to 150° C. for silicon devices.

Assignments (3)
SECURITY INTEREST Recorded Jan 5, 2026
From: EPC POWER CORP.
To: CITIBANK, N.A., AS AGENT
Reel/Frame 073367/0569 →
SECURITY INTEREST Recorded Dec 29, 2025
From: EPC POWER CORP.
To: EPIC ADMINISTRATION LLC
Reel/Frame 073329/0916 →
ASSIGNMENT OF ASSIGNOR'S INTEREST Recorded Dec 4, 2025
From: AWAL, M A
To: EPC POWER CORPORATION
Reel/Frame 073121/0478 →
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