Multi-Level Topologies for AC-DC Power Conversion

🔌 The state-of-the-art power system in the data centre utilises 400 V AC connected to the MV grid via a low-frequency transformer (LFT) and distributed power factor correction (PFC) rectifiers at the rack level, achieving an overall efficiency of approximately 97.1% from MVAC input to the rack-level 400 V/48 V DC-DC conversion. Increasing the AC distribution voltage to 690 V may enhance the overall efficiency to about 97.8 % due to reduced distribution losses, as losses in identical busbars decrease with the square of the voltage. PFC rectifiers suitable for 690 V AC can be employed with three-level topologies, maintaining high conversion efficiency. Alternatively, an 800 V DC (±400 V DC) distribution system can result in slightly lower distribution losses than the 690 V AC system. Additionally, there are other advantages to DC, such as the straightforward and efficient integration of battery energy systems.   💡 In principle, three conceptual approaches to MVAC-LVDC conversion can be considered. The first involves retaining the LFT and centralising the PFC rectifier functionality with a high-power SiC unit. This approach achieves an MVAC-LVDC conversion efficiency of approximately 98.2 % and an overall efficiency of around 97.9 %, with an estimated power density of about 0.25 kW/dm³. The second option employs robust 12-pulse rectifier systems complemented by active filters (AFs) to achieve power factor correction, forming a hybrid transformer. This partial-power-processing technique enables a high MVAC-LVDC conversion efficiency of approximately 98.5 % and an overall efficiency of about 98.2 %, with a power density estimated at 0.22 kW/dm³. Finally, solid-state transformers (SSTs) with medium-frequency transformers (MFTs) represent a fully controllable option. Current MVAC-LVDC SST prototypes have demonstrated full-load efficiencies of around 98 %, possibly reaching 98.5%, resulting in an overall efficiency of approximately 97.7 % or 98.2%. However, the power density of the overall SST system based on modular topologies tends to be comparatively lower than that of the hybrid transformer solution, despite the very high power density of the modules. #solidstate #powerelectronics #datacenters #lowvoltage #directcurrent #efficiency #powerdensity

Data centers are rapidly becoming a major driver for DC power distribution and DC microgrids. Hyperscale facilities consume 20 to 100MW each, wit most of that power ultimately delivered at ~1V at the point-of-load for xPUs and memory. To improve efficiency and to reduce distribution cost, designers push voltage levels upward, with the conversion to 1V as close to the silicon as possible. Hereby, the power conversion system architecture is of crucial importance! Across the industry, facility-scale DC distribution is converging on either +/-400VDC (Google, Meta, Microsoft) or 800VDC (NVidia). In a future architectures, these DC buses will likely be fed from the medium voltage AC grid via solid-state transformers (SSTs). Today, the power delivery from 800V to 1V is envisioned to move from 800 V → 48 V → 12 V → 1 V. A rack-level conversion from 800V to 48V, is followed by a tray- or GPU card-level conversion from 48V to 12V. The final conversion from 12V to 1V happens on the GPU card, as close to the silicon as possible. Exact voltages may vary a bit. This structure evolved from traditional AC-fed architectures. However, it has two big drawbacks: it still requires substantial copper at rack- and tray-level and it has multiple conversion stages. Both add loss and cost. Skipping a stage—for example, jumping from 800 V directly to ~12 V—sounds attractive, but creates challenges for converter semiconductors and magnetics. A multi-module series architecture may be more promising! On the high-voltage side, modules connect in series, naturally dividing the input bus (e.g., 800 V into ~100 V segments). Each module converts directly to 12V, a much more favorable design point for both semiconductors and magnetics. These modules can be integrated directly on the GPU board, minimizing the amount of copper needed to transport power within a rack. A series architecture taps into low-voltage power devices which are more more efficient and more reliable than high voltage ones. Moreover, power converter transformer ratios are less extreme which simplifies magnetics. On the flip side, a series architecture requires a more complex communication and control. But embedded digital control, and high-speed communication are becoming inexpensive, making the control challenge solvable. Power system design is ultimately about managing the “conservation of misery”. Design challenges remain, but you can choose where the burden sits. The arrival of smart, all-digital power modules unlocks new possibilities to redistribute that burden more intelligently. #DC, #800V, #microgrids, #datacenters, #nvidia

For decades, data center racks have been powered by three-phase AC, the same kind of power that runs industrial machinery. NVIDIA's next AI rack uses 800-volt DC instead, the same standard NVIDIA itself credits to the electric vehicle and solar industries. The change re-architects how electricity moves from the grid to the GPU, and three things at the silicon level had to happen first. 1️⃣ Doubling the voltage halves the current for the same power delivered, which cuts resistive losses in copper by roughly 75 percent. NVIDIA's design partners estimate copper thickness can drop by up to 45 percent across the rack. A 1MW rack needs up to 200kg of copper busbars at the legacy 54-volt distribution standard. A single 1GW data center built that way would need up to 200,000kg of copper for rack busbars alone, per NVIDIA's own technical blog. 2️⃣ The power semiconductors needed to convert 800 VDC efficiently at the rack level matured only in the last few years. Silicon carbide handles the high-voltage front-end conversion from medium-voltage utility AC down to 800 VDC. Gallium nitride handles the high-frequency stepdown from 800V to intermediate buses (50V, 12V, or 6V) feeding the GPU. Both are wide-bandgap technologies. SiC scaled because of the EV industry, where Porsche, Hyundai, Kia, and BYD have built the 800V powertrain supply chain over the last five years. GaN scaled on the back of consumer fast-charging and is now being adapted for AI infrastructure. 3️⃣ AC-to-DC conversion moves out of the IT rack entirely. In a current GB200 NVL72 or GB300 NVL72, up to eight power shelves sit inside the rack converting AC to DC. At MW scale on the same standard, those shelves would consume up to 64U of rack space, leaving no room for compute. In the 800 VDC architecture, conversion happens once at a dedicated power shelf upstream, and the IT rack receives DC directly. NVIDIA estimates this delivers up to 5 percent end-to-end efficiency improvement and up to 70 percent reduction in PSU-related maintenance costs. Power semiconductor content per rack grows substantially across this transition. SiC and GaN suppliers, high-voltage busbar and connector vendors, and rack-level DC-DC converter makers gain share. The legacy low-voltage and multi-stage AC conversion stack loses share. Most colocation facilities today operate at 10 to 30 kW per rack. The 600kW Rubin Ultra rack is more than an order of magnitude above that. The math operators are working on for 2027 deployments is whether their existing PDUs, busways, and rack power shelves are even compatible with the new spec.

Modular Multilevel Converter via EtherCAT with phase-shifted PWM The Modular Multilevel Converter (MMC) demonstrator with EtherCAT real-time communication, implemented for the purpose of investigating MMC topology suitability for medium-voltage wind turbine applications. The project focuses specifically on the EtherCAT communication architecture, synchronization method, and phase-shifted PWM modulation technique. 🔧 Decentralized submodule control on a standard industrial bus - TwinCAT on an Embedded-PC as EtherCAT Master - EtherCAT IP Core integrated into the FPGA of each submodule ⚡ Real-time performance the converter actually needs - 156 µs minimum cycle time, 5 kHz sampling rate - 20 ns jitter measured across all 18 submodules via Distributed Clocks - Phase-shifted PWM with 1 kHz, automatic capacitor voltage balancing The decentralized control follows the same modular and scalable approach as the power hardware. Doubling the submodule count scales the topology to 6 kV without redesigning the control platform. The same EtherCAT bus that runs the turbine controller runs the converter, which means seamless integration instead of a second isolated control domain. The authors' conclusion, after three years of measurements: "EtherCAT meets the requirements for a direct use in PE converter systems." PC-based Control plus EtherCAT is not just a fieldbus story. For power electronics developers, it is a way to put MMC, grid-forming inverters, and HVDC submodule control on one open, standardized, real-time platform. Research project at the Hochschule Flensburg by, Jan-Henrik Fey, Frank Hinrichsen, Prof. Dr. Regine Mallwitz. Links to the publications in the comments.

Rethinking High Voltage: Is "Stacking" the Future of Power Density? In the semiconductor sales world, the conventional wisdom for high-voltage applications (EVs, Data Centers, Solar) has usually been straightforward: match the device rating to the bus voltage. You have an 800V bus? You reach for a 1200V SiC or GaN switch. But lately, I’ve been tracking a fascinating shift in how power architects are approaching density. Instead of relying on single high-voltage switches, we are seeing more designs utilizing multilevel topologies with low-voltage GaN (100V-200V). The Commercial Logic: Even though it seems counterintuitive to use low-voltage parts for high-power systems, the math on the "performance per dollar" is compelling. Low-voltage GaN is incredibly efficient. By "stacking" these devices, designers can utilize faster switching speeds to drastically shrink the magnetics and passives—often the bulkiest and most expensive parts of the BOM. The Trade-off: As with everything in this industry, there is no free lunch. This approach increases component count and control complexity compared to a standard 2-level solution. The Question: We are at a crossroads between Simplicity (SiC/HV GaN) and Ultimate Density (LV GaN Multilevel). At what point does the gain in power density justify the added complexity of a multilevel design? Are we seeing this trend move from the lab to production in certain sectors yet? (Image below: A great example of the complexity trade-off—a 3-Level Flying Capacitor topology. Note the logic required just for the synchronous bootstrap and level shifting compared to a standard 2-level drive. The density gains are huge, but the control is definitely not trivial.) Source: EPC "GaN Power Devices and Applications - First Edition" #GalliumNitride #SiliconCarbide #PowerElectronic #AutomotiveTechnology #EV #Engineer #AI #DataCenter #Solar #RenewableEnergy

HVDC to LVDC: New Converter Topologies Promise Efficiency Gains for Green Hydrogen 🟦 1) AC or DC? The electricity grid and most devices primarily use Alternating Current (AC). However, water electrolysis, which splits water into hydrogen and oxygen, requires Direct Current (DC). Typically, renewable electricity is supplied via an AC grid and converted to DC for electrolysers, necessitating many medium voltage transformers and rectifiers. A DC distribution design could offer advantages for large-scale green hydrogen plants. 🟦 2) High Voltage Direct Current (HVDC) As HVDC-connected renewable energy sources and interconnectors increase, directly linking HVDC infrastructure with converters to medium voltage (MV) and low voltage (LV) may become viable for large-scale electrolysers operating at LVDC. For example: - Partially constructed Dogger Bank Wind Farm in the UK will use 320 kV HVDC lines to deliver offshore wind energy to the 400 kV AC grid. - In the Netherlands, 2GW 525kV HVDC lines are set to connect offshore wind farms to onshore 380kV HVAC grids by 2030, enabling electricity supply to a hydrogen conversion park in Rotterdam. However, HVDC to MVDC and MVDC to LVDC converters have not yet been fully developed. 🟦 3) Inverter, Converter or Rectifier? Converter topologies are specific arrangements of electrical components that change currents, including inverters, rectifiers, and DC/DC converters. They utilize semiconductors like silicon and silicon-carbide (SiC) to control voltage and current. Key components include diodes, thyristors, and transistors. There are three main types of converters:  1. Inverter: DC to AC conversion  2. Rectifier: AC to DC conversion  3. Converter: DC to DC conversion Transformers convert one AC voltage level to another, with advanced solid state transformers (SST) using semiconductors for rapid voltage adjustment and power factor control. 🟦 4) New Layout The new layout, as shown in the post image, has been chosen as the most promising power converter scheme due to its relatively high maturity level, suggested higher efficiency, potential for lower complexity, reduced costs, and smaller footprint. This design employs a power conversion topology that steps down the medium voltage to a level suitable for the electrolyzer module. 🟦 5) Findings: - Proposed solution may improve electricity efficiency by eliminating one conversion step. - Enhancements in power factor compensation and reduction of harmonic emissions noted. - No additional compensation (like STATCOM) used; reactive power relies on existing equipment (transformers and cables). - Electrical layout footprint is expected to occupy ~25% of a large-scale green hydrogen plant's area, with a smaller footprint anticipated. - CAPEX slightly higher, OPEX slightly lower due to efficiency gains; high uncertainty and low maturity complicate economic conclusions. Source: see post image This post is for educational purposes only.

MMC inverters, or Modular Multilevel Converters ℹ️ MMC inverters, or Modular Multilevel Converters, are advanced power electronic devices with several notable features and applications: ⚙️Architecture and Operation: An MMC consists of multiple sub-modules, each typically containing a capacitor and a power semiconductor switch. The sub-modules are connected in series to form an arm of the converter. There are usually two arms for each phase of the inverter. By controlling the switching of the sub-modules, the MMC can generate a near-sinusoidal output voltage with a very high number of voltage levels. This results in reduced harmonic distortion and improved power quality compared to traditional inverters. 🔴Advantages 1️⃣High power handling capacity: MMCs can handle very high power levels, making them suitable for applications such as high-voltage direct current (HVDC) transmission and large industrial drives. 2️⃣Scalability: The modular design allows for easy expansion and customization of the converter's power rating by adding or removing sub-modules. 3️⃣Improved reliability: The redundancy provided by the multiple sub-modules enhances the reliability of the converter. In case of a failure of one or more sub-modules, the converter can continue to operate with reduced power output. 4️⃣Reduced electromagnetic interference (EMI): The multilevel output voltage results in lower switching frequencies and reduced EMI, which is beneficial for sensitive electronic equipment and communication systems. 🔴Applications: 1️⃣HVDC transmission: MMCs are widely used in HVDC systems to convert AC power to DC for long-distance transmission and then back to AC at the receiving end. 2️⃣Renewable energy integration: They can be used to connect renewable energy sources such as wind farms and solar power plants to the grid. 3️⃣Motor drives: MMC inverters can provide high-quality power for large industrial motors, improving efficiency and reducing maintenance. 4️⃣Electric ship propulsion: MMCs are suitable for powering electric ships due to their high power density and reliability.

Why AC Is Converted to DC and Again to AC in Modern AC Locomotives Modern electric locomotives—even those drawing 25 kV AC from the overhead catenary—use a conversion chain: AC → DC → controlled AC. This is intentional and fundamental to traction control, efficiency, and reliability. 1. Incoming AC Supply Is Not Usable Directly by Traction Motors The 25 kV, 50 Hz AC supply from OHE cannot be fed directly to traction motors because: • Its voltage is too high. • Its frequency is fixed at 50 Hz. • Traction motors need variable voltage and variable frequency to control speed and torque. Therefore, the supply must be processed before being fed to traction motors. 2. AC → DC: Conversion Enables Voltage Control, Isolation, and Protection When the locomotive receives 25 kV AC, this happens: 1. Step-down transformer reduces it to a lower AC voltage suitable for power electronics. 2. Rectifier (IGBT/diode controlled) converts AC to DC. 3. This DC feeds the DC link —a stable, controllable reservoir of energy. Why DC link is essential: • Provides a smooth, constant intermediate power source. • Allows precise control of power flow to traction inverters. • Enables regenerative braking energy to be pumped back to OHE (or dissipated through resistors). • Protects traction motors and electronics from OHE disturbances. 3. DC → AC: Inverters Generate Controlled, Variable-Frequency AC The stable DC bus then goes to IGBT-based traction inverters, which produce 3-phase AC with variable voltage and variable frequency. This controlled AC drives the 3-phase induction or synchronous traction motors. Benefits: • Smooth acceleration. • High starting torque. • Continuous torque control. • Slip control and wheel creep management. • Higher adhesion and reduced wheel slip. • Regenerative braking capability. • Efficiency improvement of 15–20% over older designs. 4. Why Not Use AC Motors Directly With 25 kV AC? Three technical reasons: a) Frequency Traction motors must run with variable frequency for speed control. OHE is fixed at 50 Hz. b) Voltage Traction motors operate at a few hundred volts—nowhere near 25 kV. c) Control Advanced functions (adhesion control, torque limiting, anti-slip, regenerative braking) require electronic control, only possible with inverters operating from a DC link. 5. Energy Efficiency and Regeneration With the DC link, regenerative braking returns power: • To the traction inverter → DC link → OHE This would not be feasible with direct AC motors on fixed 50 Hz. 6. Analogy with Metro Trains, High-Speed Rail, and Modern EMUs Globally, all modern high-speed and metro rolling stock follow the same AC–DC–AC conversion chain, following same logic . 7. Industry Perspective The AC–DC–AC architecture enables locomotives like: • WAP-5,WAP-7, WAG-9 to achieve: • High adhesion (up to 0.33) • 90% overall efficiency • Full regenerative braking • Minimal maintenance on motors

Power Supply system - Rolling stock 24Kv AC Step 1: Power Collection • Pantograph collects 25 kV, 50 Hz AC from the OHE. • This passes through: • Surge arresters → protect from voltage surges. • ACPT (Air Circuit Breaker), VCB (Vacuum Circuit Breaker), EGS (Earthing Device) → provide safety and switching. • ACCT (Current Transformer) → measures current. Step 2: Traction Transformer (MTR) • The Main Transformer (MTR) steps down the 25 kV AC into different voltages: • Secondary winding → 951 V AC (1-phase) for traction. • Tertiary winding → 380 V AC (3-phase) for auxiliaries. The transformer ensures safe, usable voltage levels for propulsion and onboard systems. Step 3: Traction Power Conversion (CI Unit) 1. AC–DC Main Converter converts 951 V AC → 1800 V DC (DC Link). 2. DC–AC Motor Converter converts this DC into Variable Voltage Variable Frequency (VVVF) AC (0–1400 V). 3. This powers the 3-phase Traction Motor → drives the train. This allows smooth acceleration, braking (including regenerative braking), and speed control. Step 4: Auxiliary Power Supply (APS) • From the tertiary winding (380 V AC): 1. AC–DC Auxiliary Converter → converts to 710 V DC (DC Link). 2. DC–AC Auxiliary Converter → generates 415 V 3-phase AC. This is used for auxiliary systems like compressors, HVAC, pumps, blowers, etc. Step 5: Low Voltage Distribution • 415 V AC 3-phase → distributed via SSB (Switchboard) to: • Main compressor, HVAC, oil pump, air blower. • Auxiliary Transformer (steps down 415 V AC → 230 V AC & 104 V AC): • 230 V 1-phase AC → sockets, lights, windscreen heater. • 104 V AC → AC–DC Rectifier → 110 V DC → supplies control systems. Step 6: 110 V DC Systems • Feeds control and safety systems: • Battery charging • BCU (Brake Control Unit) • BCG • CCTV • PA/PIS, TCMS & VDUs • Fire detection • Auxiliary compressors • Ventilation fans • WSP (Wheel Slide Protection) Step 7: Negative Return • The current returns to OHE via Axle Brush → Wheel → Running Rail (negative return path). Summary: The 25 kV AC from OHE is stepped down by the transformer → converted to DC → inverted to VVVF AC for traction motors. The tertiary winding supplies auxiliary converters for HVAC, compressors, lights, safety systems, and control units. The return path is through the rails.

Why we use multi-level converters in high-power applications??? Here are some key reasons why multi-level converters are preferred in high-power applications: 1. Enhanced Voltage Levels: These converters enable the synthesis of output voltage waveforms with multiple levels, resulting in a smoother output waveform and lower Total Harmonic Distortion (THD). This is crucial for minimizing unwanted harmonics in high-power applications.   2. Minimized Switching Losses: In high-power scenarios, reducing switching losses is essential for efficiency. Multi-level converters distribute voltage across various levels, lowering the stress on individual switching devices. This leads to reduced switching losses and improved overall efficiency.   3. Reduced Electromagnetic Interference (EMI): Compared to traditional converters with higher voltage stress, multi-level converters generate less EMI. This is particularly important in high-power applications where EMI can interfere with other electronic systems, impacting overall power system reliability.   4. Lower Voltage Stress on Components: The distribution of voltage across multiple levels in a multi-level converter reduces the voltage stress on individual power semiconductor devices. This contributes to increased reliability and extended lifespan of these components in high-power applications.   The animation demonstrates the H-Bridge Cascaded Multilevel (HBCM) inverter concept, where the output voltage results from combining the outputs of multiple H-bridge modules. Each H-bridge module produces a 3-level output, and when n H-bridges are connected in series, the overall output achieves 2n+1 levels. #powerelectronics #electrical #inverter #drives #solarenergy #renewableenergy #converter #highpower