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

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

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 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