# Multi-level converter

A multi-level converter (MLC) or (multi-level inverter) is a method of generating high-voltage wave-forms from lower-voltage components. MLC origins go back over a hundred years, when in the 1880s, the advantages of DC long-distance transmission became evident.
Modular multi-level converters (MMC) were investigated by Tricoli et al in 2017. Although their viability for electric vehicles (EV) was established, suitable low-cost semiconductors to make this topology competitive are not currently available (as of 2019).

## Description

In 1999 it was already described in that motors can also be operated, but the system can also be charged without the need for an additional AC charger. A notable example is the work of the startup Pulsetrain, which is pioneering the use of MLC for electric mobility. By leveraging advancements in semiconductor/control/hardware technology, bringing MLCs closer to widespread adoption in vehicles.
Extended Battery Lifespan: MLCs can increase battery lifespan by up to 80%, primarily through pulsed charging and discharging, a process not feasible with conventional systems. By rapidly switching the current on and off, this approach minimizes lithium plating, which are common causes of battery degradation. Additionally, the ability to individually switch cells on and off offers a further potential lifespan increase of up to 60%. Combined, these mechanisms are expected to provide even greater benefits, but current estimates conservatively focus on the 80% improvement to avoid setting overly high expectations. The exact extent of these savings will become clearer as ongoing research efforts worldwide continue to advance.
Reduced Battery Formation Time and Cost: MLC streamline the battery formation process, a critical and resource-intensive step in manufacturing. This reduces costs while improving battery efficiency.
Enhanced Electric Motor Efficiency: Enhanced Electric Motor Efficiency: The holistic integration of the inverter, charger, and battery management system into a single multi-level converter (MLC) enables optimized electric motor performance. This integration ensures that the entire drivetrain operates more efficiently compared to traditional systems.
MLCs also achieve up to a 30% reduction in energy losses, particularly during partial load operation, where vehicles are not operating at full speed. This improvement is especially beneficial for everyday driving scenarios, such as city traffic and stop-and-go conditions, which represent a significant portion of typical vehicle usage. By reducing losses and enhancing efficiency in these common driving environments, MLCs contribute to a more sustainable and cost-effective electric mobility solution.
System-Wide Optimization: MLC advocates for a system-level approach to electric vehicle design, where batteries, power electronics, and motors are co-optimized. This comprehensive strategy has the potential to revolutionize electric drivetrains.
The MLC has two principal disadvantages:
* Complex Control Requirements: While the control of an MLC is inherently more complex than that of a traditional 2-level converter, many of these challenges have been resolved over recent years. Managing and balancing the voltages, state of charge (SoC), state of health (SoH), and temperature of each submodule battery is now achievable with modern computing power, which has become both affordable and efficient. Additionally, operating each battery submodule at its optimal frequency is no longer a prohibitive challenge. These advancements make MLC technology not only viable but also cost-effective when factoring in the substantial advantages it offers compared to conventional solutions.
* Lack of DC Voltage Output: MLCs lack a direct DC voltage output (e.g., 400V or 800V), commonly required for ancillary systems such as heating or air conditioning in electric vehicles. This limitation necessitates additional hardware, slightly increasing system complexity and cost.
Given these factors, MLC technology is currently best suited for applications with single-motor systems, where the absence of a fixed DC output is less critical. By integrating the batteries into the inverter design, the voltage is no longer held constant (e.g., at 400V) but varies dynamically over time, making this approach particularly effective for such application.

## Variable DC Sources as an intermediate step

An emerging variation of MLCs is the concept of variable DC sources, which offers a gradual transition toward full MLC adoption. These systems function similarly to MLCs but with a key difference: instead of switching voltages in microseconds, they adjust more slowly, over the course of seconds. This slower switching allows for much of the same hardware design, with minimal modifications. For example, the use of full-bridge configurations, common in MLCs, becomes optional, enabling the use of simpler half-bridge designs.
This approach is primarily used as a Battery Management System (BMS), allowing variable DC sources to maintain many of the advantages of MLCs, such as improved battery utilization and better energy distribution. However, it eliminates the need for a complete system redesign in vehicles. By simply swapping the battery system, OEMs can integrate this technology into existing vehicle platforms without major alterations, making it easier for manufacturers to gain experience and build trust in the technology.
While this intermediate step sacrifices some of the advanced benefits of MLCs, it retains a significant portion of their advantages, such as cost efficiency and increased sustainability. It also provides a compelling demonstration of the price attractiveness of the technology, paving the way for broader adoption and eventual transition to fully integrated multi-level systems.

## High voltage DC converters

HVDC converters typically use series connected switched capacitors blocks. The blocks are switched in or out of the circuit to form the desired waveform, typically three phase AC.

## Low voltage DC converters

Multi-level converters (MLC) are adaptable for a wide range of applications, many of which are still in the research phase:
* Mobility and Stationary Energy Storage: MLCs are expected to achieve their first major breakthroughs in electric vehicles and stationary storage systems, offering improved battery lifespan and system efficiency.
* Hydrogen Generation: MLCs can manage high currents and moderate voltages required for electrolysis through configurations such as galvanically isolated LLC resonant converters.
* Aerospace: Systems like ELAPSED explore the use of MLCs for regulating rapidly changing magnetic fields in aviation.
* Magnetic Stimulation: Academic projects utilize MLCs in medical and neuroengineering for precise magnetic field control.
* Space and Fusion: Future applications include regulating magnetic fields in space systems and nuclear fusion reactors.