Energy storage systems boost electric vehicle fast charging infrastructure and solutions

As electric vehicles (EVs) gain more and more market share and will eventually replace internal combustion engine vehicles, DC fast charging stations will replace or integrate with petrol stations, powered by renewable energies such as solar and wind. People will desire to charge their EVs in less than 15 minutes. To achieve this goal, energy storage systems (ESS) will play a critical role in the EV fast charging infrastructure. This article introduces the function of ESS in EV fast charging infrastructure and the related solutions from ADI.

As the demand for fast EV charging rapidly increases, consumer demand for EV fast charging infrastructure such as charging piles is also quite strong. Considering multiple charging piles, the charging peak power that the grid will have to locally provide is more than 1 MW. The grid can collapse in many points, or huge investments are needed to improve the transmission lines and the central power plants needed to supply a much higher base load. This load, however, is impulsive and must be merged with the intermittent energy generated by renewable sources such as solar and wind.
 
Energy storage systems can solve this problem in a simple and elegant way. We use fluids like petrol or gasses to store energy and reuse it when needed (for example, when fueling a car). With the same principle, we can store electric energy in batteries using electrons and chemistry. This energy can be then utilized to boost an EV charge to keep the grid stable by shaving the peaks of power or to provide supply in case of blackout.
 
For EVs, at home, they can be charged slowly overnight with a simple wall-box or with a few kilowatt DC charger for houses with a solar generation system together with a storage battery. When on the street, they can be charged fast at the charging piles, or superfast in future fuel stations.
 
On the other hand, together with the rising EV market, the renewable energy generation market - such as solar photovoltaic (PV) systems and wind generation systems - is still growing at a good rate, thanks to a price reduction of about 80% in the last 10 years and the push to decarbonize. Today, solar represents less than 5% of global electricity generation and is expected to be more than one-third (33%) of global electricity generation in 2050.

With a future made of intermittent loads, the EVs that need to be charged and intermittent energy sources - PV, as well as wind generation - present challenges such as how to combine these new players in the energy ecosystem, centered on the grid. Intermittent loads such as EVs will require extra dimensioning of the transmission lines to cover the higher power peak demands.
 
Solar production will change the way that central power plants operate to make sure the grid is not overcharged, and people will require easier access to electricity, and more and more self-usage of the electricity they produce at home will be with residential solar systems. To make all entities work together smoothly and to benefit from the renewable sources and zero-emission EVs, energy storage systems must come into the game to make sure we can store and reuse the electrical energy that is generated when the demand is low (for example, solar energy generated at noon to be used in the evening) and to use the extra energy to balance the grid.
 
Energy storage systems are the electrical equivalent of tanks for fuel or storage warehouses for coal. ESS can be used in multiple applications on both residential and industrial scale. In a residential application, it is simple to connect the PV inverter to the storage battery, to save and use the energy in the house or to charge the car overnight with the energy produced by the sun during the day. In an industrial- or utility-scale implementation, such as grid-connected services, ESS installations can be used for different purposes: from regulation of PV and wind sources to energy arbitrage, from backup support to black start (removing diesel generators), and, most importantly from a total cost point of view, investment deferral. In this last case, energy storage systems will be used to cover power peaks in the nodes of the grid, making sure the existing transmission lines don't need expensive upgrades.
 
Another relevant user case is the off-grid installation, where ESS enables microgrid or islands to be self-sufficient. Considering all possible applications, the ESS market will breach the 1000 GW power/2000 GWh capacity threshold before the year 2045, growing fast from today's 10 GW power/20 GWh.

The charging station has four power conversion systems all sitting on the main DC bus, rated 1000 V DC to 1500 V DC. The higher the required power, the higher the DC bus voltage. 1500 V DC represents the industry standard today and for the next 20 years. Considering the PV inverter, we see that it has a double function of a DC-to-DC converter, for the power path going from the PV panels to the DC bus, and the function of a DC-to-AC inverter, for the power path going from the PV panels to the AC bus and then into the grid. The DC-to-DC conversion stage is the most important here, since the AC-to-DC stage can also be integrated into the main bidirectional power factor correction (PFC) inverter going from the DC bus to the AC grid.
 
Considering state-of-the-art power electronics designs, the highest efficiency is reached with converters designed around silicon carbide (SiC) power MOSFETs. The comparison with silicon insulated gate bipolar transistors (IGBTs) shows an efficiency increase in the range of 5% (maximum load) to 20% (partial load). The way the SiC MOSFETs are driven is key to reach the required switching frequency needed to have the best trade-off between system design costs (driven by the MOSFETs, the coils, and the inductors) and efficiency. The designers target the switching frequency in the range of 50 kHz to 250 kHz. The requirements for the gate drivers are becoming more challenging, mainly in terms of shorter propagation delays and improved short-circuit protection.
 
Analog Devices ADuM4136 is an isolated gate driver featuring the state of the art iCoupler® technology. This isolation technology enables a common-mode transient immunity (CMTI) of 150 kV/µs to drive the SiC MOSFETs in the hundreds of kHz switching frequency range. This, together with fast fault management like the desaturation protection, gives the designer the possibility to properly drive single or parallel SiC MOSFETs up to 1200 V.
 
The isolated gate driver must be powered, and the combination of the ADuM4136 gate driver with the LT3999 push-pull controller represents a noise free, high efficiency building block to properly manage SiC MOSFETs. The LT3999 is used to control a bipolar isolated power supply for the ADuM4136. The ultralow EMI noise design of the LT3999 isolated power supply, together with the possibility to switch up to 1 MHz, enable a compact and cost-effective solution.
 
While power converters are fundamental for the power conversion paths, in energy storage systems, the key component to assure the best total ownership costs is represented by the battery managing/monitoring systems (BMS). To mitigate the system noise before it can affect the BMS performance, the stack monitor converter uses a sigma-delta topology, aided by six user electable filter options to address noisy environments. In the ADI portfolio, the LTC681x and LTC680x families represent the state of the art for battery stack monitors. The 18-channel version is called LTC6813.
 
In summary, to address the challenges of the future DC fast charging infrastructure, the critical aspects will be in the power conversion systems and in the energy storage systems. Combining the ADuM4136 isolated gate driver with the LT3999 power supply controller (for the power conversion stages designed with SiC MOSFETs) and the LTC6813 battery monitoring device (for the energy storage batteries) will greatly improve the operating efficiency of ESS.

The aforementioned ADI ADuM4136 is a single-/dual-supply, high voltage isolated IGBT gate driver specifically optimized for driving insulated gate bipolar transistors (IGBTs). Analog Devices iCoupler® technology provides isolation between the input signal and the output gate drive. Operation with unipolar or bipolar secondary supplies is possible, allowing negative gate drive if needed.
 
The Analog Devices chip scale transformers also provide isolated communication of control information between the high voltage and low voltage domains of the chip. Information on the status of the chip can be read back from dedicated outputs. Control of resetting the device after a fault on the secondary side is performed on the primary side of the device. Integrated onto the ADuM4136 is a desaturation detection circuit that provides protection against high voltage short-circuit IGBT operation. The desaturation protection contains noise reducing features such as a 312 ns (typical) masking time after a switching event to mask voltage spikes due to initial turn-on. An internal 537 µA (typical) current source allows low device count, and the internal blanking switch allows the addition of an external current source if more noise immunity is needed. In addition, the secondary UVLO is set to 12 V with common IGBT threshold levels taken into consideration.
 
Another ADI device, the LT3999, is a monolithic, high voltage, high frequency, low noise, 1A, 1MHz push-pull DC/DC driver with duty cycle control, providing isolated power in a small solution footprint. The LT3999 has two 1A current limited power switches that switch out of phase. The duty cycle is programmable to adjust the output voltage. The switching frequency is programmed up to 1MHz and can be synchronized to an external clock for more accurate placement of switcher harmonics. The input operating range is programmed with the precision undervoltage and overvoltage lockouts. The supply current is reduced to less than 1µA during shutdown. A user-defined RC time constant provides an adjustable soft-start capability by limiting the inrush current at start-up. The LT3999 is available in a 10-lead MSOP and 3mm × 3mm DFN package with exposed pad.
 
In addition, the ADI LTC6813-1 is an 18-cell battery stack monitor with daisy chain interface that measures up to 18 series connected battery cells with a total measurement error of less than 2.2mV. The cell measurement range of 0V to 5V makes the LTC6813-1 suitable for most battery chemistries. All 18 cells can be measured in 290µs, and lower data acquisition rates can be selected for high noise reduction.
 
Multiple LTC6813-1 devices can be connected in series, permitting simultaneous cell monitoring of long, high voltage battery strings. Each LTC6813-1 has an isoSPI interface for high speed, RF immune, long distance communications. Multiple devices are connected in a daisy chain with one host processor connection for all devices. This daisy chain can be operated bidirectionally, ensuring communication integrity, even in the event of a fault along the communication path.
 
The LTC6813-1 can be powered directly from the battery stack or from an isolated supply. The LTC6813-1 includes passive balancing for each cell, with individual PWM duty cycle control for each cell. Other features include an onboard 5V regulator, nine general purpose I/O lines and a sleep mode, where current consumption is reduced to 6µA.

Looking at the rapid development of EVs and renewable energy, DC fast charging infrastructure is facing multiple challenges such as high peak power supply, large grid load fluctuations, and difficult integration of intermittent energy sources. As the "buffer pool" in the electrical energy domain, ESS not only can smooth peak loads and defer grid upgrade investments, but also effectively bridge solar and wind energy to enable time-shifting of clean energy utilization, making it a critical link in supporting the stable operation of future charging networks. The solutions provided by ADI demonstrate a complete technology portfolio from power conversion to battery management. ADI's innovative solutions will continue to empower the EV ecosystem, driving green transportation from vision to everyday reality, making the convenience of "full charge in 15 minutes" compatible with sustainability.