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    Now that we've gone through some of the features of level 1 and level 2 service stations, as well, let's dive into the block diagram. We're going to spend a little bit of time on level 3 trying to understand what makes these different, how they operate, and some of the electronics behind the scenes that are required to do this high-power conversion.

    So whenever we start looking at level 3 EVSEs, again we see that the primary difference here is that these are no longer just delivering AC power. They are actually standalone, high-power AC to DC converters. The DC output is going to be fed directly into the battery of the vehicle. As such, these are going to be operating at that voltage level, which can be from 2 to 450 volts DC. And then for charging currents, up to 200 amps on some of the high speed chargers is not uncommon.

    So this enables an incredibly high charge rate for electric vehicles, and this is where we're starting to see that 30-minute fast charge time on modern EVs. However, in order to do this, the pilot signal that we discussed in the last section just simply won't cut it. In order to enable this direct communication with the vehicle, on-board charge management system is required. This is done over power line communication or CAN network.

    The different standards right now for level 3 EVSEs are the J1772 combined charging system, ChaDeMo, and Super Charger. Some of the system requirements that we can already start to see inside this system is going to be microcontrollers and digital power controllers in order to accurately control the power loops of these AC to DC and DC/DC converters. There's going to be relays and gate drivers for power flow control.

    Obviously, there will be flex gate sensors or current sensors of some kind for these highly tuned feedback systems inside the DC to DC and AC/DC stages. And then communications via CAN or PLC at a minimum. In addition, we can also expect communications with the outside world via a different link, since a lot of these are going to be public-style charging stations.

    So these are actually fairly complicated systems with multiple parts operating at the same time. Now, to help make the breakdown of this block diagram a little bit easier, I've broken it into three major sections-- the AC/DC, which will take a three-phase AC and convert it to 400 volts DC, or any other intermediate high voltage, and the DC/DC, which will take that intermediate voltage and either buck or boost it to the vehicle voltage levels required. And then additionally, a human-machine interface, which will be an overall system supervisor. Now, these designs do have an inherent limitation in how much power can be reasonably built into an AC/DC or a DC/DC stage. To mitigate this, most designs use multiple stages in parallel in order to reach the power levels required by the vehicle itself.

    Due to the level of power required inside one of these power stages, the AC to DC converter is going to be a little bit more complicated than a simple bridge rectifier and transformer arrangement. In this particular case, we're showing an active rectification arrangement, which would typically be what we call a Vienna-style active rectifier. This will typically be MOSFETs on the AC line that can be switched dynamically to enable the rectification. And then to run these MOSFETs, we're going to require some kind of a gate driver arrangement, which can either be high-voltage or isolated gate drivers.

    In order to control these effectively, a microcontroller unit capable of running all of the digital power calculations is required to operate in real time. These will typically have specific peripherals to enable the calculations to be done quickly enough to keep this running effectively. In order to run those tight control loops, some kind of current sense arrangement is required. This can be done either with isolated ADCs, or maybe a sensor that is inherently isolated like [? Hall ?] or [? Flux ?], or maybe current transformers in this arrangement. That decision is up to the specific designer of this AC/DC stage.

    Voltage sense amplification is also required here since the MCU is going to need to be able to track the sine wave, and especially the zero-crossing so that it knows when to switch the active rectifier. A simple power supply is also required in parallel with this to actually power all of the electronics, and for this, a simple flyback converter. And then multiple LDOs or buck converters to generate the different DC power lines required inside this system would be important here. This MCU must also communicate back out over to the other parts of the system, usually done via an isolated link-- either a high-speed LVDS or a CAN system.

    So the other half of this system is going to be the DC/DC converter. Just like with the AC/DC, the DC/DC is going to have a dedicated microcontroller for running all of the digital power algorithms. So in this particular case, we have a buck-boost stage using IGPTs to switch the power to the voltage required by the vehicle itself. So the output voltage is actually going to be potentially variable in this instance.

    In order to enable proper operation of IGBTs, gate drivers should be selected appropriately to match the drive current requirements, and if there's any isolation required, or again, if high-voltage gate drivers are sufficient for the particular design being done. This MCU also requires some kind of CAN peripheral on it in order to communicate with the vehicle for some of the standards or to a power line communication module that runs to the vehicle over the DC link.

    Just like in the AC side of things, for these digital power control loops, we're going to require high-accuracy current sensing via isolated ADCs or some kind of an inherently isolated sensor, as well as voltage sensor arrangements. And again, the power supply for this is required, and this can be a simple operation with a flyback converter, and then the appropriate DC voltage outputs, whether they are isolated or not.

    So as discussed in some of the previous slides, most DC charging stations are going to be the publicly accessible type. For these, they require some kind of an HMI system for the user to authenticate and interface with the vehicle charging station. So these are actually fairly complicated systems. Now that we have a very large enclosure, we can start doing a lot more interesting things with them.

    So these are going to have some type of a microprocessor. So this will be something on the range of an ARM class processor rather than the simple MCUs we had for the power stages. By using an MPU, we can enable a lot of different communication interfaces, so as to operate in parallel.

    So in particular, we can start using RS485 on multiple interfaces for Modbus communications, for console operations, and for external e-meters to measure power. These kind of interfaces can enable the system to communicate with a larger commercial building management system. Over RS232, we can have GPS for accurate time information, as well as Bluetooth for remote diagnostics. We can start enabling keypads via just simple GPIOs, fans, and then we also have isolated CAN in order to communicate with the charging module and the vehicle itself.

    These processors will also have embedded USB and RMII to enable GPRS-style communication or local Ethernet. And then we also have the media-rich interfaces such as audio, high-resolution video, and touch panels, being driven all by a single processor. In order to run this processor appropriately, the correct power supply architecture also needs to be enabled here. And this is typically done via a simple PMIC, but it can also be done in a discrete arrangement. From a system design standpoint, every one of these interfaces is going to require some kind of a physical layer translator.

    So ISO485 has its own voltage requirements. Same with 232 and USB. So the appropriate interface for each of these needs to be designed around the requirements for the particular installation design.