EV road trip from Madrid to Paris: different types of chargers and their characteristics

Pablo Blázquez, Field Application Engineer at Frenetic, will guide us through a virtual road trip from Madrid to Paris on an EV. He will show us the different types of chargers we can find on the road, providing us with different options to charge an EV during a long journey and digging into the technical details of the magnetic components on those chargers.

Figure 1: Map of the EV road trip

EV and chargers

Compared to combustion engine cars, one of the greatest challenges of electric vehicles is doing long road trips: we can find several issues such as autonomy, charging time, or the availability of chargers. The technology is evolving rapidly, but the main problem when traveling is the need of charging the car for long periods.

Which charger topologies can we find on the road?

- AC Charging stations

We have two commonly used power sources, but the most typical is to charge our EV at home: in this case we have a power supply of 120 to 220V and 12 to 16A, and we reach a maximum power of approximately 2kW. This option is used in residential applications, but, as you might know, it is not the most efficient way to charge a car. The other option are commercial chargers, where we have more power availability, as they are designed to charge these types of vehicles. The range of VAC is from 208 to 240V while the current can go up to 80A: this configuration can produce up to 20kW.

For our long road trip, we decide to charge the car with a commercial charger:

Figure 2: AC charging station

To give you an idea, charging a 24kWh battery in your house takes approximately 12 to 17 hours, while by using a commercial charger, you only need about 8 hours. Still a pretty long time, though.

- DC Charging stations

In the case of DC charging, we need to convert the AC from the network to DC power. This conversion provides a much higher voltage and current through this type of charger. We can go up to 600V and 400A, giving powers from 120kW to 240kW. This means a charging time of approximately 30 minutes for a battery of 24kWh.

Figure 3: DC charging station

This power is achieved by stacking converters and generates a very bulky configuration in comparison with AC chargers.

As we have direct DC charging, we don’t need to go through the onboard charger, and we can connect directly to the battery.

The EV trip starts: 1st charging

The usual practice for a road trip is to charge the EV at home until the battery is full and, during the trip, try to find the best options to charge the vehicle as fast as possible.

Let’s start our trip by exploring and designing the different options that we could have at home to charge an electric vehicle.

We normally use AC power to charge an electric vehicle at home: this power must be converted into energy stored in the battery through an onboard charge. If you are an average user, like in our case, you charge your EV at 7kW. This takes a long time for the car to charge, but you need all the capacity of the battery if you want to arrive at the destination as soon as possible.

All cars, both electric and combustion, are very sensitive to temperature changes, wind, and speed. The faster you go and the worst conditions we have, the range of both cars will decrease. This is a bigger problem in EVs because the charging times are higher than for combustion cars. This is getting better within time and sooner than we expect traveling with EVs will be as easy as with combustion cars.

As mentioned above, for our road trip we charge our EV at home, and this could be the design of this charger for your house. The charger is of around 7kW and we have an electrical design as shown in Figure 3. This way, we can start our journey full of charge.

For this design, we will use all the data for this simulation from TI [1].

Figure 4: AC charger

We are going to design part of the auxiliary power tree that consists of an isolated AC/DC Flyback power stage followed by several DC/DC stages. That flyback caught our attention as it´s the first component connected to the grid.

The configuration of the flyback gives us have more control of the system, as it is part of the auxiliary tree of our AC charger. This system facilitates the power delivery to the system by having a constant output voltage, as well as a constant current regulation, and it also acts as galvanic isolation for our system.

We’ve designed this component in Frenetic Online considering the design proposed by Texas Instruments. Design details here.

Figure 5: Waveform – operation point

Figure 6 – Core

Figure 7 – Winding arrangements

As you can see in the images of the simulation, it is very easy to create a functional design in Frenetic Online. In just a couple of clicks, by entering the operation point, we can have complete designs in a matter of minutes, and we can start iterating to have the optimal design for our purposes.

Let’s head north: Tesla Supercharger

For the first part of the trip, we try to maximize the autonomy by driving for around 319km. We go to Rivabellosa (Basque Country, Spain), where we have access to a Tesla Supercharger. The Tesla superchargers are already available for the use of all kinds of electric cars regardless of the brand, and we can charge our car at rates up to 150kW/h.

By the end of this part of the trip, the battery left is around 17%, meaning about 95km of autonomy. If we want to have a faster charge, we need the battery to be as empty as possible: this will help us gaining more km in the same amount of time as if we had more battery left.

This happens because when you have a lower state of the charge of the battery (SoC), it is easier for the electrons to find a place, while when it is close to full it is more difficult to find free spots. A good analogy is a movie theatre: when the first people enter the theatre, it is easier for them to find an empty place, while for the last people to enter the theatre it takes longer to find a free place.

For our charge, we start at 116kW/h, and it takes around 55mins to get to a 100%. But we don’t want to wait that long, so we charge the car for about 40 minutes and continue our trip with 95% battery. The last 5% of the battery would take 15 minutes to charge, but instead of waiting, we are ready to head to our next stop.

Figure 8: Charging in Rivabellosa

For this first day, we decide to stop in France to sleep over the night and get to Paris the next morning.

As we have access to a Tesla supercharger, we can analyse how DC chargers work.

DC Chargers

First, we should know that the grid works in AC, so we need to convert it into a fixed DC voltage. The first phase has several stacks that help the system build a higher power. The output of this phase is 800V. As we are capable of transforming AC into DC outside the car, we have to skip the onboard charger in the EV and directly connect to the battery. We will have fewer losses as we need to go through less phases.

Figure 9: DC power station module

As you can see in Figure 8, we have a huge block diagram. First, connected to the grid we have the PFC AC/DC. Then we have the power stage MOSFETs continuing with different blocks for temperature sensing, and interface for CAN. We also have isolated and non-isolated DC/DC converters for powering the auxiliary circuits.

PFC design in Frenetic Online

We focus now on the AC/DC PFC, and we draw this full design based on the one created by TI [2][3]. You can check our design here.

The AC/DC stage (PFC) converts the power from the grid into a constant voltage. With the PFC we can maintain the sinusoidal input currents.

This reference design is a 3.5-kW boost PFC regulator that operates in continuous conduction mode. The system efficiency is greater than 98% over the wide input operating voltage range from 190-V to 270-V AC under full load conditions.

As we enter the design in Frenetic Online, we select our operation point for the system. We create a waveform by entering some key values for our system and the waveform is automatically created, and we can even ask the tool to suggest some designs.

Figure 10: PFC waveform

We decide to follow the Frenetic Online suggestions to see if it can optimize the 98% efficiency of the TI design. Here we have some of the suggestions proposed by Frenetic:

Figure 11: Suggestions from Frenetic

These suggestions are a very good starting point for our design. We look at the temperature of our design and we can see that the one in TI is very similar.

Figure 12: Circuit thermal image TI

This temperature must be something between the 24ºC and the 98ºC, while in our design in Frenetic is 56ºC. It seems to be very close. For this reason, we keep the first design proposed by Frenetic with this core and windings distribution shown in Figure 13 and Figure 14.

Figure 13: Core

Figure 14: Winding

In the core, we are using powder core even though is more expensive than ferrite, because it allows us to use fewer turns. For the winding arrangement and wire selected, round wire is the selected for this design, and we have a very low current density and very few losses for the power we are using.

In terms of power and efficiency, we have 11.33W of losses in 3.5kW PFC this means that the efficiency is above 99%!

We were able to do this design in a couple of minutes and gave us an amazing result. What we’ve learned in this process:

-  We use powder core in this kind of design because we have a higher Bsat than Ferrite.

-  EV with lower SoC will be able to charge faster

-  DC chargers have a very bulky design

-  Frenetic is a fast and useful tool to create new designs and upgrade already-made ones.

Next steps of the journey

We are now eating in Rivabellosa, waiting for our car to charge and we are 949km away from Paris, we still need to go through some superchargers.

In the first stage, we did 349km to Ribavellosa superchargers, now we do 365km until the next supercharger in Bordeaux, where we can charge our battery close to a full recharge. We get to the Bordeaux Supercharger with 16% of battery left and charging the car for 45 minutes gives us a 94% SoC.

Continuing our trip from this fast charge we try to get to Tours; from Bordeaux, the last stage of the day is 355km. There, we charge the car overnight in a hotel. In hotels we usually have two options: some hotels have a wall box and others just have a standard plug in the parking. Well, we have one last option: no charger or plug at all in the parking. Unfortunately, this is the most typical case. Being cautious, we’ve selected a hotel that has at least a standard plug.

Now, as we’re tired from the long day, we can sleep for a long time and hope the car has charged enough to get to Paris.

Will it be enough? Unfortunately, not! The charge in a standard plug is 10 to 12 km per hour. In terms of power, it is 1.5kW and we have a battery of 82kW/h. We got to Tours with a 16% of battery, charged it from 8 p.m. to 10 a.m., and now we have a 42%: a very slow charge.

Waking up the next morning we decide to look for a fast AC charging station so that we can get to Paris with some battery left, as we have to do 240km and we will get there fully empty.

We move the car to an AC charger of 18kW: this gives us a higher battery percentage for the last part of the trip. We have breakfast and we take a walk in Tours for 1 hour, while the battery reaches a 64%, which is just enough for us to get to Paris with some extra charge.

Let’s have a look at what a fast AC Charger looks like and analyse one of its DC/DC converters:

Figure 15: AC charger schematic

The charger has different phases compared to a couple of DC/DC converters, so we decided to put in Frenetic some of the values of the non-isolated DC/DC power supply. The topology of this DC/DC converter is a buck, it steps down the voltage and create a constant output.

Employing the values used in Texas instruments we can make sure that our system works. [4] Check out the full design. We start the design in Frenetic by entering the operation point.

Figure 16: Waveform DC/DC buck converter

With this operation point, we decide to have some new suggestions instead of using the same components as TI. These are the suggestions that we can get:

Figure 17: Suggestions from Frenetic

We have designs that are directly offered by our software, and we also have a couple of components from the library that could fit our operation point. This could be directly used for our component. For now, let’s start iterating with Frenetic.

Starting with the suggestions from Frenetic, we select the EQ30 that initially has 19 turns, and by iterating in the design we decide to have fewer turns (14) and we reduce considerably the losses.

Figure 18: Core first iteration

As we move on to the winding, we try to reduce the number of layers that we are using, going from 5 to 3, but unluckily the wires do not fit on the winding window. We decide then to reduce again the number of turns to 12. This decision makes our wires fit in 3 layers, and we even have fewer losses! With this change, we still have good values for our core and winding.

Figure 19: Winding iteration

We can try to fit a smaller core in this case, as I have fewer turns: we use an EQ25 instead of the EQ30 and it fits perfectly!

Figure 20: Core final result

Figure 21: Winding final result

This is a very simple way to iterate our designs and it is very easy to start a design from scratch.

When we finish the charge, we move on to our last stage. In a couple of hours, we get to Paris, and we come to the end of our trip. Finishing line on the Champs-Élyseés!

References

[1] Mishra, Application Report: “L1 and L2 EV Charger Electric Vehicle Service Equipment. Design Considerations”, Texas Instruments,

[2] Ramakrishnan, J.Rangaraju; “Power Topology Considerations for Electric Vehicle Charging Stations”, Texas Instruments

[3] Ramakrishnan, J.Rangaraju; “230-V, 3.5-kW PFC With >98% Efficiency, Optimized for BOM and Size Reference Design”, Texas Instruments

[4] S.Mishra, "L1 and L2 EV Charger Electric Vehicle Service Equipment Design Considerations", Texas Instruments