In an electric race car, it is very important to have some systems to indicate to track marshals whether the car is safe to handle or not. Usually this system is a light which can be found on a high point on the chassis. If the light is green, the car is safe to handle, if it is red/orange the car might be still on a high voltage potential.
Figure 1: Indicator light on an F1 car
In Formula Student this is no different. According to the rules you must install a TSAL (tractive system active light) in the top of the roll hoop. This light will be green when the car is turned on, but the high voltage system is turned off. And will blink red when a voltage higher then 60 V is present on the high voltage bus, or when one of the accumulator insulation relays is switched on.
Figure 2: TSAL light on a Formula Student Car
The TSAL system exist out of two parts, with one PCB for each part. There is the TSAL light itself, and there is the board that drives the light.
The Light itself is a double-sided PCB that holds three RGB high power leds on each side. In total the leds can consume up to 5.5 Watts, and unfortunately not all this energy goes into light energy. The compact design made it challenging to cool the leds enough, to keep them at reasonable temperatures.
Figure 3: TSAL-light PCB
To keep the PCB from overheating, two heat pads where added so that a thermal connection can be made to the roll hoop. Thermal via’s where then required to make sure that the PCB heated up uniformly. So that as much heat as possible could be transferred to the roll hoop. In this way we where able to keep the temperatures of the leds below 100 ˚C.
To drive the TSAL light, a separate driver board was designed. This board is a pure hardware logic, because the use of a microcontroller would introduce the possibility of software errors. This is also one of the few PCBs in the car that has both high voltage and low voltage present at the same board. Due to safety reasons, and required by the rules of Formula Student, high voltage and low voltage must be isolated from each other. And when the two are present on the same board, a minimum spacing is required.
Figure 4: TSAL-Driver PCB
The TSAL driver board compares two signals, to decide which color the TSAL must have. There is a signal coming from the battery, which indicates whether all insulation relays are open or not. On the other hand, the driver board measures the voltage on the DC bus, to indicate any voltage higher then 60V. These two signals are compared with each other and drive activate the correct light. The power stage of the lights are two led drivers that can supply a continuous current to the leds, to provide an as efficient possible system.
Figure 5: TSAL-Driver overview
Another extra feature available on the TSAL driver board is a system to discharge the high voltage system of the car. This is needed because the high voltage bus has a very large capacity, needed to provide the motors with a very stable supply. When the relays of the battery are opened, it can take up to one hour for the voltage to drop below 60V. While it is a requirement that the voltage drops under 60V within 5 seconds. The discharge system makes sure this requirement is met. The system is also driven with a signal coming from the battery that indicated the position of the relays. When the relays are all open, the discharge system is activated. Which dissipates the remaining energy from the high voltage capacitors in a power resistor.
Figure 6: Discharge system
Battery voltage indicator
In the Formula Student competition, there is an extra voltage indicator on the battery. This is required because the battery is charged outside the car. This voltage indicator is activated when the voltage on the output of the battery is larger then 60V. But an important extra requirement of this indicator is that it needs to be supplied by the high voltage system itself. In other words, the led should go on when 60V is applied to the circuit, without it having an external supply. But it should also survive 600V. Which results in some demanding circuitry.
Figure 7: Battery HV-indicator light
Because the light must be supplied with the high voltage circuit, there are two solutions for this system. One is to use a resistor divider with a Zener diode, but this would result in a very large heat dissipation at 600V. The other is to build a small power supply.
For our solution we opted for a fly back DCDC converter to provide the system with the required voltage to operate, without too much heat losses. The largest challenge in designing this fly back circuit is that the system must be working between 60V and 600V. Which in it’s turn requires a rather special control chip, and a very specific transformer. On top of that, the board needs to be compact, and routing should be executed with care, to make sure that the supply doesn’t get disrupted by its own switching noise.
Figure 8: Voltage indicator PCB
The safety systems in Formula Student are very important and make sure that students can work safely on this project. Thanks to Eurocircuits, our team (Formula Electric Belgium) had the needed resources to build this safety critical systems, and drive the car (Umicore Pulse) safely on events, while passing the technical inspection without any problems.