Made a FSAE Electric accumulator safety light indicating when

high voltage is present, utilizing a 60-450V Flyback Converter

ROLE

SUMMARY

DURATION

HV Lead — Team Phantom SFU FSAE Electric

Jul 2024 - Feb 2026

ACCUMALTOR INDICATOR LIGHT

ACCUMALTOR INDICATOR LIGHT

ACCUMALATOR INDICATOR LIGHT

High Voltage PCB Design

Schematic Capture

Trouble Shooting

High Voltage Testing

LTspice

Power Electronics

  1. OVERVIEW

Formula SAE Electric requires a bright red safety light on the accumulator to indicate when high voltage (HV) is present, defined as a system voltage above 60 V. I designed a custom 60–450 V flyback converter to power the indicator light and reliably activate it at 57 V, providing a clear indication before the accumulator reaches the 60 V HV threshold.

  1. CONSTRAINTS

There are a couple difficulties associated with this project that guided it's design:

  1. Not being able to use any other low voltage sources (5V, 12V, etc.) in the car for the full 0-400V system voltage range

  2. Finding a converter that could handle the unusually wide input voltage range (0-400V), for which no compact, off-the-shelf converters exist at a reasonable cost

  1. DESIGN

3.1 CONCEPTUAL

What is a flyback converter? A flyback is a buck-boost converter that outputs a fixed voltage for input voltages greater than OR less than the output voltage.


The flyback converter should be understood by what happens when the NMOS is switched ON and OFF.


When the NMOS is ON, the primary side coil of the transformer is grounded and energy is stored in the primary coil in the form of magnetizing inductance. At the same time current is prevented to flow on the secondary and tertiary side by diodes and the reverse polarity coil winding.


When the NMOS is OFF, the magnetizing inductance in the primary discharges it's stored energy/current into the secondary and tertiary with some coil ratio which provides our power for the output.


We then use feedback to measure at what level the current at the output it to keep it there by switching very fast, 17.8kHz in my setup.



3.2 SCHEMATIC DETAILS

Specific design details:

  • A diode–zener clamp across the primary winding limits voltage spikes caused by parasitic leakage inductance when the NMOS switches OFF, preventing potentially destructive overvoltage on the NMOS.

  • The NMOS gate is driven by a Pulse-Frequency Modulation (PFM) signal from the controller IC. Switching frequency varies with output load, operating at 17.8 kHz for the selected indicator light.

  • A voltage divider sets the controller's enable threshold, turning on the converter and indicator light when the HV input reaches 57V.

  • The controller compensates for changes in the secondary-side diode forward voltage drop due to temperature rise, which is important since it's used to set output voltage. Wider PCB copper pours are used in board design to minimize temperature change.

  • A voltage divider provides feedback from the tertiary winding, regulating the output to approximately 15.8V.

  • A shunt resistor senses primary-side current and sets the maximum output current to 1A in my configuration. This can be adjusted should we require it.

  • A cartridge fuse is used on the up to 400V input sized for 600VDC, 5A current rating and 10kA interrupt rating.

  • Output filter capacitors reduce voltage ripple using a bulk capacitor supplemented by parallel bypass capacitors to reduce effective ESR.

  • A linear regulator steps the filtered 15.8 V output down to 12V for the indicator light, providing a clean supply and minimizing the impact of residual switching ripple

3.3 PCB Design

Specific design details:

  • Wide copper pours are used for high current paths, although it's overkill with my current draw

  • Thermal vias are used on flyback IC and regulator GND connections

  • 400V and HVGND come onto the board on opposite pins of the header to maintain greater than 3mm clearance according to IEC 60664-1

  • Greater than 8mm creepage is maintained between HV and LV, with slots implemented for close quarters according to IEC 60664-1

  • HVGND and LVGND planes are on the bottom layer and are also separated with 8mm creepage in the case of a HV fault and HVGND becomes HV

  • Probe touch points are used for important signals for debugging

  • An SMD indicator LED is used on output for debugging

3.3 TESTING

  • Made a test plan to test the board

  • Tested the board with up to 400V

  1. REVISIONS

4.1 REVISION 2

  • The first revision of the board I worked on and my first time designing a schematic and PCB, evidently

  • Board concept was to step down HV with a regulator to use as a reference for a comparator and use a voltage divider on HV for comparator to switch a NMOS for powering a light once 60V was reached. It also powered the light by limiting current with a resistor grid and a zener to clamp voltage

  • Problems with this board:

    • The regulator is not rated to withstand 400V

    • The zener was not rated to clamp 400V to 20V, even if you limit the current for power dissipation

    • The MOSFET was the wrong way lol

  • This was not manufactured, thankfully

4.2 REVISION 3

  • This iteration does the same sort of thing as the last but uses 2 linear regulators in parallel for powering the LED

  • This still doesn't make sense because regulators are not rated to withstand 400V

  • At least I fixed the mosfet polarity

4.3 REVISION 4

  • Learned my lesson this time by using an isolated converter module that could handle 400V

  • Ended up assembling and testing this

  • Funny thing with the board design was that there wasn't a footprint for the converter in Altium so I ended up making it from the datasheet but I didn't realize it was the bottom view so when I assembled the board, the converter was on the other side

  • Tested with 400V

  • The board worked but technically this wasn't rules legal because it would turn on the light at whatever it's minimum input voltage was, not 60V

  • We thought it would be fine since realistically our system voltage could not go below 300V because then we would be undercharging our 18650 cells and damaging them, but we decided to make another revision

  • Another problem with this design was that there were no bleeder resistors for the 22uF cap of the converter so once we tested the board, it just had 400V of charge sitting on it waiting to give someone a nasty shock, so we air-discharged it over a couple days

4.4 REVISION 5

  • This board uses a similar strategy as the previous boards where it uses a converter to step down voltage for powering the light and for using as a reference for a comparator to switch a MOSFET

  • The converter has a range of 110-450V but I saw in the datasheet a input derating curve (below) that said I could use the converter at 50V and above, just with less current, so I thought I could getaway with using the same converter

  • I could not :(

  • Tested it and it only turned on at 110V

4.5 REVISION 6

  • Solves problems of previous converters by having wide enough input voltage range to handle the full 0-400V range

  • Actually implements creepage and clearance and other HV design practices