StackFET Buck DCDC Converter

Modern EV systems requires a reliable bridge between the high voltage, dangerous tractive system, and the lower voltage traditional 12V logic rail. This was a problem that UQ Racing faced for a number of years, as previous iterations had a history of catrastrophic failures during testing. The stakes for this module were exceptionally high. It serves as the primary power source for the Accumulator Indicator Light (AIL)—a safety-critical component mandated by Formula SAE rules to signal when the 600V system is active—as well as the internal management electronics within the high-voltage battery enclosure.

Recognizing that the reliability of the entire vehicle’s safety architecture depended on this single point of failure, I took the initiative to lead a complete ground-up redesign. My objective was to move beyond "patching" the legacy hardware and instead engineer a robust, simulation-validated solution that could withstand the thermal and electrical rigors of a 600V powertrain.

The Failure Mode Analysis:

The previous design was based on the LT8316 flyback controller. During a controlled ramp-up to 450V, the IC experienced a thermal "blow-up" despite a low current draw (< 10mA). I conducted a deep dive into the failure mode by investigatng the thermal profiles of the chip, where I identified a critical oversight in the intial component selection:

• The Error: The design used the LT8316F package, which lacks an exposed ground pad for heat dissipation. 

• The Evidence: By analysing an evaluation board that uses the LT8316FE (the variant with an expposed ground pad for heat dissipation), I noted that the chp only reached 47 degrees at 600V. Using the given junction to ambient and junction to case thermal resistances, I modelled the theoretical junction temperature and concluded that localised hotspots in the non heat sink variant were likely causingg internal thermal runaway at high input voltages. 

Concept Selection:

Since the LT8316FE - the variant with a thermal ground pad - was out of stock through mainstream distributers, I performed a trade-off analysis to find a more robust solution for the FSAE environment, which eventually led me to the LNK3206GGQ by LinkSwitch. While there were other options, this was chosen as it was readily available, easily adaptable for a 600V range (most EV automotive solutions were designed for a 400V head limit), and fits within the project's budget constraint ($150). 

Why the LNK3206GGQ?

• Higher efficiency: Flyback design is superior to traditional diode ladder or current mirror designs in effciency

• Integrated Safety: Toplogy allows for the converter to easily handle the full 600V battery potential by distributing the voltage stress across an external high voltage MOSFET as well as the controller's internal switch

• Proven Reliablity: Highly recommended within online forums for buck DCDC designs, meaning more support is available in the design cycle

Design and Implementation:

Whilst going through the design stages, I decided to set out a few specifications and objectives:

• Voltage Regulation: The design must be able to step down an input of 60V - 600V to a stable 12V rail. Although it is primarily designed for the EV (600 V), I also wanted it to be modular enough to be used in the Autonomous EV (200 V) as well.

• Ultra Low No Load Consumption: Since this circuit will be connected to the tractive system battery, I wanted to minimise the drain on the battery during idle periods, so that the actual juice can be used to make the EV go further and faster instead. I aimed to keep the board under 60 degrees as per FSAE rulesets.

• Power Integrity: I targeted an output voltage ripple of below 500 mV pk-pk during standard loads due to the tolerances of the devices that this DCDC powers.

• Magnetic Stability: Since the DCDC is located within the high voltage enclosure, I wanted to ensure that the board's power inductor maintains continuous conduction mode (CCM) under full load to prevent EMI spikes that might occur which could affect the delicate electronics near it.

With that in mind, these were the specific design choices I made off this:

The Power Stage:

The Output Rectification and Feedback:

Creating the PCB:

At first, it seemed that the routing for the DCDC was fairly straightforward due to the relatively low number of components used. However, I soon ran into two significant challenges that I was unfamiliar with: high voltage isolation and thermal regulation. 

Thermal Management:

Here, my objective was to eliminate localised hotspots to prevent thermal runaways as seen on the LT8316 design, which was achieved through the following:

• Increased Copper Weight: During the fabrication process, I increased the copper weight from the standard 1.2 oz to 2 oz for all layers. This was doen to increase the total cross sectional area of the traces and planes to signicantly reduce the I^2R losses and provide more thermal mass for heat dissipation

• Polygon Pours: I used an extensive amount of copper pours around high thermal gradient components. By maximising the surface area of these planes, I was able to effectively turn the PCB itself into a passive heatsink.

• Stitching Via Arrays: According to the manufacturer documentation, the power inductor L1 and freewheeling diodes D5 and D6 are the primary heat generators. I decided to implement a high density stitching via array to thermally couple polygon pours on both the top and bottom copper layers, thus allowing heat to disspate through both sides of the board.

High Voltage Regulation

To meet strict safety regulations, I had to ensure that the High Voltage system remained galvanically isolated from the Ground Low Voltage system of the car.

• Galvanic Isolation: I controlled the output of the board through a high voltage rated reed relay. This provided physical separation between the 600V accumulator circuit and the rest of the vehicle's 12V electrical system, preventing any potential faults like shocks or shorts from leaking into the rest of the car. This also helps eliminate ground loops and common node noise from occuring

• Clearance and Creepage Prevention: Since this is a 600V board, I aimed to maintain proper air and surface clearance to prevent arcing. As per IPC-2221B standards, I implemented a 4mm across the board to maintain proper spacing

• Environmental and Electrical Protection: To further improve the dialetrics of the board and protect it against moisture and dust, I applied a conformal coating with a thickness of 3 mil. As per IPC-2221B, tihs coating allows for a tighter clearance, going from 12.7 mm minimum clearance to the aforementioned 4mm spacing. This allowed me to make the board more compact, which was essential for fitting it in the accumulator box

As such, the final board was as follows:

Testing and Future Plans

To ensure that the DCDC was reliable enough, I put the prototype board through a series of stress tests to validate my thermal and electrical design choices. This was done by using a high voltage power supply through the UQ Power Electronics Lab.

Project Takeaways

The redesign of the AIL from a failing legacy board to a stable StackFET converter was one of the most technical challenges I’ve tackled. It pushed me to look beyond basic circuit design and into the physics of component failure and thermal management.

What I learnt:

• Forensic Engineering: By analysing the previous failed design, I leanre how to perform a deep dive failure analysis. This gave me valuable experience in using datasheets as a diagnostic tool and taught me how to redesign to address these failures rather than blindly guessing

• High Voltage PCB Design: Although I have contributed on high voltage PCBs in the past, this was my first solo project meaning I had to adapt a safety first mindset. I learnt to implement different safety stanards for creepage and clearance, and how to account for galvanic isolation by using relays to ensure that proper safety protocols were followed

• Thermal Engineering: To prevent thermal runaways, I learnt how to design for heat dissipation. From increasing the copper weight of the board, to implementing high density thermal via stitching, I learnt how to use the PCB itself as a heatsink for power electronics.