Battery protection is critical to ensure both the safety of consumer devices and the longevity of the battery. This is because lithium-ion or lithium-polymer batteries can be extremely sensitive to overcharge, over-discharge, short-circuit conditions, and excessive temperature. Leading to battery degradation or even catastrophic failures like fires or explosions. To replace standard MOSFETs and improve safety in these Battery Management Systems (BMS) applications, Nexperia now offers both super-junction (SJ) MOSFETs and low-voltage bidirectional GaN FETs. But which technology is right for you?
That’s a very tricky question. Key specifications for protection FETs include low RDS(on), gate threshold voltage, and thermal resistance. As these directly impact the overall performance of the battery management system. But while these specifications are key, they really only scratch the surface when it comes to choosing between SJ MOSFETs and low-voltage bidirectional GaN FETs.
That’s because the material properties of silicon SJ FETs and gallium nitride FETs are completely different. Each comes with unique strengths, particularly in battery protection applications, where reliability, efficiency, and thermal management are paramount. And in many ways it is not a straightforward comparison. It is more like comparing apples and oranges. Let’s first look at what battery protection needs to do.
The abc’s of battery protection
In BMS systems, protection FETs have various roles. They serve as switches to control current flow during charging and discharging, disconnecting the battery to prevent overcharging or deep discharge. With multi-cell battery packs, it is also essential to ensure that all cells are charged uniformly to prevent overcharging of individual cells. In the case of overcharging, excess voltage can cause chemical imbalances, leading to excessive heat or thermal runaway. While deep discharge can reduce battery lifespan or render the battery unusable.
As protection FETs they naturally need to protect against electrical faults such as overcurrent, overvoltage, undervoltage, and short circuits. And for any battery powered device, protection circuits need to consume very little power (low quiescent current) when the device is in standby or idle mode, as excessive power draw can quickly drain the battery.
To achieve this, protection FETs are placed in series with the battery and system load, allowing the BMS to disconnect the battery in the event of unsafe conditions and prevent potential damage. Typically, this was achieved using two MOSFETs back-to-back, but it can also be achieved using a single bidirectional FET.
Key characteristics of protection FETs in BMS include:
- Low RDS(on) ensures minimal voltage drop and higher efficiency, especially important in high-current applications, where energy loss can be significant. This is key to minimizing heat generation.
- Protection FETs must handle the full range of battery voltages, including potential surges. High voltage tolerance ensures reliable operation even under extreme conditions, protecting both the FETs and the battery cells.
- Due to power dissipation during heavy loads, protection FETs must withstand high temperatures. Good thermal characteristics help maintain the system’s longevity and safety.
- Rapid switching is essential to isolate the battery in fault conditions. This response time is critical to minimize damage.
- FETs must handle high peak currents during short circuits or startup conditions without damage, making high current ratings necessary to ensure robust fault protection.
Superior SOA brings peace of mind
SOA defines the range of voltage and current within which a protection FET can operate safely without damage. Engineered specifically to expand their operating capabilities, Nexperia’s ASFETs for Battery Systems and eFuse can handle large surge currents. This makes them ideal for battery protection in systems where sudden current spikes can occur, such as during short circuits. These MOSFETs are also more forgiving when it comes to voltage and current transients, which are common in BMS. Their ability to operate safely over a broader range of conditions makes them reliable for applications requiring long-term durability.
Speed and efficiency reduce footprint requirements
GaN FETs significantly reduce both conduction and switching losses, making them ideal for applications where energy efficiency is critical, such as in portable devices where maximizing battery life is important. This also brings significant footprint savings, leading to more compact and lightweight battery protection circuits and that’s before even considering the move to a single bidirectional GaN FET rather than two back-to-back MOSFETs. And the fast switching of GaN can be advantageous in reducing power loss during protection event detection and response times, critical for fast-acting protection schemes. With the release of Nexperia’s 40 V bidirectional GaN FETs, designers now have a complementary option for battery protection.
Changing how we look at battery protection
For some applications, having the most robust and reliable BMS protection will mean design engineers may prefer Nexperia’s ASFETs for Battery Systems and eFuse. Especially for high-current, fault-tolerant designs. But for applications where space is a critical factor, 40 V bidirectional GaN FETs offer an extremely interesting alternative. One that may not have the thermal capacity of larger MOSFETs, but at the same time is extremely efficient so therefore does not generate the same heat dissipation. A major benefit in battery systems as they need to manage thermal buildup. For most battery protection circuits, the choice boils down to prioritizing efficiency and size (GaN) versus fault-handling capability and robustness (SJ MOSFETs).
