Is it time to rethink how we protect electrical circuits in home appliances, industrial systems, and motorbike voltage regulators? There are certainly reasons to question common assumptions after ST recently updated its family of high-temperature thyristors, which can reach a junction temperature of up to 150 ºC while offering a high noise immunity. The TN series covers a wide range of on-state currents, from 12 A to 80 A. However, for simplicity’s sake, this blog post will focus on one particular device: the TN5015H-6G. The device is the first to have an on-state current of 50 A and an off-state voltage of 600 V, thus representing an interesting solution to new problems.
TN5015H-6G: An Answer to the New Normal
When Current Standards Are Inadequate
The need to reevaluate how engineers guard against inrush currents stems from more demanding expectations. As engineers look to increase margins and differentiate their products, many choose to improve their design’s robustness. Users also expect that their product will weather increasingly harsher conditions, especially in high-end or niche markets. One way teams make a product more robust is by improving its overvoltage protection. Indeed, very few things are as damaging to a brand as a failure to guard against a sudden surge. For example, a TV ruined because of unreliable mains, or an industrial product failing after lightning strikes a fab, is simply unacceptable. Hence, as robustness becomes more important, surge protection becomes more critical.
The majority of certifications today guard against surges that are between 2 kV to 4 kV, depending on the application. While such values reflect most use cases, many feel they are no longer enough, especially for industrial or high-end designs. As a result, engineers increasingly develop surge protection circuits rated between 6 kV to 12 kV. Some regulatory bodies have started reviewing their standards to take this new reality into account. The problem is that companies need solutions today as designers are trying to plan for more extreme edge cases.
When Thyristors Rein in on an MOV’s Leakage Current
As soon as people start talking about surge protection, many engineers start thinking of a Metal Oxide Varistor. An MOV’s primary purpose is to absorb a massive voltage surge to prevent damages to the rest of the system. In its most basic representation, an MOV uses numerous diodes in parallel to the load. Under normal operating conditions, the MOV is highly resistant. However, if a suddenly high transient voltage appears, the diodes’ resistance drops to nearly zero, thus diverting the surge from the primary devices. It’s, therefore, an efficient way to protect sensitive components from abrupt surges.
Engineers looking to build more robust products often try to solve an MOV’s inherent shortcomings. Indeed, an MOV’s performance degrades over time or after multiple uses, rendering it useless after a while. The reason is that after every surge, or overtime, the MOV’s leakage current increases. To remedy this issue, engineers use thyristors, in series, which act as a barrier against the MOV’s leakage current. As standards and consumers try to limit a product’s environmental impact, an MOV with a short lifespan is no longer an option. As a result, engineers must use thyristors to ensure their surge protection lasts longer. Such trends explain why more and more engineers are using devices like the TN5015H-6G.
TN5015H-6G: A Solution to New Challenges
Measuring Robustness (dI/dt and dV/dt)
As engineers look to thyristors to improve the robustness of their surge protection, knowing how to size a component is critical. Unfortunately, some teams may have a hard time figuring out what technical specification to prioritize. Hence, it’s essential to understand that out of the myriad of data points, two stand out when dealing with robustness: dI/dt and dV/dt. The former measures the maximum rate of change of current when going from an off-state to an on-state. Put simply, it represents the electrical current the device can endure over a period, and it will have a direct impact on the surge that the system can manage. For example, TN5015H-6G has a dI/dt of 100 A / 100 µs, ensuring that it can handle 6 kV surge tests.
The other value that can help engineers size their device is dV/dt. This specification measures the minimum rate of change in voltage that would turn the device on. This value is significant because a low dV/dt means the device will switch on at an inopportune moment. The struggle for engineers is, therefore, to find a device that has a low-enough gate-triggering current (IGT) to be sensitive enough to turn on when a surge occurs, but with a dV/dt sufficient to be immune to noises to prevent false triggering. With an IGT of 15 mA and a dV/dt of 500 V/µs at 150 ºC, the TN5015H-6G became the solution for many engineers trying to create better surge protection.
Benefiting From a High Junction Temperature
Focusing on the protection function of the SCR, the TN5015H-6G is a fast trigger in the surge voltage ramp because of its 15 mA tuned gate. At up to 150°C, its low gate current and high surge current rating trade off is a unique feature on the market. The high junction temperature is possible because ST improved its manufacturing processes. As we increased the lithographic precision, we’ve been able to design thyristors that operate at higher temperatures than the traditional 125 ºC. It also explains why we can release TN devices certified for the automotive industry.