How to set the overcurrent protection threshold? The Insider of BLDC Controller Safety Design that Engineers Don’t show

When you deliver a carefully designed project to a BLDC controller, you inevitably feel a bit uneasy: can this guy really protect himself and the connected motor in critical moments? How are the thresholds for overcurrent and over temperature protection determined?

Firstly, regarding reliability. In modern well-designed controllers, the protection function is usually quite reliable, and its core lies in the "dual protection mechanism". The first line of defense is real-time protection at the hardware level, which is like a conditioned reflex that can respond to fatal faults such as overcurrent within microseconds and directly turn off the power transistor. The second line of defense is software monitoring, which is responsible for handling continuous overload or temperature rise. This' hard soft combination 'design forms the cornerstone of reliability. Of course, its degree ultimately depends on design standards and component quality.

So, how is the key protection threshold set? This is not a random filling, but the result of rigorous engineering weighing.

For overcurrent protection, two levels of thresholds are usually set. One is instantaneous peak protection, which is set slightly higher than the maximum starting current of the motor, but must be much lower than the absolute maximum withstand current of power devices (such as MOSFETs), in order to cope with sudden disasters such as short circuits. The second is continuous overload protection, which is closer to the rated working current of the motor and allows for brief starting shocks, but will react to long-term excessive operation. These two thresholds work together to prevent momentary damage and avoid misoperations caused by normal load fluctuations.

For overtemperature protection, the goal is to keep the power chip's junction temperature below its safe limit (typically ~150°C).  Since directly measuring the chip's internal temperature is difficult, a thermistor is placed nearby (e.g., on the heatsink) for indirect monitoring.  A critical safety margin is essential—the trigger point (often 85‑100°C at the sensor) must be set low enough to ensure the chip itself stays safe before shutting down or derating.

Some advanced controllers allow limited user adjustment of these thresholds for application‑specific tuning, though this should be done with technical expertise.

In short, a reliable BLDC controller's protection function is the result of careful design. The overcurrent and overtemperature thresholds are scientifically set after balancing performance, safety, and cost. Understanding these logics not only helps you choose the right product, but also enables you to make wiser decisions in system application and maintenance.