In the boost circuit of LED backlight drivers, the failure mode of power MOSFETs that first short-circuit and then open-circuit is a typical fault process. Initially, electrical overload causes breakdown and short-circuit inside the chip, followed by the melting of internal connecting wires due to large current, resulting in an open-circuit. This phenomenon usually indicates that the circuit has design deficiencies or has been subjected to excessive stress. Today, Hekotai delves into the underlying causes of this fault for you.

Fault Process Analysis

This damage process can be divided into two consecutive stages. First, the MOSFET experiences functional failure at the chip level, usually manifested as short-circuit between pins. This is mainly because the device bears electrical or thermal stress exceeding its design limits during operation. Common causes include:

• Abnormal high voltage generated at the moment of switch-off exceeding the device's voltage withstand rating;
• Sustained excessive current caused by abnormal load or unstable control loop;
• Switch oscillation or out-of-spec control voltage induced by poor drive signals;
• Aging of internal chip materials due to long-term high-temperature operation.Early electrostatic damage may also lay hidden dangers. At this stage, a conductive short-circuit has already formed inside the chip.

Subsequently, as the chip is short-circuited, the power supply voltage is almost directly applied to the extremely small parasitic resistance in the loop, generating a huge sustained current far exceeding the design value. This current flows intensively through the internal metal wires connecting the chip and external pins. These metal wires have a small cross-sectional area, and the enormous current will cause them to heat up rapidly and eventually melt. This breaks the electrical connection between the chip and external pins, resulting in the final open-circuit state.

Key Design Considerations and Prevention Directions

To avoid such damage, it is necessary to systematically control the stress on the MOSFET in circuit design and device selection:

1. Voltage Stress SuppressionIt is essential to accurately evaluate and suppress the voltage spikes generated across the MOSFET when it turns off. This involves optimizing transformer design to reduce leakage magnetic interference, and installing effective snubber circuits across the MOSFET to absorb high-voltage spikes, so as to limit voltage and dissipate excess energy. The rated voltage withstand value of the selected MOSFET must have sufficient margin to cover the sum of normal input voltage, reflected voltage and spike voltage.
2. Current and Thermal ManagementEnsure that the current-carrying capacity of the MOSFET (including instantaneous current) can meet the requirements under the harshest operating conditions. Precisely calculate its operating losses, and based on this and the device's thermal parameters, strictly control the internal chip temperature within the safe range through effective thermal design. High temperature is a common factor that accelerates all damage processes.
3. Drive and Layout OptimizationProvide clean, stable control signals with sufficient driving capability to avoid slow switching processes or oscillation. The circuit board layout should be optimized to minimize the loop area of the main power current, thereby reducing stray inductance. Control signal traces should be kept away from high-voltage and high-speed switching nodes to prevent interference.
4. Integration of Protection MechanismsIntegrate reliable protection circuits at the system level, such as over-current protection, over-voltage protection and over-temperature protection. These protection functions should be able to quickly turn off the drive signal when an abnormality is detected, thereby cutting off the stress path before the initial short-circuit of the chip occurs and preventing the fault from developing to the stage of melting internal connecting wires.

Conclusion

The fault of MOSFETs that first short-circuit and then open-circuit in boost circuits is essentially the result of internal connecting wires being melted by subsequent large current after the chip is broken down by electrical overload. The fundamental preventive measures lie in ensuring that the electrical and thermal stress on the MOSFET is within its absolute safe operating limits under all expected and abnormal conditions through rigorous circuit design, sufficient device parameter margins and optimized circuit board layout. System-level protection functions are the final safeguard to prevent local damage from evolving into a complete open-circuit.In-depth understanding of this damage sequence has universal guiding significance for improving the long-term reliability of switching power supply products. Hekotai not only provides high-quality MOSFET products, but also offers comprehensive technical support to customers.

If you encounter MOSFET failure problems in LED backlight driver circuit design, please feel free to contact us.