As a new type of lighting source, LED has the advantages of high efficiency and energy saving, long working life, etc., and has been widely used in various lighting scenarios such as LED display, automotive electronics, and life lighting. LED constant current drive characteristics require a specific AC-DC constant current drive power supply, in order to improve the power efficiency, LED drive power supply generally adopts a single-stage PFC topology.
Figure 1 Single-level PFC topology
As shown in Figure 1, the topology of the single-stage PFC does not have a high-capacity electrolytic capacitor between the rectifier bridge at the input end and the high-voltage MOSFET, and when encountering a lightning surge, the surge energy is easily transmitted to the MOSFET, and the high-voltage MOSFET VDS is prone to overvoltage, and the MOSFET is prone to avalanches. In switching power supplies, R&D engineers require MOSFETs to occur as little or no avalanches as possible.
An avalanche is a phenomenon in which the voltage on the MOSFET exceeds the rated withstand voltage of the drain source and breakdown occurs. Figure 2 shows a schematic diagram of the safe working area of the 600V MOSFET, and the red line in the figure shows the right boundary of the safe working area. When an avalanche occurs, the voltage across the drain exceeds the rated BVDSS and is accompanied by current flowing through the drain, at which point the MOSFET operates on the right side of this boundary, beyond the Safe Operating Area (SOA).
Figure 2 A 600V MOSFET Safe Operating Area (SOA)
The avalanche measurement circuit and waveform shown in Figure 3, once beyond the safe operating area, the MOSFET power consumption will increase significantly. The figure on the left shows the standard circuit of the avalanche test, and the right picture shows the running waveform during the avalanche. MOSFET in the shutdown due to the VDS voltage is too high and into the avalanche state, during the avalanche of the drain source voltage current exists at the same time, the instantaneous power consumption generated by it reaches several KW, which will greatly affect the reliability of the entire power supply. And during the avalanche, it must be ensured that its channel temperature does not exceed the rated channel temperature, otherwise it is easy to cause the device to fail overtemperature.
Fig. 3 Avalanche measurement circuit and waveform
So, how can we avoid avalanche destruction during MOSFET applications? From the perspective of surge protection, power engineers can increase the specifications of the AC input surge protection components, so that the AC front-end device protection elements absorb most of the surge energy and reduce the surge residual pressure, such as increasing the size of the varistor, selecting the lower residual pressure pressure and increasing the RCD surge absorption circuit. In addition, it is also possible to choose a mosFET with a higher current ID or a higher withstand voltage specification to avoid avalanches on its own, but a higher ID often means a higher cost, so choosing a higher withstand voltage specification becomes a simpler, safer and cost-effective solution.
Figure 4 800V MOSFET surge test waveform
CH2 VDS (Blue 200V/div)
CH3 IDS (purple 1A/div)
Figure 5 650V MOSFET surge test waveform
Figure 4 shows the waveform that occurs in the surge using an LED driver power supply with a withstand voltage of 800V MOSFETs. Figure 5 shows the waveform of the 50W LED drive power supply with a 650V MOSFET withstand voltage when the surge occurs, and the surge test conditions are exactly the same.
The moment when the VDS waveform suddenly rises in Figure 4 and Figure 5 is the moment when the surge comes, the maximum voltage of VDS in Figure 4 is 848V, and the maximum voltage of VDS in Figure 5 is 776V. The energy consumed by the avalanche comes from the surge, and there is a large leakage source current flowing through the 650V MOSFET shutdown in Figure 5, and the avalanche phenomenon is more obvious. In Figure 4, the 800V MOSFET has a current of 0 during shutdown, and no avalanche occurs, indicating that the surge energy transmitted to the device is not enough to cause an avalanche, so it can be seen that the use of an 800V withstand voltage MOSFET in the power supply greatly improves the surge safety margin and avoids avalanches in the device.
In applications such as LED driver power supply and industrial control auxiliary power supply, when designing surge protection, power engineers can choose MOSFETs with higher withstand voltage, and devices with high pressure resistance can block the surge energy at the AC input, so that the MOV protection device of the input port can absorb it, avoiding the MOSFET from exceeding the safe working area. This can greatly improve the surge protection ability of the whole power supply.
As a green lighting product in the 21st century, LED is replacing the traditional light source in large quantities. Relying on the huge LED market, domestic devices have great potential to replace imported brands in the field of SJ MOSFET. Combining the needs of the market and customers, Wei'an continues to innovate in product processes and packaging, and is more complete in product series and specifications for the common voltage resistance of more than 800V in the field of LED lighting.
Table 1 List of main specifications of Vian 800V SJ-MOSFET
In the above table, 03N80M3, 05N80M3 can be applied in auxiliary power supplies with higher surge requirements, such as the 3-5W auxiliary power supply input of the Industrial 380VAC input. Compared with the 650V specification, the surge capacity is significantly improved, and compared with the 2N80 specification of the planar process, the on-resistance is significantly reduced, and the temperature rise and efficiency are significantly improved.
WAYON is a circuit protection component and power semiconductor provider. WAYON has always adhered to the core values of "customer-oriented, technology-oriented, and hard work". Committed to leading the market through technological innovation, and strive to become the world's leading brand of circuit protection components and power semiconductors.