Secondary breakdown and protection of power transistors

TP-IP4220CZ6 5V 0.35PF breakdown voltage 6V SOT

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Overview:
With the continuous development of electronic technology and the advent of new components, switching power supplies are increasingly favored by everyone because of their small size, light weight, high efficiency, and wide adaptability to grid voltage, in most fields. It replaces traditional linear power supplies and is widely used in different electronic products.

In most switching power supplies, the power switching transistor operates in a high-voltage, high-current, high-frequency pulse state, and the opening and closing under such conditions can cause a large impact on the transistor. Secondary breakdown is one of the important causes of transistor damage. To design a high-performance, high-reliability switching power supply, it is necessary to clearly understand the knowledge and avoidance measures of the transistor's secondary breakdown.

This paper analyzes the phenomenon and causes of secondary breakdown of transistors, and introduces the application of buffer circuits and other related transistor protection measures in combination with the design and production of switching power supplies.

1 Causes of secondary breakdown Secondary breakdown is mainly caused by excessive local temperature in the device body. The reason for the increase in temperature is caused by thermal imbalance when forward biasing and by avalanche breakdown during reverse bias.
Because the thermal resistance of the transistor is unevenly distributed throughout the inside of the tube, in some weak areas, the temperature rise will be higher than other parts, forming a so-called "hot spot", local temperature causes local current to increase, and current increases cause temperature to rise. This cycle up to a critical temperature, causing breakdown of the tube.
The secondary breakdown caused by avalanche breakdown is a phenomenon in which the local temperature is too high due to the excessive current density at some points due to an avalanche breakdown, which causes a negative resistance effect and a local temperature too high.

2 Measures to avoid secondary breakdown Turn-on and turn-off loss are important factors affecting the normal operation of the switching device. In particular, transistors are prone to secondary breakdown during dynamic processes. This phenomenon is directly related to switching losses. Therefore, reducing the switching loss of the self-shutdown device is an essential measure for proper use of the device. There are two ways to reduce losses:
(1) Turn off the transistor at the lowest possible collector-emitter voltage (Vce);
(2) Turn off the transistor during the rise of the emitter voltage to minimize the emitter current. Introducing a snubber circuit is one of the ways to achieve the above objectives.

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