How to avoid diode​ overload and damage

How to avoid diode overload and damage

diode, as a fundamental electronic component device, are known by all engineers to have a single conductivity. According to the classification of semiconductor materials, they can be divided into silicon diode and germanium diodes; According to the classification of application occasions, it can be divided into rectifier diodes, detector diodes, switch diodes, and voltage regulator diodes.

Dissipative power

Dissipative power, also known as the maximum allowable dissipative power PCM of the collector, refers to the maximum dissipative power of the collector when the transistor parameters do not exceed the specified allowable value. It is the difference between the total active input power and the total active output power of the power grid components or the entire network at a certain moment. Under linear conditions, the calculation of the dissipation power of conduction is relatively simple, PD=I2R, perhaps PD=U2/R; In the on/off state, the calculation is relatively complex.

The dissipated power of the diode is related to the promised temperature savings. The maximum allowable operating temperature for silicon diodes is 150 ℃, while for germanium, the maximum allowable operating temperature is 85 ℃. The temperature of semiconductor operation is limited. When the practical power increases, the temperature savings will also increase. When the temperature savings reach 150 ℃, the current power is the maximum dissipated power. Of course, there is also a certain connection between the dissipation power and the size of the packaging. Generally, equipment with larger packaging has a relatively larger maximum dissipation power, and the most common is that high-power equipment has a large volume and large area of heat dissipation metal surface.

The dissipation power of a specific model of diode is related to test conditions, such as ambient temperature and heat dissipation conditions. In general, the maximum dissipation power tested is at 25 ℃. As the ambient temperature increases, its maximum dissipated power will decrease. Due to the smaller thermal conductivity temperature difference under this condition, for example, at 25 ℃, a diode's dissipated power can reach 1W, and at 75 ℃, the dissipated power may become 0.4W. The maximum dissipation power agreed upon is related to the heat dissipation conditions. The better the heat dissipation conditions, the higher the dissipation power. At the same ambient temperature, the dissipation power is 1W. After adding a heat sink, the dissipation power may become 1.7W.

One parameter that characterizes the heat dissipation method is thermal resistance. Thermal resistance is an inductive parameter that reflects the ability to obstruct heat transfer. Thermal resistance, similar to resistance in electronics, is a reference quantity that reflects the size of "resistance ability". The smaller the thermal resistance, the stronger the heat transfer ability; On the contrary, the greater the thermal resistance, the smaller the heat transfer ability. From an analogical perspective, heat is equivalent to current, temperature difference is equivalent to voltage, and thermal resistance is equivalent to resistance. During this period, thermal resistance Rja: The total thermal resistance of the chip's heat source junction to the surrounding cooling air, expressed in ℃/W, indicating the temperature difference between the two ends of the thermal conductivity at 1W.

From it, it can be seen that its dissipation power is PD=200mW, and its thermal resistance is 625 ℃/W. These two quantities were tested at an ambient temperature of 25 ℃, welded to the FR-4 raw material PCB.

The dissipated power is related to the ambient temperature, and the higher the temperature, the smaller the dissipated power. The dissipated power of 1N4448HWS is related to the ambient temperature as follows:

At 0-25 ℃, the dissipated power remains constant at 200mW. At 25-150 ℃, it decreases linearly until it reaches 150 ℃, and the dissipated power is 0. At this temperature, the silicon  can no longer operate.

In the planning process, people pay more attention to the temperature during equipment operation to ensure a safe operating range. Taking 1N4448HWS as an example, under an ambient temperature of 25 ℃ and a practical power of 100mW, its temperature is 25+625 * 0.1=87.5 ℃, and it can operate normally; When the practical power is 200mW, the temperature is 25+625 * 0.2=150 ℃. At this point, the maximum temperature for temperature regulation has been reached, which is relatively risky and should be avoided.

In terms of heat transfer of diode, the primary consideration is PD and thermal resistance Rja. The former is the maximum dissipated power, which cannot be exceeded in practical operations. The latter is a heat transfer resistance parameter that reflects the heat transfer ability of different diode. When using diodes, not only forward current, reverse withstand voltage, and switching time should be considered, but also dissipated power should be considered.