Guidelines for selecting transistor parameters

After the transistor works in the circuit for a period of time, the circuit components will generate heat (including the heat generated by the transistor itself), and the temperature will continue to rise, causing the hFE of the transistor to increase, the switching performance to deteriorate, and the secondary breakdown characteristics to decrease. Conversely, this further increases the heat generation of the transistor, leading to a vicious cycle that ultimately causes the transistor to break down and burn out. Therefore, reducing the heat loss of the transistor itself is an important measure to improve the reliability of transistor use.

The experiment shows that the power consumption of the transistor in the off state is very small; The dissipation of conduction state accounts for a certain proportion, but there is not much room for change. The dissipation of transistors mainly occurs during the transition period from saturation to cutoff and from cutoff to saturation, and is closely related to the selection of circuit parameters and the rise time tr and fall time tf of the transistor.

In recent years, the energy-saving lamps and specialized transistors for electronic ballasts introduced by the industry have fully paid attention to reducing the switching losses of products. For example, the domestically produced BUL6800 series products have significantly improved the switching loss performance of the products on the basis of optimizing the MJE13000 series products.

In addition, controlling the parameters of the magnetic ring is also beneficial for controlling losses. Because changes in magnetic ring parameters can cause changes in transistor Ib, affecting the rise and fall time of the transistor. Overdriving of a transistor can cause severe heating and burning of the transistor, while insufficient driving of the transistor may result in instantaneous breakdown and damage during cold start of the transistor.

Magnification factor hFE and storage time ts

The hFE parameter of a transistor is related to the storage time ts. Generally, transistors with larger hFE also have larger ts. In the past, people's understanding of ts and measurement instruments for ts were relatively lacking, and people relied more on hFE parameters to select transistors.

In the on/off state, the selection of hFE is usually understood as follows: firstly, hFE should be as high as possible to obtain the maximum operating current with the least base current, while providing the lowest saturation voltage possible, so as to reduce losses in both the output and drive circuits.

However, if considering the switching speed and current tolerance, the maximum value of hFE is limited; Secondly, Chinese manufacturers used to prefer devices with smaller hFE, such as transistors with hFE ranging from 10 to 15, or even 8 to 10, which were once very popular (later, due to the popularity of capacitor triggered circuits in base circuits, the value of hFE increased). A smaller hFE value resulted in a lower saturation depth, which was beneficial for reducing transistor heating.

In fact, the saturation depth of a transistor is influenced by two factors, Ib and hFE. Therefore, by adjusting the parameters of the magnetic ring and winding, as well as the base resistance Rb, the saturation depth can also be reduced.

At present, energy-saving lamps and specialized transistors for electronic ballasts launched in the industry attach great importance to controlling storage time. Due to the long storage time ts, the oscillation frequency of the circuit will decrease, and the working current of the whole machine will increase, which can easily cause damage to the transistor. Although the parameters of choke inductance and other components can be adjusted to control the overall power of the machine, the discreteness of TS will result in poor product consistency and reduced reliability. For example, in a quartz lamp electronic transformer circuit, transistors stored for too long may cause the circuit to oscillate at frequencies below the output transformer's operating limit, resulting in saturation of the magnetic core at the end of each cycle. This causes the transistor Ic to peak at each cycle, ultimately leading to overheating and damage to the device (Figure 3).

If the storage time difference between two transistors on the same circuit is too large, the upper and lower half waves of the operating current of the whole machine will be severely asymmetric, and the transistor with heavy burden will be easily damaged, resulting in more harmonics and electromagnetic interference in the circuit.

Practical use has shown that strict control of storage time ts and appropriate adjustment of the entire machine circuit can reduce the dependence on hFE parameters. It is worth mentioning that, with a certain chip area, the characteristics of the transistor, current characteristics, and withstand voltage parameters are contradictory. The Chinese market once used BUT11A to make 220V40W electronic ballasts, based on the high BVCEO and BVcbo values. However, currently, in the vast majority of electronic ballast circuits, it is no longer necessary to excessively choose the voltage parameters of the transistor.