Selection guide for parameters of energy-saving lamps and electronic rectifier transistors

In the past, people's requirements for the parameters of transistors used in energy-saving lamps and electronic ballasts were not clear. In addition to the conventional parameter requirements such as BVceo, BVcbo, Iceo, hFE, Vces, and Ic, low-frequency tubes only have characteristic frequency requirements (generally on the order of several megabytes). However, the characteristic frequency is a requirement for linear amplification of sine waves and is not the same concept as the switching parameters of the transistor in the switching working state. In addition, due to limitations in knowledge level and domestic hardware conditions, people are prevented from effectively controlling and identifying the parameters of lamp triodes. This article attempts to summarize the selection of triodes for energy-saving lamps and electronic ballasts in order to fully understand the damage mechanism of triodes during application. The complete power margin curve reduces the thermal loss amplification factor hFE and storage time ts of the triode. The complete power margin curve. The power margin (SOA) is an area surrounded by a curve (Figure 1). When the voltage and current applied to the triode When the coordinate value exceeds the curve range, the transistor will undergo power breakdown and be damaged. In practical applications, some switching power supply line loads are inductive. After the triode is turned off, the self-inductance potential reverse peak voltage generated by the inductive load is added between the CE poles of the triode. The triode must have sufficient SOA, BVceo and BVcbo values ​​to Withstand such counter-pressure. It must be noted that currently, generally transistor manufacturers do not have the conditions to test SOA. Even qualified semiconductor transistor manufacturers have the ability to test this indicator. However, what the instrument tests is often only the value at the boundary point of the safe working area, not the All about the SOA Curve. In this way, it may happen that two pairs of transistors with exactly the same SOA value at one point are damaged during actual use on the line, while the other pair is not damaged. Therefore, in the process of selecting triodes for lamps, you must find the complete SOA curve provided by the device manufacturer. Reduce the heat loss of triodes. At present, energy-saving lamps and electronic ballasts generally use circuits in which the upper and lower tubes are turned on in turn. The self-inductance potential reverse peak voltage generated by the inductive load is discharged through the conduction tube, so the SOA value of the triode at room temperature is generally felt. It is not very sensitive in energy-saving lamps and electronic ballast circuits. Reducing the heat loss of triodes has attracted widespread attention in the industry. This is because the secondary breakdown tolerance of triodes decreases as the temperature increases (Figure 2). After the transistor has been working in the circuit for a period of time, the circuit components will generate heat (including the heat of the tube itself). The rising temperature will cause the transistor hFE to increase, the switching performance to deteriorate, and the secondary breakdown characteristics to decline. In turn, this further increases the heat generated by the tube, and this vicious cycle eventually causes the transistor to break down and burn out. Therefore, reducing the heat loss of the triode itself is an important measure to improve the reliability of the triode. Experiments show that the power consumption in the off-state of a transistor is very small; the dissipation in the on-state accounts for a certain proportion, but there is little room for change. Transistor dissipation mainly occurs in the transition period from saturation to cutoff and from cutoff to saturation, and is closely related to the selection of line parameters and the rise time tr and fall time tf of the transistor. In recent years, the industry has introduced special transistors for energy-saving lamps and electronic ballasts that have paid full attention to reducing the switching losses of the products. For example, the domestic BUL6800 series products have greatly improved the switching loss performance of the products on the basis of optimizing the MJE13000 series products. In addition, controlling the magnetic ring parameters is also beneficial to controlling losses. Because changes in the magnetic ring parameters will cause changes in the triode Ib, affecting the rise and fall times of the triode. Over-driving the transistor can cause serious heating and burning of the transistor, while under-driving the transistor may cause instantaneous breakdown damage when the transistor is started in a cold state. Magnification hFE and storage time ts. The hFE parameter of a triode is related to the storage time ts. Generally, a triode with a larger hFE also has a larger ts. In the past, people’s understanding of ts and the measurement instruments for ts were relatively lacking. People relied more on hFE parameters to select triodes. . In the switching state, the selection of hFE usually has the following understanding: First, hFE should be as high as possible in order to obtain the maximum operating current with the least base current and at the same time give the lowest possible saturation voltage, so that it can simultaneously Reduce losses in output and driver circuits. However, if the switching speed and current tolerance are taken into account, the maximum value of hFE is limited; secondly, Chinese manufacturers once tended to use devices with smaller hFE, such as transistors with hFE of 10 to 15, or even 8 to 10 It was very popular for a time (later, due to the popular use of capacitive trigger lines in the base circuit, the value of hFE increased). The smaller the value of hFE, the smaller the saturation depth, which is beneficial to reducing the heat generation of the transistor. In fact, the saturation depth of the transistor is affected by two factors, Ib and hFE. Therefore, the saturation depth can also be reduced by adjusting the magnetic ring and winding parameters, and the base resistance Rb. At present, the energy-saving lamps and special transistors for electronic ballasts introduced by the industry pay great attention to the control of storage time. Because the storage time ts is too long, the oscillation frequency of the circuit will decrease, and the increase in the operating current of the whole machine will easily cause damage to the triode. Although the choke inductance and other component parameters can be adjusted to control the power of the entire machine, the discrete nature of ts will make the product less consistent and reduce reliability. For example, in a quartz lamp electronic transformer circuit, a transistor with too long a storage time may cause the circuit to oscillate at a frequency below the operating limit of the output transformer, causing core saturation at the end of each cycle, which causes transistor Ic to appear at the end of each cycle. spikes, eventually causing overheating and damage to the device (Figure 3). If the storage time of two transistors on the same line is too different, the upper and lower half-waves of the operating current of the whole machine will be seriously asymmetrical, the triode with the heavy burden will be easily damaged, and the line will produce more harmonics and electromagnetic interference.