Transistor parameter selection guide for energy-saving lamps and electronic rectifiers
In the past, the positioning of the parameters of transistors used in energy-saving lamps and electronic ballasts was not clear. In addition to the conventional parameter requirements of BVceo, BVcbo, Iceo, hFE, Vces, Ic, etc., the low-frequency tube only requires the characteristic frequency (generally in the order of several megabytes). However, the characteristic frequency is a requirement for the linear amplification of the sine wave, and it is not a concept with the transistor switching parameters in the working state of the switch. In addition, due to the limitation of knowledge level and domestic hardware conditions, people are prevented from effectively controlling and identifying the parameters of the lamp triode. 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 in the application process.
Complete power tolerance curve
Reduce the heat loss of the transistor
Magnification factor hFE and storage time ts
Complete power tolerance curve
The power tolerance (SOA) is an area surrounded by a curve (Figure 1). When the voltage and current coordinate values ​​applied to the transistor exceed the curve range, the transistor will break down due to power breakdown. 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. Withstand such back pressure.
It must be noted that at present, the general transistor manufacturers do not have the conditions for testing SOA. Even the qualified semiconductor transistor manufacturers have the ability to test this index, but the instruments often test only the values ​​at the boundary points of the safe working area, not All of the SOA curve. In this way, there may be two pairs of transistors with exactly the same SOA value at one point. During use on the actual circuit, one pair of transistors is damaged, while the other pair is not damaged.
Therefore, in the process of selecting the triode for the lamp, we must find the complete SOA curve provided by the device manufacturer.
Reduce the heat loss of the transistor
At present, energy-saving lamps and electronic ballasts generally use the upper and lower tubes to work in turn. The inverse peak voltage of the self-inductance generated by the inductive load is discharged through the conduction tube, so it is generally felt that the SOA value of the transistor at normal temperature is Not very sensitive in the ballast circuit. However, reducing the heat loss of the transistor has caused widespread concern in the industry, because the secondary breakdown tolerance of the transistor is reduced as the temperature increases (Figure 2).
After the transistor works in the circuit for a period of time, the circuit components will heat up (including the heating of the tube itself). The rising temperature will cause the transistor hFE to increase, the switching performance will deteriorate, and the secondary breakdown characteristics will decrease. In turn, it further promotes the heat generation of the tube, and this vicious cycle eventually leads to the breakdown of the triode and burns. Therefore, reducing the heat loss of the transistor itself is an important measure to improve the reliability of the transistor.
Experiments show that the power consumption of the off-state of the transistor is very small; the dissipation of 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 cut-off and from cut-off to saturation, and it has a great relationship with the choice of line parameters and the rise time tr and fall time tf of the transistor.
In recent years, the industry ’s energy-saving lamps and electronic ballast dedicated transistors have fully paid attention to reducing the switching losses of products. For example, 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 also helps control losses. Because the change of the magnetic ring parameters will cause the change of the transistor Ib, affecting the rise and fall time of the transistor. Transistor overdriving can cause the transistor to overheat and burn, and insufficient transistor driving can cause instantaneous breakdown damage when the transistor starts cold.
Magnification factor hFE and storage time ts
The hFE parameter of the triode is related to the storage time ts. Generally, the triode ts with a large hFE is also larger. In the past, people's understanding of ts and measuring instruments of ts are relatively lacking.
In the switching state, the selection of hFE usually has the following understanding: first, hFE should be as high as possible, so as to obtain the maximum operating current with the minimum base current, and at the same time give the lowest possible saturation voltage, so that it can be at the same time Reduce losses in output and drive circuits. However, if the switching speed and current tolerance are taken into consideration, the maximum value of hFE is limited. Second, Chinese manufacturers have tended to choose devices with smaller hFE, such as transistors with hFE of 10 to 15, or even 8 to 10. It was very popular at a time (later, due to the popular use of capacitive trigger circuits in the base loop, the value of hFE has increased). The smaller the value of hFE, the lower the saturation depth, which is conducive to reducing the heating of the transistor.
In fact, the saturation depth of the transistor is affected by two factors, Ib and hFE, so 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 triodes dedicated for electronic ballasts introduced by the industry all pay great attention to the control of storage time. Because the storage time ts is too long, the oscillation frequency of the circuit will drop, and the increase in the working current of the whole machine will easily cause damage to the transistor. Although the choke inductance and other component parameters can be adjusted to control the power of the whole machine, the discreteness of ts will make the consistency of the product poor and the reliability decrease. For example, in a quartz lamp electronic transformer circuit, a transistor with a storage time that is too large may cause the circuit to oscillate at a frequency lower than the operating limit of the output transformer, resulting in saturation of the core at the end of each cycle, which makes the transistor Ic appear in each cycle The spikes eventually cause the device to overheat and damage (Figure 3).
If the storage time difference between two transistors on the same line is too large, the upper and lower half waves of the working current of the whole machine will be severely asymmetric, the heavy transistor will be easily damaged, and the line will also generate more harmonics and electromagnetic interference.
Practical use shows that strict control of storage time ts and proper adjustment of the complete circuit can reduce the degree of dependence on hFE parameters. It is also worth mentioning that in the case of a certain chip area, the transistor characteristics, current characteristics and voltage resistance parameters are contradictory. The Chinese market used BUT11A as a 220V40W electronic ballast. The starting point is that BVceo and BVcbo have high values. However, in most of the electronic ballast circuits, there is no need to select the voltage parameter of the transistor too high.
Figure 3: Ic waveform diagram of two transistors with different storage time for the same line.
First, the complete power tolerance curve. The power tolerance (SOA) is an area surrounded by a curve (Figure 1). When the voltage and current coordinate values ​​applied to the transistor exceed the curve range, the transistor will break down 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. Withstand such back pressure.
It must be noted that at present, the general transistor manufacturers do not have the conditions for testing SOA. Even the qualified semiconductor transistor manufacturers have the ability to test this index, but the instruments often test only the values ​​at the boundary points of the safe working area, not All of the SOA curve. In this way, there may be two pairs of transistors with exactly the same SOA value at one point. During use on the actual circuit, one pair of transistors is damaged, while the other pair is not damaged.
Therefore, in the process of selecting the triode for the lamp, we must find the complete SOA curve provided by the device manufacturer.
Second, reduce the heat loss of the transistor
At present, energy-saving lamps and electronic ballasts generally use the upper and lower tubes to work in turn. The inverse peak voltage of the self-inductance generated by the inductive load is discharged through the conduction tube, so it is generally felt that the SOA value of the transistor at normal temperature is Not very sensitive in the ballast circuit. However, reducing the heat loss of the transistor has caused widespread concern in the industry, because the secondary breakdown tolerance of the transistor is reduced as the temperature increases (Figure 2).
After the transistor works in the circuit for a period of time, the circuit components will heat up (including the heating of the tube itself). The rising temperature will cause the transistor hFE to increase, the switching performance will deteriorate, and the secondary breakdown characteristics will decrease. In turn, it further promotes the heat generation of the tube, and this vicious cycle eventually leads to the breakdown of the triode and burns. Therefore, reducing the heat loss of the transistor itself is an important measure to improve the reliability of the transistor.
Experiments show that the power consumption of the off-state of the transistor is very small; the dissipation of 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 cut-off and from cut-off to saturation, and it has a great relationship with the choice of line parameters and the rise time tr and fall time tf of the transistor.
In recent years, the industry ’s energy-saving lamps and electronic ballast dedicated transistors have fully paid attention to reducing the switching losses of 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 also helps control losses. Because the change of the magnetic ring parameters will cause the change of the transistor Ib, affecting the rise and fall time of the transistor. Transistor overdriving can cause the transistor to overheat and burn, and insufficient transistor driving can cause instantaneous breakdown damage when the transistor starts cold.
Three, magnification hFE and storage time ts
The hFE parameter of the transistor is related to the storage time ts. Generally, the transistor with a large hFE has a larger ts. In the past, people's understanding of ts and measuring instruments for ts are relatively lacking. People rely more on hFE parameters to select the transistor.
In the switching state, the selection of hFE usually has the following understanding: first, hFE should be as high as possible, so as to obtain the maximum operating current with the minimum base current, and at the same time give the lowest possible saturation voltage, so that it can be at the same time Reduce losses in output and drive circuits.
However, if the switching speed and current tolerance are taken into consideration, the maximum value of hFE is limited. Second, Chinese manufacturers have tended to choose devices with smaller hFE, such as transistors with hFE of 10 to 15, or even 8 to 10. It was very popular at a time (later, due to the popular use of capacitive trigger circuits in the base loop, the value of hFE has increased). The smaller the value of hFE, the lower the saturation depth, which is conducive to reducing the heating of the transistor.
In fact, the saturation depth of the transistor is affected by two factors, Ib and hFE, so 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 triodes dedicated for electronic ballasts introduced by the industry all pay great attention to the control of storage time. Because the storage time ts is too long, the oscillation frequency of the circuit will drop, and the increase in the working current of the whole machine will easily cause damage to the transistor. Although the choke inductance and other component parameters can be adjusted to control the power of the whole machine, the discreteness of ts will make the consistency of the product poor and the reliability decrease. For example, in a quartz lamp electronic transformer circuit, a transistor with a storage time that is too large may cause the circuit to oscillate at a frequency lower than the operating limit of the output transformer, resulting in saturation of the core at the end of each cycle, which makes the transistor Ic appear in each cycle The spikes eventually cause the device to overheat and damage (Figure 3).
If the storage time difference between two transistors on the same line is too large, the upper and lower half waves of the working current of the whole machine will be severely asymmetric, the heavy transistor will be easily damaged, and the line will also generate more harmonics and electromagnetic interference.
Practical use shows that strict control of storage time ts and proper adjustment of the complete circuit can reduce the degree of dependence on hFE parameters. It is also worth mentioning that in the case of a certain chip area, the transistor characteristics, current characteristics and voltage resistance parameters are contradictory. The Chinese market used BUT11A as a 220V40W electronic ballast. The starting point is that BVceo and BVcbo have high values. However, in most of the electronic ballast circuits, there is no need to select the voltage parameter of the transistor too high.
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