The term "bipolar" refers to a common switching transistor that has two PN junctions, typically used as a switch in power supplies, line output stages, and S correction circuits in color displays. Unlike analog amplification, the requirements for its switching behavior are quite different. The switching characteristics of such transistors are not generally described by parameters like fT or fa.
In a switching power supply, the output voltage is regulated based on the duty cycle of the transistor's on-off state. Here, the transistor functions as a switch, with a small base current controlling the collector current through its amplification capability. When the collector current reaches saturation, the transistor is considered "on," and when it drops to zero, it is considered "off."
However, the transition between on and off states is not ideal. There is a saturation voltage drop (VCES) when conducting, and even when off, there is a small leakage current (ICEO). Compared to an ideal switch, a transistor does not turn on or off instantaneously with the base control signal; instead, it goes through a transitional process.
To study this transient behavior, a standard is set: when the collector current reaches 90% of its maximum saturation value, it is considered turned on. When it drops to 10%, it is considered turned off. This time interval is used to evaluate the switching performance of the transistor.
Transistors operating in switching mode have different requirements compared to those in linear amplification. In amplification mode, the collector current (Ic) must be precisely controlled by the base current (IB), maintaining a stable linear relationship. In contrast, during switching, the base current should quickly bring the collector current up to its maximum value without any delay. However, due to the nature of the transistor's IC-IB characteristic curve, which is not perfectly vertical, there is always a rise and fall time involved.
Additionally, the basic amplification principle of a bipolar transistor also involves some inherent delays. While higher fT and fa values indicate better high-frequency performance, they do not directly reflect the switching characteristics. Instead, the on-time (ton) and off-time (toff) are more commonly used to describe how fast a transistor can switch.
On-time is defined as the time it takes for the collector current to rise from 0 to 90% of its saturation level after the base pulse is applied. During this time, there is a delay before the current starts to increase, caused by the charging of the emitter junction capacitance. The faster the charging, the shorter the delay.
Similarly, when the drive pulse turns off, the transistor does not immediately stop conducting. There is a storage time (ts) during which the stored charge in the base region dissipates, followed by a fall time (tf) as the collector current decreases. The total off-time is the sum of these two periods.
The on-time and off-time contribute to the conduction and switching losses of the transistor. During these times, the transistor is in the active region, leading to increased voltage drop and power dissipation. A similar concept applies to diodes, especially their reverse recovery time, which is critical in high-frequency switching applications.
Diodes also have on/off times, but the most significant factor in switching power supplies is their reverse recovery time. If the diode cannot recover quickly enough, it may cause short circuits in the circuit. Different types of diodes, such as power frequency rectifiers, fast recovery diodes, and Schottky diodes, have varying recovery times and are suited for different applications.
Schottky diodes, for example, have extremely fast switching times, typically between 50 and 100 ns, making them ideal for low-voltage, high-current applications. Their forward voltage drop is much lower than that of standard PN junction diodes, although their reverse voltage rating is generally limited to around 40V.
Understanding the switching behavior of transistors and diodes is essential for designing efficient and reliable switching power supplies. Proper selection of components and careful design of driver circuits can significantly improve the performance and efficiency of the system.
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