The term "bipolar" refers to a standard switching transistor that has two PN junctions. It is commonly used as a switch in power supplies, line output stages, and S-correction circuits in color displays. The requirements for the transistor's switching behavior are quite different from those in its analog amplification mode. Instead of using parameters like fT (transition frequency) or fa (gain-bandwidth product), the switching performance is typically described through other metrics.
In a switching power supply, the output voltage is maintained by controlling the duty cycle of the transistor’s on-off state. In this role, the transistor acts as a switch, where a small base current controls the collector current through the transistor's amplification property. When the collector current reaches saturation, the transistor is considered "on," and when it drops to zero, it is considered "off."
However, the on/off states of a transistor are not ideal. There is a saturation voltage drop (VCES) when it is 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—it takes time to transition between states.
To evaluate the switching performance, 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. The time required for this transition is used to measure the transistor’s switching speed.
Transistors operating in switching mode have different requirements than when they are in linear amplification mode. In amplification, the collector current must be precisely controlled by the base current, maintaining a stable linear relationship. In switching mode, the base current should be sufficient to drive the collector current directly to its maximum value without any delay. However, due to the inherent characteristics of the transistor, such as the slope of the IC-IB curve, this ideal behavior cannot be achieved instantly.
The switching action also involves a certain amount of time for charge storage and dissipation. During the "on" phase, the base region accumulates charge, which needs to be removed during the "off" phase. This leads to storage time (ts) and fall time (tf), both contributing to the overall switching loss.
In addition, the reverse recovery time of diodes plays a critical role in switching power supplies. Diodes do not turn off immediately when the forward voltage is removed—they take time to recover, which can cause issues at higher frequencies. Fast recovery diodes and Schottky diodes are designed to minimize these effects.
Schottky diodes, for instance, have extremely short switching times (typically 50–100 ns) and low forward voltage drops (around 0.3–0.5 V), making them ideal for low-voltage, high-current applications. However, their reverse voltage rating is generally limited to below 40V, so they are best suited for specific power supply designs.
Understanding the switching characteristics of transistors and diodes is essential for optimizing the efficiency and performance of switching power supplies. Proper selection of components, along with well-designed driver circuits, ensures faster transitions and reduced losses.
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