Understanding High Switching Loss and Low Conduction Loss in BJT Transistors

When analyzing the performance of bipolar junction transistors (BJTs), it is crucial to understand the concepts of high switching loss and low conduction loss. These characteristics are inherently tied to the upstate and downstate transition of the transistor, which significantly affects its overall efficiency and application in various electronic circuits.

Why Do Transistors Have High Switching Loss?

Any transistor, whether it's a BJT or a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET), consumes significant power when operating in the active region. The key factor affecting the switching loss is the time it takes for the transistor to transition between the on and off states. This transition is quantified by the turn ON time (Ton) and turn OFF time (Toff).

During these transitions, the transistor experiences high transient power dissipation, especially when there is a simultaneous presence of high voltage and current. This rapid changing state can lead to substantial power loss, which is significantly higher in BJTs compared to MOSFETs. The slower switching speed of BJTs results in longer time spent in linear regions during each switching cycle, thereby dissipating more power.

Why Do MOSFETs Have Lower Switching Loss?

MOSFETs outperform BJTs in terms of switching speed and therefore in switching losses. A MOSFET can switch very quickly, handling numerous transitions per second. This rapid switching capability minimizes the transient power dissipation, leading to lower switching losses.

Comparison of Conduction Losses: BJTs vs. MOSFETs

Conduction losses refer to the power dissipated by the transistor while it is in the on state. Unlike switching losses, which are related to the transient conditions during transitions, conduction losses are determined by the steady-state conditions when the transistor is fully on.

BJTs are minority carrier devices, meaning they rely on minority carriers to conduct current. This characteristic causes the BJT to turn off more slowly. Consequently, BJT spends more time in the linear region during each switching cycle, leading to higher conduction losses compared to MOSFETs.

MOSFETs, on the other hand, behave more like a constant resistance when on. This means that at low current levels, a MOSFET can often dissipate less power than a BJT, due to its lower resistance (RDSon). However, as the switched voltage increases, the RDSon of a MOSFET also increases, leading to higher conduction losses.

At very high currents, the constant voltage drop across a BJT remains lower than the voltage drop in a MOSFET due to its resistance. Therefore, even at high current levels, a BJT might still exhibit lower conduction losses compared to a MOSFET.

Social and IGBT Considerations

Insulated-Gate Bipolar Transistors (IGBTs) are popular for applications involving very high currents. IGBTs combine the high voltage handling capability of a BJT with a MOSFET-like gate structure, which significantly improves their switching speed and reduces switching losses. Despite this, IGBTs still maintain a lower on-state voltage drop compared to MOSFETs, making them the preferred choice in applications where low conduction losses are critical.

Conclusion

The performance of BJTs in terms of switching and conduction losses is highly dependent on the switching speed and the current levels involved. While BJTs generally have higher switching losses and conduction losses, they still have their advantages, particularly in applications where high voltage and low conduction losses are essential. On the other hand, MOSFETs offer lower switching losses but may have higher conduction losses, especially at high voltages.

FAQs

What causes high switching loss in BJTs? How do MOSEFTs reduce switching losses? When are IGBTs preferred over BJTs and MOSFETs?

Keywords

BJT Transistor, Switching Loss, Conduction Loss