H-Bridge Circuit Operation: Why Do Some Transistors Saturate While Others Cut Off?

✨ The discussion explains the operation of an H-bridge circuit focusing on why some transistors saturate while others cut off. Bipolar junction transistors (BJTs) in the H-bridge act as switches: a transistor saturates (fully on) when the base-emitter voltage exceeds approximately 0.7 V with correct polarity, and cuts off (off) otherwise. The top transistors (PNP) conduct when driven by a positive signal, while the bottom transistors (NPN) conduct when connected to ground. The circuit uses complementary pairs where one transistor in a pair saturates while the other cuts off, controlled by input signals pulling bases high or low through resistors and intermediate transistors. Saturation is desired to minimize power dissipation in the transistor, as partial conduction (active region) causes voltage drop and heat. However, in some H-bridge designs, transistors may not reach full saturation due to voltage limitations at emitter and collector junctions, resulting in a compromise between simplicity and efficiency. The discussion also covers the thermal implications of switching transistors, suggesting that MOSFETs are often preferred for high current applications due to their low on-resistance and better efficiency, while IGBTs are suitable for high voltage scenarios. The importance of consulting datasheets for saturation voltage (Vce(sat)) and power dissipation (Ptot) is emphasized. Additional points include the forward biasing of both emitter-base and collector-base junctions for saturation, and the practical observation that in some circuits, transistors never fully saturate but operate near an equilibrium point. The use of complementary push-pull bipolar transistors is also noted for driving MOSFET gates efficiently, where full saturation is less critical for switching speed.
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