A saturation control loop for a BJT or IGBT in a switching power supply

摘要

The base drive to a switching transistor Q1 in a switched mode power supply is adjusted in dependence on the sensed collector voltage so that the transistor can conduct the required load current without becoming saturated. Saturation would delay turn-off and increase turn-off switching loss. The transistor is switched on quickly by a large initial base current 410 to reduce turn-on switching loss but the base drive is then reduced. The left plate of the collector voltage sensing capacitor 400 is clamped to earth 406 during the turn-on process. This node is then allowed to float so that collector voltage variations can be sensed by the leaky peak detector 1202. The base drive may be adapted in dependence on the measured turn-off time in a previous cycle (figure 35). Collector dv/dt may be controlled during turn-on and turn-off (figure 31). The on-period may be ended early if the collector voltage exceeds a threshold.