Turn On characteristics of a Power Transistor
Turn On characteristics of a Power Transistor:
- The exact level of minority carrier densities (and depletion region widths) required for proper switching is determined by the collector current and biasing collector voltage during switching, both of which are determined by external circuits.
- The rate at which these densities are attained is determined by the base current waveform. Therefore, the switching characteristic of a power transistor is always specified in relation to the external load circuit and the base current waveform shows a clamped inductive switching circuit with a flat base drives.
Some simplifying assumptions have been made to draw these waveforms are
- The load inductor has been assumed to be large enough so that the load current does not change during Turn ON period.
- Reverse recovery characteristics of D have been ignored.
- All parasitic elements have been ignored
- Before t = 0, the transistor (Q) was in the “OFF” state. In order to utilize the increased break down voltage (VCBO) the base-emitter junction of a Power Transistor is usually reverse biased during OFF state. Under this condition only negligible leakage current flows through the transistor. Power loss due to this leakage current is negligible compared to other components of power loss in a transistor. The entire load current flows through the diode and VCE is clamped to VCC (approximately).
- To turn the transistor ON at t = 0, the base biasing voltage VBB changes to a suitable positive value. This starts the process of charge redistribution at the base-emitter junction. The process is akin to charging of a capacitor. Finally at t = td the BE junction is forward biased. The junction voltage and the base current settles down to their steady state values. During this period, called the “Turn ON delay time” no appreciable collector current flows. The values of iO and VCE remains essentially at their OFF state levels.
- At the end of the delay time (td ON) the minority carrier density at the base region quickly approaches its steady state distribution and the collector current starts rising while the diode current (id) starts falling. At t = tdON tri the collector current becomes equal to the load current (and id becomes zero) IL. At this point D starts blocking reverse voltage and VCE becomes unclamped. tri is called the current rise time of the transistor.
- At the end of the current rise time the diode D regains reverse blocking capacity. The collector voltage VCE which has so far been clamped to VCC because of the conducting diode “D” starts falling towards its saturation voltage VCE (sat). The initial fall of VCE is rapid. During this period the switching trajectory traverses through the active region of the output characteristics of the transistor. At the end of this rapid fall (tfv1) the transistor enters “quasi saturation region”.
- Turn ON time can be reduced by increasing the base current. However large base current increases the quantity of excess carrier in the base and collector drift region which has to be removed during Turn Off.
- The Turn ON delay time can however be reduced by boosting the base current at the beginning of the Turn ON process. This can be achieved by connecting a small capacitance across RB. This increases the rate of rise of VBE & iB. Therefore, Turn ON delay time decreases. However, in steady state iB settle downs to a value determined by RB & VBB and no adverse effect on the Turn OFF time is observed.