AN1889 datasheet(17/33 Pages) STMICROELECTRONICS

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AN1889

ESBT driving circuit

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dynamic saturation, while the capacitor CB plus the resistor RB supply the correct level of

base current during all the on state. It is evident that the real advantage of the proposed

circuit is the possibility to set VB (affecting the all ON stage) independently from the voltage

needed (VB) to get the desired current spike.

It is worth noticing that the proposed circuit provides also an energy recovery function,

which minimizes the power needed to supply the base. The smaller capacitor stores all

charges coming from the base during the storage time, and gives them back to the base

during the next turn-on.

In a practical driving circuit VB can be set in order to obtain IB1 = -IB3, while VB must provide

the right saturation level for the most stressful working condition (that is the highest IC). This

condition is characterized by the flat zone where the base current IB = IB2, shown in

Figure 6, is practically constant and fixed according to the gain of the transistor used at

maximum current. This sort of "fixed voltage driven" method may cause some problems at

low load conditions and in general all the times the device works at low current level, in fact

in this case the storage time will be longer (over saturation effect), with consequent worse

turn-off performance.

It is clear that a proportional base biasing could positively impact the device performance at

lower current too. In the driving circuit reported in Figure 7 a current transformer associated

with the base of the ESBT implements a proportional base biasing and provides two further

advantages: firstly designer does not need to get a constant voltage anymore, secondly, the

current transformer provides an IB current with the same shape as the collector current.

Figure 7.

ESBT proportional driving circuit

AN1889 датащи(PDF) 17 Page - STMicroelectronics