AN3400 datasheet(9/78 Pages) STMICROELECTRONICS

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AN3400

Bipolar transistor operating modes

Doc ID 018840 Rev 1

9/78

Bipolar transistor operating modes

Bipolars switching in the resonant circuit have three different operating modes in normal

working conditions: re-circulating, conduction and transition phases.

During the re-circulating phase, the transistor isn't yet in conduction state and passes from a

first inactive state in which its B-C junction diode is activated to allow the re-circulation of

load inductor demagnetization current ILP shared with the external diode in anti-parallel to

the device itself, to a second inactive state in which the external diode is turned off but both

of the two B-E and B-C junctions are forward biased in order to complete the residual

demagnetization of the Lp inductor until the forcing re-circulating current ILP, that imposes

the negative collector current, reaches the zero value. In conduction mode, the two B-E and

B-C junctions of the transistor continue to be forward biased and the bipolar transistor

conducts a positive magnetization current for the inductive load. The turn-off mechanism

occurs in three stages: “storage time”, or time spent to extract the storage in excess and

recombine the charge stored on the base, “current fall time”, in which the collector current

passes from 90% to 10% of its maximum value, and “rise time” of the collector-emitter

voltage. The transition mode occurs during the voltage rise time and represents a sort of

“dead time” phase in which both bipolars are substantially inactive since only the B-C and

B-E junction capacitances of both devices are interested in being charged/discharged in

reverse bias. This phase anticipates the re-circulating phase preliminary to the conduction

phase of the complementary device. During the turn-off process, the dynamic operation

point of the transistor moves through three different operating regions on the current-voltage

IC-VCE characteristic: “hard saturation region”, “quasi saturation region” and “active region”.

●

Hard saturation region: at the first instance of turn-off switching time, the base-collector

junction is forward biased and the base region and collector drift region are both in

high-level injection condition. An excess carrier distribution fills the collector drift region

and the storage time is just the time required to remove/extract this charge stored in

excess. Also the contribution of charge decay due to recombination, depending on the

minority-carrier lifetime, influences this phase. During the storage time process the

base-emitter voltage does not change immediately from its forward bias value VBESAT,

due to the excess minority carriers stored in the base region, and also the

collector-emitter voltage remains constant. The extent of storage time is dependent not

only on the amount of excess charges remaining in the collector drift region but also on

the external driving. Excess minority carriers are removed from the base region and

base-collector junction at a constant rate determined by the negative base drive

voltage, as well as the base drive resistance, and proportional to the reverse base

current IBoff slope. So the excess charge at the base-collector junction begins to reduce

due to charge removal to make up for the reverse base current and the collector current

continues to increase.

●

Quasi saturation region: after the storage time, the forward biased base-collector

junction is out of high-level injection and the remaining charges stored in the base

become insufficient to support the transistor in the hard saturation region. Therefore, at

this point the transistor enters quasi saturation region in which the depletion region

begins to expand while a voltage appears across it and the collector-emitter voltage

starts rising with a small slope. After having removed the charges in the drift region

holding the bipolar in the quasi saturation region, the transistor enters the active region.

●

Active region: crossing the active region, the charges stored in the base region are

insufficient to support the full negative base current so the base-emitter voltage starts

falling negatively and the negative base current starts reducing. Correspondingly, the

stored base charges can no longer support the full load current through the collector so

that also the collector current decays exponentially resulting in current fall time required

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