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ESBT: theory and evolution

AN1889

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ESBT: theory and evolution

The "emitter switching" concept was widely investigated a few decades ago with the aim of

improving the trade-off between switching and conduction losses mainly in high voltage

applications.

Figure 1.

ESBT symbol and equivalent circuit

The configuration can be easily implemented by using discrete components and basically

consists in a high voltage Power Bipolar Transistor driven by a low voltage Power MOSFET,

the two devices result connected in cascade configuration, as shown in fig.1. It is clear that

the structure requires two supplying sources: one to ensure the necessary current to the

base of the power bipolar transistor and the second to drive the gate of the Power MOSFET.

Practically, the Power Bipolar Transistor is biased with a constant voltage source between its

base and ground while a PWM controller could directly drive the gate of the Power

MOSFET.

The On condition is guaranteed just by switching on the Power MOSFET. Being the On

voltage drop on the Power MOSFETs negligible compared the VCE(sat) of the power bipolar

transistor, we can consider as a first approach the emitter of the power bipolar transistor

grounded.

The driving circuit associated to the base supplies the current needed to saturate the power

bipolar transistor, so that the main conduction losses are those related to the VCE(sat) plus

the losses on the input of the power bipolar transistor itself. As a figure of merit, for devices

rated at 1200V we can note that the current density (and consequently in reverse

proportionality the output voltage drop) on Power Bipolar Transistor is 10 times bigger than

that of an equivalent high voltage Power MOSFET.

Starting from the ON-state and switching off the Power MOSFET, the drain current falls

instantaneously down to zero, so that the output current changes its path to the ground

through the base of the transistor itself.

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