AN1889 STMicroelectronics, AN1889 Datasheet - Page 2

• Manufacturer Part Number: AN1889
• Description: ESBT STC03DE170 IN 3-PHASE AUXILIARY POWER SUPPLY
• Manufacturer: STMicroelectronics

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 cascode 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 V
as a first approach the emitter of the power bipolar transistor grounded as shown in fig.2a.
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 V
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 (see fig.2b).
Figure 2: ESBT Switching Operation
Being the negative base current equal to the collector current, the resulting turn-off time is by far lower
than any traditional power bipolar transistor and comparable with that of a high voltage Power MOSFET.
In fact, thanks to the floating emitter configuration, the high value of the reverse base current results in a
fast removal of the charges stored in the base, achieving both reduced storage time and, most important,
the structure results virtually free of that tail current that characterizes all power bipolar based devices. It
is worth to be noted that the configuration gives an extra safety margin in reverse safe operating area, by
increasing the ruggedness versus the secondary breakdown, in fact since the emitter is open the
phenomenon of crowding current under the emitter finger (with possible creation of hot spot) is practically
absent.
2/25
Fig2a. ESBT equivalent switching-on circuit
V
[ V ]
G A T E
V
T H
t [ s ]
CE(sat)
of the power bipolar transistor, we can consider
V
[ V ]
V
G A T E
T H
Fig2b. ESBT equivalent switching-off circuit
t [ s ]
CE(sat)
plus the losses on the