AN1889
Application note
ESBT
STC03DE170HV in 3-phase auxiliary power supply
Introduction
The need to choose a high value of the fly-back voltage is well known to power supply
designers when efficiency and high duty cycle become important requirements. Three-
phase auxiliary power supplies, starting from a bulk voltage of 750 V, require theoretically
power transistors with a block voltage capability higher than 1200 V. Practical considerations
linked to better efficiency and safe margin may impose to choose devices with even higher
breakdown voltage (i.e. 1500 V, 1700 V). Looking at the power switches currently available,
while power bipolar transistors are strongly limited in switching frequency operation, Power
MOSFETs show a much lower current capability, which may limit their use to low power
applications. Recently available in the market, the ESBTs (Emitter Switched Bipolar
Transistors) represent a valuable and cost effective alternative for all those applications
where high voltage and high switching frequencies are a must.
This application note describes the realization of a 50 W 3-phase auxiliary power supply by
using the STC03DE170HV as main switch for the fly-back converter. Particular attention has
been given to the transformer design as well as to the ESBT driving circuit requirements.
August 2007
Rev 3
1/33
www.st.com
Contents
AN1889
Contents
1
2
ESBT: theory and evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Application description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1
2.2
Pre-design requisite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Fly-back transformer design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3
4
5
6
7
Output circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Clamping circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
ESBT driving circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Current transformer core selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
PWM driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.1
7.2
7.3
7.4
7.5
Primary side regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Control loop compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Switching frequency and max duty cycle setting . . . . . . . . . . . . . . . . . . . 22
Current sensing and limiting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
ESBT gate drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8
9
10
11
Start-up network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
PCB layout and list of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2/33
AN1889
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
ESBT symbol and equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
ESBT switching operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
ESBT cross section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
SMPS simplified block schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Simplified fly-back schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
ESBT driving circuit and relevant waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
ESBT proportional driving circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
ESBT proportional waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
ESBT current transformer driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
ESBT equivalent circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
ESBT fly-back schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Steady state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Turn-on behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Turn-off behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Turn-off losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Steady state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Turn-on behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Turn-off behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Turn-off losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Steady state at minimum voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Steady state at maximum voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3/33
ESBT: theory and evolution
AN1889
1
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
C
C
B
B
G
G
S
S
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 V
CE(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 V
CE(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.
4/33
AN1889
ESBT: theory and evolution
Figure 2.
ESBT switching operation
[V]
V
GATE
V
TH
[V]
V
TH
V
GATE
t
[µs]
t [µs]
ESBT equivalent switching-on circuit
ESBT equivalent switching-off circuit
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. Cascade configuration can be implemented in a single four or
five pins package either as hybrid or as a monolithic single chip solution that combines a
vertical NPN Power transistor and a low voltage standard MOSFET. The choice of a suitable
value of thickness and resistivity for the collector drift layer coupled with the most
appropriate edge termination allows in principle to design devices with blocking voltage up
to 3.5 KV. The integration of a low voltage Power MOSFET inside the monolithic structure
had represented the main challenge in designing the ESBT. In particular the power
MOSFET has been integrated inside the emitter region of the Power bipolar stage realizing
a sort of a sandwich structure: thanks to the adopted solution the silicon area depends only
by the BJT size in spite the whole switch is formed by the series connection of the two
devices.
Figure 3.
ESBT cross section
5/33
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