AN2252 Datasheet, PDF (1/21 Pages) STMicroelectronics – The flyback converter is a popular choice in applications where the required power

AN2252
Application note
Zero-voltage switching and emitter-switched bipolar transistor
in a 3-phase auxiliary power supply
Introduction
The flyback converter is a popular choice in applications where the required power is
normally less than 200W. The main reasons explaining its popularity are its simplicity, low
cost and high efficiency for a small number of active components.
In switching converters power loss is caused by power dissipation within the parasitic
elements of both passive and active components. Power loss in passive components can be
reduced by selecting suitable passive components and carefully designing the transformer.
Power loss in active components can be improved by selecting suitable active components
and making sure that they are used correctly.
Power loss generated by active components can be divided into two categories:
● conduction loss
● switching loss
The aim of the proposed zero-voltage switching control is to reduce switching loss (in this
application, the primary switch turn-on loss). The zero-voltage switching control also greatly
reduces the EMI generated by primary switch turn-on.
Conduction loss is generated with the device fully turned-on, by the voltage drop across the
conducting device. The proposed use of the Emitter-Switched Bipolar Transistor (ESBT) as
the primary switch reduces conduction loss efficiently. Moreover, with its low saturation
voltage and fast switching capability compared to an IGBT or a bipolar junction transistor
(BJT), the ESBT is well suited for this use. These characteristics are essential in
applications where a high breakdown voltage capability is required.
The reference board presented in this Application Note gives a solution of a power supply
for 3-phase applications like inverters for induction motors, welding machines, UPS etc. Very
commonly in this kind of applications, the neutral line is not available or its use is not
allowed, and only phase-to-phase voltage is available. The nominal European phase-to-
phase voltage is 400VAC. Taking into account a ±20% tolerance, the rectified input bulk
capacitor voltage can reach up to 680VDC. The zero-voltage switching topology requires a
reflected flyback voltage equal to the input bulk capacitor voltage. For this reason it is
necessary to use a switch which will accept at least 1500V and exhibits a low conduction
loss during the ON-time. The high-voltage MOSFET switches rated for this voltage, available
on the market today are rather expensive due to their large die size. The ESBT, thanks to its
low voltage drop, high speed, square reverse bias safe operating area, smaller die size and
lower price is well suited for use as a high-voltage power switch.
November 2006
Rev 2
1/21
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