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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Contents
AN2252
Contents
1
2
Theory of ESBT and quasi-resonant operation . . . . . . . . . . . . . . . . . . . 4
Application circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1
2.2
Operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Circuit operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2.1
2.2.2
Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Transformer design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.3
PCB Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3.1
Evaluation and measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3
4
5
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2/21
AN2252
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.
Flyback converter switching cycle - primary switch voltage . . . . . . . . . . . . . . . . . . . . . . . . . 5
Zero voltage turn-on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Non zero voltage turn-on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
ESBT’s internal schematic and symbol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Power transformer dimensions and winding arrangement . . . . . . . . . . . . . . . . . . . . . . . . . 11
Current transformer dimensions and winding arrangement . . . . . . . . . . . . . . . . . . . . . . . . 12
Assembly schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Picture of the converter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Converter efficiency versus output power. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Converter switching frequency versus output power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Primary switch collector voltage, gate voltage and base current at full load and minimum input
voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Primary switch collector voltage, gate voltage and base current at full load and maximum
input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Primary switch collector voltage, gate voltage and base current at 10% load and minimum
input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Primary switch collector voltage, gate voltage and base current at 10% load and maximum
input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Detailed view of the primary switch base current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Detailed view of the primary switch base current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Proportional base current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Detailed proportional base and collector current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3/21
Theory of ESBT and quasi-resonant operation
AN2252
1
Theory of ESBT and quasi-resonant operation
As mentioned in Introduction, the application studied in this Application Note implements
zerovoltage switching (ZVS). This principle of operation is also known as quasi-resonant or
valley switching.
These names come from the waveform shape of the voltage across the primary side switch
during or just before switch turn-on.
Figure 1.
shows the switch voltage, which is the sum of
V
IN
, the DC bulk capacitor voltage, and V
flyback
, the reflected voltage across the primary
winding.
The winding voltage depends on the state of the switch and the amount of magnetizing
energy stored in the magnetic circuit of the transformer. One switching period can be divided
into three basic areas determined by the state of the primary switch and the output diode
conduction: the ON time, the OFF time and the DEAD time areas.
●
●
The “ON time” area corresponds to the time during which the primary switch is on and
the transformerfs magnetizing inductance stores energy.
During the “OFF time”, the primary switch is off and the magnetizing inductance energy
is discharged through the conducting output diode to the output capacitor. A ringing
voltage of amplitude V
spike
also occurs during this phase. It is generated by the layout-
related track inductance and by the leakage inductance created by the imperfect
magnetic field coupling between the transformer primary and secondary windings. The
ringing voltage amplitude is controlled and limited by a clamp circuit.
The “DEAD time” starts once all the stored magnetizing inductance energy has been
discharged to the output capacitor. It is called “DEAD time” because neither the primary
switch nor the output diode is conducting. So there is no energy transfer between the
primary side and the secondary side. The primary winding voltage during this phase is
resonating and has a cosine waveform starting from a voltage equal to the OFF-time
plateau voltage. The DEAD time is used for the initiation of the next switching cycle,
only there is no energy conversion, which is why this concept is called QUASI resonant,
in comparison with pure resonant converters where the resonance of the primary
current or voltage is the means of energy conversion. The voltage waveform has a
negative slope and approaches or can even cross zero. The suitable moment to turn
the primary switch on again is when the voltage across the primary switch is lowest.
The shape of the switch voltage waveform at this point evokes a valley. This is why
quasi-resonant or zero-voltage switching is also called valley switching. The resonance
frequency during the DEAD time is determined by the magnetizing inductance and
parasitic capacitance. The parasitic capacitance consists of the primary switch
capacitance, the transformer winding capacitance, the inter-winding capacitance, the
capacitances of the diodes located in the secondary, auxiliary and clamp circuits
transformed to the primary side. The PCB tracks also generate some parasitic
capacitance depending on the layout.
●
4/21
AN2252
Figure 1.
Theory of ESBT and quasi-resonant operation
Flyback converter switching cycle - primary switch voltage
The zero-switching operation mode was selected because it has a higher efficiency and
produces less EMI. The benefits of using zero-voltage turn-on for the primary switch can be
clearly seen by comparing
Figure 2.
and
Figure 4.
The switch voltage obtained with zero-
voltage turn-on has a higher waveform and the drain or collector current is lower.
Figure 2.
Zero voltage turn-on
5/21
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