Transcript PSU_Part4_IsolatedDCDC - Renesas e

Application information
Power supply unit (PSU)
Part 4…Isolated DC/DC (types, operations, sales guide, etc.)
Renesas Electronics Corporation
General Purpose Systems Marketing Dept.
General Purpose Systems Division
Marketing Unit
Sep. 2011
Rev.1.0
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Introduction to Part4
 Purpose
This course provides basic knowledge of power supply units
 Objectives
 Learn about types of isolated DC/DC
 Learn about isolated DC/DC operation
 Learn about the sales guide for isolated DC/DC ICs
 Contents
 21 pages
Learning Time
 30 minutes
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Types of isolated DC/DC
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Is isolated DC/DC IC same as PWM
IC?
 ICs for isolated DC/DC converters have a variety of
names, including
1. PWM ICs
2. AC/DC ICs
3. Main converter ICs
etc.
 In this material,we will use“ Isolated DC/DC IC”.
◊ The generic term "PWM IC" is unsuitable to use for "resonance
converter ICs", which do not have duty control.
◊ "AC/DC" refers to the "function" of an AC adaptor or power supply unit.
As the ICs are used for circuits that convert from DC to DC, the
expression "AC/DC ICs" is not appropriate.
◊ With "main converter ICs", it is not clear what is main and what is sub.
◊ DC/DC ICs used on the secondary side that do not use a transformer
are called a "non-isolated DC/DC ICs”, or also called a "chopper ICs."
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Types of isolated DC/DC
 The major types are shown in below
 There are other types besides these, such
as push-pull, RCC, etc., but we will not talk
about them here.
Feedback circuits are not shown in the
figures.
 Which type to be use in PSU is decided
mainly by power range.
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Flyback (1)
This is the simplest isolated DC/DC converter.
Please note the reversed directions of the primary and secondary
windings of the transformer. (starting position of winding is indicated
by green dot)
 When the MOSFET turns on,
current flows to the primary side
of the transformer as shown by
the red arrow.
 Voltage appears on the secondary
side of the transformer and tries
to flow current in the direction of
the blue arrow.
 This voltage is the reverse of the
diode direction, so no current
flows on the secondary side.
Flyback converter
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 Therefore, power input to the
primary side is stored in the
transformer as a magnetic field.
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Flyback (2)
 When the MOSFET turns off, the
primary side current of the
transformer turns off.
 The energy stored in the
transformer generates voltage
to generate current in the
direction of the blue arrow on
the secondary side, and the
capacitor is charged.
Flyback converter
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 The IC monitors the output
voltage using a feedback circuit,
(not shown in the left fig.) and
controls the duty of the MOSFET
gate pulse to regulate output
voltage.
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Forward (1)
 Please note that the direction of the primary and secondary
windings of the transformer are the same, and that there is a
inductor on the secondary side.
 When the MOSFET turns on,
current flows to the primary
side of the transformer as
shown by the red arrow.
 Current flows in the secondary
side of the transformer as
shown by the blue arrow,
passes through the inductor,
and charges the capacitor.
 Energy is stored in the
inductor during this period.
Forward converter
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Forward (2)
 When the MOSFET turns off, a voltage trying to flow current in the
direction of the yellow arrow appears in the secondary winding of the
transformer, but because it is in the reverse direction of the diode, no
current flows in the secondary winding of the transformer.
 On the other hand, using the
energy stored in the inductor,
current flows as shown by the
blue arrow, charging the
capacitor.
 Since most of power input to
the primary side is transmitted
to the secondary side while the
MOSFET is on (the transformer
does not store much energy),
a smaller transformer than
flyback can be used.
Forward converter
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 Forward is suitable for higher
power applications than flyback.
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Full-bridge
 When Q1 and Q4 are on and
Q2 and Q3 are off, current
flows as shown by the red
arrow.
 When all Q1 to Q4 are off, no
current flows.
 When Q1 and Q4 are off and
Q2 and Q3 are on, current
flows as shown by the blue
arrow.
Full-bridge converter
 The IC monitors the output
voltage and controls the duty of gate pulses for each pair of MOSFETs
to regulate the output voltage.
 Since voltage V1 is added to the primary winding which changes
polarity alternately, transformer utilization rate is high in full-bridge.
 For high efficiency, the secondary side diode is replaced by a MOSFET
and synchronous rectification is performed (R2A20121, 124A support
synchronous rectification).
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Soft switching
 Soft switching is switching when the MOSFET source-drain
voltage or current is zero or close to zero.
 Soft switching is available for each type of isolated DC/DC
(next page).
 Soft switching can reduce the amount of heat and noise
generated, so it is used for situations where high efficiency
and low noise are important.
Example of zero current switching
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Example of zero voltage switching
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Soft switching
Flyback and Half-bridge examples.
 Flyback: By monitoring ZCD winding in the transformer, turns the
MOSFET from off to on at the same time secondary side current
becomes zero (zero current switching: ZCS). -> This type is called a
(flyback) quasi-resonant converter.
 Half-bridge: Connect an inductor and a capacitor in series to the
transformer, and make them resonate. The shape of the current
waveform flowing through the transformer approaches a sinusoidal wave,
reducing noise. -> This type is called an LLC resonant converter.
Quasi-resonant converter (flyback)
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LLC resonant converter (half-bridge)
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Full-bridge soft switching
(1)
 The figures on the right show the
phase shift method.
 In phase shift, each MOSFET (Q1-Q4)
are turned on and off independently.
 When Q1 and Q4 are on, current
flows as shown in fig. a.
 Then, Q4 is turned off, the Q4
capacitor is charged and the voltage
of the capacitor becomes V1, and the
Q3 capacitor discharges to 0 (fig. b).
 At this time, Q3 is turned on (fig. c).
 Next, Q1 is turned off (fig. d).
 The Q1 capacitor is charged and
becomes V1, and the Q2 capacitor is
discharged to 0.
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Full-bridge soft switching (2)
 Q2 is turned on next, the current is as shown in fig.
e, which is the opposite of fig. a on the previous
slide.
 After this, performing Q3 off -> Q4 on -> Q2 off ->
Q1 on will return to fig. a on the previous slide.
 Each MOSFET carries out ZVS (zero voltage
switching), so efficiency improves.
 The phase of the gate signal is shifted and the
period when Q1 and Q4 or Q2 and Q3 turn on
simultaneously is controlled.
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List of types of isolated DC/DC
(): Old product
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Sales guide for isolated DC/DC
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Sales guide
Renesas is focusing on promoting full-bridge phase shift
IC in isolated DC/DC area.
Full-bridge phase shift (soft switching) with
synchronous rectification
• R2A20124A (improved version of R2A20121)
• R2A20121
Suitable for over 1 kW systems, such as
• Servers
• Base stations
• Office routers
• Rapid charging stands for electric and hybrid
vehicles (not in-vehicle)
Please promote for the above applications.
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Sales guide (documents and tools)
The following items intended for customers are available.
Please use them.
One-page guide:
One-page summary of IC features
Presentation:
customer.
Overview of ICs. Can provide to
Datasheet:
Explanation and specification of IC
Application notes:
Explanation of evaluation board,
design examples summarized
Excel sheet:
For calculating circuit constants of
external parts (R2A20124A)
Evaluation boards:
Boards used for evaluation (all may
be lent out) (R2A20124A)
IC samples:
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Supplement: Feedback circuit of
isolated DC/DC converter
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Feedback circuit and photocoupler
 Typically, a feedback circuit
uses a photocoupler (right
figure).
 A photocoupler is used to
isolate the feedback signal.
 The output voltage is divided
by resistances R1 and R2, and
inputted to the IC FB
(feedback) pin via the
photocoupler.
 The IC controls the duty of the
gate pulse of the MOSFET by
monitoring FB signal.
 Output voltage can be
adjusted by changing the
values of R1 and R2.
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Example of feedback of
a flyback converter
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Thank you
Thank You
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Renesas Electronics Corporation.
©2011. Renesas Electronics Corporation. All rights reserved.