TI E2E Community

Menu

Search through millions of
questions and answers

User

Menu

Search
through millions of questions and answers

User

TI
E2E Community

Support
forums

Amplifiers

Applications

Processors

Clock and Timing

Data
Converters

Development
Tools

Digital Signal Processors
(DSP)

DLP®
Products & MEMS

Embedded Software

Interface

Isolation

Logic

Microcontrollers

OMAP™ Applications Processors
(Read Only)

Power
Management

RF & Digital
Radio

Sensor Products

Other Analog &
Touch

Wireless
Connectivity

/etc
Support

Blogs

Analog Wire

Analog: The real
world

Behind the
Wheel

ConnecTIng
Wirelessly

Enlightened: The TI
DLP® Blog

Fully Charged

Industrial
Strength

Launch Your
Design

Learn E2E

Motor Drive &
Control

MSP Low-Power
Plays

On the Grid

Power
House

Precision Hub

The Process

Think.
Innovate

TI Live @...

Tools Insider

University
Program

Groups

Corporate
Citizenship

TI University
Program

Russian E2E (сообщество
E2E)

Japanese E2E (日本語コミュニティ)

Learn E2E

Launch Your
Design

Motor Drive &
Control

Videos

More

Cancel

Blogs

Power House

Sign In to Post

Cancel

Vijay Choudhary86929

Apr 1, 2014

LM5019

Fly-Buck

buck
converters

layout

LM5018

LM5017

PCB

wide
VIN

RSS for
posts

Share
this

Options

Cancel

Related

Fly-Buck converter PCB layout tips

Tweet

Synchronous buck converters have gained acceptance as isolated bias supplies
in communications and industrial markets. Isolated buck converters, or Fly-Buck™
converters as they are commonly called, have a coupled inductor in place of
the buck converter inductor which is used to create isolated outputs in addition
to the non-isolated buck output. Each isolated output only needs a winding, a
rectifier diode, and an output capacitor. Multiple semi-regulated isolated, or
non-isolated, outputs can be generated using this topology in a simple and cost
effective manner.

There are some key differences in the current flow in a buck and a Fly-Buck
converter. The switching current loops in a buck converter are well known and
are shown in Figure 1. The input loop consisting of input bypass capacitor,
VIN pin, high and low side switch, and the ground return pin carries
switching currents. This loop should be optimized with minimum trace lengths and
loop area for a quiet operation. The output loop consisting of low side switch,
inductor, output capacitor and ground return path carries essentially a DC
current with low ripple. While it is important to keep all current flow paths
small for low dc drops, low losses, and low regulation error, the area of this
loop is not as important as the input current loop.

Figure 1. The current loops in a
buck converter. VIN loop is a high di/dt loop.

The Fly-Buck converter primary side appears similar to the buck converter as
shown in Figure 2. The VIN loop here is a high di/dt loop just like
in a buck converter. The VOUT1 loop however has very different
current flow then in a buck converter. In addition to the primary inductor
magnetization current, this loop also has the reflected current from the
secondary windings. The reflected current has only the leakage inductance of the
coupled inductor in its path and therefore has much higher di/dt than the
magnetization current of the inductor. It is therefore important to minimize the
loop area of VOUT1 loop as well. For the same reason, the secondary
output loop consisting of secondary inductor winding, rectifier diode, and
secondary output capacitor also needs to be minimized because it has high di/dt
current flowing through it.

Figure 2. Fly-Buck
converter has two high di/dt loops on the primary. All the secondary
loops are high di/dt.

Another important point to remember when laying out a Fly-Buck converter is
to realize that the secondary winding also has a switch node. This secondary
switch node (SW2) is high dv/dt node and has a voltage transition of VIN*N2/N1.
It is customary therefore to keep the SW2 trace area small to keep it from
radiating noise.

A layout example incorporating the guidelines in this article is shown in
Figure 3. The high di/dt loops on the primary and secondary sides are minimized
as are the switch node areas.

Figure 3. A Fly-Buck layout based on LM5017 to minimize the di/dt loops and the high di/dt SW1,2
node areas.

Resources:

Read an application note: “Designing
an Isolated Buck (Fly-Buck) Converter”

Watch a video: “LM5017 Fly-Buck Multi-Output Demo”

Find out more about TI’s wide VIN products here

Read an article in How2Power: “Synchronous Buck Converter Enables Multiple Bias Rails And
Input Voltage Sensing For Isolated Applications”

Sign
in to Comment

Chris Belting

Vijay, I've been looking at the applications information for this topology
and have a couple of questions.

In a few of write-ups and the video, the inductor current is shown during
the turn-off period as exponentially rising in the secondaries and falling in
the primary.

1. Can you explain the reason for this exponential shape?

2. How does the load current affect the shape?

In those related documents there is a discussion regarding the control
method used and cautions that certain types of control are not suitable but
that the Constant On Time control implemented by the LM5017 is well suited to
this topology.

3. Are there papers, books or application notes providing additional
guidance or deriving the control-to-output transfer function for this topology
especially if one were to use peak current-mode control with for example the
LM21305?

Thanks

Apr 13, 2014

More

Cancel

Vijay Choudhary86929

Hi Chris,

#1 Please refer to application note "Designing an isolated buck (Fly-Buck)
Converter" for this waveform. In short, during the off time "TOFF" the primary
and secondary output capacitors are connected through the leakage impedance of
the transformer. This results in a LC charging current from COUT1 to COUT2.
This current is superimposed on the primary buck current. That is what looks
like the LC current. If the leakage is small enough, the current will
ultimately start decreasing and the IL1 waveform will turn back towards zero
(from a negative value).

#2 The higher the load current the higher the LC charging current (COUT1 to
COUT2). Accordingly the IL1 current will look steeper and will go more
negative.

#3 We have analyzed Fly-Buck mostly with COTs. My main point was that
because COTs are easier to design than voltage or current mode parts from a
stability point of view, their is no significant change is the design process
between a buck and a flybuck. Unfortunately at this point I don't have any
document that explains the transfer function of a Fly-Buck.

Apr 15, 2014

More

Cancel

TI E2E™ Community

Support
Forums

Blogs

Videos

Groups

Site Support
& Feedback

Settings

TI E2E™ Community Groups

TI
University Program

Make the
Switch

Microcontroller
Projects

Motor Drive
& Control

Other Communities

Deyisupport

Designsomething.org

beagleboard.org

TI
on Element 14

TI
on TechXchangeSM

Other Technical & Support Resources

WEBENCH® Design Center

Product Information
Centers

Technical
Documents

TI Design Network

TI
Technical Articles

TI
Training

All content and materials on this site are provided "as is". TI and its
respective suppliers and providers of content make no representations about the
suitability of these materials for any purpose and disclaim all warranties and
conditions with regard to these materials, including but not limited to all
implied warranties and conditions of merchantability, fitness for a particular
purpose, title and non-infringement of any third party intellectual property
right. TI and its respective suppliers and providers of content make no
representations about the suitability of these materials for any purpose and
disclaim all warranties and conditions with respect to these materials. No
license, either express or implied, by estoppel or otherwise, is granted by TI.
Use of the information on this site may require a license from a third party, or
a license from TI.

Content on this site may contain or be subject to specific guidelines or
limitations on use. All postings and use of the content on this site are subject
to the Terms of
Use of the site; third parties using this content agree to abide by any
limitations or guidelines and to comply with the Terms
of Use of this site. TI, its suppliers and providers of content reserve the
right to make corrections, deletions, modifications, enhancements, improvements
and other changes to the content and materials, its products, programs and
services at any time or to move or discontinue any content, products, programs,
or services without notice.

Follow Us

TI Worldwide | Contact Us |
my.TI
Login |
Site Map | Corporate
Citizenship |
m.ti.com (Mobile Version)

TI is a global semiconductor design and manufacturing company. Innovate with
100,000+ analog ICs and
embedded processors, along with software, tools and
the industry’s largest sales/support staff.

© Copyright
1995-2015 Texas Instruments Incorporated. All rights reserved.
Trademarks |
Privacy
Policy | Terms
of Use