Flyback
Wizard Documentation:
The wizard dialog
should be run before doing anything else. The dialog is nonmodal. As
long as you don't modify the winding structure by moving or adding windings,
you can go back and make changes. If you move windings around, the dialog
will lock you out. It assumes that you are building a transformer coupled
Flyback Power Supply. The data on the top and left side has been set to
a reasonable default. It should be changed based on your design specifications
and preferences. Here's what the entries mean:
These
Core Selections are also Available using the Core Tab:
Family:
A list of core families in the database.
Vendor: A list of vendors that supply the
Family and Material.
Material: The magnetic core materials available
from the selected vendor
Design
Parameters on the Left Side:
Frequency:
Power Supply Switching frequency.
Efficiency:
Fraction of input power supplied to the outputs
do not include output voltage drops, they should be included in the
output voltage specification.
RatioIsat/Ipk:
Defines the current, Isat, at which the core may
saturate. The maximum PWM current should be set below this value. When
MD adjusts the gap to calculate inductance, the inductance goes up with
reduced gap until the core begins to saturate, then the inductance goes
down. MD is allowed to maximize this inductance so that reduced inductance
(saturation) may begin slightly above Isat.
Primary
Duty Ratio: The fraction of a switching cycle
that the input switch is turned on.
Secondary
Duty Ratio: The fraction of a switching cycle
that the output switch is turned on. It is assumed that all secondaries
switch at the same time. While this is not strictly true, it gives reasonable
results for sizing the transformer. Simulation data can improve accuracy.
Percent
Ripple: The change in current during the TurnOn
time divided by the peak current at the end of the charging cycle expressed
as a percentage.
Primary
Power: A calculated value that is the sum of load
power divided by efficiency.
Winding
Data:
Winding:
An integer number that corresponds to the Spreadsheet winding number.
You create new windings using the Insert Button.
Voltage: The Power Supply DC Voltage.
Current: The Power Supply DC Current.
Turns Ratio: A computed value based on
previously entered data.
Buttons:
Previous:
Decrements the winding number.
Next: Increments the winding number.
Delete: Deletes the present winding.
Insert: Inserts a winding after the current
winding.
Close:
Hides the dialog.
Apply:
Fills in the
Spreadsheet with requirements if the Lock Design isn't checked. If the
"Lock Design" box is checked, the spreadsheet AC and DC currents
are updated. This allows you to design at one operating point and evaluate
performance at other operating points. Most commonly, the losses over
a range of input voltages can be examined.
How
it Works: The
wizard uses the mathematical relations that define a Flyback converter
to compute the input data needed by Magnetics Designer. MD need the Peak
current, the AC current, DC current and required minimum inductance for
each winding. In addition it needs to get the Flux Swing set to half wave
and fills in the frequency and Duty Ratio. You can change the default
core family, its magnetic properties and vendor. When you have finished
entering the data, press apply and MD will go through its auto design
process and the wizard will check its lock box. You can return to the
wizard and change input voltage and press the apply button to see how
the operating point affects your design. With the lock box checked, the
Wizard doesn't vary structural parameters, only the Currents that affect
the power dissipation.
Background:
Flyback Power Supplies have a large set of free parameters. In particular,
the Primary Duty Ratio and Percent Ripple can be combined to yield a myriad
of design possibilities. As the ripple is reduced, the high frequency
copper losses are also reduced at the expense of transformer size. When
the ripple is increased to 100%, the Power Supply operates in discontinuous
conduction mode and you can specify the output duty ratio. In discontinuous
conduction mode, the output switch will be turned off when the input switch
turns on, reducing switching losses at the expense of increased stress
in nearly all other power components. Your design is constrained by the
availability of technology and its cost. Even so, there are most likely
a number of design approaches that have satisfactory outcomes. The SMPS
wizard allows you to explore these combinations and to get detailed Spice
simulation models and check out the effect of the wizard input data on
important design parameters.
Flyback Power Supplies
alternate between transformer primary and secondary conduction. The nonconducting
winding is exposed to the fields of the conducting windings and the gap
field. The nonconducting winding(s) will then develop eddy current losses.
These losses quickly increase with frequency. From a time domain perspective,
a great deal of loss occurs in the current switching interval. Magnetics
Designer accounts for these losses when the <Apply Using Fields>
button is pressed. If the windings are barrel wound and extend over the
length of the stack, then the <Apply Using Fields> button will add
the gap loss term. If you experience a sharp increase in loss, you should
move the wire at least 3 gap lengths away from the gap (or move the gap
away from the wires if possible). Breaking the gap into a number of small
segments is an effective way to reduce eddy current losses. In the limit,
a powdered toroid eliminates gap loss. For core materials with distributed
gaps, you can effectively eliminate gap loss by pressing the options button
and setting the gap field to 1p. Litz wire is always on option; however,
you can generally make the wire diameter smaller by increasing the strands
and by reducing the ripple current. It's probably worthwhile to make several
designs with differing ripple current, primary duty ratio and frequency
in order to get a feel for the boundaries of an acceptable design. Optimizing
the transformer without considering the effects of Power Supply free parameters
could result in an unnecessarily costly design.
Another significant
Power Supply loss occurs because of leakage inductance. This loss is on
the order of Leakage*Ipeak^2*Frequency. Optimized high frequency magnetic
devices tend to under fill the available winding area because the increased
loss experienced by large diameter wire. The automatic designs produced
by MD may fail to fill out a layer. You can increase the number of strands
to fill a layer and possibly reduce the wire diameter (increase the AWG
index). In doing this, you will reduce the leakage inductance as well
as the gap induced eddy current losses. You can use the under fill to
move the winding away from the gap, using the Bobbin tab, <Bobbin Thickness>
entry.
Hints:
Turns ratio quantization may affect the accuracy of the turns ratio and
winding inductance. To improve accuracy, increase the <Min Turns>
button data input. It controls the minimum turns on the primary. Set the
<Round Coef.> button value to .5 for best accuracy.
To get windings to
extend across the bobbin, increase strands and decrease wire diameter.
When the number of layers is less than or equal to an integer (usually
1.0), you have the right input. You can enter a number for the Wire AWG
and press <Enter>. The recalculation will be done and the Wire AWG
data will be high lighted so you can repeat the procedure watching the
Number of Layers data. Magnetic devices made in this manner will have
the lowest leakage inductance. You can similarly increase the number of
strands to extend the width of a winding.
Reduce increased temperature
rise using <Apply Using Fields> by increasing the bobbin thickness
or use the Field Analysis Options dialog to move the gap by setting gap
location = 95 (center at 95% of the length).
Use smaller diameter
wire (the higher voltage winding) nearest to the gap. This winding will
have the largest proximity and gap field. Using smaller wire reduces these
losses because smaller wire conducts better at higher frequencies than
larger wire.
Avoid using a single
gap under the windings. The gap related eddy current losses increase with
fewer gap segments.
Working
through an example:
 Start
MD and select the inductor tab. Then press the SMPS Wizard button.
 Enter
150 for Voltage then press insert.
 Enter
5.5Volts and 10Amps for the new winding.
 Press
Apply.

Press the bobbin tab, notice that the first winding extended into a
second layer.
 Go
back to the inductor tab and increase the Wire AWG to 22 to get 1 layer.
 Now
increase the secondary wire AWG to 16 and increase its strands to 4
get 1 layer.
 Press
Apply using fields, note the temperature increase.
 Go
to the bobbin tab and increase thickness to center the winding.
 Go
back to the inductor tab and Apply using fields.
The increased spacing
increased leakage inductance, reducing eddy current loss, saving .4 Watts.
To make this design
work; the strands need to lie flat in a single layer, they can't be twisted.
Alternatively, you could have used Litz wire with its increased cost and
handling difficulties.
Now go to the IsSpice
tab and copy the netlist from the copyright notice to the end. Then open
the FlybackTemplate3005.dwg and paste the models into the X11 subcircuit.
To do that, select the TranSubckt configuration and double click on X11.
Then double click on the .SUBCKT data field. Then select the part of the
model to be replaced and paste the new model in. That's a shortcut that
avoids making new library entries when you are doing whatif analyses.
In the model you can see several important parameters. First, Lmag is
the primary side switched inductance and the Efwd2 gain (.1154 is the
turns ratio). You will need to modify the main circuit parameters for
the Avg, AvgSubck and AvgSubcktLs configurations. You can also keep the
schematic symbol up to date with the correct turns by selecting the transformer
and choosing Edit Symbol in the Edit menu. After correcting the symbol
nomenclature, select replace on drawing from the File menu.
After running a transient
simulation, you can measure the AC and DC currents in the windings and
update the inductor spreadsheet to refine your loss predictions.
Buck
Regulator Wizard Documentation:
The wizard dialog
should be run before doing anything else. The dialog is nonmodal. The
dialog is nonmodal. As long as you don't modify the winding structure
by moving or adding windings you can go back and make changes. If you
move windings around, the dialog will lock you out. It assumes that you
are building a transformer coupled Buck Regulated Power Supply. The data
on the top and left side has been set to a reasonable default. It should
be changed based on your design specifications and preferences. Here's
what the entries mean:
These
Core Selections are also Available using the Core Tab:
Family:
A list of core families in the database.
Vendor: A list of vendors that supply the
Family and Material.
Material: The magnetic core materials available
from the selected vendor
Design
Parameters on the Left Side:
SMPS
Type: Choose
one of the 3 topologies.
Forward:
The topology is halfwave, it has DC current and lets the core relax
back to its residual flux.
PushPull:
Centertaps primary and secondary windings. It has symmetric flux swing
and DC current in its windings.
Bridge:
Requires full wave switching on primaries and secondaries. There is
no DC current.
Frequency: Power Supply Switching frequency,
for PushPull and Bridge converters, the PWM frequency is twice the
transformer frequency.
Efficiency:
Fraction of
input power supplied to the outputs; it does not include output voltage
drops, they should be included in the output voltage specification.
Primary Duty Ratio: The fraction of a switching
cycle that the input switch is turned on.
Secondary Duty Ratio: The fraction of a
switching cycle that the output switch is turned on. It is assumed that
all secondaries switch at the same time. While this is not strictly
true, it gives reasonable results for sizing the transformer. Simulation
data can improve accuracy.
Percent Ripple: The change in current during
the TurnOn time divided by the peak current at the end of the charging
cycle expressed as a percentage.
Primary Power: A calculated value that
is the sum of load power divided by efficiency.
Winding
Data:
Winding:
An integer number that corresponds to the Spreadsheet winding number.
You create new windings using the Insert Button. For PushPull, there
will be a pair of windings for each power supply input or output voltage.
Voltage:
The Power
Supply DC Voltage
Current:
The Power Supply DC Current
Turns
Ratio: A computed
value based on previously entered data
Switched
Inductor:
A computed value that you should use in the output stage
Buttons:
Previous:
Decrements
the winding number.
Next:
Increments the winding number.
Delete:
Deletes the present winding.
Insert:
Inserts a
winding after the current winding.
Close:
Hides the dialog.
Apply:
Fills in the Spreadsheet with requirements if the Lock Design isn't checked.
If the "Lock Design" box is checked, the spreadsheet AC and
DC currents are updated. This allows you to design at one operating point
and evaluate performance at other operating points. Most commonly the
losses over a range of input voltage can be examined.
How
it Works: The wizard uses the mathematical relations that define
a Buck regulated DCDC converter to compute the input data needed by Magnetics
Designer. MD needs the Average voltage, the AC current and DC current
for each winding. In addition it needs to get the Flux Swing set to half
wave for Forward the topology and fills in the frequency and Duty Ratio.
You can change the default core family, its magnetic properties and vendor.
When you have finished entering the data, press apply and MD will go through
its auto design process and the wizard will check its lock box. You can
return to the wizard and change input voltage and press the apply button
to see how the operating point affects your design. With the lock box
checked, the Wizard doesn't vary structural parameters, only the Currents
that affect the power dissipation.
Background:
Buck regulators have fewer free parameters than the Flyback
topology because the switched inductor isn't included with the transformer.
When comparing Forward and Flyback designs, the Flyback transformer will
be bigger and/or lossier. The percent ripple can be combined with duty
ratio to yield a myriad of design possibilities. The design point is usually
for the highest load power and it usually has ripple less than 100%. The
tradeoff between ripple and stress is nearly the same as discussed for
the Flyback designs. The Forward topology also carries the concern for
transformer saturation during startup. You can multiply the ratio of
(saturation flux density) / Bac times the Edt value in the spreadsheet
to get the turnon Voltsecond capacity. By adding a gap to reduce the
magnetizing inductance, you can use the PWM sensed current to limit the
turnon voltseconds (Lmag = Vdt / Imax). Frequently the minimum gap does
the trick  this gap also adds slope compensation!
 Start
MD and select the transformer tab. Then press the SMPS Wizard button.
 Enter
28 for Voltage then press insert.
 Enter
5.5Volts and 10Amps for the new winding.
 Change
the on duty ratio to .5 and the ripple to 25%
 Press
Apply.

Press the bobbin tab, notice that the windings aren't spread across
the bobbin.
You should spread
windings across the bobbin by increasing the strand in each winding to
6 which reduces the leakage inductance from 96nH to 32nH. For low voltage
designs, the turnoff over shoot is limited to Vthreshold*Leakage/Lsource.
Low leakage inductance can eliminate the need for snubber components.
Press Apply using
fields; note the temperature didn't change much. That characteristic of
barrel wound transformers when the windings spread all the way across
the bobbin.
To make this design
work; the strands need to lie flat in a single layer, they can't be twisted..
Now go to the IsSpice
tab and copy the netlist from the copyright notice to the end. Then open
the Forward Template, FwdTemplate.dwg and paste the models into the X2
subcircuit. To do that, select the TranSubckt configuration and double
click on X11. Then double click on the .SUBCKT data field. Then select
the part of the model to be replaced and paste the new model in. That's
a shortcut that avoids making new library entries when you are doing whatif
analyses. In the model you can see several important parameters. You also
need to make L2, the switched inductor 5u as shown in the SMPS wizard.
Run the TranSubckt
configuration using the tran 2m simulation setup.
You will need to modify
the main circuit parameters for the Avg and AvgSubck configurations by
changing the turns ratio, N=.6667, Lmag=26u and L=17.5u. You can also
keep the schematic symbol up to date with the correct turns by selecting
the transformer and choosing Edit Symbol in the Edit menu. After correcting
the symbol nomenclature, select replace on drawing from the File menu.
Notice that the magnetizing inductance provides slope compensation of
MC=.15 for the Forward_ID PWM model you need to enter MC=.15; however,
the Forward_I PWM model calculate this effect using the Lmag paramter.
After running a transient
simulation, you can measure the AC and DC currents in the windings and
update the inductor spreadsheet to refine your loss predictions.
