.The stacking dialog allows you to choose how each output winding is to be electrically connected in relation to one another. Windings are often connected to one another in series with the output voltage increasing to improve cross regulation. The term "stacking" is used to describe how the windings are stacked on top of each other producing successively higher output voltage with each added turn. There are different electrical connections (stacking configurations) available:
AC Stacked
DC Stacked
Floating
Please see the PI Expert Suite topic Appendix A: Designing Multiple Output Flyback Power Supplies for more information on each of these stacking configurations.
Consider a 2 output transformer winding: a 5 V winding with 5 Turns and a 12 V winding with 12 Turns. AC stacking is the process when the 12 V output shares its first 5 Turns with the 5 V winding. The balance 7 turns is exclusive to the 12 V winding.
AC stacking provides better cross regulation than floating outputs where the 5 V and 12 V windings do not share any turns. This is because in an AC stacked configuration, the lower voltage winding (which is usually the regulated winding) holds part of the voltage very tightly regulated. In the above example, variations in the 12 V winding are reduced by approximately 50% due to the fact that the voltage generated across the first 5 Turns is very tightly regulated. The only variation on the 12 V winding due to load variations will arise as a result of imbalances in the upper 7 turns.
DC stacking is the process when the top winding (higher voltage) is returned to the DC regulated side on the lower winding. DC stacking offers all the advantages of AC stacking, but a slightly better regulation than AC stacking and, due to the addition of diode drop, can better center the output voltage. However, it is a less efficient circuit since all of the higher voltage output current also flows in the lower voltage output diode.
A floating connection (also known as independent connections) is a connection when there is no sharing of turns between two output windings. Although floating windings do not offer the same regulation advantages as AC or DC stacked windings, floating windings offer galvanic isolation between outputs. An example of a preferred use of floating output connection is for driving high side and low side drivers.
As discussed in the Optimization Overview help topic, during the optimization process numerous output stacking configurations are examined. AC Stacking is the most preferred configuration since it gives good cross regulation across the load range. However, if your specified tolerances cannot be met, the software will also try to use a combination of AC Stacked, DC stacked, and floating windings to best meet your specifications.
If you do not wish to use DC Stacking during optimization, you can disable it by unchecking the Enable DC Stack box in the Stacking dialog. You can also disable DC Stacking by default on all new designs by using the Design Defaults tab of the Preferences dialog.
The Type list allows you to specify the stacking configuration. For example, you might want a winding floating so that the winding is galvanically isolated from the others. PI Expert gives you the flexibility to manually select your preferred stacking configuration and then optimize the design using the stacking configuration you have chosen. Optimization returns the best possible design results using only the stacking configuration specified.
You can modify the stacking configuration by selecting another value from the Type list. The row where you change the stacking type is marked with .
Depending on the number and voltage of the windings available, your choice of connection types will change for each output winding. For example, if you have only two positive outputs and you wish to make one winding floating, you must actually make both windings floating as it doesn't make sense to AC stack only one output. In this example, if you do not configure both windings as floating, PI Expert will present you with an error message and a suggested fix.
You can also choose the connection type for negative windings; however, your choices are limited to a Common Return (RTN) or Floating. PI Expert also supports multiple outputs with the same output voltages and will make the windings floating as necessary to meet your specifications.
You can restore the automatically selected stacking type by clicking Restore Defaults.
It is also important to note that the software does not allow you to DC Stack a winding on top of the main output. PI Expert issues an error in this case and advises you on how to fix the problem.