Outputs

Use the Outputs Specification form to specify outputs and their parameters:

• Add... - click to  add a new output.

• Remove - click to remove the selected output.

• Edit... -  click to edit the selected output.

• Peak Loads  - select if you wish to design the supply for peak loads.

• LED Driver - changes the Operation Mode to CV/CC to design LED driver power supply.

• Total Peak Power - displays the cumulative peak power. If the Peak Loads check box is selected and you have added at least two outputs, you can click the calculator icon to enter the total peak power that the supply is expected to deliver.

• Continuous Power - displays the cumulative power of the specified outputs.

• Operation Mode - select whether the design operates in CV Only (Constant Voltage) mode, CV/CC (Constant Voltage/Constant Current) mode, or in CC Only (Constant Current) mode. Use the CV/CC mode if the supply is a battery charger or requires fixed output current limit.

• CC Threshold - for CV/CC mode specifies the threshold voltage at which the supply enters CC mode. Click the calculator icon to type a custom threshold value.

• Use SR on Main Output - Specifies whether to use or not synchronous rectifier on main output. If you select NO then the output diode will be used by default.

• Additional SR Output - Available only if you select YES for Use SR on Main Output. For multiple output designs you can choose positive output as additional output with sync rectifier. The new SR is connected to the SR on Main output. This action changes the output winding interconnection to Floating.

Note: For LinkSwitch-PH and LinkSwitch-PL families you only can add a single output.

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Adding Outputs

To specify an additional output, click Add.... PI Expert allows up to six DC outputs to be specified. The main output must have a positive voltage. A negative voltage is only allowed for one output that must not be the main output.

When the Peak Loads check box is selected, you need to specify the peak load values: output current and current duty cycle. The current duty cycle is important because it affects average primary current and thus core selection, turns ratio, output diode, and other various components and operating parameters. The current duty cycle is also important when considering thermal constraints of the system including, device, transformer, and output diodes.

In the Output Voltage Tolerance group you can enter a target output voltage tolerance either by percent or by entering the absolute values. Do this by clicking the calculator icon next to your desired tolerance specification. The output voltage tolerance is considered during multiple output optimization and can affect the choice of transformer core (primarily due to winding window area), turns ratio and output diode. Entering very tight output voltage tolerances may result in an optimization failure.

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Editing Outputs

To edit an output:

1. Select the output you wish to edit by clicking anywhere in its row

2. Clicking Edit... or double click anywhere in the output row.

3. Make the desired changes to the output.

4. Click OK.

PI Expert calculates the power for that output based on the voltage and current entered.

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Selecting the Main Output

The main output usually requires the tightest regulation. PI Expert automatically chooses the lowest voltage output that is above 2.5 V and makes it the main output. However, PI Expert may choose the second smallest output of the outputs above 2.5 V to be the Main Output if the current is greater than two times that of the smallest output voltage. This happens because in most applications the lowest output voltages must be the tightest regulated as they often power microprocessors. Smaller voltages are often derived via a linear voltage regulator or secondary PWM IC if tight regulation is needed on these types of outputs.

If you want to change the main output, select another output from the drop down Main list. The accuracy of the main output is determined by the feedback scheme used and the precision of the reference used (a TL431 will produce greater output voltage accuracy than a Zener diode reference).

The cross regulation (accuracy) of the other outputs will rely on the turns ratio of the other outputs to the main output. The proximity of  the voltage of an additional output to the desired value can be affected by that turns ratio since integer numbers of turns must be used in the winding of practical transformers. PI Expert considers your tolerance specifications during multiple output optimization and chooses the best possible turns ratio for each output to meet your specifications.

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