The power transformer design is perhaps one of the most critical step in designing a converter, especially one that uses a Secondary-Side Regulated (SSR) flyback configuration. Fairchild’s Flyback/Forward PWM design tool helps make quick work or designing and prototyping the power transformer. The tool walks you through the steps of configuring an SSR flyback converter, including magnetic core selection, bobbin, wire size, winding method, and more. The tool also includes a Simplis simulation model that lets you verify circuit operation once the design is done, and you can issue a Bill of Materials (BOM) based on the circuit parameters.

You can access the Flyback/Forward PWM tool from the main page of the Fairchild Semiconductor website (www.fairchilsemi.com). Using the dropdown menus along the top of the page, navigate from Tools & Support to Design Tools, and then to Flyback/Forward PWM. This is what we’re calling the SSR design tool.

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 1

 

The next screen is the log-in page for “My Fairchild Account.” Once you’re logged in, you’ll see the first page of the design tool, shown below.

 

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 2

The left-hand side of the screen shows the nine steps in the design process.

  1. Design Requirements
  2. Controller Selection
  3. Core & Turns
  4. Windings
  5. Power Semiconductors
  6. Output Filters
  7. Control
  8. Snubbers & Input
  9. Efficiency

The right-hand side of the screen shows the first page for Step 1: Design Requirements. We’ll go through each step page by page.

Step 1. Design Requirements

1a. Input

Enter the type of input (AC or DC), and specify the input voltage range. If the input is AC, enter the line frequency.

1b. Output DC

Enter the number of outputs on the load side. If there are multiple outputs, enter the output voltage and current for each one.

1c. Output Regulation

 

Enter the requirements for output ripples, over-shoots, and dips during dynamic load conditions.

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 3a

1d. Operating Conditions & Input Capacitor

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 4

Enter the expected efficiency and the input capacitance, based on the input voltage ranges.

Step 2: Controller Selection 

2a. Choose Controller + MOSFET

Enter the maximum drain-to-source voltage for the primary-side FET, and the  maximum current flowing through it. The SSR tool calculates and lists the suitable ICs.

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 5

2b. Magnetizing Inductance, Vcc

Enter the number of turns of the primary side winding on the magnetic core. The SSR tool calculates the magnetizing inductance.

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 6

2c. Semiconductor Voltage Stresses

Using the parameters entered in 2b, the SSR tool calculates the voltage, current, and temperature rise for the switching components.

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 7

Step 3: Core & Turns 

3a. Core Selection

Enter the parameters for the magnetic core, including the maximum flux density, window factor, and the current density for the winding wires.AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 8

3b. Bobbin Selection

Based on the core selected, the SSR tool recommends a bobbin.

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 9a

3c. Turns

Using the input and output voltages, the SSR tool calculates the transformer turns ratio.

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 9b

3d. Core Gap & Losses

Using the switching frequency and other parameters, the SSR tool calculates the core loss.AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 10

Step 4: Windings

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 11a

4a. Wire

The tool calculates the minimum and maximum sizes for the winding wire, along with the skin effect depth.

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 11b

Figure 11: Windings: Primary Windings

4b. Primary Windings

The tool calculates the maximum diameter of the wire and the AC-to-DC resistance ratio for the primary side winding.AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 12

4c. Main Output

The tool calculates the maximum diameter of the wire plus its AC-to-DC resistance ratio for the secondary side.AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 13

4d and 4e are missing since only one output was selected. The 4d and 4e are for multiple outputs

4f. Auxiliary Winding

The tool calculates the parameters.

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 14

4g. Transformer Shielding

Enter the specifications for the transformer. The tool will generate a schematic.AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 15

4h. Transformer Schematic

Electrical parameters are displayed on the schematic.

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 16

Step 5: Power Semiconductors

5a. Diode Derating

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 17a

5b. Main Output Diode

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 17b2

5e. MOSFET Losses

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 18

 Step 6: Output Filters

6a. Main Output Filter

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 19

Step 7: Control

7a. Power Stage Characteristics

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 20

7b. Feedback Compensation

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 21

7c. Loop Gain

Explain the criteria stability analysis for the flyback converter. Plot gain and phase plot for the design.

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 22

7d. Controller Setup

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 23

Step 8: Snubbers & Input

8a. Primary-Side RCD Snubber

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 24

8b. Secondary-Side RC Snubbers

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 25a

8c. AC Input circuit

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 25b

Step 9: Efficiency 

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 26

9a. Efficiency Plot

The SSR tool plots the efficiency of the flyback converter, comparing the percentage load to the input line voltage.  The SSR tool also generates loss structures for the converter.

AEP3094 Fairchild Intro to Flyback Converter Tools Part 2 Figure 27

 

Related Links:

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http://www.fairchildsemi.com/support/

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http://www.fairchildsemi.com/support/design-tools/