SDAA053 July 2025 UCG28824 , UCG28826 , UCG28828

4 Design Examples

When designing for non-USB-PD applications, or in case the output voltage is not 20V, the TR pin needs to be programmed differently than the default 20V output, and to set the appropriate output OVP level.

A universal input, a 12V output design and a 24V output design are used to demonstrate the design process. The transformer is firstly designed to optimize the power stage performance. The RT pin then can be programmed according to the transformer design.

For a universal input, 12V output design, the transformer turns ratio is normally around 10:1, which reflects 120V to the primary side when the output voltage is regulated at 12V. This provides enough margin for the integrated GaN HEMT voltage stress. Additionally, 10:1 turns ratio is outside of the transformer turns ratio setting in the data sheet.

For the 12V output, the desired OVP level is normally 20% above the regulation level. Therefore, the output OVP is calculated as

Equation 1.

V_{O V P} = 12 V \times 1.2 = 14.4 V

With 14.4V OVP level, the output voltage reflecting to the primary side becomes

Equation 2.

V_{r e f l e c t} = V_{O V P} \times N_{P S} = 14.4 V \times 10 = 144 V

According to Table 3-2, 3rd column, the TR pin setting is 5.23kΩ, which gives OVP setting at 150V reflected output voltage. The real OVP accomplished can be calculated as

Equation 3.

V_{O V P} = \frac{V_{r e f l e c t}}{N_{P S}} = \frac{150 V}{10} = 15 V

From this example, when TR pin is set to 5.23kΩ. According to Table 3-1, the turns ratio setting is 6 instead of the real turns ratio of 10.

For a universal input, 24V output design, the transformer turns ratio is normally around 5.5:1, which reflects 132V to the primary side. The 5.5:1 turns ratio is also outside of the transformer turns ratio setting in the data sheet.

For the 24V output, we can set the OVP at 30% above the regulation level. Therefore, the output OVP is calculated as

Equation 4.

V_{O V P} = 24 V \times 1.3 = 31.2 V

With 31.2V OVP level, the output voltage reflecting to the primary side becomes

Equation 5.

V_{r e f l e c t} = V_{O V P} \times N_{P S} = 31.2 V \times 5.5 = 171.6 V

According to Table 3-2, 3rd column, the TR pin setting is 20.5kΩ, which gives OVP setting at 171.9V reflected output voltage. The real OVP accomplished can be calculated as

Equation 6.

V_{O V P} = \frac{V_{r e f l e c t}}{N_{P S}} = \frac{171.9 V}{5.5} = 31.25 V

From this example, when TR pin is set to 20.5kΩ. According to Table 3-1, the turns ratio setting is 6.875 instead of the real turns ratio of 5.5.

From these examples, when the output voltage is not 20V, the transformer turns ratio setting through TR pin can be completely different than the turns ratio implemented in the transformer. The TR pin needs to be set based on the output OVP needs, instead based on the transformer turns ratio implementation.