3 Auxless Sensing Technology

Traditionally, the Flyback converter uses an auxiliary winding on the Flyback transformer to provide the controller's biasing voltage. Due to the variable output voltage, the controller's biasing voltage changes together with the output voltage. The large voltage range on the biasing causes higher power loss and requires more external components to regulate the biasing voltage. UCG2882x uses self-biasing technology, directly harvesting energy from the power stage, solving the design challenge associated with the variable output voltage for USB-PD.

Additionally, UCG2882x eliminates the transformer auxiliary winding through auxless sensing technology. As illustrated in Figure 3-1, with integrated voltage divider, the switch node voltage is sensed. Through signal post precessing, the input voltage and the reflected output voltage can be derived from the sensed switch node voltage.

Because the output voltage regulation is achieved through the feedback loop on the secondary side, the output voltage sensed on the primary side is only needed for protection purposes. When sensed on the primary side, the auxless sensing technology only detects the reflected output voltage through the transformer, instead of the absolute value of the output voltage. Therefore, the transformer primary side to secondary side turns ratio is needed to calculate the absolute value of the output voltage.

In UCG2882x, the transformer turns ratio is set through a programming pin TR. The TR pin setting is summarized in Table 3-1. When design for USB-PD application, with universal AC input (85V to 264V), the transformer turns ratio is normally within this range and designer can select the corresponding setting to match the transformer turns ratio. With the turns ratio setting, the output over voltage protection threshold is set at 25V, which is appropriate for the 20V output in USB-PD applications.

However, for a non-USB-PD application, or when the output voltage is not 20V, the transformer turns ratio can be outside of this range, Sometimes, even when the turns ratio can be setup to match the design, the 25V OVP can be inappropriate, some setting changes are required.

As discussed earlier, UCG2882x senses the reflected output voltage, together with the programmed transformer turns ratio, the output voltage OVP can be set to 25V. Instead of calculating the output voltage by dividing the reflected voltage by transformer turns ratio, UCG2882x uses the TR pin setting to change the primary side OVP threshold, so equivalently achieves the same output OVP level with different turns ratios settings. Follow that principle, a third column is added, Table 3-1 turns into Table 3-2, which is the same as Table 7-1 in UCG2882x data sheet. The TR pin is really an output OVP setting pin. Instead of setting the absolute value for the output OVP level, the TR pin sets the output OVP level based on the reflected output voltage.

The principle of primary side OVP using auxless sensing is illustrated in Figure 3-2.