The UCC28780EVM-021 EVM that was used to perform this experiment originally had an ATL431 shunt reference with a biasing resistance of 34 kΩ. Once test probes were installed, the average cathode voltage, V_KA, under a 40 W load was measured to be 16.9 V. When a new shunt reference was soldered onto the board for testing, a new bias current had to be established. Equation 3 was used to calculate the new required biasing resistance, R_bias, where 16.9 V was assumed as the cathode voltage V_KA for simplicity. I_KA(min) was pulled from the new devices data sheet, and V_out was found using Equation 1. The cathode voltage, V_KA, changes according to the programmed output voltage due to the required feedback current to set that output voltage. The TLVH432 has a lower internal reference of 1.24V, meaning that the programmed output voltage is much lower, so R₂ was decreased to 10 kΩ to change the programmed output voltage to 19.855 V. The TLVH432's cathode voltage, V_KA, was measured at 18.27 V under a 40 W load, which was then used in Equation 3 to calculate the bias resistance, R_bias. The biasing resistance and biasing current provided for each shunt reference for a 40 W load are shown in Table 4-1.

The following scope images show the steady state performance of each shunt reference under the three-load conditions.

Figure 4-3 TL431 No-Load

Figure 4-5 TL431 40 W Load

Figure 4-7 TLVH432 20 W Load

Figure 4-9 ATL431 No-Load

Figure 4-11 ATL431 40 W Load

Figure 4-13 TL431LI 20 W Load

Figure 4-15 ATL431LI No-Load

Figure 4-17 ATL431LI 40 W Load

Figure 4-4 TL431 20 W Load

Figure 4-6 TLVH432 No-Load

Figure 4-8 TLVH432 40 W Load

Figure 4-10 ATL431 20 W Load

Figure 4-12 TL431LI No-Load

Figure 4-14 TL431LI 20 W Load

Figure 4-16 ATL431LI 20 W Load

Vout​​ was measured as the expected programmed value for each of the plots shownand V₁ was measured as a slightly lower value than V_out. V_Anode and V_KA change under different load conditions to modify the secondary side feedback current, I_{FB(secondary)} , through the optocoupler diode. This change in V_KA also decreases the biasing current, I_bias, being provided to the shunt reference under larger loads, which is why I_bias was established as I_KA(min) at the full-load condition.