LM5023: LM5023 OVP

Hello Max, 

Thank you for your interest in the LM5023 QR flyback controller. 

There is a design procedure in the LM5023 datasheet starting on page 21.  Some of the earlier calculations are for determining transformer inductance, turns-ratios, Ipk, etc. 

The calculation set for R1 begins at equation (41) on page 24, where Naux has been previously chosen (here, Naux is the ratio of Aux turns to Primary turns Na/Np). 
Your choices for IQR, Naux and other parameters may be different from the design example, but the procedure is the same.  The value for R1 is determined by equation (45).

This value is then used later at equation (51) to determine R2.  Vaux_ovp is chosen using equation (49), where Vaux_ovp is the value of Vaux when Vo is set to the voltage where you want OVP to be triggered.  For your 19-V output, you may wish to choose Vo_ovp to be 22V, for example, or some other value. 

The schematic diagram in Figure 14 is copied from the LM5023 EVM.  Resistors R1 and R2 are not necessary for normal operation, but were added to the EVM design to bleed down the bulk capacitor voltage after AC power is removed.  This is simply a precaution to reduce the high voltage hazard to people evaluating the EVM, who may handle the board shortly after turning it off and may forget that the bulk caps retain charge when the converter is not running.  
You don't have to use R1 and R2 if you don't want to. 

Regards,
Ulrich