UCC28710: Meaning of equation 7 on the datasheet

Hello Itoh-san,

Thank you for your interest in the UCC28710 controller. The K_AM term accounts for the change of primary peak current in the AM section of the device's control law (see Figure 16 on page 14 of the datasheet).

This term is squared because in a DCM flyback, power is delivered according to this equation: Pout = 1/2*Lm*fsw*Ipk^2.
(There is an efficiency term involved, too, but I left it out to keep it simple.)
This equation says that power equals an amount of magnetic energy transferred at a certain repetition rate (frequency).

Equation 7 can be rearranged as Psb /fmin = Pout/fmax/Kam^2.
This relates the standby energy 1/2*Lm*Ipk_min^2 to the full power energy 1/2*lm*Ipk_max^2 with the factor Kam.
Cancelling terms, it reduces to Ipk_min = Ipk_max/Kam.

At no load, each switching cycle still delivers a fixed minimum amount of energy to the output. However, the frequency cannot be reduced to zero or the primary bias power will deplete and the control will turn off. So it will be necessary to maintain a minimum internal load to keep the control operating and regulate the output voltage even with no external load. Equation 7 is a rough approximation to help determine that minimum load, along with Equation 8.

I hope this explains why Kam is squared.

Regards,
Ulrich