As shown in Figure 44, the output power good LED1 and series resistor R18 form an output pre-load or minimum load. This pre-load is necessary in order to maintain regulation at no load, or when the power converter output is disconnected from the load system. Magnetic regulation relies on sensing the output voltage during switching cycles, so it is necessary to maintain a certain minimum switching frequency fSW(min) in order to continue sensing the output voltage. However, generating switching cycles at fSW(min) transfers energy to the output, which requires some load on the secondary-side to absorb this energy and prevent the output capacitors from being charged out of regulation. The minimum energy transferred at fSW(min) depends on the choice of magnetizing inductance LPRI and current sense resistor RCS.
In order to ensure that the control loop operates at a frequency above the minimum switching frequency, fSW(min) (to ensure that the loop has adjustment range up/down as required to maintain regulation), the recommended minimum pre-load is at least twice the value calculated in Equation 64.
The required value of R18 can then be calculated, assuming a forward voltage drop of 1.8 V for the LED:
Use the next lower E24 value of 8.2 kΩ. For a design without an LED, a pre-load resistor of similar value is still required across the output voltage.