Easy Definition of Fixed-Frequency & Variable Duty Cycle

The fixed-frequency switching regulator varies the duty cycle of the pulse train in order to change the average power. The fixed-frequency concept is particularly advantageous for systems employing transformer-coupled output stages. The fixed frequency permits efficient design of the associated magnetics. Transformer coupling also has advantages in single and multiple voltage-conversion applications. The fixed-frequency regulator will establish a dc current through the inductor (for increased load conditions) to maintain the required load current, with minimal ripple current. The single-ended and transformer-coupled configurations are illustrated in Figure 5-26.

Figure 5-26. Switching Voltage Regulator Configurations

These types of switching regulators can thus be operated with high efficiency to provide low-voltage, regulated outputs from a high-voltage, unregulated supply or vice versa. The switching frequency should be established at the optimum value for the switching components of the supply (transformer, switching transistor, inductor, and filter capacitor). High frequency operation is distinctly advantageous because the cost, weight, and volume of both Land C filter elements are reduced.

However, the frequency at which the effective series resistance of the filter capacitor equals its capacitive reactance is the maximum allowable frequency.

Operation above 20 kHz is desirable to eliminate the possibility of audible noise. Choosing an operating frequency that is too high will result in power switching transistor losses as well as “catch” diode losses. The higher cost of these high performance components must be balanced against the reduced cost, size, and weight of the Land C components when determining the optimum frequency for a specific application.