Dead time is the term used to describe the ensured OFF time of the PWM output during each oscillator cycle. It is used to ensure that even at maximum duty cycle, there is enough time to reset the magnetic circuit elements, and prevent saturation. The dead time of the UCC280x PWM family is determined by the internal 130-Ω discharge impedance and the timing capacitor value. Larger capacitance values extend the dead time whereas smaller values results in higher maximum duty cycles for the same operating frequency. A curve for dead time versus timing capacitor values is provided in Figure 9-10. Increasing the dead time is possible by adding a resistor between the RC pin of the IC and the timing components, as shown in Figure 9-11. The dead time increases with the discharge resistor value to about 470 Ω as indicated from the curve in Figure 9-12. Higher resistances must be avoided as they can decrease the dead time and reduce the oscillator peak-to-peak amplitude. Sinking too much current (1 mA) by reducing R_T will freeze the oscillator OFF by preventing discharge to the lower comparator threshold voltage of 0.2 V. Adding this discharge control resistor has several impacts on the oscillator programming. First, it introduces a DC offset to the capacitor during the discharge – but not the charging portion of the timing cycle, thus lowering the usable peak-to-peak timing capacitor amplitude. Because of the reduced peak-to-peak amplitude, the exact value of C_T may require adjustment from UC3842 type designs to obtain the correct initial oscillator frequency. One alternative is keep the same value timing capacitor and adjust both the timing and discharge resistor values because these are readily available in finer numerical increments.

Figure 9-10 Minimum Dead Time vs C_T

Figure 9-11 Circuit to Produce Controlled Maximum Duty Cycle

Figure 9-12 Maximum Duty Cycle vs R_D for R_T = 20 kΩ