6.1 Programming DELAB, DELCD, and Adaptive Delay Set (ADS)

The UCC2895-EP allows the user to set the delay between switch commands within each leg of the full-bridge power circuit, according to the formula from the data sheet:

For this equation, V_DEL is determined in conjunction with the desire to utilize (or not utilize) the ADS feature from:

Figure 9 shows the resistors needed to program the delay periods and the ADS function.

Figure 9. Resistors Needed In Programming

The ADS allows the user to vary the delay times between switch commands within each of the converter’s two legs. The delay-time modulation is implemented by connecting ADS (pin 11) to CS, GND, or a resistive divider from CS to GND to set V_ADS. From the previous equation for V_DEL, if ADS is tied to GND, V_DEL rises in direct proportion to V_CS, causing a decrease in t_DELAY as the load increases. In this condition, the maximum value of V_DEL is 2 V. If ADS is connected to a resistive divider between CS and GND, the term (V_CS – V_DS) becomes smaller, reducing the level of V_DEL. This decreases the amount of delay modulation. In the limit of ADS tied to CS, V_DEL = 0.5 V and no delay modulation occurs. In the case with maximum delay modulation (ADS = GND) when the circuit goes from light load to heavy load, the variation of V_DEL is from 0.5 V to 2 V. This causes the delay times to vary by a 4:1 ratio as the load is changed.

The ability to program an adaptive delay is a desirable feature because the optimum delay time is a function of the current flowing in the primary winding of the transformer, and can change by a factor of 10:1 or more as circuit loading changes. Reference [1] delves into the many interrelated factors for choosing the optimum delay times for the most efficient power conversion and illustrates an external circuit to enable ADS using the UC2879. Implementing this adaptive feature is simplified in the UCC2895-EP controller, giving the user the ability to tailor the delay times to suit a particular application, with a minimum of external parts.

[1] L. Balogh, "Design Review: 100W, 400 kHz, DC/DC Converter With Current Doubler Synchronous Rectification Achieves 92% Efficiency," Unitrode Power Supply Design Seminar Manual, Unitrode Corporation, 1996, Topic 2.

Figure 10. Resistors Needed For Programming

Figure 11. UCC2895-EP Timing (No Output Delay Shown)

Figure 12. Block Diagram

6.2 Circuit Description

Figure 13. Oscillator Block Diagram

Figure 14. ADS Block Diagram

Figure 15. Delay Block Diagram (One Delay Block Per Output)