SLUAAH6 November 2021 UCC27424 , UCC27511 , UCC27517 , UCC27524 , UCC27531 , UCC27614 , UCC27624 , UCC27624-Q1
Figure 5-1 shows a simplified block diagram of the PSFB circuit where MOSFETs Q1, Q2, Q3 and Q4 form the phase shifted full-bridge on the primary side of the transformer T2. Q1 and Q4 are switched at 50 % duty and 180 degrees out of phase with each other. Similarly, Q2 and Q4 are switched at 50 % duty and 180 degrees out of phase with each other. The PWM signals for the half-bridge Q2 – Q3 are phase shifted with respect to those for second half-bridge Q1 – Q4. The amount of this phase shift dictates the amount of overlap between diagonal FETs, which in turn determines the amount of energy transferred.
Synchronous rectification is used for high output current isolated DC-DC to converters, where output diodes are replaced with MOSFETs to replace diode forward conduction losses with MOSFET conduction losses. This is because the voltage drop across a MOSFET, RDS(on), is significantly smaller than the diode forward voltage drop and therefore results in lower losses and higher efficiency.
Figure 5-2 shows the converter input (400VDC), output (12VDC) using the UCC27614 outputs (CH4_U5_OUT) driving the synchronous rectification MOSFET to achieve efficient VDS switching times (CH1). Figure 5-3 shows the converter’s output voltage and current with a gate drive voltage of 12V to drive the synchronous rectification MOSFET.