Part Number:UCC28950
Hi all,
One customer used UCC28950 to design a power with 48V input, 12V output and maximum output current 35A. Set the clock frequency 400KHz, namely, switch frequency is 200KHz. The plane transformer and inductance are used in this design.Both the primary MOSFET and the rectifier MOSFET adopt TI's CSD19534 and withstand voltage of 100V.
1. Replace the rectifier MOSFET (full wave rectification) with Schottky before joining the synchronous rectification, and strictly deduct ZVS parameters.Due to the maximum current limitation of shottky, the maximum output current is 6A. Under the light load condition, the test result looks stable, the advanced arm is fully realized ZVS, and the lagging arm is not fully realized ZVS.
2. The no-load input current is 20mA under the schottky rectifier condition;The efficiency with 2A load is 86%. Output voltage ripple is less than 1%. The critical output current for CCM and DCM is 3.5A. When the output current reaches to the 4.4 A (wholly go into the CCM), synchronous signal OUTE and OUTF are enabled, secondary side driver is LM5111-1, output is stable.
3. Replace schottke with CSD19534 (two CSD19534 parallel, no RC absorption circuit). When the output current is added to 2.7A, 4 MOSFET of the output rectifier breakdown, and G,S and D are all short. After recovering the schottky rectifying tube, when the circuit starts up without load, an output voltage can be found, while when the circuit starts up with 0.1A load, output voltage is zero. So there may be some device damaged at the primary side.
UCC28950 synchronization Settings are as follows:
RAEFHI does not install, which means CS and ADELEF open the way;RAEF is set to 0, which is ADELEF pin grounded.We analyze that the sync signal is not related to CS, but do they need to be relevant?The DATASHEET is not clear.Look at the reference design RAEFHI is generally several megohm.
Question: Is the above analysis reasonable? Does RAEFHI have to be installed? Or is there another reason? It's best to give the principle.Thank you
Best regards!