Part Number: UCC3813-1
I am working with a UCD3138 hard switching full bridge evaluation module (UCD3138HSFBEVM-029). It uses a UCC3813-1 for the 3 housekeeping supply rails. Schematic available in the User Guide (http://www.ti.com/tool/UCD3138HSFBEVM-029#technicaldocuments). The flyback has a regulated 11V rail for the bridge gate drivers, an unregulated isolated 11V rail for the sync fet gate driver, and a 6V unregulated isolated rail from which 3.3V for the main controller is derived.
Issue #1: We are trying to synchronize the UCC3813 controller to an external signal. First off, I noticed the flyback was switching at 100kHz even though the Rt and Ct chosen should set the switching frequency to 200kHz per the datasheet. I found this odd. After successfully synchronizing the UCC3813 I found the flyback had a switching frequency of half my external sync signal. What's causing this? Is there something left out of the datasheet regarding the -1 variant of the UCC3813?
Issue #2: The datasheet suggests driving a 50 ohm resistor placed between Ct and ground with a 1V 100ns pulse width sync signal. This did not work for us. The datasheet states RC can also be driven directly by a pulse train. We removed Rt and Ct and applied our external sync signal directly to RC on the UCC3813. We are driving RC with a function generator that is more than capable of handling the low impedance state of RC while held high. What we notice is proper synchronization when the external sync signal positive pulse width is between 100-900ns (4-36% duty cycle @ 400kHz). If the external sync signal's duty cycle is increased above 35% we see the flyback output rails start to droop. This droop becomes detrimental nearing 45% DC and even causes the 3.3V LDO to dropout before reaching a duty cycle of 50%, causing the entire EVM to hiccup. Is this behavior to be expected? That datasheet does not specify this could be problematic but it may indeed be a limitation of the UCC3813. Any insight would be much appreciated.
For now we will focus on using a one-shot to reduce the pulse with for the UCC3813, but we want to verify if this behavior makes any sense or if something may be very wrong here.
Thanks again,
Scott