Tool/software:
Dear TI Team,
I am currently using the UCC256613 in an LLC converter design and have encountered the following issue:
The output voltage is set to 255 V, but during operation the actual output voltage fails to reach this target. Additionally, the IC switching frequency does not decrease properly, remaining around 862 kHz, as shown in Figure 1.
At this time, the TEST pin settings are:
-
Minimum Frequency for IPPC Operation: 112.2 kHz
-
Integrator Time Constantvm: 257 ns
(See Figure 2)
However, after modifying the TEST pin configuration to the following:
-
Minimum Frequency for IPPC Operation: 591.6 kHz
-
Integrator Time Constantvm: 80 ns
(See Figure 3)
The switching frequency decreases to approximately 382 kHz.
-
Why does increasing the Minimum Frequency setting allow the actual switching frequency to decrease ?
This seems to contradict the expected behavior of a “minimum frequency” setting. -
Why doesn’t the IC decrease frequency when VFB is expected to be at its maximum (i.e., no optocoupler current / Iopto = 0)?
I’m using a voltage feedback configuration. Since the output voltage has not reached the target, the optocoupler is off (Iopto = 0), and VFB should be internally at its maximum. According to the IPPC logic, this condition should drive the IC to decrease switching frequency to boost the output voltage. However, the frequency remains high and does not respond as expected.
