The UCC25800-Q1 transformer driver has two levels of overcurrent protection (OCP).

• The first level (OCP1) triggers if the current through the low-side MOSFET exceeds programmed threshold I_OCP during its on-time in each switching cycle for 2.1 ms. Refer to OCP Threshold Setting for OCP1 threshold programming details.
  • OCP1 detection is based on only the low-side MOSFET current, when the SW pin current flows into the SW pin
• The second level (OCP2) triggers if the current in either the high-side or low-side MOSFET exceeds 5 × I_OCP for 100 ns.
  • The OCP2 threshold is set significantly above OCP1 threshold to allow the unit to cope with heavy load surges for a short duration, or during the start-up to charge the large output capacitor. If OCP2 is exceeded, it indicates that there is a serious fault in the system. OCP2 tracks OCP1 so that events like output overload can still trip OCP2, even if the current limit is set well below the maximum current limit of the transformer driver.

• During soft-start
  • The OCP1 is disabled
  • The OCP2 threshold is fixed at its maximum value of 5 A
• After soft-start
  • OCP1 is enabled, with the threshold I_OCP equal to the programmed value
  • OCP2 threshold becomes 5 times of the programmed I_OCP level.
• The OCP1 overcurrent timer is implemented as an up-down counter to ensure that the repetitive short over-current events as well as a sustained 2.1-ms over current trigger the OCP.
  • OCP1 overcurrent timer counts up if the SW current crosses I_OCP for longer than 100 ns in each switching cycle
  • OCP1 overcurrent timer counts down if the SW current does not cross I_OCP for longer than 100 ns in the entire switching cycle
  • The internal counter for OCP1 overcurrent timer counts up in 2.1 ms from 0 to the trip threshold and counts down in 180 ms from the trip threshold down to 0.
• OCP2 detection has an analog filter which filters out pulses of less than 100 ns.

The transformer driver imposes a restart time of 100 ms before restarting from overcurrent protection to maintain the RMS current in the transformer driver below its limit. The OCP behaviors are illustrated in Figure 8-10 and Figure 8-11.

Figure 8-10 OCP1 protection and recovery behavior

Figure 8-11 OCP2 protection and recovery behavior