* Question
What three conditions does the UCC28610 need to start the next cycle modulator?
* Answer
The UCC28610 from Texas Instruments is a quasi-resonant flyback controller designed for high-efficiency, low-standby AC/DC power supplies.
Its control logic uses a cycle-by-cycle modulation scheme, and before it can initiate the next switching cycle, three critical conditions must be satisfied.
These conditions ensure that energy transfer, timing, and control remain stable throughout the converter’s operation.
1. Transformer Demagnetization Must Be Complete
Before the next MOSFET turn-on event, the transformer core must be fully demagnetized — meaning the secondary current has decayed to zero.
The controller monitors the demagnetization signal via the auxiliary winding.
When the auxiliary winding voltage rises above the demagnetization threshold (VDEMAG), it confirms the core has reset.
Purpose:
Ensures all stored magnetic energy from the previous cycle has been delivered to the load, preventing core saturation and enabling valley switching for higher efficiency and lower EMI noise.
2. The Control Voltage (VCOMP) Must Exceed the Minimum Threshold
The COMP pin provides the internal control voltage output from the error amplifier, which represents the system’s power demand.
The UCC28610 compares VCOMP to an internal minimum threshold (VCS_MIN).
If VCOMP is above this level, it indicates that output regulation requires a new energy pulse.
If it is below, the controller holds off switching — entering skip-cycle mode at light load to save power.
Purpose:
Prevents unnecessary switching during low-load or standby conditions, improving efficiency and reducing standby power consumption.
3. The Maximum Off-Time (TOFF(MAX)) Must Expire
The controller imposes a maximum off-time limit to control the lowest switching frequency.
Even if the first two conditions are met, the UCC28610 waits until the internal frequency clamp timer expires before allowing the next turn-on.
Purpose:
Maintains frequency stability across wide load and input ranges and prevents the switching frequency from dropping too low under heavy load or high-line operation.
Summary Table
Condition | Signal/Parameter | Purpose |
1. Transformer demagnetized | Auxiliary voltage above VDEMAG | Ensures full energy transfer and valley switching |
2. Control voltage above minimum | VCOMP > VCS_MIN | Indicates load demand and avoids idle switching |
3. Maximum off-time expired | TOFF(MAX) timer complete | Keeps switching frequency within safe range |
Conclusion
In summary, the UCC28610 initiates the next modulation cycle only when:
The transformer is fully demagnetized,
The control voltage exceeds the minimum threshold, and
The maximum off-time period has elapsed.
This coordinated control ensures high efficiency, low EMI, and stable quasi-resonant operation, making the UCC28610 ideal for flyback power supplies in industrial, communication, and consumer applications.
COMMENTS