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UCC28730-Q1: Does the UCC28730 *NEED* the UCC24650 to start, or will it run alone?

Part Number: UCC28730-Q1
Other Parts Discussed in Thread: UCC28730, UCC24650,

Every schematic I see with the UCC28730 always has a wake-up UCC24650 on the secondary side.

Is this needed? Will the flyback work without it?

For example if I have a diode, capacitor as normal and a resistive load on the secondary, will the circuit start up once the UVLO threshold is reached (like a 'normal' flyback controller)?


  • Hi, Mike:

    Thanks for your interested in UCC28730-Q1. 

    UCC28730-Q1 is a PSR flyback controller so it can be used without UCC24650 and works as a normal flyback basically. 

    However the feedback response as PSR mode, which it only can sense voltage through auxiliary winding. And the fs is quite low when it operates as sleep mode to save more power consumptions. Based on this, the output voltage would drop a lot once there is a step loading on output. And it is not acceptable to lots of application. Extra output capacitors are needed to improve this.

    You may refer the test results as below.  (or you may find this through this link:  Achieving High Power Density and Ultra Low Standby Power in Flyback )

    So that's the reason UCC24650 is recommend to be used in every UCC28730's design. It helps to reduce output capacitance and provide good transient response.  If you are looking for a stable and low standby power solution, UCC28730 and UCC24650 would be a right solution to you.