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UC2843: Fixed Frequency flyback can it be both CCM and DCM?

Joshua Schroeder
Prodigy 80 points

Part Number: UC2843

What are the ramifications of allowing a continuous conduction fixed frequency flyback power supply go from Continuous to Discontinuous operation?  

I've inherited a design where the load on the CCM fixed frequency power supply was significantly overestimated.  As a result the flyback is crossing into DCM operation under actual low load conditions.  Tehere really isn't adequate time to go back to the drawing board to redesign the circuit.  Is this actually Bad?  

All the information I've found seems to assume a supply is designed to be either CCM or DCM.   What issues do I need to look with a supply operating in both modes?  Could there be serious issues that hide right at the threshold between the two operating modes?

  • 0
    TI__Expert 3095 points
    Hi Joshua Schroeder,
    It is quite acceptable for the fixed frequency flyback to go from CCM to DCM in normal operation. In fact I would say it is quite normal for a CCM Flyback to drop into DCM operation at some point when the load becomes very light.
    Two things that you should look out for:
    1) The gain between duty cycle and voltage ratio (Vout/Vin) changes significantly as you move deeper into DCM and becomes a function of load. You should check that your control loop is still able to regulate properly when lightly loaded.
    2) You may notice some erratic operation as you step over the boundary between CCM and DCM. This is normal and it happens because there is some non-linearity at this boundary due to the idle ring 'resonance' that occurs during the DCM period.
    The control loop design should focus on CCM operation, then just check to see what the duty cycle and effort must be to achieve the lightest load required and make sure that the control loop is able to achieve the required duty cycle.
    I hope this answers your question.
    Thanks
    Joe Leisten