• State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 63 subscribers
  • Views 1252 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Cross Regulation issue in Flyback converter

Makarand Gadgil
Prodigy 10 points

Hi,

 

We have designed UCC2803MDREP based triple output Flyback regulator. The main output (master) is +5V, 5A, The other two (slave) outputs are +/-15V and 750 mA.

During testing, we observed that we need some minimum load on +5V output in order that there is no load regulation issue at +/-15 V.

 At present, this minimum load requirement on +5V rail is 2.7 A to achieve the above condition. We want to keep it bare minimum say 1 A.

Please advise how this can be achieved. Is there any modification to be done to control loop or any other circuit modifications are required?

 

Thanks,

 

Makarand Gadgil

 

 

 

  • 0
    Genius 3295 points
    Makarand,

    It's hard to know for sure without actually knowing the circuit component values, but it sounds to me that your Flyback is changing to discontinuous mode DCM when the load drops to 2.7A. DCM means that the current on the secondary side of the transformer is reaching zero before the switching period ends. This action changes the DC gain of the Flyback.

    The Flyback transformer is actually a coupled inductor. Energy is stored in the primary inductor while the primary side switch is on. Once the primary side switch turns off, the energy is transferred to the secondary side inductor. The inductors must be large enough so that they store and release their energy from primary side to secondary continuously, i.e., the currents in the inductors never reach zero.

    When the Flyback converter goes into DCM the primary side voltage varies over the switching period due to the energy being fully discharged in the primary side inductor. A varying primary side voltage results in a varying magnetizing flux (fm) that corresponds to a varying or falling magnetizing current which is shared between all active windings. This action degrades the converter's cross regulation. The details of this are explained very clearly in a TI appnote "Under the Hood of Flyback SMPS Designs" by Jean Picard.

    You have several options -none of them are very easy.

    You can replace the output diode with a MOSFET and run in synchronous mode. This is actually the best option since you not only improve cross regulation but you simultaneously improve the converter efficiency. You can increase the transformer primary and secondary inductance. You can increase the switching frequency so that inductors do not fully discharge over the switching period. It's the ratio of transformer's inductance compared to the switching period that determines when the converter transitions from CCM to DCM. However that will increase switching losses, core losses, and copper losses in the transformer. You might also consider using linear regulators on the auxiliary outputs.

    -Chuck