UCC28C43: Transformer selection

Jagannathan,

First, could you please share schematic of your Flyback? Also, please provide the output power demands of the Flyback and confirm the input voltage range (around 24V I guess). 

Depending on how much you deviate from recommended turns ratio, it might affect efficiency or capabilities of the Flyback converter. For more details, schematic and information requested above needs to be provided.

Depending on your power capabilities, you can consider using an Isolated DC/DC converter if your power demands are low. For example, UCC1424O-Q1 is a good option.

Thank you

Hi.

This is the design that I'm currently referring to: https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=D0AE5E2AE3EF9EA5. This uses UCC28C55-Q1 instead of UCC28C43-Q1. This way I'm able to choose between primary or secondary side regulation.

The required max output power is around 120W at 24V. I was not able to find any off the shelf modules that fit the required specifications and also fits my budget.

Thanks

Jagannathan,

Regarding the turns ratio selection, I recommend following the Design Procedure of the Datasheet to make sure the values are well selected. Assuming your Flyback will operate in DCM, VIN_min=24V (minimum value was not specified but I assume is close to 24V), then D_min=0.5, assuming Vf_Dsec is ~0.7V, then following DS equation, Nps is slightly lower than 1. Webex claims Ns2tpNp=0.97. My guess is that it means Np is primary winding, Ns2 is secondary/output winding and Np/Ns2=0.97V. 

Please, follow same verification value for Aux windign turns ratio.

Thank you

Hi,

The design that Webench gave me, operates in CCM and not DCM. Will the design procedure be valid for CCM as well? The duty cycle is around 35% which is too inefficient if it were a DCM design. 

Also, in the datasheet, we fix a minimum duty cycle, but, I don't see how the controller will know that minimum duty cycle (say 80%). What component in the circuit is set for this purpose, which prevents the controller from dropping its duty cycle below 80%?

Thanks

Jagannathan,

-Nps equation applies for CCM as well as it comes from volt-second balance theory on the transformer magnetizing inductance.

-80% in the DS is the duty cycle value at VIN_min value (see table below). DS explains that Nps should be designed at VIN_min (worst case), to be able to provide the correct output voltage at VIN_min

-Regarding on how to controller set this value, it all depends on the control stage. In this case, the DS typical application schematic shows PSR (primary side regulation), meaning that the output voltage is sampled across the aux winding of transformer through a resistor voltage divider connected to FB pin of the controller.

Thank you

Hi Manuel,

DS says that Dvin_min = 80%, based on which they choose the Nps.

Can I use the transformer's Nps to arrive at Dvin_min? Should it be between a range of values? So far I do not have a design requirement that involves Dvin_min. 

Thanks

The flyback is boost derived topology. The ideal large signal gain (DC transfer function) of of a boost (non-isolated flyback):

VOUT/VIN=1/1-D --> VIN=VOUT*(1-D)

From this you can see that as VIN is decreasing, D is increasing and as VIN is increasing, D is decreasing. VIN and D are inversely proportional so that DMAX is happening at VIN_MIN. Please look at TI Power Stage Designer as another tool for understanding the operation of power topologies. Attached are some additional TI resources for understanding flyback converter operation and transformer design.

2308.slup338.pdf

1803.slup339.pdf

Regards,

Steve

Hi Steve,

I went through the resources. I understood how 80% is the maximum duty cycle for this design. My question was, say I choose Dvin_min as 60%, I would arrive at Nps = 3.8. Is there a reason why I should choose 80% instead of 60%? Can I choose any value (As it only affects Nps)? If there is a limit, what decides that? In a way the choice of Dvin_min = 80%, fixes Dvin_max = 13%. Are there any tradeoffs?

Thanks