UCC28701: UCC28701

robin gangopadhya18

Part Number: UCC28701
Other Parts Discussed in Thread: UCC28780

We have several modules using UCC28701 controller. For isolated applications in a small vol of the package, all low power.

We use SLUC408b to get design values.

Consider an application: input dc, min 375, type=380v, max=385V, core-cross section is 59 mm sq.

Output: 5v with 4 amp nominal.

For no output load, we can have a static load of suitable current.

But we get Lm extremely high even with 132 kHz selection.

Other values also seem extreme.

Do we conclude that this controller cannot be used for this level of input DC voltage? As you can notice, I am trying to explore if have to use UCC28780 for this purpose. But that too seems to give doubtful results with DC input. For UCC28780, we already have other modules designed, core cross-section is 108 mm sq.

Any help will be highly appreciated. We are almost going to have to go for pcb fab as we speak.

-robin5v3a_unprotected_28701.xls

I am suspecting we are inputting something wrong here.

Any help will be appreciated.

Pl see the xl spreadsheet.

-robin

over 5 years ago

0 Eric Faraci over 5 years ago

TI__Expert 8210 points

Robin

First off I would like to confirm that UCC28701 can work with a 375-385VDC input to 5V/4A output.

This excel calculator you shared is unlocked. We do not recommend unlocking the tool since this can cause functions to be improperly modified, which will cause results to become incorrect. We recommend that you use the latest excel calculator here http://www.ti.com/lit/zip/sluc408 rather than the version you currently have.

When I reviewed the excel calculator you attached I noticed the following issues

The input voltage is RMS AC, not DC. If you have DC input you need to reduce the input voltage by square root of 2, since the calculator assumes a diode peak wave rectifier is used
The maximum switching frequency of 132kHz is too high. 120kHz the abs min maximum frequency, we do not recommend setting a frequency above this since all UCC28701 devices are not guaranteed to be able to operate above this frequency
The ideal calculations for the following parameters are much different than the actual value used. It’s recommended to use values as close to the actual as possible to insure the design operates as expected. Later calculations depend on the actual value used, so the actual values could be causing some undesired behavior

NP/NS turns for the transformer – ideal row 40, actual used in calculations row 54
Primary Magnetizing L – ideal row 36, actual used in later calculations row 56
Current Sense Resistor RCS – ideal row 93, actual used in calculations row 94
VS resistor RS1 – ideal row 136, actual used in calculations row 137
VS resistor RS2 – ideal row 138, actual used in calculations row 139
RLC resistor – ideal row 153, actual used in calculations row 154

Please use the latest excel calculator I shared the link to above and make the following corrections to see what parameters in your design need to change. We also have a SPICE simulation model http://www.ti.com/lit/zip/slum455 that you can use to verify the operation before you start testing your hardware.

Best Regards,

Eric

0 robin gangopadhya18 over 5 years ago in reply to Eric Faraci

Mastermind 6296 points

Eric

Relief! Thnx for confirming that 5v3 amp can be controlled by UCC28701

Your detailed review is highly appreciated.

One thing I noticed- maybe wrong- the max peak current in the output diode is 14 amps. This I thought would cause problems in small size diodes we can use.

But let me redo all calculations using the new version- then send you feedback on this.

Yeah, for some reason, I forget now, a while ago, I had to use some tricks to unsecure the file.

I always keep the original version( protected) & for using design values, I get them from the protected version.

-robin

Power management

Power management forum

UCC28701: UCC28701