WEBENCH® Tools/LM5161: Wrong VinMin evaluation

Stefano Infante

Part Number: LM5161

Tool/software: WEBENCH® Design Tools

Hello,

I am trying to start a Webench design using LM5161, but I always get the same error: VoutPrim should be less than 0.5VinMin

My parameters are as follows:

LM5161 in a flybuck topology

VinMin = 45 V

VinMax = 67.2 V

Vout = 22 V

Iout = 0.4 A

IoutNom = 0.2 A

Tmax = 60 °C

Use ceramic capacitors and shielded inductors

Changing Iout, Tmax and options has no effect. The only way I have to let it work is selecting a Vout < 18 V, as if VinMin was the default 36 V disregarding my input.

Is it a bug or am I wrong in some way?

Thanks in advance.

Regards,

Stefano

over 6 years ago

0 Vishwanath Tigadi over 6 years ago

TI__Intellectual 1710 points

Hi Stefano,

The Webench is particularly designed such that it will fail the design for your input conditions. We're looking into why this is set this way and will keep you posted.

Regards,

Vishwanath

0 Vishwanath Tigadi over 6 years ago in reply to Vishwanath Tigadi

TI__Intellectual 1710 points

Hi Stefano,

We likely put a limit in there such that Vout must be less than 0.5*Vin. We did this to ensure decent load regulation on isolated output.

This a valid failure for your input conditions.

Regards,

Vishwanath

0 Stefano Infante over 6 years ago in reply to Vishwanath Tigadi

Expert 2320 points

Hi Vishwanath,

in my application Vout is 22 V and VinMin is 45 V, hence Vout is always less than 0.5*Vin. So i don't think this should be an issue. Maybe there is something more, is there?

Regards,

Stefano

0 Vishwanath Tigadi over 6 years ago in reply to Stefano Infante

TI__Intellectual 1710 points

Hi Stefano,

There are other elements to consider here, for example, the power FET Rds, the transformer parasitic resistance etc., these cause voltage drops across them. even tough your Vout is regulated at 22V, the the primary side of the transformer sees a higher voltage.

I hope this clears your query.

Regards,

Vishwanath

0 Stefano Infante over 6 years ago in reply to Vishwanath Tigadi

Expert 2320 points

Hi Vishwanath,

by playing with input and output voltages I am able to set an output voltage higher than 18 V, but in order to get a 20 V output I must select a 50 V minimum input. Are those parasitic parameters that influent on the design?

Thanks and regards,

Stefano

0 Vishwanath Tigadi over 6 years ago in reply to Stefano Infante

TI__Intellectual 1710 points

Hi Stefano,

VoutPrimary is not just a function of parasitic resistances, transformer turns ratio and feedback resistances, but also the ripple injection circuit calculations. Hence it is turning out to be greater than 22.5V. It is recommended to increase VinMin for 22V application.

Regards,

Vishwanath

0 Stefano Infante over 6 years ago in reply to Vishwanath Tigadi

Expert 2320 points

Hi Vishwanath,

ok, thanks for your help.

Stefano

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WEBENCH® Tools/LM5161: Wrong VinMin evaluation