Webench AC power design seems off

Dan Andersen

Other Parts Discussed in Thread: LM317

When using an AC input in webench of 18V-30V (that's RMS) to provide a DC out at 3.3V and 500mA, webench gives the LM317 (an LDO) as the only viable solution. This seems wrong on a number of levels, namely the power dissipation that the LDO would see at a rectified 30Vrms input (~42V rectified DC) to 3.3V out at 500mA works out to almost 20W of dissipation (!!). The max voltage level input of the LM317 is 40V, which also seems low for this requirement.

Using rectified DC values (and the DC version of webench) appears to give reasonable results (switchers). Why is webench recommending an LDO when using the AC version?

Thanks,

Dan

over 7 years ago

0 Wanda Garrett - WEBENCH Design Center over 7 years ago

TI__Intellectual 2360 points

Hi Dan,
Thanks for reporting your finding. We're checking into it.

0 Wanda Garrett - WEBENCH Design Center over 7 years ago in reply to Wanda Garrett - WEBENCH Design Center

TI__Intellectual 2360 points

Hi Dan,
The AC-input solutions shown in WEBENCH Power Designer include the input rectifier section. This configuration has been done for many of the high-voltage devices, but not for the low-voltage ones.

The result you see from your query indicates that there are no solutions in WEBENCH Power Designer that meet your requirements (with the "AC" input specified). I agree that the presentation of this information isn't super helpful, and we are working on a better display. It is rare that this view is seen, given the wide variety of solutions supported now.

To see WEBENCH solutions that could handle your input voltage range, you could enter the rectified voltage range (min to max) and identify it as "DC."

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Webench AC power design seems off