Flybuck - flyback Excel Calculator shows positive outputs when entering negative

Elijah Gould

Hi, I'm using TI's flybuck - flyback Excel calculator (www.ti.com/.../flybuck-flyback-design-calc) to calculate values for a split rail ±12v output flybuck-boost from an 11.5-42v input. When entering -12v for the primary output, and +12 for the secondary, the calculator's bar graph on the flybuck-boost shows positive voltages for both the outputs. If I enter -12v for both primary and secondary, the calculator inverts the primary output, so it ends up as +12v and the secondary is -12v. Any ideas why this might be? A TI engineer's blog post (e2e.ti.com/.../should-you-use-buck-or-inverting-topology-for-fly-buck) states it's best to have a negative primary, not secondary. Thanks. -Elijah

over 9 years ago

0 Sourav.Sen over 9 years ago

TI__Intellectual 2320 points

Hello Elijah,

I cannot access the link to the 'Fly-Buck-FlyBack design calculator' that you have attached in your email. Can you please get me the right link?

Regards,
Sourav

0 Elijah Gould over 9 years ago in reply to Sourav.Sen

Prodigy 80 points

Hi Sourav,

Apologies for that, the correct link is

Thanks,

-Elijah

0 Elijah Gould over 9 years ago

Prodigy 80 points

Anyone have any suggestions? I can design the board as per the schematic in the design calculator, but I don't know if I'll end up with + or - 12v!

0 Sourav.Sen over 9 years ago in reply to Elijah Gould

TI__Intellectual 2320 points

Hello Elijah,

I have followed up regarding the calculator and we are going to have a revised version for the calculator soon.

However, just to clarify, in the Fly-Buck-Boost application, the primary (or the non-isolated side) output voltage is always negative. The bar chart in the calculator only shows the magnitude of the primary side voltage at present, hence the confusion.

Hope this clarifies.

Regards,

Sourav

0 Elijah Gould over 9 years ago in reply to Sourav.Sen

Prodigy 80 points

Thanks very much for your reply Sourav, I thought that should be the case with a fly-buck-boost, but the bar graph was confusing!

Thanks again,

-Elijah

0 Elijah Gould over 9 years ago in reply to Sourav.Sen

Prodigy 80 points

Hi Sourav,

I've found another bug in the calculator: Again with fly-buck-boost configuration it's impossible to change any of the resistor or cap values in the Compensation section. Any chance of a fix? (Or the password to override the restriction)

Thanks.

-Elijah

Power management

Power management forum

Flybuck - flyback Excel Calculator shows positive outputs when entering negative