Flyback with Parallel diodes or parallel secondary_and_diodes?

[cupoftea]

Hi,
LM5021 datasheet page 19 shows flyback with two output diodes. What is the percentage sharing improvement like with this method, compared to just paralleling the secondaries directly and paralleling the diodes directly ?

LM5021

https://www.ti.com/lit/ds/symlink/lm5021.pdf?ts=1718136308907&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM5021

 

[KlausST]

Hi,

there are documents discussing the "paralleling diodes" problem.
It is well known that thermal runaway reduces the benefit of "paralleling".

Also: what benefit do you see in paralleling diodes?
* reduced power loss?
* reduced forward voltage?
* increase current rating (mind both diodes are in one case)

*****

two coils, two diodes:
The current sharing here works way better. Not only because of the series "static" coil resistance that provide a better sharing,
but also the thermal runaway is - at least partly - compensated by the coil. The coil is copper with a rather high PTC behaviour, while the diodes have (bad) NTC behaviour.

One can not give general numbers ... they need to be calculated individually.

Klaus

 

[Easy peasy]

this is what the data sheet shows:

 

[cupoftea]

Thanks, to be honest it looks gimmicky to me now....i mean, why dont they just use one coil and one diode, or one coil one diode and a heatsink....or one coil, and a sync rect on the diode.
They have the hassle of 2 coils and each to be equally coupled to the pri.

 

[KlausST]

Hi,

maybe because of skin effect.
Or they want the option to connect both coils in series.

Klaus