Why not point out inrush disaster for LT1681 Two Tran Fwd controller?

[treez]

Hi

The page 1 schem (and all other schems) in the datasheet of the LT1681 Two Transistor Forward controller show an input LC filter.

If a 70V input voltage is “hot plugged” into this, then it will ring up to some 140V, thus overvoltaging the LT1681, and possibly destroying it…Since the chance of this happening is quite likely, why don’t the schems show the necessary inrush limitation circuits that will be essential for use with the LT1681?

LT1681 datasheet
https://www.analog.com/media/en/technical-documentation/data-sheets/1681f.pdf

 

[d123]

Hi treez,

What does soft-start pin do? Page 6 OVLO section says it (moving words around a bit) 'gracefully' prevents input transient events alongside SS function.

Do datasheets normally show inrush-limiting add-ons? I haven't seen any that do. Typical application is like tutorial schematic: it all looked so clear-cut until I discovered I needed to add five subcircuits or components to actually make the thing functional, all of which solutions cause their own problems... How many people are only too familiar with: I just need to add one more component and it will work, and then I'll just need to add one more component to make that one work, and then add just one more... aaaarrggghhhh...!

 

[treez]

Thanks yes , if OV Input is detected then it stops switching so that the Boost pin doesn’t get overvoltaged..ie, stays below 90V……that’s better than nothing…however, if 72v gets hot plugged to the input then it will ring up to above 90V, and the OVLO wont be able to do anything about it…the chip will get damaged. The soft start gets re-introduced after an overvoltage shutdown….ie instead of hiccupping straight back on it comes back on gracefuly.

 

[FvM]

Don't agree with the presumptions. Many DC/DC applications don't involve a risk of high dV/dt hot plug and can use a LC filter without dedicated protection circuit.

 

[treez]

...though i must admit i went to a place where they were killing LED2000 LED drivers because every now and then the production staff would hot plug the 12v offtheshelf power supply...instead of connecting it and then powering up...

https://www.st.com/resource/en/datasheet/led2000.pdf

 

[Easy peasy]

For any IC, where a maximum input voltage is stated, it is up to the designer to ensure that external influences do not exceed the rating ...

a small L followed by a large C will generally not ring up very much, compred to a larger L followed by a smaller C ...

 

[treez]

Also, if the power supply rise time can be slowed, so that it takes more than an LC period to get up to regulation voltage, then that also reduces overshoot ringing quite considerably. Really, if the vout rise time of the input power supply can be limited to 2*T, then the overshoot ringing is very significantly reduced. (where T = the period of the LC ring).
Would you agree?

 

[treez]

Page 10 of the following is even worse...

https://www.analog.com/media/en/technical-documentation/data-sheets/8312fa.pdf

...that FET will get blitzed whenever that circuit is turned on at mains peak 240VAC...the LC ringing will overvoltage the power FET.......thats touted as a pretty complete schem....the inrush limitation should be shown there, wouldnt you agree?

 

[Easy peasy]

The output rise time has little to do with it, unless the converter starts IMMEDIATELY the power is applied, if there is a gap in the timing ( as there usually is ) the input filter is essentially unloaded and will ring up to a level decided by the resistance of the input wires etc ...

 

[Easy peasy]

per your page 10, actually the volt rise at switch on is determined by Lin and Cout,

thus even at 265Vac switch on at pk mains ( 375Vpk) it is somewhat unlikely the o/p cap will exceed 400V ( it's rating ) as the Zo = 1.732 ohms, and if the series R was half of this 860m-ohm you would just be OK. Better to have an R or PTC shunted by a relay though.

Peak I ( worst case, ext R = 0 ) is 375 / 1.73 = 216 amps

for 0.5 ohms external, you lose ~ 80V due to the (reduced) current ...

LT spice to model and find limit of ext R for safe cold start at pk mains ...

 

[treez]

Thanks yes, i missed the output caps....concerning the slow start to avoid high LC ringing, ...sorry i meant as in the attached LTspice simulation...top circuit with slower rise in voltage, has far less voltage overshoot