low temperature startup problems

[buenos]

low temperature startup

hi

what happens when a circuit is not rated to operate or startup at -40C ?

I have a configuration:

DCDC-----------------------------------1.8V
               I          I        I
              IC1        IC2      IC3

The 1.8V rail does not come up even with 1second timeout. I used an enviromental chamber, set to -40C. If i set to -25C, it starts up fine.
The DCDC converter is rated to operate from -40C to +85C, as the datasheet says.

Is it possible that one of the ICs loading the power rail make a short circuit when they are too cold?

 

[karesz]

low temperature startup

I canot belief it, but isnt inpossible..
What abot DC/DC conv. tests only on dummyload, & put pls a current meter or fuses in your testcircuit?
After an chambertest is the function of circuit regenerated?
I guess sooner on an capacitor...
K.

 

[buenos]

low temperature startup

the board is not really a test circuit. it is a very dense computer motherboard.
the problem only occures much below -25C, i have tested at -40C.

karesz, magyarul esetleg?

 

[AdvaRes]

Re: low temperature startup

Is it possible that one of the ICs loading the power rail make a short circuit when they are too cold?

What is sure, It is not a problem of short circuit.
The problem is that some components can't operate correctly below -25 °C.
Try to find them.
Test the DCDC component alone If it works fine then I suggest you to start with PLL.

 

[karesz]

low temperature startup

Buenos magyaros!
termeszetesen. de nemtom, hogy itt szeretik-e a nem közerthetö nyelvezetet_mint pl. az angolom sem annyira az...

As AdvaRes & I sayed too: check pls the components pieceweise, you can install some wires into your board (DUT) & will see what is wrong.
K.
For me is important to know: will your circuit regenerate over -25Celsius?

 

[buenos]

low temperature startup

if i increase the temperature to -25, it will start up fine.

 

[karesz]

low temperature startup

Hi josagos!
Pls read PM!
K.

 

[karesz]

low temperature startup

Hi buenos!
Do you have some "cold sprays"?
I think its will be very helpful for you if you can make cooler some parts of your electronics...
I.e. "Kältespray" from Konatkt Chemie, & may be some others too, can give you for the sprayed components up to -40...-50 Celsius...
Other possibility is the PCB self!
Its real that some vias/holes are problematics trough temperature cycling....
K.

 

[dick_freebird]

low temperature startup

When you power up large digital circuits there can be a large
"hump" to get over, especially if there is bus contention
or uninitialized logic. MOSFETs conduct best when cold. You
may just be hung up on a current limit?

Perhaps you could ramp an external supply and look at
the profile of current vs voltage, and verify that the current
capability is adequate -at all conditions-.

A foldback scheme which further reduces Iout when Vout
is not near regulation, will only aggravate this behavior.

 

[buenos]

low temperature startup

hi

when the DCDC doesn't start up, the voltage doesn't rise even a littlebit. i checked with an oscilloscope. second time when it starts, it takes few milliseconds but nicely/monotonely rises to final value.
if there was an overcurrent, then i would expect a ramp to an intermediate voltage like 0.6V then switch off. this didnt happen on my board.

 

[karesz]

Hi buenos,
What is progress?
K.

 

[buenos]

hi

we replaced the dcdc converter and now it works.
also we tried another board and that was already working.
i guess i will not waste any more time on this. its not really design engineering, i think.

 

[karesz]

K.