High capacitance load driving

[hm_fa_da]

Hi,

I'm using a step up booster chip to convert 2-3V to 4V for GSM module.
I have to use high caps on module power lines, around 3000uF. with extreme low ESR, less than 10m ohms.
and my power supply has high internal resistance ! around 1ohm.
I can't change supply internal resistance or load caps value.

Now the problem is that the booster chip can not start and lock on 4V when input is less than 2.5V, i know it's because of high current drawn at start from input which makes high voltage drop.
But if the output of step up chip be disconnected from load (3000uF caps with low ESR), then it can start fine and after a while, connecting to load, continues to work well down to even 2V with GSM module on ...
The booster chip has a fixed 10 ms soft starting time.
I think i may use a load switch chip on output, on load way, so first i start booster chip and after it is locked and stabilized on 4V, then i turn on the load switch and so on ...

Do you think this is good solution ?
which load switch do you recommend ?
won't the high cap load with low ESR, damage the load switch ?!
I saw some load switches with slew rate time controllable, is it suitable ? how should i calculate the right slew rate time according to high load caps and load switch ratings ?

Thanks

 

[FvM]

Utilizing switcher soft start feature is the usual solution.

 

[KlausST]

Hi,

The benefit of a switching power supply is, that you don´t need high value capacitors.
Often - if they are used at the SMPS output - they just cause problems. As you see. Do you see a benefit in them?
If you just need an energy reservoir, then I recommend to put a big capacitor at the input side of the SMPS.

Calculation: 3000uF charged from 0V to 4V within 10ms needs an average current of 1.2A. This may be more than the modem will draw.
If disconnecting the modem makes it work, then you are at the edge of safe operation.
I assume any change of some system parameters (battery age, battery type, temperature...) can lead to malfunction.

*******

And you say that the power supply output resistance is 1 ohms. Why is this?
(this often is combined with power loss = heating = low efficiency)

If this was my project I´d reduce the capacitor values and improve the SMPS.

*****

But for sure I recognize that you wrote you can´t change this.

Klaus

 

[hm_fa_da]

Hi,

The benefit of a switching power supply is, that you don´t need high value capacitors.
Often - if they are used at the SMPS output - they just cause problems. As you see. Do you see a benefit in them?
If you just need an energy reservoir, then I recommend to put a big capacitor at the input side of the SMPS.

Calculation: 3000uF charged from 0V to 4V within 10ms needs an average current of 1.2A. This may be more than the modem will draw.
If disconnecting the modem makes it work, then you are at the edge of safe operation.
I assume any change of some system parameters (battery age, battery type, temperature...) can lead to malfunction.

*******

And you say that the power supply output resistance is 1 ohms. Why is this?
(this often is combined with power loss = heating = low efficiency)

If this was my project I´d reduce the capacitor values and improve the SMPS.

*****

But for sure I recognize that you wrote you can´t change this.

Klaus

Thanks for your comment, The SMPS chip can't handle surge load transients good enough, i tested different solutions, best one was using CAPS directly on GSM module power line as close as possible.
Another reason is the leakage current of the caps when device is in standby mode, if i use it at the input side, i need load switch to disconnect it in standby mode.
The 1ohm resistor is for limiting short circuit current for some safety factors according to ATEX rules. however i'm still not sure about the necessity of it as i asked the question ( max permitted short current circuit in atex) in another topic.