[sdiy] BUCK stepup converter module... output goes way too high on startup !!!

[rburnett at richieburnett.co.uk]

As Gerry said make sure that you are satisfying any minimum load 
requirement in its spec.

Having said that, this is pretty ugly behaviour for any boost converter! 
  I've done a bit of work on boost converters in the past and it looks as 
if the designer might not have got the control loop compensation right 
for when the boost converter goes into continuous-current mode.  This 
happens at power up when it tries to rapidly charge the output reservoir 
capacitor to the target output voltage.  Something is obviously very 
wrong in the control loop at this time because it is overshooting the 
15V target by a large amount!

Your attempt at introducing a "soft start" is a good idea, but rather 
than doing this by preceding the converter with an RC circuit this is 
usually done by adding a capacitor to a dedicated soft-start pin on the 
SMPS controller chip.  If you can tolerate it taking a bit longer for 
the output to reach the target voltage at power up, increasing the 
soft-start time is usually a good way of limiting voltage overshoot when 
it gets there.

Given that you're seeing some pretty nasty output overshoots at power 
up, I'd be a little bit concerned that the converter might also do a 
similar thing (gross voltage overshoot) after a momentary short-circuit 
or overload at the output.  So, it might be worth checking for this if 
the downstream circuitry is at risk of being damaged by the voltage 
overshoot and the chance of a fleeting short-circuit can't be ruled out!

-Richie,



On 2024-02-16 17:04, The SynthiMuse via Synth-diy wrote:
> Hi Jean-Pierre
> It might be a load issue. Most buck regulators need a minimum load of
> about 5% to 10% of their max rating.
> It could be that you're not loading it appropriately and it's
> struggling to establish on startup.
> Not a solution, but if you give it an appropriate resistive load, it
> might give you an insight in to the problem.
> Best wishes
> Gerry
> 
> On Fri, 16 Feb 2024, 16:50 Jean-Pierre Desrochers,
> <jpdesroc at oricom.ca> wrote:
> 
>> Hi all,
>> 
>> I’m using this type of stepup voltage regulator to generate +15vdc
>> and +30vdc
>> 
>> From an incoming single +9vdc power adapter.
>> 
>> These BUCK’S converters generate very good regulated voltage
>> outputs but I noticed
>> 
>> that on power up they ‘need’ to rise up to around +48vdc
>> 
>> Before settling down to their trimmed voltages.
>> 
>> Well these 2 voltages will supply a DSP module @ +15vdc (not more
>> !!!)
>> and a dual opamp TL072 that has a maximum allowed supply voltage of
>> +/- 18vdc (36vdc overall).
>> 
>> I tried putting a 16v zener diode in parallel with the trimmed
>> +15vdc
>> 
>> But the BUCK goes wild (input current rise up to around 2.5 amp and
>> stays there !!)
>> 
>> I also tried to put an RC circuit at the input (33ohms + 100uf) of
>> one of these BUCKs
>> 
>> But again the modules rise its input current to ‘try’ stabilize
>> the regulation..
>> 
>> Even with 100ohms resistors and 16v zeners.. the BUCKs won’t start
>> and finalise their outputs (inrush current installs).
>> 
>> So.. I’m stuck with this problem..
>> 
>> Any possible solutions ???
>> 
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