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Driving a Buck-Boost Converter With A Switching Supply

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RickR

Thu Nov 01 2018, 05:22PM

Registered Member #93 Joined: Thu Feb 09 2006, 04:11PM
Location: Houston, Texas
Posts: 64

I've purchased a couple Buck-Boost converters to make variable output power supplies: https://www.allelectronics.com/item/udc-10/buck-boost-converter-7-amp/1.html. I was planning on driving the converters using switching power supplies.

Are there any problems using switching supplies versus rectified transformer supplies to drive the converters? I seem to recall that using a switching supply to drive a converter that uses switching technology itself can lead to instability or some other problematic condition in the driving supply.

Is anyone aware of problems like this? I'd really like to use switching supplies to avoid the extra weight of a transformer supply.

Many thanks,

- Rick

Patrick

Thu Nov 01 2018, 06:52PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

RickR wrote ...
Are there any problems using switching supplies versus rectified transformer supplies to drive the converters? I seem to recall that using a switching supply to drive a converter that uses switching technology itself can lead to instability or some other problematic condition in the driving supply.

Yes, load sharing and feed back loop instability problems could appear. What is your specific requirement ? There are many smps's out on amazon and ebay th a t may do what you need with isolation.

Edit: are you meaning to run multiple sub ps's off one parent ps ?

RickR

Thu Nov 01 2018, 09:14PM

Registered Member #93 Joined: Thu Feb 09 2006, 04:11PM
Location: Houston, Texas
Posts: 64

Well basically what I'm trying to do is just put together a modest (read cheap) bench variable power supply that's able to source 5-30 (continuous) volts or so at 3 to 5 amps (wattage depends on voltage/current setting).

I only plan to run one converter from one switching supply.

It sounds, though, like I'm headed to using a transformer + rectifier + filtering for the source supply.

Thanks for the info.

Plasma

Fri Nov 02 2018, 02:46AM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268

You could use decoupling capacitor between the smps and DC converter, 1uf and 100uf in parrellel should fix it,I'm not sure why the different values

johnf

Fri Nov 02 2018, 09:44AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

Using capators is not recommended as they slow the feedback loops and cause positive feedback. take a look at how forward converters work and use a series inductor to slow isolate your converters from each other.
I t pays to know what frequencies each differing unit uses close /similar is to be avoided as each control loop now is intereacting

Sulaiman

Fri Nov 02 2018, 09:46AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Judging by the physical size of the inductor, that converter operates at 100's kHz or MHz,
so I'd expect your main problem will be rfi.
Ferrite on the power lead(s) between the two smps should be enough to prevent any harmonic interactions.
It is common practice to pass power from smps to smps(s).

A combination of capacitance values is used to maintain a low impedance across all frequencies of interest,
because each capacitor has its own 'resonance' frequency due mainly to intrinsic, lead and interconnection inductances.

RickR

Mon Nov 05 2018, 01:27AM

Registered Member #93 Joined: Thu Feb 09 2006, 04:11PM
Location: Houston, Texas
Posts: 64

Thanks to all for your replies.

I have just a few extra questions.

How are the instabilities likely to manifest themselves? Am I likely to see varying output voltage, current, or both? Is it likely the variations will be large, say 10-30%, smaller, say 5-10%, or is it impossible to say without more information?

Unfortunately, I'm unable to provide the working frequencies of either the supply or the converter at the momant, although I might be able to find a reference design based on the chip(s) used to drive the converter.

Finally, intuitively (to me at least), I could see how using series inductors would help stabilize the driver/converter comfiguration. The problem is that I have next to no experience in designing feedback loops, so maybe capacitors are the way to go.

Does anyone have any suggestions about the values of inductors/capacitors to use for first-cut experimentation?

Thanks again for all the input so far.

- Rick

Sulaiman

Mon Nov 05 2018, 02:35AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Many modern smps operate at a combination of frequencies to spread the noise across a spectrum to pass emi tests :(
You can probably receive the smps switching on any nearby a.m. radio.
The actual frequency is unimportant,
and you will probably not have a problem
if you do then add MnZn ferrite beads until the problem goes away,
or buy/wind/find a small MnZn core inductor.

Patrick

Mon Nov 05 2018, 06:56PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Is there a reason you can't use a smps from amazon ?

RickR

Tue Nov 06 2018, 03:36PM

Registered Member #93 Joined: Thu Feb 09 2006, 04:11PM
Location: Houston, Texas
Posts: 64

Well, if there are cheap (~$50-$60), variable voltage (5-30 volt continuous)/current (3-5 amp continuous) switching supplies from Amazon, I haven't seen them. Oops - I take that back! I found at least one that is around the upper limit of my price range: This one. I apologize, it's been a while since I've looked.

But, since the thread is started, let me ask this last question: If I were trying to drive a Mazilli or some other switching circuit with that supply, would the same comments that I've received in this thread still hold? I'm guessing the answer is yes, but just wanted to settle the question once and for all.

Thanks again for all the help.

- Rick

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