DC-DC Module acting weird

Hi Everyone,

I'm having an issue with some cheap BUCK modules that aren't doing what I've expected.

Power source:
unregulated 24VAC output of a main transformer.
Tested, the transformers unloaded output is about 28VAC.

Power Requirements:
8.5V @ 120mA (Completed circuit at max continuous draw)
(There is a 5V linear reg in my circuit.)

Module in use: LM2596HVS

This appears to be the same module.

Cap. Ratings are the same.
My Inductor is a 470uH, the one in the listing is 330uH.

From reading the switcher's datasheet, my needs appear well within the stated Specs.

Abnormal Behaviour:

The module works fine for a time. Eventually (1 - 3 Hours), it shuts itself down.

The shut-down appears controlled. If I remove the source supply and reattach, the module starts to function again.

When I first remove the source supply, I briefly get the output back on and working. Lasts maybe 1/2 second, before running out the Caps, presumably.

I've worked the issue to the module itself - My rectification, fuse, and other safety circuitry is not the problem.

Testing has been done with more than 1 module.

My last tests were powering a standalone module from a stable 40VDC, to simulate the expected peak from the transformer, while attaching a dummy load to draw a steady 130mA.

This has eliminated the problem as being from my circuit.

I have finger checked the temperature on the module while its running. After at least 30 min, there is no significant heat. THe LM2596HVS itself is only slightly warm. I believe this was a stable temperature, as I checked many times and it always felt the same.

Any assistance would be appreciated.

I'm open to using some other sort of voltage dropping method.
As the current need is quite low, I thought maybe to try to use a Zener or voltage divider, but haven't done the math yet, as I'm still hoping to make this module work.

Thanks!

PS - My circuitry works fine continuously using USB power. I didn't have any issues until I tried to attach to the actual supply.

Hi,

Power source:
unregulated 24VAC output of a main transformer.
Tested, the transformers unloaded output is about 28VAC.

Have you converted the 24AC to DC?
Have you filtered the DC supply?

Can you please post a copy of your power circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

The elcheepo DC-DC inverters are a bit of a mixed bunch.
I bought an elcheepo lot and found that the current rating of the inductor was not up to the spec of the converter.

At 130mA load you would expect some sort of decent performance.

Thanks.. Tom... :slight_smile:

TomGeorge:
Hi,
Have you converted the 24AC to DC?
Have you filtered the DC supply?

Hi Tom,

Yes, I did, as mentioned in my initial post:

I’ve worked the issue to the module itself - My rectification, fuse, and other safety circuitry is not the problem.

I’ve attached the power section of my design.

To reiterate, I’m having the same issue supplying DC voltage with a steady dummy load. The problem is on the module.

Your schematic shows no filter capacitor after the rectifier bridge. It is required, typically several thousand uF.

OP's schematic:

Fair enough.
I was relying on the on-board input caps of the switching module. When I did my initial testing, additional Caps didn’t seem to have any noticeable effect.

This could explain some issues, however the problem is acting more like a thermal shutdown than anything else. Any ripple on the input shouldn’t be causing these issues.

As previously mentioned, I have also tried using the module completely on its own, using the output from a very stable DC supply that has a quite a bit of output capacitance. This is connected directly to the module input. The load used for this test is a steady 130mA.

I can’t see how a circuit in a very steady state would suddenly change.

28VAC when rectified and filtered gives 39.2VDC, which is uncomfortably close to the 40V maximum input voltage of a LM2596. I'm not saying it is the cause of your problem, but it would concern me.

Also, you said it is a cheap converter, I think that says it all; buy a more expensive one.

Which is why I’m using a LM2596HVS module.

It has a higher input range.

The problem does exist at lower voltages too. 30 Vin had issues.

15 Vin didn’t seem to have trouble, buy only tested for 4-5 hours.

Being ebay purchase i would double check if lm2596 is in fact HV version.

Add 47-200nF ceramic capacitor across input and output of the module, as close to lm2596 as possible, unless already there.

Too low load might be a problem, try with 1 A load.

Which is why I'm using a LM2596HVS module.

Ah, OK. Doesn't say that on the schematic, although I can see you mentioned it in your OP.

Sunkmail:
Which is why I’m using a LM2596HVS module.

It has a higher input range.

The problem does exist at lower voltages too. 30 Vin had issues.

15 Vin didn’t seem to have trouble, buy only tested for 4-5 hours.

That would be because you had no filter capacitor across the DC output of your rectifier.
Have you fitted 2200uF or 4700uF to smooth the DC.
Have you measured the DC with the filter capacitors fitted?
Tom.... :slight_smile:

TomGeorge:
That would be because you had no filter capacitor across the DC output of your rectifier.
Have you fitted 2200uF or 4700uF to smooth the DC.
Have you measured the DC with the filter capacitors fitted?
Tom.... :slight_smile:

With virtually no load(130mA on output = 50 ma input) capacitor on module is more than enough.
I have run similar modules same as OP with 2 A output and no capacitors other than those on module.

Plus OP said he tested with stabilised DC supply too, read before commenting.

Hi,
Note;

100uF input cap.
Datasheet recommends 470uf input and output.
Also 68uH inductor, that looks like a 33uH.
Also 3 x 0.1uF bypass caps.

So converter possibly not built to recommended specs.

Tom… :slight_smile:

LM2596HV.PDF (1.21 MB)

I did a comparison between the datasheet and the modules I have:

  • EDIT - For some reason my linked images aren’t showing up. I’ll attach them at the bottom.

Sorry

Here is one of my modules - a Slight variant of the others linked:

and here is the schematic as best as I could reverse engineer it:

Here is the schematic as suggested by the datasheet, layed out the same as the one above:

So a few differences …

Obviously the caps at the input and output are different.

As my testing supply is clean and my load is constant, I don’t think it is causing my issue, though something to keep in mind for once this issue is sorted out.

Value of R1

The datasheet says this should be between 1k & 5k. 330R is low.

(R2 on the datasheet is a constant resistance to create a 5V output, I just left the POT symbol)

Inductor in different, 47uH Vs the suggested 68uH.

C2 is in a different place than the datasheet suggests. (Would this do anything when positioned on my module?)

The Diodes have different part numbers, but the one used should be at least as good, if not better.

I’ll see if I can find anything in the datasheet on possible effects of the parts that are different.

I was doing this at the time Tom posted his comments. :slight_smile:

I can try adding additional filter caps, and increasing the R1 value shouldn’t be too much of a pain.
I doubt I have any appropriate inductors laying around though.

The point of the filter capacitor is to store energy during the long time when the rectified AC input voltage is lower than the voltage needed to drive the converter. Typically this is 5 milliseconds or so. At 1A load 1000µF is needed to keep voltage droop to 5V during this period. dV/dt = I/C

For 120mA and only 8V output you can afford something like 20V droop, indicating the filter cap needs to be only 30µF. 0.12A / 30µF = 20V / 0.005s

You problem with it failing is almost certainly over-voltage.

MarkT:
The point of the filter capacitor is to store energy during the long time when the rectified AC input voltage is lower than the voltage needed to drive the converter. Typically this is 5 milliseconds or so. At 1A load 1000µF is needed to keep voltage droop to 5V during this period. dV/dt = I/C

For 120mA and only 8V output you can afford something like 20V droop, indicating the filter cap needs to be only 30µF. 0.12A / 30µF = 20V / 0.005s

You problem with it failing is almost certainly over-voltage.

Since my last post, I've incrementally tried the following:

  1. Replace R1 from 330R to 3k3, to be more inline with datasheet

  2. Replace Pot with 20k resistor - Sets output to 8.5V - Had to remove the pot to get at R1 Anyway

  3. move the cap that isn't on the datasheet to across input, per the datasheet.

  4. Add input Cap - 1000uF @50V

  5. Add output Cap - 1500uF @ 35V

No significant improvement.

Though, It seems to trigger whatever shutdown at a more consistent point now.
If I try an input voltage greater than 37.5VDC it turns off within 15s.

So, yeah, probably an overvoltage problem. Therefore, probably counterfeit chips?

Assuming that 35V is stable - I had a test running at least 4 hours at 35V without issue ...

Is there an easy way to reduce the input voltage by a few volts
?

I already have these modules, and I have made a case that fits them, so I would rather not abandon them completely.

two ideas spring to mind ....

  1. a few silicon diodes in series (not efficient in space or part count, but easy to bodge)

  2. Some sort of Zener arrangement

  3. Voltage Divider (I only need 120 mA, so not too wasteful)

I'm leaning towards #2. I have lots of 5v1 zeners kicking around from an old project.

I'm not even sure how to look this up ...

Is there a way to use a zener to reduce a voltage By a certain amount?

(Rather than TO a certain amount)

Haven't had the chance to do any experiments, and (so far) google has come up empty.

Any suggestions?
Solutions or a subject to research?

Thanks

Therefore, probably counterfeit chips?

Chips are OK but the design and PCB layout is crap. Designing a proper switch mode power converter is hard. It is a specialist skill. This is just a poor implementation of a data sheet design. You also need a good quality inductor as well.

I have had a few of these and they don’t really perform well at all when trying to draw current of about 700 mA. You need a good scope to these the output and see what it is doing. They are prone to oscillating at specific current / voltage combinations, as are most switch mode supplies.