Too low resistance load in SMPS

varunme said:

The load is 12V 500W 41.67A heater.

Click to expand...

'Heater' implies resistive element. Usually this can run on AC.
Perhaps you can get by with a plain transformer instead of SMPS?

Or does it absolutely need DC?

-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

Edited to add:

Or in case you can use non-isolated...

* Choke or coupled inductor (5 to 50 mH suitable only if your load is unchanging)(needs power factor correction)

* Autoformer

The question is: Why does it restart?

Overtemperarture?
Mains voltage breakdown?

.. I mean: under normal conditions (see SMPS datasheet) it never should restart.

Also: check whether the SMPS is able to start under full load?

Klaus

BradtheRad said:

'Heater' implies resistive element. Usually this can run on AC.
Perhaps you can get by with a plain transformer instead of SMPS?

Or does it absolutely need DC?

Click to expand...

yes, for safety we are running on 12v dc, can't use transformer, as it is adding cost and cause magnetic problems, we are using in medical device

KlausST said:

The question is: Why does it restart?

Overtemperarture?
Mains voltage breakdown?

.. I mean: under normal conditions (see SMPS datasheet) it never should restart.

Also: check whether the SMPS is able to start under full load?

Klaus

Click to expand...

SMPS is not producing over temperature, nor voltage breakdown in voltage logger.
The SMPS supplier have made the point the low resistance may be the reason.


Actually the symptom is, the system works for around 15minitues and it goes off (Actually not a restart).
If I switch off and switch on after some time it works after one or two attempts.

I have tried with higher resistance load and it works under 80% load.

I have tried with 100w same heater too, result was same

varunme said:

The SMPS supplier have made the point the low resistance may be the reason.

Click to expand...

Makes no sense to me.
If it is specified for 12V / 83.3A then it needs be able to handle load resistance down to 144 mOhms.

I´d ask them how the SMPS can be specified to deliver 83A while it has problems even with 36A.
It´s just Ohm´s law.

Klaus

Suggest using CV mode switching power supply

KlausST said:

Makes no sense to me.
If it is specified for 12V / 83.3A then it needs be able to handle load resistance down to 144 mOhms.

I´d ask them how the SMPS can be specified to deliver 83A while it has problems even with 36A.
It´s just Ohm´s law.

Klaus

Click to expand...

Either:
1) The person you talked to at the mfr doesn’t know what they‘re talking about. Or:
2) The mfr lied about their load capacity.

barry said:

Either:
1) The person you talked to at the mfr doesn’t know what they‘re talking about. Or:
2) The mfr lied about their load capacity.

Click to expand...

They are asking to add series diode to increase resistance,

varunme said:

They are asking to add series diode to increase resistance,

Click to expand...

Maybe you could also try tying a banana to the power cable.

I suspect you did not buy this SMPS from a reliable manufacturer.

A heater load will show a resistance swing as its temperature runs up.
Nameplate current is probably "steady operation" and cold start can
be well higher (depends on element tempco). A 100% margin may be
"iffy".

But syndrome sounds like the heater has been run up to temp when
the problem starts.

There are different short circuit protection schemes and one point of
variation, is whether the protection is "real time" or "latched". If you
have a latched SC protection and are operating close to trip limit, a
"noise event" can put you over and leave you locked out. This can be
preferable for consumer equipment, preventing a "motorboating"
behavior that is possible with real-time SC protection (which will clear
itself, and then repeat if need be). Requiring manual intervention is a
feature, if you are broadly exposed to the plaintiff's bar.

That's -a- thing; whether it's -the- thing wants some poking.

You could also look at the load voltage and make sure it is not prone
to instability (either continuous oscillation, or anomalous overshoots),
which could crest into "short circuit detect" territory. Just because it
says 12VDC doesn't mean that's what you get.

This method is simpler than SMPS although it requires care in selecting values. Reduce mains AC via choke or coupled inductor.

Small Henry value is feasible because of high current draw. Add power factor correction. In this manner you draw 4A (average) from mains yet the load receives 36 A.

Add full diode bridge in order to obtain DC. Choke filtering maintains voltage at a low level. In this way it performs differently from capacitive smoothing.
Add capacitive filtering if desired.

Load should be unchanging and always attached. If load is absent then voltage soars alarmingly.

Put a fan on your power supply . . . .

barry said:

Either:
1) The person you talked to at the mfr doesn’t know what they‘re talking about. Or:
2) The mfr lied about their load capacity.

Click to expand...

I agree with the other posters, especially barry here. So long as you stay within the rated specifications of the PSU, the resistance of the load is irrelevant. The suggestion of adding a series diode in particular is silly.

A bit more info on your application and test setup would probably help determine the reason for the behavior. Perhaps your heating element is actually heating the PSU itself, causing its overtemperature protection to kick in?

varunme said:

yes, for safety we are running on 12v dc, can't use transformer, as it is adding cost and cause magnetic problems, we are using in medical device

Click to expand...

As an aside, this does not make sense, at least according to medical device safety standards I'm familiar with. Is your PSU isolated? If so it has a transformer in it. Not sure what you mean by "magnetic problems".

Hi
Can you make clear your test case ?
I guess you connected 0.33R to power module then turn on AC input.
If you do like that, the power started with heavy load. Each SMPS power module limited peak output current or output capacitor or soft-start time, if these condition are not met, controller will consider has something fault like shorted-circuit and try to start-up. Because, when SMPS start, it need to charge output capacitor and 0.33R load, so the peak output power that time is higher than the limitation, module will reset/restart.
Can you try to start SMPS before, then plug load 0.33R.
If module can run normally, it mean it is still OK. Maybe, the design don't use current mode control.
If module can't run, it maybe faked its output power. You can try to use higher load as 1R, then reduce step by step to see what real output power is. Or its current sense circuit has some problem.
Brg!

Taihung said:

Hi
Can you make clear your test case ?
I guess you connected 0.33R to power module then turn on AC input.
If you do like that, the power started with heavy load. Each SMPS power module limited peak output current or output capacitor or soft-start time, if these condition are not met, controller will consider has something fault like shorted-circuit and try to start-up. Because, when SMPS start, it need to charge output capacitor and 0.33R load, so the peak output power that time is higher than the limitation, module will reset/restart.
Can you try to start SMPS before, then plug load 0.33R.
If module can run normally, it mean it is still OK. Maybe, the design don't use current mode control.
If module can't run, it maybe faked its output power. You can try to use higher load as 1R, then reduce step by step to see what real output power is. Or its current sense circuit has some problem.

Click to expand...

This has absolutely nothing to do with what the OP posted. OP has already stated that it runs fine for 15 minutes and THEN fails.

I was suffering from illness, get back to work, will keep updating

I have tried two setups, one setup with diode and another without diode.
Both the systems ( with and without diode SMPS) switched off in 15min

I tried adding two parallel diodes in series with heater, but the SMPS switched off in 15min,

By the way, heater is tungsten heater

Tested with two brand SMPS, the second one, branded costly one restarts in a moment,
but the cheaper one switch off after 15min.

The heater manufacturer also suggests transformer PSU, but they told they have inhouse R&D which can develop an SMPS to suit this particular heater.(by correcting output power) ( they also telling this kind of heaters behaves like this).

Hi,

Please give clear and complete informations:

varunme said:

Tested with two brand SMPS, the second one, branded costly one restarts in a moment,
but the cheaper one switch off after 15min.

Click to expand...

So now in total 3 SMPS?
* a cheap one
* brand one 1
* brand one 2

What does "restarts in a moment" mean?

varunme said:

The heater manufacturer also suggests transformer PSU, but they told they have inhouse R&D which can develop an SMPS to suit this particular heater.

Click to expand...

Without additional information .. this sounds like nonsense.

varunme said:

they also telling this kind of heaters behaves like this

Click to expand...

What are "this kind of heaters"?
What does "behaves like this" mean?
****

If two different SMPS show the same behaviour ... it's likely that the problem is not caused by the SMPS at all. Thus we need to discover the whole setup.

All in all the given informations are vague, makes a detailed help impossible.
If you still need help, then provide clear informations:
* exact type of heater, with link to datasheet
* exact type if each SMPS, each with link to datasheet, each with clear information how it behaves
* sketch of wiring
* photos of the whole test circuit, where we can see which wire goes where
* how do you test it? What test equippent do you use?
* additional informations like (expected) tungsten temperature

Klaus

refer to post #12