Testing SMPS Transformer

[sdancer75]

This SMPS transformer outputs 3 voltages +5V, +12V and -12V. I get correctly the +5 but in case of +12V/-12V I get only about ~8V that is fluctuating when load is attached.

Anyway, I would like to ask, if it is normal to get ~0.2ohm (continuity) among GND and +12V/-12V ? It seems that GND/+12/-12 are connected internally. Is that correct?

Measuring directly on the PCB transformer output pins +12/-12, I can also confirm that they have continuity with circuit ground.

Regards,

[tautech]

I think you will find the load you are placing on the 12V rails exceeds the design rating.
Possibly the 12V rail caps are failing and can no longer handle higher currents as their ripple rating spec and/or ESR has degraded.
Looking at the image it seems the SMPS regulation is only on the 5V rail and it also appears to be designed for much higher currents.

[Jwillis]

GND is a centre tap for the 12V taps . The +12v and - 12V  on the transformer is sort of misleading . You don't get those +- voltages until after rectification. The 2 diodes D8 and D9 rectify the 2 voltages into positive and negative rails .

[T3sl4co1l]

Wow, impressed that they labeled the transformer pins on the board!

Yeah, if you're not getting 12V, it's because the diodes or filter chokes (the small colored axial components) are dead, or the filter caps need replacing (they don't look dead, but maybe worth a measure?), or because the load is wrong.  This type of supply looks to be the sort that will happily allow you to brown out those outputs, they'll just go to whatever as long as the priority (+5V) output remains stable.  If you aren't getting enough on the 12V outputs, try loading the 5V output as well (this will cause the supply to throttle up -- the underlying issue is cross-regulation), or just start over with a new supply that's rated for the load you're putting on it.

(The opposite is true of cross-regulation as well, if the 5V is heavily loaded and 12V lightly, it will end up quite a bit higher, maybe 15 or 18V even.)

Tim

[sdancer75]

I think you will find the load you are placing on the 12V rails exceeds the design rating.
Possibly the 12V rail caps are failing and can no longer handle higher currents as their ripple rating spec and/or ESR has degraded.
Looking at the image it seems the SMPS regulation is only on the 5V rail and it also appears to be designed for much higher currents.

Hi,

Thanks for your answer.

No, the load in metered with an adjustable electronic load its ok with that. I also checked ALL secondary parts of the SMPS (capacitors [esr & capacity], diodes, resistors, optocoupler and TL431 programmable reference regulator). They found ok.

So, I concentrated my attention in the transformer and the main power control ic (VIPER22A).

Anyway, the main question is remaining unanswered. Should the GND pin of the transformer have continuity with +12V pin?

Regards,

[sdancer75]

GND is a centre tap for the 12V taps . The +12v and - 12V  on the transformer is sort of misleading . You don't get those +- voltages until after rectification. The 2 diodes D8 and D9 rectify the 2 voltages into positive and negative rails .

The problem is that in diodes D8 & D9 never get the rectified voltage since this is grounded immediately thru the GND pin of the transformer. Am I correct?

Check the attached photo.

[sdancer75]

Wow, impressed that they labeled the transformer pins on the board!

Yeah, if you're not getting 12V, it's because the diodes or filter chokes (the small colored axial components) are dead, or the filter caps need replacing (they don't look dead, but maybe worth a measure?), or because the load is wrong.  This type of supply looks to be the sort that will happily allow you to brown out those outputs, they'll just go to whatever as long as the priority (+5V) output remains stable.  If you aren't getting enough on the 12V outputs, try loading the 5V output as well (this will cause the supply to throttle up -- the underlying issue is cross-regulation), or just start over with a new supply that's rated for the load you're putting on it.

(The opposite is true of cross-regulation as well, if the 5V is heavily loaded and 12V lightly, it will end up quite a bit higher, maybe 15 or 18V even.)

Tim

Thanks Tim,

I answered some of your questions just above. I just want to add, that +5V is tested with electronic load without problems.

Regards,

PS : Where exactly on the PCB do you place the "the small colored axial components" ?

[magic]

The problem is that in diodes D8 & D9 never get the rectified voltage since this is grounded immediately thru the GND pin of the transformer. Am I correct?.

The "continuity" that you see is the secondary windings of the transformer. It is probably alright, particularly if both windings measure equal. You will get similar result on the 5V winding too. SMPS transformers are small and so are their resistances, most of the time.

Maybe the caps are dried out, maybe you are loading the ±12V outputs above their design capacity. As others already said.

[sdancer75]

The problem is that in diodes D8 & D9 never get the rectified voltage since this is grounded immediately thru the GND pin of the transformer. Am I correct?.

The "continuity" that you see is the secondary windings of the transformer. It is probably alright, particularly if both windings measure equal. You will get similar result on the 5V winding too. SMPS transformers are small and so are their resistances, most of the time.

Maybe the caps are dried out, maybe you are loading the ±12V outputs above their design capacity. As others already said.

Hey thanks,

As I already said, I verified that all caps are ok. I measure them out of the circuit for both capacity and ESR.

Maybe, I was not very clear about the real question. It doesn't matter that the resistance between ±12V and GND are too low. That's ok and I understand that since as you already have said, the SMPS transformers are small and so their resistances.

My concern is that the GND pin of the transformer is connected directly with the PCB Ground (I attached a photo above with the DMM that proves that).

So, as the +12V/-12V pins of the transformer are also connected with the GND pin, the rectified voltage from the transformer goes all straight to the PCB's ground. Is that correct? If yes, the D8/D9 diodes are useless in this case. Where am I wrong here?

Regards,

[magic]

These are the secondary windings of the transformer. The output voltage isn't shorted out by them, it is created in them. Like in every single transformer.

One end of each winding is connected to ground because the voltage has to be generated with respect to ground.

I mean, every transformer is made like that. It's just a piece of wire, wrapped around a piece of iron. The diode (or many diodes, in other configurations) ensures that current flows only in the desired direction. In this case, positive pulses on the transformer secondary charge the capacitor and negative pulses are blocked from discharging it. Vice-versa for the negative output.

[sdancer75]

These are the secondary windings of the transformer. The output voltage isn't shorted out by them, it is created in them. Like in every single transformer.

One end of each winding is connected to ground because the voltage has to be generated with respect to ground.

Oh! Thanks!! Didn't know that. So, the transformer is fine in this case.

Best Regards,

[T3sl4co1l]

Yes, the circuit will most likely be a centertapped winding, with one diode pointing forward to make +12V, the CT grounded, and the other diode pointing backwards to make -12V.  A single winding can't be used (see: full wave doubler circuit) because of how the flyback supply is phased.  It is better to use two windings of opposite phase, and rectify those.

I can't tell you the designators of the "colored axial components" because they're hidden, but they'll be L-something and look to be right in front of the "104" ceramic disc caps in the picture.

Tim

[sdancer75]

Yes, the circuit will most likely be a centertapped winding, with one diode pointing forward to make +12V, the CT grounded, and the other diode pointing backwards to make -12V.  A single winding can't be used (see: full wave doubler circuit) because of how the flyback supply is phased.  It is better to use two windings of opposite phase, and rectify those.

I can't tell you the designators of the "colored axial components" because they're hidden, but they'll be L-something and look to be right in front of the "104" ceramic disc caps in the picture.

Tim

If I measure the voltage just after the d8/d9 diodes output will I get about 12v? If not, what it will be the problem?

[tautech]

If Tim is right and those are small inductors it's likely they only 100mA rated so higher draw off after them will see lower voltages.
Some clues on the 12V ratings can be deduced from the size of the wire in the transformer compared to the 5V winding.

[magic]

If I measure the voltage just after the d8/d9 diodes output will I get about 12v? If not, what it will be the problem?

I will simply describe how it works. The primary side receives feedback on whether the 5V output is too low or too high and drives the primary winding accordingly. The 5V secondary winding ends up producing about 5.7V pulses to get 5V after the diode. The 12V secondaries are designed with more turns so that their pulses are about 12.7V and about 12V is produced after the diodes. But there is no feedback from there, so if voltage sags, it sags. The primary never increases transformer drive to correct for it because it doesn't know.

Try running with zero load on 12V. You should see 12V after the diode, across the first capacitor, and also after the inductor, across the second capacitor, and on the output pin. With increasing current draw, voltage will steadily keep decreasing.

[Jwillis]

The diodes after the transformer on the -12 , +12 and 5 Volt are Half wave rectifiers . https://en.wikipedia.org/wiki/Rectifier . Judging by the size of the 12v rectifier diodes  they are only 1 amp diodes meaning that the Maximum forward current that can pass through those diodes is 1 Amp before failure. So the Maximum total output Watts is around 12 Watts at best but probably more like 10W . I would even go as far as saying that  your supply is only about 5 Watts to prevent  catastrophic failure of other components . The transformers output current would be less than what the diodes would be able to handle by design  . I suspect  theirs only about a half of and Amp or 500mA available current at your output at best . But as Tim says it probably less than 100mA ..Loading the output greater than what is available at the transformer would cause a dropout of voltage . Drawing more current than what a transformer can handle causes the field on the transformer to collapse  which lowers the voltage . This creates  more heat creating more resistance which further decreases the efficiency of the transformer .  Also having half wave rectifiers  the amount of capacitance just isn't enough  to keep up with a high load. And as others have pointed out , If those capacitors are failing it makes the problem with loads much worse.

On the 5 volt rail the size of the diode is probably 5 Amp .But the same issues  would apply as the 12 Volt rails . So their would probably be only 2 to 3 amps  available on the 5 Volt rail.

[sdancer75]

If I measure the voltage just after the d8/d9 diodes output will I get about 12v? If not, what it will be the problem?

I will simply describe how it works. The primary side receives feedback on whether the 5V output is too low or too high and drives the primary winding accordingly. The 5V secondary winding ends up producing about 5.7V pulses to get 5V after the diode. The 12V secondaries are designed with more turns so that their pulses are about 12.7V and about 12V is produced after the diodes. But there is no feedback from there, so if voltage sags, it sags. The primary never increases transformer drive to correct for it because it doesn't know.

Try running with zero load on 12V. You should see 12V after the diode, across the first capacitor, and also after the inductor, across the second capacitor, and on the output pin. With increasing current draw, voltage will steadily keep decreasing.

Hi,

Today I found some time to test the voltage on diodes D8/D9 with ZERO load.

I found them about ~8.5V. So I get directly from the transformer and after the rectifying diodes D8/D9 about ~8.5V.

So, is something wrong with the transformer itself?

Regards,

[magic]

Put some load on 5V and see what happens to ±12V.

[sdancer75]

Put some load on 5V and see what happens to ±12V.

Hi,

With 1A Load to +5V I got for +11.66V and -13V.

Seems to be ok, what do you think? How can I explain that?

[magic]

It's magic

Load causes 5V to sag, this is detected by the controller, more energy is pumped into the transformer, all outputs go up. The thing is clearly optimized to operate with some particular load on 5V.

Look up what "flyback converter" is.

If you aren't getting enough on the 12V outputs, try loading the 5V output as well (this will cause the supply to throttle up -- the underlying issue is cross-regulation)

[drussell]

Put some load on 5V and see what happens to ±12V.

With 1A Load to +5V I got for +11.66V and -13V.

Seems to be ok, what do you think? How can I explain that?

That is the expected behavior.

Why would you think it would act differently?

The voltage feedback to the SMPS controller is taken from the 5V output.  If you have very light or no load on the 5V, the controller will be running the whole supply at idle.  If you try to draw power from only the 12V outputs, the controller will still be idling the whole power supply and you will get virtually no output on the 12V rails.

These types of supplies with only one source of voltage feedback of a multi-output supply without any form of follow-on secondary regulation have the quirk that you must be using some power from the main output (in this case 5V) in balance with any level of power needed from the additional supply voltages that don't provide voltage feedback.  You need to size the designed output current capabilities correctly for a given specific application for this type of supply to function correctly in practice in cases like this or else you need a supply with more complex regulation scheme for that application.

[sdancer75]

Thanks to all.

So, to sum up, this smps is working just fine. Am I correct?

Regards,

[magic]

It is as bad as it was meant to be and no worse