SMPS problems in a data bus

Hi

I have an automation network spread over a power and data bus that is about 1000 m (yards) long. Most of the equipment is is 3 panels spaced along this bus.

The power supply is provided by battery backed switched mode power supplies that output 13.8 volts - one in each panel.

Total current drawn is approx 5 Amps which should be shared between the power supplies.

I have recently replaced one of the power supplies with a smps rated at 12v (adj) 45 W (**broken link removed**). I adjusted the output to 13.8 v using a 35w bulb as a dummy load.

However, when I wire the power supply onto the bus, I find it is not providing any current. The power supply is not faulty as it will light the bulb when removed from the bus.

I have monitored the other power supplies and they are providing all the current. If I switch them off, the new power supply takes over.

Does anyone know what is going on here ?

Thanks

D

Re: SMPS problems

If the rail voltage is higher than the one you adjusted on the error amplifier then it 's possible that it will do nothing unil the voltage goes down and then, of course, it takes over ..
Try to slightly lower it's output voltage ..

Regards,
IanP

Re: SMPS problems

Sharing power when you have several supplies in parallel is tricky. A slight difference in the output voltage will cause huge variations in the output currents.

To overcome this, power sup[lies that are meant for paralleling (either for increased power or for redundant operation, which means one P/S can take ove if another fails) usually employ a forced current-sharing bus. This is just an extra wire that connects together all the supplies. The voltage on this wire is a function of the current supplied by the P/S with the highest voltage. Hoever, the other supplies in the system monitor the line and each increases its output slightly, in order to deliver more current. In the end, all power supplies share the current almost equally. For more on this, go to the TI website and look for "load sharing". Here is one chip that does this. https://focus.ti.com/docs/prod/folders/print/uc3907.html
Also, read this document **broken link removed**

In your case, you could possibly modify the new power supply by using this chip
https://focus.ti.com/lit/ds/symlink/ucc29002.pdf, especially if your power supply has remote sensing (which I doubt). However, adding remote sensing to a power supply is easy, basically a couple of resistors added to the feedback loop.

Anyway, check the system to see exactly how the load sharing is currently implemented on the supplies that work and then post again, maybe I can suggest something.

Re: SMPS problems

Thanks guys

I suspected it would be a voltage related thing.

I'll let you know how I get on.

Re: SMPS problems

Good reading ;-)

Now - my power supplies are based around the Power Integrations TopSwitch204 **broken link removed** - see attached circuit diagram.

The TI UC3902 or 29002 look interesting - as does the LM5080 from National.

However because I don't have a current sense input on the power supply, I will need to modify the power supply so I can adjust the output voltage. I.e. increase it until the output current is balanced.

Back to the drawing board.

Re: SMPS problems

Simply connect one 100 ohm resistor in series with each R4 and R5. The one in series with R4 is connected at the top (between the output and R4) and the one in series with R5 is at the bottom (between GND and R5). The junction point between R4 and the 100 ohm resistor is your new S+ input. The junction point between R5 and the lower 100 ohm resistor is your new S- input. You just added sense inputs to your existing power supply, which will allow you to increase its output voltage somewhat.

Now you can simply follow the schematic in the datasheet of the UCC29002 and implement current sharing. As simple as that.

Re: SMPS problems

You're the man ;-)

Thanks a million.