Load Sharing of Power Supply, UC28710D

Hello Solomon

I may have given you misleading information just now -  and I will have to think about this a little more -

The problem is that the PMP 8363 doesn't have either remote sensing or a trim input - which means that the load share controller cannot adjust the output voltages of the module outputs being shared.

I'll think about this a little more and see if I can come up with a sensible solution

Regards
Colin

Thank you Colin for coming back to me. I will await for your answer. Thank you.

Hi Solomon
I missed the fact that the PMP8363 has no remote sensing or trim inputs so the UCC29002 load share controller cannot be used in this case.
I think you have two simple options
1/ Hard wire the two PSUs in parallel. You won't get load sharing but the output currents will add. The PSU with the higher output voltage will supply the load current when the load is less than 2A and the other PSU will operate at no load. As the current is increased, the first PSU will go into current limit - this will happen at approx 2.2A. As the load current increases further, the second PSU will supply the added current. This method is simple and will work providing you are able to accept the unequal load sharing. Increasing the output impedance of the PSU by adding a resistor can help load sharing at the expense of voltage regulation and power loss in the resistor, although this method does work it won't give you good load sharing

2/ Redesign the transformer to allow operation at higher currents.

I tried to think of ways to parallel two PSUs but this controller is a primary side regulated device and the schemes I looked at all got very complicated and frankly it would be simpler to redesign the transformer.

What would I do? I'd look at hard wiring the PSUs in parallel - this will give you a quick solution if you can tolerate the unequal current sharing. If not, I'd change the transformer - simpler and lower overall cost too.

There is a good summary of the techniques available in the document at www.ti.com/.../slup207.pdf;tisearch=Search-EN-Everything

Apologies again for leading you astray in my first response.

Regards
Colin

Good Day Colin,

I found an IC from another supplier which does exactly what I need it to do: LTC4370. It is basically a two-supply current balancing controller. I was about to design a circuit from scratch now, when I planned the layout, I thought to myself that just maybe someone else might have already done it. And somebody did. Please take a look at it and advise whether you have a similar IC. I Love to Support TI, because you support me!

Thank you,

Solomon 

Thank you Colin,
I was busy looking at N-MOSFETs when I received your reply. In the nick of time! I will take a look at them now and order some samples to test.
Thank you once again!
Solomon

Good Day Colin,

As you can see, I have not given up yet...on TI. I looked at my problem slightly different. TI is making a ORing FET Controller IC: LM5050.

Looking at it, the LM5050 use almost the same setup as the LTC4370, but one need one LM5050 for each power supply. The IC is small and cost effective enough to make this choice. 

NOW: The LM5050 senses the output voltage and effectively changes the forward voltage drop of the "ideal" OR- "diode" (The NMOSFET used in this case), by changing the Gate Voltage of the NMOSFET. This is exactly what the LTC4370 is doing. So, by applying the parallel scheme of the LM5050 the load current should be shared almost equally among the two power supplies. The great thing is that one can continue to add power supplies in parallel with this setup, whereas the LTC4370, can only handle two power supplies.

Will you please confirm my understanding?

Even if the load current sharing is with 10% error, it will still be an acceptable solution. 

Thank you,

Solomon Smit

Hello Solomon
Interesting. Let me think about this - I may need to talk to some colleagues in Santa Clara so it may be a day or so before I get back to you.
Regards
Colin

Thank you Colin,

I was about to order a few LM5050 Evaluation boards to test it out. Maybe someone at TI has already tested it out. I am positive that it would work, provided that the voltages of the two PSUs are within range of the Forward Voltage drop of the body diode of the NMOSFET. If it is too far out, than it is just OR'ing. In our case, we have made testing equipment to test each power supply to be between 4.9V to 5.2V. Thus, it will be OR'ing and Current balancing to some degree.

Thank you,

Solomon 

Solomon
This is what I got back.
'The LM5050-x will control gate drive to maintain some minimum drain-to-source voltage (i.e. Vin – Vout) through the Rds(on) of the mosfet. See the ‘Vsd(reg)’ spec in the datasheet. However, the LM5050-x min/max limits for ‘Vsd(reg)’ are a bit on the ~sloppy~ side. So I would not expect two LM5050-x stand-alone devices match very well.

Since the gain of the Vsd(reg) comparator is relatively high, the behavior of the gate control at in this condition tends to be a bit choppy, as the gate discharge (off) is very fast and the gate charge (on) is much slower, so the Vsd(reg) gate drive looks like a saw-tooth. Some users reported this Vsd(reg) gate drive behavior, mistakenly, as an oscillation.

Unlike LTC4370, there is no allowance for variations between two different input voltages. The LM5050-x is always OR-ing controller so a difference of approx. >28mV, Vsd(rev), between two input supplies will force the lowest supply to zero current.

Finally, there is no way (that’s obvious to me) to connect multiple parallel LM5050-x circuits together so that they are aware of their relative current share.

All-in-all, I seriously doubt that LM5050-x can be used effectively to balance currents from multiple input supplies.

The good news is that multiple LM5050-x EVMs can be wired up to test this out. '

It may be possible to degrade the gain with some well chosen resistors between the LM5050 and MOSFET gate.

If I had to summarise this - the best thing to do is to get some LM5050 EVMs and see how they actually work.

Let me know how you get on.
Regards
Colin