LM74700-Q1: Expected behaviour when trying to share current between parallel channels.

Hi Johan,

Thanks for reaching out!

Can you please follow the steps in the below thread to suggest solution for your requirement.

https://e2e.ti.com/support/power-management/f/196/t/792247?LM74700-Q1-Current-Shared-Power-OR-ing

Regards, Rakesh

Hi Rakesh

Thanks for your reply.

I have sent a email to karikalan@ti.com, describing our system requrements.

Is that ok or should I have sent it to you?

/Johan

Hi Johan,

That is ok. 

Thanks, Rakesh

Hi Johan,

I have received your email and will respond to it by early next week.

Let us know if you are looking for a quick response.

Regards,

Kari.

Hi Kari

Thanks for looking in to this.

We are wokring on the hardware design of this unit and the solution of the power supply redundancy/sharing strategi kind of affects all parts of the design.

So of course we need to decide about this as soon as possible.

But early next week is fine.

Regards

/Johan 

Hi

When the power supply is closely matched in-terms of its output voltage, then both the supply will share the load curent.

22mV tolerance is available, but if the difference between two supplies is more than 22mV, then only the supply with the highest voltage will supply all of the current.

For load sharing, TI has load share controller ICs such as UCC39002 and TPS2411 or LM74700, refer to the application note:

http://www.ti.com/lit/an/slua550/slua550.pdf

Regards,

Kari.

Hi Kari

From a quick view in the app note it looks like the current sharing solution presented depends on power supplys with remote sense.

Also we need automotive grade parts for this design.

From the link that Rakesh initially gave me, you stated that:

"Yes it is possible to use LM74700-Q1 in current sharing ORing.
But we would need add some more discrete circuits along with LM74700-Q1."

The discrete circuits along with LM74700-Q1 you mentioned, is it a load share controller like UCC39002? or is there any other way to increase those 22mV max voltage difference?

Regards

/Johan

Hi Johan,

Yes, to use UCC39002 load share controller along with LM74700-Q1 for load sharing, access to the sense/feddback lines of power supply are required.

We with addition of discretes, we can have LM74700-Q1 sharing laods for 2 power supply scenario.

Can you let us know if you are looking for sharing loads of two supplies and the load current range, power supply output voltage tolerance.

Regards,

Kari.

Hi Kari

Yes we are looking for a load sharing solution with 2 supplies.

We have two power suppy connections on two different connectors (Vbat_A and Vbat_B). Both comes from a 24V battery. It might be the same battery but it might also be two different batteries.

The maximum load current is 30 A per connection and is limited by the cable dimesions rather than the power supply or the components. The total maximum load current of the system (Vbat_AB) is 60 A and in that case we must share the current between the connectors/cables. The minimum load current might be a very small sleep current of hundreds of uA, buit in that case it is not critical that we share the current.

I am looking at using a low Rdson MOSFET with about 1 mOhm Rdson, for example https://www.onsemi.com/pub/Collateral/NVBLS1D1N08H-D.PDF

It would be great if we could accept for example 500 mV difference between the supplies and still share the current. Or at least increase the 11 mV, that is stated in the datasheet, to what is achievable.

I am alos worried about what happens whan you connect one connector at a certain time and than connect the other connector a bit later?

The first channel will turn on it's MOSFET but what happens when the other connector is connected and the voltage comes from the same battery with the same voltage?

According to the datasheet it needs a 50 mV voltage difference to turn on the MOSFET. Will the second channel then always be off if the supplies are not applied at the same time?

Regrads

/Johan

Hello Johan,

Current sharing with 30A load current and 500mV difference between supplies would mean 15W of power (0.5*30A) would need to be dissipated and this is lot of power.

The current sharing method using discrete with LM74700-Q1 is recommended for low currents 1-2A range.

The 11mV threshold can be shifted to 22mV using some resistor network on cathode, but increasing beyond that would be tough with LM74700-Q1.

TPS2411 would be more suitable here because its reverse threshold is programmable via external resistor. But TPS2411 is not auto qualified.

Do you require an auto qualified part?

Regards,

Kari.

Hi Kari

Thanks for clarifying the limitations of sharing current with LM74700-Q1.

Yes we need all components to be automotive qualified.

As you say it is not a good idea to burn 15W just to obtain current sharing. And that would not be allowed by the application that controls the outputs that consumes the current.

We would need to supervise the inputs and not allow high load current unless the two supplies are within an acceptable range. in the case of dual battery sources, some external equipment would need to balance the voltage levels of the batteries. But initially before the battery levels are balanced we need to have functionallity (but it might be reduced in current capability).

There are 3 important things.

1. We need to be able to detect a broken power supply cable on any channel (and also short to GND wich is probably easier)

2. We need reverse polarity protection on each channel.

3. We need current sharing when total load current exceeds 30A

I still think LM74700-Q1 is a possible solution but I have some concerns

1. How can addition of discretes increase the allowed voltage differrence between the channels? (can you please show an example with discretes)

2. How is the second channel turned on if the connectors are not connected at the same time? (is it when the cable drop caused by the load on channel 1 is above 50 mV?)

3. Can we detect that no current flows in one of the channels (indicating a broken wire or to large difference in voltage between the channels)

4. Is there any risk of "oscillation" between the channels caused by load current transients for example (that might be the main reason for allowing 500 mV voltage difference in my mind???)

I have got myself two dev boards with LM74700-Q1 so I might be able to modify them with discretes to allow largre voltage difference, If you could advise me with an example with how this is done.

Regards

/Johan