Battery on power supply output so as to help service load transients?

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Hi
We are doing a 12kW output power supply with....
Vin = 700VDC
Cable resistance = 7.08 Ohms
Vout = 48VDC
Its an SMPS made with DCDC modules.

The load will have transients which are likely to be from no_load to 130% overload and vice versa.
Overload is treated with an output current clamp, but this takes some time to act (approx. 150ms).

In order to service these transients, (without the power supply tripping out) we are thinking of having a lithium battery simply connected to the output. (We need one there anyway to act as a power fail power source).

Is this concept of using a battery on a power supply output a common way of meeting heavy load transients as described?

There won’t be any interfacing circuitry (other than an initial inrush cct so that the battery doesn’t inrush to the power supply output caps upon connection) . The battery will start off being fully charged, so will only take a “trickle” charge from the power supply.
We are thinking of picking a ~47V battery so that it settles to 48V and just gets trickle charged as the power supply supplies the load.

 

[Diver300]

Is this concept of using a battery on a power supply output a common way of meeting heavy load transients as described?

Yes. Car batteries do that. Even after starting the engine, the alternator can't always keep up, so the battery takes over.

There are three conditions that I can think of where that happens. Firstly the load can be just too large, due to heaters, big fans or power steering. Secondly, when loads are applied, the alternator takes time to respond, so the battery is used. Thirdly, some cars will deliberately turn off the alternator briefly to reduce engine load.

In your application, I would be concerned about making sure that the power supply couldn't overcharge the batteries, or charge them too fast, or the cells could get out of balance.

 

[Flyback]

Thanks yes

In your application, I would be concerned about making sure that the power supply couldn't overcharge the batteries,

Yes, this is our worry....and this is why we feel we should pick a battery with a fully charged voltage of 50V..do you agree?..as follows for more detail..

......So to summarise, and to be more precise, We are doing a 12Kw power supply with vout = 47.18V at max load.
Though At 10% load, the vout is 49.45V.

That is, the vout rises linearly as the load current falls. (this is the modus operandi of the DCDC modules and facilitates paralleling).

The problem is that there will be no_load to full_load transients, and so we need a battery to be on the output to help handle these. (the battery is also needed for power supply failure management).

What nominal voltage of lithium battery would you pick for this?

I am thinking that we can obviously not pick a battery voltage that’s less than 49.45V, because it would get worn out by the constant trickle charging?
Therefore, we must pick a battery of say 50V. However, this means that it will get discharged down to 47.18V (as most of the time the product will be on about maximum power) …..which means it will not last long as batteries need to be returned regularly to their nominal voltage in order to last a decent lifetime?
(It must be a lithium battery as small size is essential.)

 

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...So, to summarise, we are doing a ~12kW power supply that comprises 42 DCM3623T50M53C2T00 modules in parallel…each one outputting 50V and 6.7A at maximum load.

The load is motors (BLDC + Inverter).

Battery in parallel with load
The load also has a 72Ah Lithium Battery in parallel with it. There is no Battery Management System between Battery and Load or between Battery and Power supply connection. Usually, the load runs directly off the battery with no power supply…But we are adding a power supply so that the load can run for longer…

The problem we face is that we are not allowed to add a Battery Management System.

Situation of Battery overcurrent chargeing
As such, there is a chance that the power supply could start putting more than 90A into the Battery ,which is not permitted. (could overheat it).
The reason that this could happen is that the DCM3623T50M53C2T00 DCDC modules decrease their Vout with Iout. As such, a lengthy period on say, half load, following a previous lengthy period on full load , would make the power supply vout rise and potentially put more than 90A into the Battery.

Solution: Hack DCM3623T50M53C2T00 to make it constant Vout
…We can solve this by buying DCM3623T50M53C2T70 DCDC modules. These have a constant vout over the load current range. However, we have already bought DCM3623T50M53C2T00 DCDC modules, and cannot afford to now buy the ’70 version.

Therefore, our contractor has elected to “hack” the DCM3623T50M53C2T00 DCDC modules so as to make them behave like the DCM3623T50M53C2T70 modules. (ie, have constant vout vs iout) .This will be done as in the attached. As you can see, a microcontroller constantly reads the load current, and outputs a voltage to the DCM3623T50M53C2T00’s TRIM pin so as to keep the vout at 50V, no matter what is the load current.

Feedback loops fighting each other
…The problem with this is that we also have a current clamp connected to the TRIM pin. (This is to stop any module from overly hogging current in the parallel array). The challenge is to set the feedback loop bandwidth of the microcontroller system to a bandwidth such that it doesn’t fight with the feedback loop of the current clamp. There is already an internal feedback loop inside the DCM3623T50M53C2T00 to regulate vout as described. So this is quite a lot of feedback loops we now have, all closed on essentially the same thing….

Would you agree that we must set the microcontroller’s feedback loop bandwidth to be 10x slower than the current clamps bandwidth? (The current clamp’s feedback loop bandwidth already had to be reduced in order to stop it fighting with the internal feedback loop of the DCM3623T50M53C2T00)

DCM3623T50M53C2T00 datasheet:
https://www.vicorpower.com/documents/datasheets/DCM3623x50M53C2yzz_ds.pdf

DCM3623T50M53C2T70 datasheet:
https://www.vicorpower.com/documents/datasheets/DCM3623x50M53C2y7z_ds.pdf