Vicor Maxi DC-DC Converter with Backup Battery

[iruraz]

Hi all,

I purchased Vicor 's Maxi converter module which output 's maximum value can be 13.2Vdc :

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I intend to use it with 12Vdc lead-acid backup battery. If DC-DC converter 's output would be about 13.8-14.2Vdc , there is no problem about the design.

Now , I am confused about connection. How should I connect Vicor 's output with 12Vdc backup battery and load? As far as I know, DC-DC should hold battery voltage on charged value, it can be 12.8Vdc in this case.

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What do you think about this solution (in the below schematic)?

There is a zener diode which is 12.8V and 5W.

Maximum current which is flow over Rs resistor:

Max. Cur = 5W/12.8V = 0.39A --> ~400mA

Minimum value for Rs:

Min. Rs = (13.2V - 12.8V) / 0.4A = 1 ohm

power dissipation for Rs:

P = (0.4A) x (0.4V) = 0,16W

In this case auxiliary (backup) battery behaves like a load until it charges. It charges between 0-400mA at 12.8V. I know it is very slow charging but time is not important for me in this application.

I would like to know your advices.

Regards.

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[KlausST]

Hi,

Picture1)
there is no "charging" circuit.
You don´t care about charge current.

Is the vicor module built to avoid reverse current? Or does it discharge your battery until it´s dead?

Picture2)
charging current is limited here. But a simple resistor may work or may not.
Usually lead acid batteries are charged with constant current - constant voltage.


*******
13.8V may be OK for standby voltage. 14.2 may be too high for long time battery voltage. Reduced lifetime.
Also try to implement an under discharge protection unit.

Klaus

 

[iruraz]

@Klaus,

Picture 1 shows the problem. No Vicor module does not avoid the reverse current. But its output voltage is always higher than backup battery. Its output is adjustable but its maximum value is 13.2V. And I configure it to 13.2V with trim up resistor.

Picture 2 shows my solution and I would like to know your advices for it. Yes charging current is limited and it is slow charging. Vicor 's module behaves as voltage source and its output remains at 13.2V. I intend to hold battery voltage at 12.8V with zener diode and limit the current with Rs resistor.

Backup battery works for a while when DC-DC failed. Other times DC-DC supplies the system.

Regards.

 

[KlausST]

Hi,

But its output voltage is always higher than backup battery.

Then you don´t need a backup battery ;-)

Picture2 shows a cheap solution with it´s disadvantages:
* Slow charge
* no cI cU charging characteristic
* high power dissipation during charging in Rs (depends on values)
* high power dissipation with fully charged battery in zener (depends on values)
* no reverse current protection
* no under discharge protection.

*****
At least a schottky series diode in DCDC output is necessary to avoid discharge. Maybe you have to adjust output voltage because of diode forward voltage drop.

A simple constant current or charge current limiting circuit can improve power dissipation and charge time.
It depends on how much effort you want to spend and on specifications like current and battery size.

Klaus

 

[iruraz]

@Klaus,

Thanks for your reply and advices.

You are right about charging characteristic and high power dissipation.

I added a schottky diode (D2) output of DC-DC after battery charging path (Rs and Zener branch). Because If I add it before charging path diode forward voltage it drop the charge voltage and decrease the charge current.

This is temporary solution and I need basic and cheap solution for a while.

Regards.

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[KlausST]

Hi,

D2 does not prevent from discharge, because now discharge is via Rs.



Klaus

 

[iruraz]

D2 does not prevent from discharge, because now discharge is via Rs.
Klaus

Does Zener keep backup battery 's voltage at 12.8V?

Regards.

 

[KlausST]

Hi,

Does Zener keep backup battery 's voltage at 12.8V?

Only when DCDC = ON (charging),
not during DCDC = OFF (discharging)

Klaus

 

[iruraz]

not during DCDC = OFF (discharging)

But forward voltage for schottky is serious problem for me. DC-DC is about 600W and I need high current rated (over 40A) schottky and their Vf is about 2V in general.

In my scenario DC-DC is off when it is failed. DC-DC charges backup battery and supply the system until it is broken.

Regards.

 

[KlausST]

Hi,

it would be easier if you give all specifications at once.
That´s why i wrote

...and on specifications like current..

For sure it is different on a 100mA system or a 100A system. How can we know?

***************

For 40A i recommend to use an "ideal diode circuit" with mosfet.
Current controlled MOSFET gate drive can lower voltage drop to 100mV and less.

There are ICs for automatic battery backup switching.

***************

if you can´t find an IC that suits your idea, thenyou can build it with an OPAMP.

Decide how to go on.


Klaus

 

[iruraz]

Hi,
it would be easier if you give all specifications at once.

For sure it is different on a 100mA system or a 100A system. How can we know?

***************

For 40A i recommend to use an "ideal diode circuit" with mosfet.
Current controlled MOSFET gate drive can lower voltage drop to 100mV and less.

There are ICs for automatic battery backup switching.

***************

if you can´t find an IC that suits your idea, thenyou can build it with an OPAMP.

Decide how to go on.
Klaus

Sorry, you are right about specifications. I should have added them after question.

Input voltage range: 265-275Vdc
Ouput voltage: 13.2Vdc
DC-DC converter is 600W

D1 (reverse current diode) should have very low Vf and high current rating (over 40A).

I could not proper schottky for this task, finding and providing switching IC is difficult in short time.

I seek solution in accordance with your recommendation "ideal diode circuit" with MOSFET and OPAMP.

Thanks for your interest.

 

[KlausST]

Hi,

Look for: dual diode DSSK 80-0045B
Maybe one diode from battery, the other from DCDC, cathode to load.
Or both paralleled.

or single diode: MBRB4030G

Klaus

 

[iruraz]

Look for: dual diode DSSK 80-0045B
Maybe one diode from battery, the other from DCDC, cathode to load.
Or both paralleled.
or single diode: MBRB4030G

Hi Klaus,

Thanks for your advices.

Regards.