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Current regulator

Thread starter #1
Hello,

I am trying to design a current regulator(5A,42V) for a Battery(39.2V,5Ah). I came across a voltage regulator LM2575HV. I am thinking to use 5 in parallel so that I can get the required current. So is this a good idea or should I go for a LT1170HV and use it?

I am uploading the block diagram and LM2575HV application circuit as well. Please have a look and help me.

One last question. If we are using a voltage regulator as a current regulator, is the output voltage going to be the same as the input voltage or changes according to the Topology(Buck or Boost etc)?
 

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audioguru

Well-Known Member
Most Helpful Member
#2
If the battery is dead then the output from the current regulator will be close to 0V then the voltage will rise as the battery charges. The battery limits the charging voltage.
What chemistry is the battery? Lead-acid, Ni-MH or Lithium?
 

ronsimpson

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#3
is the output voltage going to be the same as the input voltage
If there is no battery, the output voltage will be slightly below the input voltage. Some battery charges also regulate output voltage. Like the Guru said, the battery limits the voltage.
 

dr pepper

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#6
If you incorporate your current limiting section into this kind of circuit you could just have 1 2575/6 and an outboard fet.
Voltage regulation would also be a good idea in case the battery goes bad, or least so you dont get a spark when you connect the batt to the o/p cap with 30v on it.

http://www.aca-vogel.de/DCDC-Converter/8AfromLM2575_en.html

The Irf9450 used is rated at 100v which doesnt give loads of headroom, so either use a higher voltage Fet (which will have a higher Rds and therefore dissipation) or use a snubber zener or a Rc circuit to limit ringing.
 

MrAl

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#7
Hello,

I made almost the exact same circuit 9 years ago using the LM2576-ADJ version which has a lower feedback voltage requirement.

The catch about the feedback voltage requirement is if the output falls very low for some reason the current limit may no longer work. That is because the feedback voltage has to be above the min requirement of the chip in order to limit current.

My application was to charge a 4.2v Li-ion battery so i added a diode in series with the output in order to raise the min voltage coming from the chip itself to a higher voltage so that i would most likely always get some current limit. Since your output is 10 times higher than that of mine, it may work ok to use a 5v output chip but the -ADJ version is always better.
 

crutschow

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#8
5A is a very high charging current for a 5Ah NiMh battery.
Unless you monitor the battery temperature, it will likely overheat the battery and damage it.
 

audioguru

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#9
Why not use a charger IC that is designed to charge the Ni-MH battery you have?? The charger IC is made with important features like detecting a full charge then shutting off so that the battery is not damaged by over-charging.
Many battery charger ICs have an input for a temperature sensor so that the battery is not over-heated.
 
Thread starter #11
ronsimpson If we are going to use these Ic's in parallel is there going to be a problem because of the voltage of one IC inducing to the feedback of the other? or Since we are using these as current regulators there should be no problem since we can keep the current sources in parallel?
 

dr pepper

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#13
Sorta.
You would need to incorporate the current limit portion of the circuit you posted with the link I referred to as the circuit in the link doesnt have current limit, just voltage regulation.
 

MrAl

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Most Helpful Member
#14
Hi,

NiMH is a little more tricky to get right when charging with highish current.

Now a 5 amp charge current for a 5AHr battery is a charge rate of only 1 C which is something that is easily done with modern chargers, BUT when charging with that level of current (1C) you do have to have a reasonable charge termination method in place.
Some of the acceptable methods include delta V and delta T, with delta V the most common.
Delta V means monitoring the cell voltage and watching for a small DROP in voltage.
Delta T means monitoring the cell temperature and watching for a certain RISE in temperature over ambient.

Monitoring delta V is often used because the cell is already connected to the circuit so checking the voltage is somewhat easy to do and reliable. Delta T on the other hand means monitoring temperature and that means a probe that touches the case of the cell at all times has to be reliable enough to be sure it always touches the case and never comes loose.

Charge levels of up to 5C can be found in commercial chargers, but i think 1C is the max that should be used. When i got a 5C charger a long time ago one of the things i did was modify it down to a 2.5C charger so it was easier on the cells. Yes it's slower to finish but the cells dont get as hot.
 

AnalogKid

Well-Known Member
#15
I am thinking to use 5 in parallel so that I can get the required current. So is this a good idea
On paper, yes; but not in the real world.

kpr is correct. Paralleling linear regulators in a way that evenly distributes the output current among multiple circuits is tricky. Doing it with switching regulators is more tricky. Doing it with regulators designed from the ground up to be stand-alone rather than grouped will be difficult. Better to come up with a circuit that has only one control chip for the entire output load.

When the battery is nearly discharged, the entire 40 V or so will appear across the regulator circuit. Make sure the components are rated for this voltage level; even better, rated for 25-50% more than the worst-case voltage level.

ak
 

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