MAX713/MAX712 in series

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kein0r

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Hi

Also i read this forum anonymous from time to time this is my first post and im hoping for your help.
I want to build a charger that can charge 4 times 10 cells of NiCd cells with the max713/max712. The problem is that those cells are all in series so i need 4 power supplies serving 4 max713/max712. So far no problem (at least i hope so ).
The problem arises when i want to control the charging process with a microcontroller (pic18f1220) since i dont have a common ground. My idea is to use 4 optocoupler (4n33) attached to the fastchg pin of the max713 with a 470 ohm resistor.
The pic will then be powered with a 7805 by the lowest power supply.

Another thing is about the charging transistor. I want to charge those nicd cells with c/2 or c/4 which is 2.5A or 1.25A. I want to use the switch mode operation as described in the datasheet (last page) and max713 evaluation kit. Are those two (three) npn/pnp transistors critical or can i use any general purpose npn/pnp transistor (like bc556, bc546)?

My power supply will be around 21V for each charger. According to the datasheet the v+ should be between 4.5V and 5.5V but all examples drive the v+ pin either via a resistor or a current limiter diode.

Last question: What is the transitor attached to fastchg in the switch mode example for?

Very last question: The load is in std-by when beeing charged. Do i need to disconnect the load from the nicd cells when i charge them? it just drains a few mA.

Sorry for the amount of questions, but i need a few suggestions before i can continue with this project.

Links:
https://datasheets.maxim-ic.com/en/ds/MAX712-MAX713.pdf
https://datasheets.maxim-ic.com/en/ds/MAX713SWEVKIT.pdf
 

Yuck! Maxim! Go to nxp.com and look up the BQ2000 series parts.

D.
 
I will try to answer your questions.
In Fig 19, page 16, the NPN-PNP transistors, Q1 & Q2 are general purpose types, so most substitutes should work. The FET must be rated at or above the 2.5 amp charging rate.
The max voltage input is 20 volts and you only need 19 volts to charge 10 cells in series, so reduce the input with series diodes or put a load on the source that pulls it down to near 19 volts.
The data sheet tells how to calculate the series resistor to safely power the V+ pin.
Q2 in Fig 7, page 11, is a shunt to reduce the current into the battery during trickle charge so as not to overcharge the battery. Note that the output pin has a bar over the "fastchg" which means "not fastchg"
You should disconnect the load during trickle charge, otherwise the load can be left connected.
 
cadstarsucks said:
Yuck! Maxim! Go to nxp.com and look up the BQ2000 series parts.

D.
Click to expand...
A search of the NXP site for BQ2000 came up empty.
What do you have against Maxim?
 
Russlk said:
I will try to answer your questions.
Click to expand...
thank you very much, since i usually do only microprocessors this one is getting a bit hard for me

Russlk said:
In Fig 19, page 16, the NPN-PNP transistors, Q1 & Q2 are general purpose types, so most substitutes should work. The FET must be rated at or above the 2.5 amp charging rate.
Click to expand...
that sounds good, thx
Russlk said:
The max voltage input is 20 volts and you only need 19 volts to charge 10 cells in series, so reduce the input with series diodes or put a load on the source that pulls it down to near 19 volts.
Click to expand...
I dont understand this. The datasheet tels me that i need 1.9 x cell_count + 2 volts for switch mode which is 21V. According to the data sheet only with a cell count :gtoet: 11 a special circuit is needed for the vbat+ pin.
Russlk said:
The data sheet tells how to calculate the series resistor to safely power the V+ pin.
Click to expand...
yep, saw this. Just wanted to be sure that it also works with 21V.
Russlk said:
Q2 in Fig 7, page 11, is a shunt to reduce the current into the battery during trickle charge so as not to overcharge the battery.
Click to expand...
The question was related to the q4 on page 16, where a transistor (q4) is atteched to fastchg just for driving a led? or is this transistor just functioning like a "not" gate?
Russlk said:
Note that the output pin has a bar over the "fastchg" which means "not fastchg"
Click to expand...
I saw this. Since the 4m33 optocoupler is rather picky regarding the current for the internal led, my question was rather if the fastchg pin can directly drive the 4n33 via a resitor.
Russlk said:
You should disconnect the load during trickle charge, otherwise the load can be left connected.
Click to expand...
??? should i or *must* i disconnect it.

Another question that came today is: if i use a hexfet power mosfet with an internal bootstrap diode (like the irf 4905, link to datasheet below) can i ommit the d1 diode in the schematic?

Links:
https://www.irf.com/product-info/datasheets/data/irf4905.pdf
 
Ron H said:
A search of the NXP site for BQ2000 came up empty.
What do you have against Maxim?
Click to expand...

Same as PIC and more... Price/Performance and availability. Maxim never discontinues anything, they just don't produce anything but their latest and greatest more than once a year.

Lead times on "active" parts can be 60wks, and then you will pay double what the alternative costs.

D.
 
Dear Mr Sucks,

Why do you answer the same post 3 times? The poster you quoted only typed two sentences and only one was a question!

Mike.
 
can i please ask you kindly to stick to the topic. I have to choose the parts which are available to me.
As mentioned in the previos post I still have a few things i dont understand in the schematic of the datasheet and are asking for your help/hints.
 
If you are going to operate at input over 15 volts, I recommend the cascode circuit, Fig 5 on page 9. If you are operating above 20 volts, you must use the cascode circuit.

The resistor works with any voltage. The objective is to set the supply current to the chip at 8 mA.

Q4 on page 16 is an emitter follower. Its purpose is to reduce the voltage applied to the fastchg pin. The emitter voltage is 0.7 volts below the base, or 4.3 volts.

In the circuit on page 16 the fastchg pin is sinking 4 mA. Probably it can sink more, limited by the power disapation of the internal transistor.

Whether the load is disconnected during trickle charge depends on the current drawn by the load during that time. If the load current exceeds the trickle charge current, the cells will eventually discharge.

You cannot omit either D1 or D2 and they must be schottky or fast silicon types. The internal diode of the FET is not a good diode for switching use.
 

Thx very much, that cleared most of my questions. I will post the schematic when its ready.
 
Took me a while but heres the first result. The Board is ready and i will post some further results when the board is running.

The Charger should now be capable of charging 3 times 10NiCc Cells which are connected in series. So i should now have a charger that is capable of charging my 57V Akku.
The two molex connector go to a microcontroller which monitors the charging process via the optocouipler and the FASTCHG pin and controlls the relais.
 

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