current limiting using top243 or top245

[djc]

how to limit the current while charging the battery and using top 243 or top 245. So that initialy when battery needs large current switcher should not trip.

 

[neddie]

I've found that a lot of switcher circuits are not suitable for battery charging as they go into a "Burst" mode when they reach
current limit , which is NOT what you want when charging a battery. One solution which works quite well is the old resistor/pnp transistor arrangement.
Attached is a picture.
I1 is your load (Battery). How the circuit works is. When the battery draws too much current the volt drop across R1 causes the transistor to start turning on. This then adds current into your feedback circuit raising the voltage on your
feedback pin. The regulator then "thinks" is output voltage has climbed too high and reduces the output voltage , and lowering the current. This circuit can normally be modified in some way to work in a lot of applications. Not all switchers like it and
they become slightly unstable , but it's simple enough to throw together and try.
Just watch the power dissipation in R1. Your switcher feedback resistors are probably going to be different values (not 1 and 10k:0) )
You can also add a fairly large resistor , say 47K in series with Q1's collector. Sometimes helps with stability.
cheers
Neddie

 

[djc]

Thanx neddie,

That's a great help. However can you please tell me how to implement Constant current and constant voltage mode in this circuit?

 

[neddie]

Constant voltage mode is set by the Topswitch output voltage. As long as you are below the current limit
the output voltage will remain constant. (within reason :0) )
To make the constant current mode work you need to put a pot across R1 , so you can set the current.
It's not an optimal circuit for this , but it can work. Unfortunately you need to set R1 for the minimum
current. The pot can then be turned it increase the current limit. R1 can dissipate quite a lot of heat unfortunately.
Another option is to use a high-side current monitoring circuit. This can give you an output voltage that is proportional
to the current , but it can use a much smaller sense resistor.ZXCT1009 comes to mind , but a opamp cct could do the job.
This voltage could then be scaled and fed into the FB input as before. It IS going to need a little experimentation , there is no elixir here. Get out the breadboard and start experimenting.
Cheers
Neddie.

 

[picgak]

how to limit the current while charging the battery and using top 243 or top 245. So that initialy when battery needs large current switcher should not trip.

Series pass transistors Dissipate too much heat and efficiency is very poor. use top switch
Normally in top switch voltage feedback is used. Instead of that, sense the output current and give feedback and set the required current, also use a over voltage sense at the output to shut down the Top switch incase the battery is removed while the circuit is on

 

[neddie]

There are no series pass transistors in this application ?

 

[djc]

it seems to be very complicated. I have been told not to use high side current sensing or opamp or microcontroller.

 

[neddie]

It's hardly complicated , maybe 4 components!
If you are not however allowed to use high side current sensing / micro / opamps , then it's going to get even more complicated.
Does that mean you can use transistors and low side sensing?

 

[djc]

yes transistors and low side sensing can be used and may be single opamp. but if op amp is not used then will be better. While feeding current through transistor in circuit u posted do we have to isolate the feedback pin is it necessary , because same feedback pin is being used for voltage of 300 v.Because SMPS is designed that way. 240V i/p and 300V to switcher.

 

[neddie]

OK , just had a look at the TOP242-245 pdf.
I think I can see a solution here. Attached is a circuit.
Looking at FIG1 of the datasheet , the "Typical application".
How the circuit works under normal conditions. When the output voltage exceeds the Zener voltage current starts to flow
through the opto-coupler diode. This turns on the opto-coupler transistor , which it used by the TOP243 to regulate the
output. If the output voltage climbs , more current flows in the opto diode and the transistor turns on more. This is sensed by the 243 and it will reduce the output voltage to where it should be.
What the current limit circuit does is force more current through the opto diode when current limit is reached. This will have the same effect as if the voltage has gone too high. The 234 will drop the output voltage. I've moved the resistor to the ground side of the opto(diode) and the current limit circuit effectively puts a resistor in parallel with it.
Hope this makes sense.
Please note I have not built this circuit , I'm just theorizing on it operation. You must get the breadboard out :0)
Neddie

 

[djc]

IN ABOVE CIRCUIT zener diode will conduct when voltage across it is above rated votage. However what about current managing. Because initially when battery is discharged it asks for more current more than SMPS's rating.So how come ZD is going to manage that.

Can we use LM317 as current regulator. So that when it is connected to battery, whenever battery asks for more current LM317 gives only I=Vref/R. Vref is fixed to be 1.25v, so will it hep.

 

[neddie]

If the battery is completely flat and you don't even get to regulation , then yes the circuit as it stands won't work properly.
You can still I'm sure use the principle.
Move the zener to the other side of the opto , leave the resistor at the top , connected to the output.
Put the current sensing transistor across the zener. When the current limit is reached , regardless of the output voltage (within reason of course , the cct won't work at 2V!! ) the transistor will turn on and sink current through the opto. R3 determines the current limit.
R5 can be pretty small or maybe even left out. Under normal operation , not in current limit , the transistor is off and plays no
part in the circuit.
P.S I didn't put a resistor inline with the transistors base in the picture. I'd put say 100 ohms in series with the base.

P.P.S You can use a LM317 as a constant current source if you want , but it's going to get HOT.

 

[djc]

If R3 determines the current limit, what is the roll of R4.

 

[neddie]

R4 is my "load" , that is where your battery would be.

 

[djc]

What is Vref for?
And is base of transistor getting voltage from battery?
I have 4 pin optocoupler, so where Vref wil be connected.
In the circuit battery is connected between SMPS o/p and current sensing circuit.
Can battery i.e R4 be connected after SMPS and current sensing transistor circuit?

- - - Updated - - -

In the circuit i understand that negative terminal of battery is connected to current regulation circuit i.e transistor. So will it regulate the current ......
please clear this point.

 

[neddie]

This is just a sample circuit , you have to make your work in the same way. I have not seen your circuit , I'm just
giving you suggestion of what might work.
Take R4 out of the circuit , your battery goes there. The negative of the battery goes to R3.
Size R3 for your current limit. +- 0.6A / ohm.

- - - Updated - - -

Yes , but you must build it and test it!!!

 

[djc]

Hi neddie,

This is the final circuit.
here we can use PNP transistor too. Thanx for the help.
Can you suggest me the technique for current limiting in Mosfet. When mosfet is ON voltage across it is 300V and when it is OFF what is the lossless way to limit the current.

 

[neddie]

Why have you got 2 zener diodes in your circuit?
You can't use a pnp transistor in this circuit , it won't work.
If you do the high side monitoring , you could use a pnp to monitor current .
What mosfet are you talking about ?

 

[djc]

The lower side ZD is also mentioned in circuit you have posted. Upper side ZD gives 1.2V for optocoupler LED. There it may be more than 1 ZD.
Switcher is TOP250.

 

[neddie]

I don't think you know how the regulation circuit works!!
There is only supposed to be 1 zener , at the top or bottom , but not both.
In your setup , when the output voltage reaches the combined voltages of the 2 zeners and the forward drop
of the opto diode , something is going to burn out!!!
There is no resistor to limit the current.
"Upper side ZD gives 1.2V for optocoupler LED" ????