High Current Power Supply

[malik_zohaib]

hi guys i need help to modify the design a 40amp 13.8v psu. i want to ask some questions about this schematic:


this circuit is for 40 amp.
my question is 2n3055 can handle 15amps then why there are 5 of them(tr3-tr7)??
can i use only one for 10amp??
please help me, i am confuse!!!!!!!!:?:

 

[dfullmer]

One reason may be the input voltage from the transformer may be high enough such that the drop across each transistor may be very high. At current the amount of heat that will be disipated by each transistor current times the voltage drop equal watts disipated by each transistor. The wattage may exceed a single devices ability to disipate. So a lower current through several may handle more disipation than one single alone.

dfullmer

 

[malik_zohaib]

in the above design the transformer o/p is 16v and of 50amps.
so what you suggest for 10amps psu?? can i remove the one or two transistor?

 

[dfullmer]

If you use three devices to distribute the load, you will need to disipate about 30Watts per transistor at full current of 40 amps. Make sure you have good heat sink for each that will keep them all within their respective operating temperature.

dfullmer

 

[mrarslanahmed]

this will divide the load current ....

may be one transistor was not available that could withstand 40Amps current .... so the designer used 5-6 transistor so that our all current delivering capacity of the power supply will remain same....

---------- Post added at 13:06 ---------- Previous post was at 13:04 ----------

this sort of design is very much common on locally made UPS .....
the manufacturer use lot of switches in series or in parallel to deliver power......

 

[BradtheRad]

In addition to the above wise suggestions...

Customary wisdom says not to use a device at more than half its rated capacity.

Therefore the project uses five 2N3055 so each delivers 8 amps. This gives a cushion for the unexpected, such as marginal heatsinking, excessively high voltage from the transformer, heavy load, leaky filter caps, accidental short circuits while the enclosure is open, etc.

Notice that each also has a low ohm resistor in the emitter leg. This is to balance current flow through the five transistors. Thus eliminating the risk that any one transistor might be so out of spec with the others that it could hog current and burn up.

 

[mrarslanahmed]

if these resistor are to balance the current flow is it good to add fuse instead of resistor so if there is any high spike in current or voltage fuse should blow off....?

 

[malik_zohaib]

hi BradtheRad i want to design 10-16amp. if i only use tr2-tr4 is it all right without changing the rest of the design?? 8,8 amps each transistor??

 

[mrarslanahmed]

the extra transistor is for to be at the safe side...if your load with draws more the rated current .....

 

[BradtheRad]

if these resistor are to balance the current flow is it good to add fuse instead of resistor so if there is any high spike in current or voltage fuse should blow off....?

In that case if one fuse were to open up it would result in sending greater burden onto the remaining transistors. And you would not necessarily realize something was wrong. Additional components could get ruined.

Better to use one fuse for the entire circuit. That way if it blows there can be no further harm to components. You will be instantly aware that a problem occurred.

---------- Post added at 15:50 ---------- Previous post was at 15:29 ----------

hi BradtheRad i want to design 10-16amp. if i only use tr2-tr4 is it all right without changing the rest of the design?? 8,8 amps each transistor??

Yes that will work.

Some components can have reduced ratings to suit the reduced current draw. Such as F1, T1, D1.

The current-limiting networks can be modified as well.

TR2 is a 2N3055 yet it appears to have a low-power role, which is to provide bias to the power transistors. If you use only two power transistors then you probably can use a less beefy transistor for TR2.

 

[malik_zohaib]

thanks.
one more thing, is it ok if i short (and remove r18 and r20) +out with +s, and -out with -s as i am not using remote sense.????also suggest me the low power tr2.
thanx again.

 

[BradtheRad]

Yes, according to the labels you can use links of plain wire for R18 and R20 if you don't use remote sense.

TR2 can probably be a general purpose medium power NPN. By using a T0220 case it will be easy to attach a heat sink.

I do not think TR2 should be a low power transistor. In other words not in a small case. It has to carry 100mA or more, to turn the 2N3055's full on. It will also get a lot of heat conducted from the 2N3055's which are close by and very hot.

It is important that the transistors are attached to the heat sink so that it does not create a path for short circuits. TR2 may require an insulating pad beneath it.

 

[malik_zohaib]

thanks a lot guys.
i will test the circuit and let you know if i need any help.