2n3055 saturation

[Damien]

Hi there

I am wondering if someone out there knows how much the 2n3055 requires to be fully on (saturated)

Having a look at the datasheet is says - Collector−Emitter Saturation Voltage − VCE(sat) = 1.1 Vdc (Max) @ IC = 4 Adc

Does this mean that it is saturated at 4a 1v? What if the voltage is at 12v or higher?

4 ampres at 1v is 4w so does this mean that it only requires a total of 4w (at any voltage) ?

I need to know this as I am driving a pair of them with the output of an lm317 variable voltage regulator and want to know if the 1.5a supplied by this i.c is enough to make the 2n3055 fully open.

If these 2n3055 transistors require more power to open them up would it be better to put lm317's in parallel to get more power or better to use a couple of mosfets or jfets after the lm317 as a pre amp?

Any help appreciated I have asked about this on a previous post but stopped getting replies perhaps I was asking too many questions.

Regards-

Damien

 

[JWHassler]

That means: 'When the load requires 4 Amps DC, the voltage drop across the collector-emitter will be 1.1V maximum."
Voltage drop will be at saturation: Vce can't get any lower for increase in Ibe at the given Ic.
Whether the LM317 will be able drive it to saturation depends on the load: the current-gain drops to 5 at Ic = 10A




What are you doing?

 

[Damien]

Hey Hassler thanks for your reply.

So from this I can kind of gather that as current through the 2n3055's increases their gain decreases is that right?

So to drive a pair of 2n3055's at their maximum (15 X 2 = 30a) with one lm137 with a maximum output of 1.5a. Will this be enough current at the bases of the 2n3055's to get the full 30 amps from a pair of 2n3055's?

Someone suggested putting a couple of resistors at the bases and at the outputs between the 2n3055's to make sure I am getting equal loading. Is this correct? If so, what value resistors? I would have imagined that any resistance into the bases is just going to limit current going into the bases which I'm probably running short on anyway.

What am I doing? Trying to make a linear regulated power supply with fixed variable voltage.

Appreciate your input.

regards -

Damien

 

[BobK]

The short answer is no. You would need a gain of 20 to get 30A collector current with 1.5A base current, and that is not likely at 15A current each, since the datasheet says it falls to 5 at 10A collector current.

Bob

 

[Damien]

To get the full 30a's feeding into the collector & leaving the emitter on two 2n3055's -

Q1. How much current will I need at the bases in total to get 30 amps from collector to emitter of two of these transistors?


Q2. Are there any resistors needed for load sharing?

Q3. Does the gain in all B.J.T transistors drop as current from collector to emitter increases? (in common collector/emitter follower configuration)

It is my understanding (correct me please if I'm wrong) that

Gain value X Base current = Collector to emitter current.

So for example, If collector to emitter gain in a 2n3055 is 20 @ 4 amps then it will require 1/20'th of 5 amp's at the base?

IF - collector to emitter gain in a 2n3055 is 5 @ 10 amperes collector to emitter then it will require 1/5th of 10 amp's at the base? so...2 amperes?

IF - Base current is 250ma then 5 amperes will flow from collector to emitter. ?

IF - Base current is 2 amperes then 10 amperes will flow from collector to emitter ?

How much amperes at base for 15 amps to flow from collector to emitter? I'm guessing 3 or there abouts at base?

Sidenote: I have plenty of heat sinking.
I will be using this power supply as a lab power supply for experiments & also to power some ham radio equipment.


Thanks again -

Damien

 

[Damien]

To get enough base current to the 2n3055's to pull 30amp would this work ok?
I currently only have one lm317 to the bases of the two 2n3055's but this would increase the current capability to the bases to 4.5a if I am right?

 

[BobK]

What are you actually trying to do? Are you trying to make a regulated 30A supply? If so, saturation is not a question you should be asking about. The transistors cannot regulate when they are saturated, they must be operated in the linear region to regulate a voltage. This is why you typically need 2 to 3V above the regulated voltage for input, this is the minimum Vce to put the transistor in the linear region.

Bob

 

[Damien]

I think I understand what you're saying & need to look more into the different functions of transistors to get a proper understanding of their different modes.

When I say saturate I guess I just meant (full power).

I just want to make sure that I'm not bottlenecking the capability of the transformer by not supplying enough input current to the bases of the power transistors.

I'll be possibly using this thing to power ham radio's and or charge lead acid batteries & want the transistors to pass as much current as the transformer can supply.

Perhaps it is working fine then? I'm seeing a rise & fall in voltage as I change the voltage to the bases of the power transistors.

The reason for me posting asking for advice is because I had several people point out to me that in order for the 2n3055's to fully open up (I thought this meant saturate) they required more than 4 amps of base current in order to pass the full 30 amps split between two transistors. Since my lm317 can only supply 1.5 amps, it made me think that this would limit the power supplies current passing capability.

I hope all that made sense.

 

[BobK]

I don't understand why you are trying to use a voltage regulator to supply base current. If you are trying to make a regulated supply, you would need to supply the current through a error amplifier.

bob