Transistor alternative.

[davelectronic]

Looking for some advice for a follower emitter pass transistor in a power supply circuit. Can i use an MJ2955 PNP transistor in place of a TIP2955 PNP transistor ? The main reason is it being a TO3 package and would fit better the hardware layout of the project case.

Thanks for reading, any help appreciated. 

[Audioguru]

Didn't you compare the datasheets?
The chips are the same but the TO-3 metal case of the MJ2955 can dissipate a max of 115W if its case is cooled somehow to 25 degrees C but the plastic case of the TIP29555 can dissipate only 90W.
So if they both use the same heatsink then the MJ2955 will have its chip temperature farther away from its max allowed temperature.

[davelectronic]

Thanks for your reply. The only difference I can see is HFE min is IC MA max is 5@10Amps instead of 20@4 Amps for the TIP2955, and frequency is 4 MHz for the TIP2955, and 3MHz for the MJ2955, also the MJ2955 has a maximum power of 115 watts, where the TIP2955 is 90 Watts. Is this suitable to use the MJ2955 instead ? Thanks for reading.

[Zero999]

You haven't provided enough information.

[Gyro]

The Hfe of the MJ2855 is quoted at 10A Ic vs 4A for the TIP2955. As Hfe tends to drop off with increasing current the two transistors gains are probably roughly the same in practice (EDIT: probably the same die), the different Ic points simply reflect the sort of typical currents that the two packages are likely to be used at.

The lower FT at 3MHz is unlikely to be a problem in a power supply (drastically higher FT...or Hfe can occasionally lead to stability problems in marginal designs).

I think what Hero999 is trying to say is what are you driving the 2n3055 with? - You need to confirm (to yourself at least) that your driver can provide enough base current for the output current required.

[davelectronic]

Thanks for helping me. My use of the MJ2955 is for a high current power supply, which in the schematic uses the TIP2955. The number of transistors is 6x the TIP2955, but this is where I wanted to use the MJ2955 instead. In the schematic the base resistor is 100 ohms 0.5 watts, and each emitter has a ceramic power resistor in series with it, the value is 5 watts 0.1 ohms for each ceramic resistor. The power supply uses a 7812 linear voltage regulator for output regulation, and the pass transistors carry the current. But as said in the schematic the transistors used are the TIP2955's, this is where I wanted to know if its feesable to use the MJ2955 transistors instead for the 6x power transistors used in the circuit. Thanks again for your help.

[Zero999]

What's the maximum power required for the transistors to dissipate?

What are you using for a heat sink?

Post the schematic.

[davelectronic]

As I'm mobile at the moment, I can't post the schematic, but the link to the circuit is below.
Thanks for your help.
http://www.zen22142.zen.co.uk/http://www.zen22142.zen.co.uk/Circuits/Power/1230psu.htm

[Richard Crowley]

Your link is broken.  404 Not Found.
It should be: http://www.zen22142.zen.co.uk/Circuits/Power/1230psu.htm

You seem to be concentrating on the published specs for the transistors, while the actual issue seems to be broader than that.  Specifically, the total dissipation and especially the heat sinking.  If you want to use the smaller, plastic-package parts while maintaining the power rating, you could simply add more transistors in parallel to achieve an equivalent performance.  But you said that you wanted to use the smaller plastic package parts because of space which makes me think that you are trying to stuff a 10-pound circuit into a 5-pound sack. I suspect the larger scope of the mechanical design may be the real issue.

[Kleinstein]

Using the transistors in TO3 package (MJ2955 or 2N2955) instead of TO218 is OK.

However the circuit has some problems by itself:
0.1 Ohms Enitter resistoance are barely enough for current balancing. Stabilitiy against oscillation might be an issue - the lm7812 is build to work in this circuit. It is likely OK with a well behaved load, but can be oscillating if wires are layed out the wrong way or with capacitive load. The Transistors will create a awfull lot of heat likely too much even for the TO3 version. At something like 15-20 V drop transistors should not deliver much more than 3 A each, especially if current sharing is not perfect due to rather small resistors. A 35 amps rated transformer will only deliver something like 20 A at rated RMS current with this simple rectifier / capacitor combination.

[dom0]

Get some decent TO-264 transistors, they are easier to mount, especially in numbers, and tend to be more modern compared to TO-3s. Also all modern high performance heat sinks are made for flat / surface packages and not designed for TO-3s. Especially channel heat sinks and of course CPU heat sinks, a viable and low-cost choice for hobbyist use.

At dozens of Amperes of output current power losses pop up everywhere. Consider an active rectifier instead of a Si diode rectifier, which will have a few dozen Watts power loss alone.

The linked circuit probably works, but protection is marginal at best. Whether a normal 30 A fuse could save the transistors is rather doubtful and would depend on the exact output catastrophe.

By the way, check your applications' requirements, maybe you don't need a linear supply and can do with a much lower cost and higher efficiency COTS SMPS.

[T3sl4co1l]

Eh, instead of copy and pasting I'll just link my reply here.. http://www.dutchforce.com/~eforum/index.php?showtopic=43888

Tim

[davelectronic]

 Thank you to the genuine members for your replys. I have built already a two transistor version using the TIP2955, and so far it works well up to 8 Amps. I was going to attempt the 30 Amp version, but would have preferred the MJ2955 transistor package. I know the circuits not that complex, but for a begginer its one for me to taste it and see, if you get what I mean. So all I was looking for is can the MJ2955 be used instead of the TIP2955 transistor. Thanks again for reading and help given.

[davelectronic]

So if anyone can let me know, is it a viable option to use the MJ2955 in place of the TIP2955 transistor in a follower emitter circuit as a current carrying series pass element ? Thanks in advance if you can answer my question, much appreciated.

[Richard Crowley]

The frequency response is of no consequence in a DC circuit.
The gain is of virtually no consequence in an emitter-follower circuit.
You have plenty of base current available to drive the pass element transistors in that circuit.
The biggest issue is how much heat it will generate with your max load*, and whether your heat sink is sufficient.
You can add more parallel transistors to spread out the current (reduce the heating in each device).

* That is YOUR max load, not the circuit design max current.

[davelectronic]

Thanks for the replys. I would close the input voltage to much less, to avoid heating issues, around 15 Volts  for the transformers secondary. I can see by the replys its far from the perfect circuit with regards efficiency, although I would still like to try the 30 Amp version with some added input and output protection. As a newbie I'm curious to see it working in real time practical experiment if you like.

From the replys here on the eevblog forum, and the kind members on the Dutch force electronics forum, it seems the MJ2955 will work in place of the TIP2955 transistor. So thank you all from me for the replys on both forums. Appreciated.