Question regarding pass transistors

[davelectronic]

pass transistor

It makes no difference in voltage or current any of these exhibit the same characteristics any one of these placed in your circuit design will do the same job, despite current ratings, i would use the last of these three, its the most hardy, but the other two are fine. LM317T / LM338T / LM338K. Dave.

 

[jackorocko]

Another option may be to look at a SMPS.

Even if you want the output to be vaa a linear regulator, you might consider a SMPS pre-regulator to drop the input voltage tot he linear regulator to (say) 5 volts above the output voltage.

This also gives more protection in short circuit conditions.

You have given me a good idea. I have a 18V laptop power supply that must be SMPS as with no load it has almost zero voltage output. You won't get that with a linear supply. Is this what you mean and then use this output to drive a linear voltage regulator of say 12V?

Of course, you may wish to avoid a SMPS for noise reasons, in which case switching the secondary tap at the same time as changing output voltage is almost as good (and much simpler)

Would you mind explaining this a little further? I am not sure I understand what you are talking about.

 

[jackorocko]

It makes no difference in voltage or current any of these exhibit the same characteristics any one of these placed in your circuit design will do the same job, despite current ratings, i would use the last of these three, its the most hardy, but the other two are fine. LM317T / LM338T / LM338K. Dave.

I didn't see a graph in the LM317 stating that with increased voltage difference current drops off. Is this supposed to be assumed or is it just the reality of how a VR works? I haven't really read a lot of datahseets and haven't had to much real world experience with components, just mostly theory up to this point. That is why I ask so many questions.

 

[(*steve*)]

You have given me a good idea. I have a 18V laptop power supply that must be SMPS as with no load it has almost zero voltage output. You won't get that with a linear supply. Is this what you mean and then use this output to drive a linear voltage regulator of say 12V?

Not really. A pre-regulator is a regulator that comes before your main regulator. The idea is that it roughly regulates the input voltage so that the main regulator doesn't have to dissipate so much heat (due to a large voltage differential)

If you're using a pre-regulator then having one which isn't linear is obviously the key.

Would you mind explaining this a little further? I am not sure I understand what you are talking about.

There are several ways of doing this. The first is to employ a SMPS that has it's voltage set to be the output voltage of your main regulator plus (say) 5v.

Another way is to simply change the taps on your transformer so that when you have a 3V output you use a (say) 6VAC winding, and when your output voltage rises to say 5V, an 8VAV winding is used, when the output voltage hits 7.7V, a 12VAC winding is used, and so on...

 

[jackorocko]

I have this transformer, it is a transformer out of a car battery charger with a 50A start setting. It also has a winding for 6V. Seeing as how the output is center tapped and I have a primary winding for 6V wouldn't that suggest the transformer has a +12 to -12 output?

I haven't tried to hook the transformer up yet. I just was going through my stuff when I remembered I have this thing.

It has 4 primary winding's. The charger was capable of 12V@2A 12V@50A 12V@10A 6V@10A, the output is center-tapped(right side in the picture)

 

[davelectronic]

pass transistor

The three voltage regulators have almost the same charectaristics, i cant see how an smp will make any differance still using a VR, the regulator will still have heat issues with the voltage diff, ie 3.3-24 volts with air cooling you can have reasonble currents across the range, its the voltage diff that causes current issues if you add one to an excisting smp's or go full linear, in genral your circuit will work fine with an LM338K, smp's with add on VR will work, but issues with heat remain. Dave.

 

[davelectronic]

pass transistor

The LM338K will offer the best temp over the range of volts you want to cover due to its max current 5 amps and its sturdy build quality. on that site you left a link for you could use a 2N3055 npn transistor for 10 amps coupled with the 338K TO3 your get near max current across the range add thermal cooling with a temp switch thats auto control of the switch and your supply will do 2.5v to 24v with little or no problem the axial thermal switch cooling will allow the best option. Dave.

 

[jackorocko]

The LM338K will offer the best temp over the range of volts you want to cover due to its max current 5 amps and its sturdy build quality. on that site you left a link for you could use a 2N3055 npn transistor for 10 amps coupled with the 338K TO3 your get near max current across the range add thermal cooling with a temp switch thats auto control of the switch and your supply will do 2.5v to 24v with little or no problem the axial thermal switch cooling will allow the best option. Dave.

My regulated voltage would be the Vbe - 0.7, now the question is what is the max Vbe for the 3055 and what Ib would I need to get say ~10A?

edit: After looking up the MJE3055, the Vbe is only 5V.That doesn't seem like it would work out to well for a PSU with a select-able V range of 3.3. to 24V. The TIP3055 only has Vbe of 7V http://www.onsemi.com/pub_link/Collateral/TIP3055-D.PDF

What am I missing here? The link I posted to says the V range would be from 4.5 to 15V. Nothing is making sense here at all.

 

[davelectronic]

pass transistor

All these high power transistors can be used in power supply's, the choice is up to you, the 2N3055 NPN is a favorite very sturdy transistor as are most of the commonly used high power transistors. Dave.

2N3055
SILICON NPN TRANSISTOR
n SGS-THOMSON PREFERRED SALESTYPE
n NPN TRANSISTOR
DESCRIPTION
The 2N3055 is a silicon epitaxial-base NPN
transistor in Jedec TO-3 metal case. It is intended
for power switching circuits, series and shunt
regulators, output stages and high fidelity
amplifiers.
INTERNAL SCHEMATIC DIAGRAM
June 1997
ABSOLUTE MAXIMUM RATINGS
Symbol Parameter Value Unit
VCBO Collector-Base Voltage (IE = 0) 100 V
VCER Collector-Emitter Voltage (RBE = 100W) 70 V
VCEO Collector-Emitter Voltage (IB = 0) 60 V
VEBO Emitter-Base Voltage (IC = 0) 7 V
IC Collector Current 15 A
IB Base Current 7 A
Ptot Total Dissipation at Tc £ 25 oC 115 W
Tstg Storage Temperature -65 to 200 oC
Tj Max. Operating Junction Temperature 200 oC
1
2
TO-3
1/4
THERMAL DATA
Rthj-case Thermal Resistance Junction-case Max 1.5 oC/W
ELECTRICAL CHARACTERISTICS (Tcase = 25 oC unless otherwise specified)
Symbol Parameter Test Conditions Min. Typ. Max. Unit
ICEV Collector Cut-off
Current (VBE = -1.5V)
VCE = 100 V
VCE = 100 V Tj = 150 oC
1
5
mA
mA
ICEO Collector Cut-off
Current (IB = 0)
VCE = 30 V 0.7 mA
IEBO Emitter Cut-off Current
(IC = 0)
VEB = 7 V 5 mA
VCEO(sus)* Collector-Emitter
Sustaining Voltage
IC = 200 mA 60 V
VCER(sus)* Collector-Emitter
Sustaining Voltage
IC = 200 mA RBE = 100 W 70 V
VCE(sat)* Collector-Emitter
Saturation Voltage
IC = 4 A IB = 400 mA
IC = 10 A IB = 3.3 A
1
3
V
V
VBE* Base-Emitter Voltage IC = 4 A VCE = 4 V 1.5 V
hFE* DC Current Gain IC = 0.5 A VCE = 4 V Group 4
IC = 0.5 A VCE = 4 V Group 5
IC = 0.5 A VCE = 4 V Group 6
IC = 0.5 A VCE = 4 V Group 7
IC = 4 A VCE = 4 V
IC = 10 A VCE = 4 V
20
35
60
120
20
5
50
75
145
250
70
hFE1/hFE1* DC Current Gain IC = 0.5 A VCE = 4 V 1.6
fT Transition frequency IC = 1 A VCE = 4 V 2.5 MHz
Is/b* Second Breakdown
Collector Current
VCE = 40 V 2.87

 

[jackorocko]

Yes davelectronic, but even in those stats you provided, the base-emitter Voltage is max of 7V. If I was to build a circuit like the one I posted using the TIP3055 as the only pass transistor, then the Regulated voltage would have to be the TIP3055 base voltage minus the voltage drop across the B-E junction. So that means the output voltage could never be anymore then 7V - 0.7V = 6.3V

So either I am completely missing some very valid point, or the schematic and the math posted on that site is WRONG!

 

[jackorocko]

On second thought, is the Veb rating in the datasheet the reverse breakdown voltage of the e-b junction? If that is the case then what would the max b-e voltage be?

 

[Resqueline]

Don't confuse Veb with Vbe.
The first is the reverse bias breakdovn voltage and has no importance here.
The second is the forward bias and may be 0.6V - up to 1.4V for power transistors on full current.

 

[jackorocko]

Don't confuse Veb with Vbe.
The first is the reverse bias breakdovn voltage and has no importance here.
The second is the forward bias and may be 0.6V - up to 1.4V for power transistors on full current.

Yeah I kinda figured that out finally. But what is the max voltage one can apply to the base before it breaks down, like the Vce voltage listed in the datasheet? They list the Vce as 70V before the transistor will fail. There has to be an equivalent Vbe, right?

 

[(*steve*)]

The base-emitter junction is almost always forward biased. As such, in a class A amplifier it is always carrying some small current.

The reverse breakdown is typically quite low because reverse bias only happens in pretty limited circumstances. Even when it does happen, it's rarely more than one or two diode junction's worth.

 

[jackorocko]

I am still not sure how that answers my question? I am not trying to be rude, but all I want to know is what is max voltage one can apply to the base-emitter junction and why is this info not in the datasheet?

edit, I am not curious about the voltage drop across the junction. I want to know if I used the circuit in the first link I posted with a TIP3055 as the pass transistor, How much voltage can I put on the base, since my regulated voltage is gonna be Vb - Vbe if I am not mistaken.

 

[(*steve*)]

You never apply a voltage to the base (unless perhaps there is an emitter resistor).

You always have some resistance between the voltage and ground (typically in series with the base) to convert the voltage into a current.

A BE junction is like a LED. it's current, not voltage that you should be thinking about. Whatever Vf you get is variable and depends on heaps of factors.

edit: it's also why you would use the second circuit rather than the first. The first one will vary the voltage by Vbe, which will also vary under load, temperature, phase of moon, etc... And you wouldn't use a fixed voltage regulator like that, you'd use a variable one because they are optimised for a low (and constant) ground current.

 

[jackorocko]

Ok, so then that schematic is wrong in just about every way? If I understand you correctly, this schematic, the first one http://talkingelectronics.com/projects/200TrCcts/101-200TrCcts.html#58

Because if what your saying is correct, then there is no way for there to be a regulated voltage at the emitter side of the transistor? <-- and if this isn't correct then how does it work because I am not understanding it. It seems like I have hit a mayor mental block. I am literally dumb founded

 

[(*steve*)]

The first circuit is essentially the same as placing a resistor/zener connected to the base of a transistor. In this case the load is effectively the major resistance which limits base current (due to negative feedback).

Regulators such as this are poor because the zener is less than perfect, and the transistor's Vbe varies.

The circuit you point to uses the regulator as a fancy zener, but does nothing to improve on the inadequacies of the transistor.

The other circuit has the transistors inside the feedback loop for the regulator and should have an output nearly as stable as the regulator itself.

 

[davelectronic]

pass transisistor

There are many power supply circuits people as individules have put together on the net, no power supply is 100% perfect, slight drift in volts, mili or micro, or volt, same for current, loaded heavy the supply might drop a little, the maths work out if you allow a senseable tolorance, + or - % depending on components design, many factors involved, input, load line regulation parameters. the maths is exact, the practical often has some slight deviation, you allow for this, ajust or adapt or except. Dave.

 

[davelectronic]

pass transistor

The pass transitor circuits have power gain in mind, and not super critical line load regulation. In practice the slight deviation in volts current rairly cause issues. after all your powering projects, kits, your own design, and there will be a lower and upper tolerance in those as well, but will run happily inside the tolerance parameters. on a final some supply designs have tighter tolerance than others, what do you want ? your future use of the supply will serve well, if tolerance is allowed for. Dave.