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Guest SM2GXN

2N3055 Vce(sat)

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Guest SM2GXN

Hi!

I was reading the data sheet for the well known 2n3055 and I found that Vcesat) at IC=10Adc is equal to 3Vdc.

If used in a smps can I just simply calculate the power dissipation (no load) with Pd=3*10=30W?

If total Pd has to be calculated fall and rise time has to be taken into account as well as  duty cycle and lot more I guess.

For mosfet transistors I can't see or understand any value similar to Vce(sat) take a BUZ11 as an example.

As stated in the data sheet BUZ11 has Rdson 0.03 ohm, there is nothing like Rceon for the 2n3055. Can it simply be calculated with Vce(sat)/IC?

This might just be alot of foolish thoughts but I had to ask :)
Be kind I'm a newbie  ;)

Bjorn

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Guest Alun

As you've correctly pointed out, MOSFETS don't have a saturation voltage, they have a drain-source resistance instead.

So the power dissipation (ignoring switching losses) is simply I2R

so, for your transistor:
R = 0.03
I = 10
So:
102*0.03 = 3W
The switching losses will depend on the frequency and the speed of the transistor, and I don't have my notes available at the moment so I can't tell you how to calculate it.

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Hi Bjorn

mosfets and transistors are quite different as you've seen, but in a switching configuration you could equate the two parameters, ie, Vcesat = 1.1v@4A is equal to a resistance of 0.28ohms and Rds(on) = 0.03ohms

Didn't Liverpool do well

Nothing foolish about the thoughts

Ed

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Guest SM2GXN

Thank's for reply Alun and EdwardM!

Glad to see that my thoughts and calculations was close to reality  :).
One thing that comes to my mind is: why is 2N3055 used so frequently in smps?
Regarding to the data sheet Vce(sat) 3Vdc at 10Adc it has to dissipate lots of power compared to many other transistors used in smps. Have seen some caramp smps schematics on the web containing 2N3055.

I read in a post that someone actually preferred the 2N3055 in smps design, why so?
Pricing or availability?

Bjorn

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Hej Bjorn,

Welcome to this forum!

Do you really see many 2N3055 as switchers? I think they are more likely to be found in linear systems. Mosfets are more common in switchers and even other bipolar like the BUV types. It’s not even a very good switcher but cheaper than mast any of the rest.


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Guest SM2GXN

Hej Ante!

Thank's Ante and it's nice to be here  :)

Yes I do mean 2N3055 as switchers, have to find the link but of course as you believed not many :).
Yes I know that the 2N3055 is mostly used as series transistor in linear power supplies.

The only thing I've found right now was a discussion about car amp power supply
inverter circuit with multivib transistors and a whole bunch of 2N3055.
http://www.electronics-lab.com/forum/index.php?topic=348.14
Maybe I've mixed things up  :-[

Yesterday I borrowed a book at the library called the art of electronics by Horowitz & Hill.
I will read the chapter about switchers in general and behaviour of transistors as saturated switches.

Have to run the sauna is still hot :)

Bjorn

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sauna, luuverly ;D

as you've seen the vce(sat) of the '3055 equates in resistance terms to 10 times greater than that of the BUZ11, however there is a slight trade-off in that the gate-source capacitance of of the BUZ has to be charged up before anything useful can be done and this means power wasted, I still lean toward FETs in dc-dc converters

I really must build myself a sauna

Ed

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Hi Ed,

I agree, once you have solved the drive circuit part for Mosfets they are way ahead. They are much cheaper compared to bipolars now than 15 years ago and there are some very good readymade IC:s for driving them too.

Hej Bjorn,

There have been a few discussions here about bipolar switching and not only DC-DC but I think mostly about AC-DC (inverter) which turned out very good in the end. I have done a few experiments with switchers and learned a few things while these threads have been running.   

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Guest Alun

The only advantage BJTs have is switching speed at high frequencies the switching losses can be lower with BJTs despite their higher saturation voltage and higher driving current.

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Hi Alun,
I disagree. Power BJTs are much slower than power Mosfets. The car DC-DC power supply posted with many 2N3055 transistors operates at an audio frequency and requires a huge transformer and big filter caps. The transformer's vibrating core laminations probably make an audible whine.
Here is an article with two similar SMPS circuits for powering a 350W car amp. They use power Mosfets, a tiny transformer and small filter caps:
http://www.sound.westhost.com/project89.htm

post-1706-14279142260909_thumb.gif

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Guest SM2GXN

Hi EdwardM!

Yes sauna is a good thing :) you get clean and come down with a beer or two.
Build a sauna it isn't that hard, you might have some empty space in the basment but remember regarding the liquids vine is banned only beer  ;D

Glad I asked about Vce(sat) I wasn't sure for switching apps.
Many good answers on the subject, I'll continue reading about smps and when I'm finnished I will be back with a project, there is actually one I'm thinking of.
The plan is to build a 24-volt high current smps but as I said I have to go through some reading.

Hi Ante!

Yes I read a discussion about a switching power supply using Tl494, interesting.
An excellent way to learn by reading those discussions but in the end one will have to grip the soldering iron otherwise it's more or less useless.
Think I will try to play a little with the TL494, I got some old PC smps I can pick components from but right now I'm in tube business reparing an old HRO receiver from the late 1930s.

Bjorn




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Guest Alun

audioguru,
I've heard that before, and it may be true for switching power supplyies,  but why are BJTs used more than MOSFETS in power RF applications?

I've never seen a 100MHz class C MOSFET amplifier before most use BJTs.

I always thought the gate capacitance starts to slow things down at higher frequencies and at some point BJTs overtake MOSFETS in speed.

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Guest SM2GXN

Hi Alun!

I new I had a link that could fit into what you are talking about, not switching apps this is RF.
Check the linearity aspects which is one of the more important factors if you compare fet and pipolar. But you are right for rf amplifiers pipolar transistors "is" widely used.
Check the pdf file: http://rfwireless.rell.com/pdfs/AN_860_D.pdf


Bjorn

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Guest Alun

That sounds very interesting, I've done some research and MOSFETS are used more at lower RF frequencies <3MHz but I haven't seen anthing about using them at VHF and UHF before.

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Hi Alun,

Yes the RF is sinusoidal and not square wave as far as I know. Maybe you could say that CW is digital though!  ;D ;D
Most SMPS runs in the 40 to 200 kHz range.

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Guest Alun

CW RF is not digital or square wave because for example if a 27MHz transmitter uses squarewave then you get lots of harmonics at 2*27MHz, 3*27MHz, 4*27MHz . . . up to a point deturmined by the rise and fall times.

Yes it's true most SMPS use 40 to 200KHz but the highest I've seen is 3MHz.

By the way to get round the gate capacitance in a MOSFET you connect an inductor in paralell to form a tuned circuit to prevent the drive requirements from being too high.

EDIT:
Sorry I forgot to say that CW (continious wave) is always a sine wave, there's only one fundermental frequency present.

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