Vce,sat vs Rds,on

Hello,

From this datasheet for the TIP31A,
https://www.onsemi.com/pub/Collateral/TIP31A-D.PDF

I see that Vce,sat is 1.2V with Ic=3A.

To calculate the equivalent of Rds,on (if I were to use a mosfet instead), would the TIP31A give a resistance of R=V/I = (1.2V) / (3A) = 0.4 ohms at saturation?

Thanks,

Michael

 

[Tim Wescott]

On Mon, 15 May 2017 08:36:40 -0700, mrdarrett wrote:

Hello,

From this datasheet for the TIP31A,
https://www.onsemi.com/pub/Collateral/TIP31A-D.PDF

I see that Vce,sat is 1.2V with Ic=3A.

To calculate the equivalent of Rds,on (if I were to use a mosfet
instead), would the TIP31A give a resistance of R=V/I = (1.2V) / (3A) =
0.4 ohms at saturation?

Yes and no. Vce_sat tends to be fairly constant in a BJT over a wide
current range, while Vds in a FET goes as current * Rds_on. So for a FET
operating _at the exact same current_, yes, you can do the Rds_on
calculation. Otherwise -- no.


--
www.wescottdesign.com

 

What Tim said but in simpler terms. There is no comparison between the two because a bipolar transistor, when saturated it acts as a forward biased diode. A saturated MOSFET acts like a low ohms resistor.

So how much resistance does your forward biased diode have ? How much voltage drop does your resistor have ? As you can see the two are not directly comparable.

 

[John Larkin]

On Thu, 18 May 2017 12:32:42 -0700 (PDT), jurb6006@gmail.com wrote:

>What Tim said but in simpler terms. There is no comparison between the two because a bipolar transistor, when saturated it acts as a forward biased diode.

Not really. It pretty much acts like a resistor with a little DC
offset from the base current into Re. Vce-sat can be low millivolts.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

"Not really. It pretty much acts like a resistor with a little DC
offset from the base current into Re. Vce-sat can be low millivolts. "

So does a diode, so the difference is almost moot.

 

[John Larkin]

On Thu, 18 May 2017 16:06:46 -0700 (PDT), jurb6006@gmail.com wrote:

"Not really. It pretty much acts like a resistor with a little DC
offset from the base current into Re. Vce-sat can be low millivolts. "

So does a diode, so the difference is almost moot.

But a forward biased diode might have .6 volts of drop or more. And
it's exponential, not ohmic.




--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

On Thursday, May 18, 2017 at 4:44:30 PM UTC-7, John Larkin wrote:

On Thu, 18 May 2017 16:06:46 -0700 (PDT), jurb6006@gmail.com wrote:

"Not really. It pretty much acts like a resistor with a little DC
offset from the base current into Re. Vce-sat can be low millivolts. "

So does a diode, so the difference is almost moot.

But a forward biased diode might have .6 volts of drop or more. And
it's exponential, not ohmic.




--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

All I really want is to control a pump motor. I suppose I just need the lowest resistance between the drain and source, or collector and emitter, within reason; I ain't paying $50 for a 0.1 micro-ohm semiconductor thingie

Michael

 

On Thursday, May 18, 2017 at 2:02:30 PM UTC-7, John Larkin wrote:

On Thu, 18 May 2017 12:32:42 -0700 (PDT), jurb6006@gmail.com wrote:

What Tim said but in simpler terms. There is no comparison between the two because a bipolar transistor, when saturated it acts as a forward biased diode.

Not really. It pretty much acts like a resistor with a little DC
offset from the base current into Re. Vce-sat can be low millivolts.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

I just realized - if I'm controlling a small DC pump motor from a breadboard, which takes 22 AWG wire, that's 0.016 ohms per foot... so anything below 16 mOhm for Rds,on is wasteful, huh...

Michael

 

[John Larkin]

On Thu, 18 May 2017 16:52:20 -0700 (PDT), mrdarrett@gmail.com wrote:

On Thursday, May 18, 2017 at 4:44:30 PM UTC-7, John Larkin wrote:
On Thu, 18 May 2017 16:06:46 -0700 (PDT), jurb6006@gmail.com wrote:

"Not really. It pretty much acts like a resistor with a little DC
offset from the base current into Re. Vce-sat can be low millivolts. "

So does a diode, so the difference is almost moot.

But a forward biased diode might have .6 volts of drop or more. And
it's exponential, not ohmic.




--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com


All I really want is to control a pump motor. I suppose I just need the lowest resistance between the drain and source, or collector and emitter, within reason; I ain't paying $50 for a 0.1 micro-ohm semiconductor thingie

Michael

A bipolar will need a bunch of base current. A fet won't.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[rickman]

On 5/18/2017 8:05 PM, John Larkin wrote:

On Thu, 18 May 2017 16:52:20 -0700 (PDT), mrdarrett@gmail.com wrote:

On Thursday, May 18, 2017 at 4:44:30 PM UTC-7, John Larkin wrote:
On Thu, 18 May 2017 16:06:46 -0700 (PDT), jurb6006@gmail.com wrote:

"Not really. It pretty much acts like a resistor with a little DC
offset from the base current into Re. Vce-sat can be low millivolts. "

So does a diode, so the difference is almost moot.

But a forward biased diode might have .6 volts of drop or more. And
it's exponential, not ohmic.




--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com


All I really want is to control a pump motor. I suppose I just need the lowest resistance between the drain and source, or collector and emitter, within reason; I ain't paying $50 for a 0.1 micro-ohm semiconductor thingie

Michael

A bipolar will need a bunch of base current. A fet won't.

A bipolar only requires 0.7 or so of base voltage. Not all FETs will
operate with that low a gate drive. The part needs to be picked for the
application regardless of the type.

--

Rick C

 

[rickman]

On 5/18/2017 7:59 PM, mrdarrett@gmail.com wrote:

On Thursday, May 18, 2017 at 2:02:30 PM UTC-7, John Larkin wrote:
On Thu, 18 May 2017 12:32:42 -0700 (PDT), jurb6006@gmail.com wrote:

What Tim said but in simpler terms. There is no comparison between the two because a bipolar transistor, when saturated it acts as a forward biased diode.

Not really. It pretty much acts like a resistor with a little DC
offset from the base current into Re. Vce-sat can be low millivolts.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com


I just realized - if I'm controlling a small DC pump motor from a breadboard, which takes 22 AWG wire, that's 0.016 ohms per foot... so anything below 16 mOhm for Rds,on is wasteful, huh...

Or a wire resistance lower than the Rds is wasteful. Don't worry so
much about the exact number. Find a part that will do the job and not
cost a lot. The Digikey search engine is a great selection tools.

Do you know the other parameters you need to meet?

--

Rick C

 

"But a forward biased diode might have .6 volts of drop or more. And
it's exponential, not ohmic. "

Pretty much like a saturated bipolar, eh ?

Bipolars used to have drops like that but they seem to have made alot of progress in that area. Saturation used to be defined by (with caveats) the base voltage being higher than the collector voltage with respect to the emitter. I don't know what they say now, the old way works.

They also now have Schottkey diodes.

Now if you took and measured a Vf diode like you would a Zener, you might be able to estimate a "knee impedance". It is just instead of like 12V it is at 0.16V or so. The effect still exists mathematically.

But we are talking about the tween here. Saturated is a different story.

But anyone who saturates a transistor does not want to operate in "knee" mode. they want it saturated, they want it saturated.

 

"And
it's exponential, not ohmic. "

Exponential is not the correct word here.

 

[George Herold]

On Thursday, May 18, 2017 at 9:33:58 PM UTC-4, jurb...@gmail.com wrote:

"But a forward biased diode might have .6 volts of drop or more. And
it's exponential, not ohmic. "

Pretty much like a saturated bipolar, eh ?

You do know that if you drive an npn on hard (saturate) you can
get Vce to be less than ~0.1 V (Depending on all sorts of other things.)

George H.

Bipolars used to have drops like that but they seem to have made alot of progress in that area. Saturation used to be defined by (with caveats) the base voltage being higher than the collector voltage with respect to the emitter. I don't know what they say now, the old way works.

They also now have Schottkey diodes.

Now if you took and measured a Vf diode like you would a Zener, you might be able to estimate a "knee impedance". It is just instead of like 12V it is at 0.16V or so. The effect still exists mathematically.

But we are talking about the tween here. Saturated is a different story.

But anyone who saturates a transistor does not want to operate in "knee" mode. they want it saturated, they want it saturated.

 

[John Larkin]

On Tue, 23 May 2017 06:47:14 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Thursday, May 18, 2017 at 9:33:58 PM UTC-4, jurb...@gmail.com wrote:
"But a forward biased diode might have .6 volts of drop or more. And
it's exponential, not ohmic. "

Pretty much like a saturated bipolar, eh ?

You do know that if you drive an npn on hard (saturate) you can
get Vce to be less than ~0.1 V (Depending on all sorts of other things.)

Millivolts. And in the inverted switch configuration, microvolts or
even negative.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[George Herold]

On Tuesday, May 23, 2017 at 2:21:55 PM UTC-4, John Larkin wrote:

On Tue, 23 May 2017 06:47:14 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Thursday, May 18, 2017 at 9:33:58 PM UTC-4, jurb...@gmail.com wrote:
"But a forward biased diode might have .6 volts of drop or more. And
it's exponential, not ohmic. "

Pretty much like a saturated bipolar, eh ?

You do know that if you drive an npn on hard (saturate) you can
get Vce to be less than ~0.1 V (Depending on all sorts of other things.)

Millivolts. And in the inverted switch configuration, microvolts or
even negative.

Grin... I had to build it on my bench supply to be sure.
It went down below 50 mV... but I was being conservative.

By inverted do you mean switching C and E?

George H.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[George Herold]

On Tuesday, May 23, 2017 at 3:20:33 PM UTC-4, George Herold wrote:

On Tuesday, May 23, 2017 at 2:21:55 PM UTC-4, John Larkin wrote:
On Tue, 23 May 2017 06:47:14 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Thursday, May 18, 2017 at 9:33:58 PM UTC-4, jurb...@gmail.com wrote:
"But a forward biased diode might have .6 volts of drop or more. And
it's exponential, not ohmic. "

Pretty much like a saturated bipolar, eh ?

You do know that if you drive an npn on hard (saturate) you can
get Vce to be less than ~0.1 V (Depending on all sorts of other things.)

Millivolts. And in the inverted switch configuration, microvolts or
even negative.
Grin... I had to build it on my bench supply to be sure.
It went down below 50 mV... but I was being conservative.

By inverted do you mean switching C and E?

here,
http://www.edn.com/electronics-blogs/living-analog/4457607/Inverted-mode-switches-

GH. (I don't think I have any 2N2222's.)

George H.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

"You do know that if you drive an npn on hard (saturate) you can
get Vce to be less than ~0.1 V (Depending on all sorts of other things.) "

Yes, but it still acts more like a diode than a resistor.

 

[John Larkin]

On Tue, 23 May 2017 19:52:16 -0700 (PDT), jurb6006@gmail.com wrote:

"You do know that if you drive an npn on hard (saturate) you can
get Vce to be less than ~0.1 V (Depending on all sorts of other things.) "

Yes, but it still acts more like a diode than a resistor.

No. The collector acts more like a resistor than a diode.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Tue, 23 May 2017 12:20:30 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Tuesday, May 23, 2017 at 2:21:55 PM UTC-4, John Larkin wrote:
On Tue, 23 May 2017 06:47:14 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Thursday, May 18, 2017 at 9:33:58 PM UTC-4, jurb...@gmail.com wrote:
"But a forward biased diode might have .6 volts of drop or more. And
it's exponential, not ohmic. "

Pretty much like a saturated bipolar, eh ?

You do know that if you drive an npn on hard (saturate) you can
get Vce to be less than ~0.1 V (Depending on all sorts of other things.)

Millivolts. And in the inverted switch configuration, microvolts or
even negative.
Grin... I had to build it on my bench supply to be sure.
It went down below 50 mV... but I was being conservative.

By inverted do you mean switching C and E?

Not exactly.

https://dl.dropboxusercontent.com/u/53724080/Parts/Transistors/Inverted_NPN.JPG

At Ib=0, Vout is zero.

As Ib increases, Vout gets close to V+, like a regular saturated
transistor. But more Ib makes Vout keep increasing, to exactly V+ or
eventually a bit above V+.

I built a bunch of 16-bit DACS when I was a kid, using this switching
mode into an R-2R ladder network of wirewound resistors, with the Ib's
of the first few MSBs trimmed for exactly zero saturation.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics