learning about transistor equivalent to R_ds_on

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In this data sheet for the TIP31A:

http://www.fairchildsemi.com/ds/TI/TIP31A.pdf

The spec is given:
VCE(sat) * Collector-Emitter Saturation Voltage IC = 3A, IB = 375mA:
1.2 V

Does this mean then that the resistance between the collector and
emitter is (1.2V) / (3A) = 0.4 ohms, when the transistor is carrying
3A through the collector to the emitter?

If so, that's a bit higher than the Rds_on for the IRF530, at 0.1 ohm.

Thanks,

Michael
 
mrdarrett@gmail.com wrote:
In this data sheet for the TIP31A:

http://www.fairchildsemi.com/ds/TI/TIP31A.pdf

The spec is given:
VCE(sat) * Collector-Emitter Saturation Voltage IC = 3A, IB = 375mA:
1.2 V

Does this mean then that the resistance between the collector and
emitter is (1.2V) / (3A) = 0.4 ohms, when the transistor is carrying
3A through the collector to the emitter?

If so, that's a bit higher than the Rds_on for the IRF530, at 0.1 ohm.
Click to expand...
Pretty much. But the BJT needs a whole lot of base current to keep it
there and it needs a long time to come back out of saturation. IOW if
you want to switch something at a reasonable efficiency you are usually
better off with a FET.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
 
On Fri, 20 Jun 2008 14:39:20 -0700 (PDT), mrdarrett@gmail.com wrote:

In this data sheet for the TIP31A:

http://www.fairchildsemi.com/ds/TI/TIP31A.pdf

The spec is given:
VCE(sat) * Collector-Emitter Saturation Voltage IC = 3A, IB = 375mA:
1.2 V

Does this mean then that the resistance between the collector and
emitter is (1.2V) / (3A) = 0.4 ohms, when the transistor is carrying
3A through the collector to the emitter?
Click to expand...
---
Yes
---

If so, that's a bit higher than the Rds_on for the IRF530, at 0.1 ohm.
Click to expand...
---
Oh, well...
---

Thanks,
Click to expand...
---
Yer welcome.

JF
 
Joerg wrote:
mrdarrett@gmail.com wrote:
In this data sheet for the TIP31A:

http://www.fairchildsemi.com/ds/TI/TIP31A.pdf

The spec is given:
VCE(sat) * Collector-Emitter Saturation Voltage IC = 3A, IB = 375mA:
1.2 V

Does this mean then that the resistance between the collector and
emitter is (1.2V) / (3A) = 0.4 ohms, when the transistor is carrying
3A through the collector to the emitter?

If so, that's a bit higher than the Rds_on for the IRF530, at 0.1 ohm.


Pretty much. But the BJT needs a whole lot of base current to keep it
there and it needs a long time to come back out of saturation. IOW if
you want to switch something at a reasonable efficiency you are usually
better off with a FET.

You can pull the base negative when you turn off; this sucks charge out
Click to expand...
of the base, thereby speeding things up considerably. Zetex has app
notes, and bipolar transistors that could kinda sorta compete with
MOSFETs for overall circuit efficiency.

Dunno if they managed to keep it up, though -- switching FETs have
gotten better since the last time I really scrutinized the Zetex line.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
 
On Sat, 21 Jun 2008 11:07:25 +1000, "Phil Allison"
<philallison@tpg.com.au> wrote:

mrdarrett@gmail.com

In this data sheet for the TIP31A:

http://www.fairchildsemi.com/ds/TI/TIP31A.pdf

The spec is given:
VCE(sat) * Collector-Emitter Saturation Voltage IC = 3A, IB = 375mA:
1.2 V

Does this mean then that the resistance between the collector and
emitter is (1.2V) / (3A) = 0.4 ohms, when the transistor is carrying
3A through the collector to the emitter?

If so, that's a bit higher than the Rds_on for the IRF530, at 0.1 ohm.


** You have just discovered why power MOSFETS have been preferred over BJTs
for high current switching applications for the last 20 years or so.
Click to expand...
---
Indeed, but there are always those applications where Vbe(sat)for a
BJT at <= 0.3V will beat out a MOSFETs Rds(on).

JF
 
On Jun 20, 7:26 pm, "Phil Allison" <philalli...@tpg.com.au> wrote:
"John Fields pedantic ASD fucked fuckwit "



Indeed, but there are always those applications where Vbe(sat)for a
BJT at <= 0.3V will beat out a MOSFETs Rds(on).

** Vbe (sat ) ??

...... Phil
Click to expand...

Yeah, and sat. at 0.3 (vs. 0.7) V ??

....... Michael
 
mrdarrett@gmail.com wrote:

"Phil Allison" <philalli...@tpg.com.au> wrote:
"John Fields pedantic ASD fucked fuckwit "


Indeed, but there are always those applications where Vbe(sat)for a
BJT at <= 0.3V will beat out a MOSFETs Rds(on).

** Vbe (sat ) ??

...... Phil

Yeah, and sat. at 0.3 (vs. 0.7) V ??
Click to expand...
Yes, *Vce (sat)* can easily be that low.

Graham
 
On Sat, 21 Jun 2008 12:26:46 +1000, "Phil Allison"
<philallison@tpg.com.au> wrote:

"John Fields pedantic ASD fucked fuckwit "


Indeed, but there are always those applications where Vbe(sat)for a
BJT at <= 0.3V will beat out a MOSFETs Rds(on).



** Vbe (sat ) ??
Click to expand...
---
Oops, trypo... Meant Vce(sat).

Thanks :)

JF
 
Tim Wescott wrote:
Joerg wrote:
mrdarrett@gmail.com wrote:
In this data sheet for the TIP31A:

http://www.fairchildsemi.com/ds/TI/TIP31A.pdf

The spec is given:
VCE(sat) * Collector-Emitter Saturation Voltage IC = 3A, IB = 375mA:
1.2 V

Does this mean then that the resistance between the collector and
emitter is (1.2V) / (3A) = 0.4 ohms, when the transistor is carrying
3A through the collector to the emitter?

If so, that's a bit higher than the Rds_on for the IRF530, at 0.1 ohm.


Pretty much. But the BJT needs a whole lot of base current to keep it
there and it needs a long time to come back out of saturation. IOW if
you want to switch something at a reasonable efficiency you are
usually better off with a FET.

You can pull the base negative when you turn off; this sucks charge out
of the base, thereby speeding things up considerably. Zetex has app
notes, and bipolar transistors that could kinda sorta compete with
MOSFETs for overall circuit efficiency.

Dunno if they managed to keep it up, though -- switching FETs have
gotten better since the last time I really scrutinized the Zetex line.
Click to expand...
Yes, but don't get too close to the abs max negative Vbe or the
transistor will literally degrade. Most FETs can safely be swung to -15V
with gusto. Do that with a BJT ... phssst ... *BANG*.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
 
On Fri, 20 Jun 2008 17:26:47 -0700, Joerg
<notthisjoergsch@removethispacbell.net> wrote:

mrdarrett@gmail.com wrote:
In this data sheet for the TIP31A:

http://www.fairchildsemi.com/ds/TI/TIP31A.pdf

The spec is given:
VCE(sat) * Collector-Emitter Saturation Voltage IC = 3A, IB = 375mA:
1.2 V

Does this mean then that the resistance between the collector and
emitter is (1.2V) / (3A) = 0.4 ohms, when the transistor is carrying
3A through the collector to the emitter?

If so, that's a bit higher than the Rds_on for the IRF530, at 0.1 ohm.

Pretty much. But the BJT needs a whole lot of base current to keep it
there and it needs a long time to come back out of saturation. IOW if
you want to switch something at a reasonable efficiency you are usually
better off with a FET.
Click to expand...
Looking at the _typical_ (rather than max spec) curve for Ic/Ib=10,
Figure 2, it shows under 0.5V for Vce at Ic=3A and, by definition,
Ib=0.3A. So the typical is more like 0.5V/3A or 1/6th (.17) Ohm. The
IRF530 data sheet I looked at lists typical .14 Ohm and max .16 Ohm.
But the max spec on the TIP31A appears worse, as you both agree about,
and there is no question about the BJT's base drive power for Ic=3A,
which even using typical figures from the curve is about 1.05V*0.3A or
more than 300mW, by itself.

Jon
 
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