Darlington pair and Rbe resistors

[atferrari]

In this last (long) weekend, not having handy any Darlington transistor I implemented this pair with discretes to PWM control a current up to 720 mA.

Having found the TIP41 hot to touch I recalled the necessity to facilitate the discharge of the internal capacity to reduce the rising time of the edge measured at the TIP's collector. Not sure of how to calculate that, I went by experimenting with progressively lower values.

From an initial rising time of 20 usec I came down to 3,1usec. Not bad.

Comments anyone?

 

[AnalogKid]

Looks good to me. You can get some guidance by looking at the darlington transistors in the TIP line to see what resistors they use. I don't remember the collector current for a TIP41, but there probably is an equivalent darlington version.

Also, for possibly faster turn off, experiment with a small cap in parallel with R2. You might find that the cap lets you have a larger R2 for the same timing performance as your current resistor with no cap.

ak

 

[MikeMl]

Why use a Darlington when a NFET does so much better?

 

[atferrari]

Now I understand why at certain moments I do not know in what forum I am...! We seem to go and come back all together!!

The crudest way to distinguish quickly one from the other is: one has chat room the other has an ebook. Go figure.

 

[AnalogKid]

There are other ways to tell them apart...

 

[atferrari]

There are other ways to tell them apart...

Sure there are but I keep mine on and focus on my hobby only...

 

[audioguru]

A TIP41 transistor is very slow, almost as slow as a 2N3055. Try a BD13x transistor that has a gain-bandwidth that is about 38 times higher.

 

[Tony Stewart]

TIP41's are medium power rated but ON SEMI's & Fairchild's spec has surprising low hFE of 15 min, 75 typ at 25'C and

 

[Tony Stewart]

While Sanken's 2SD2390 TRANS NPN DARL 150V 10A TO-3P Darlington has DC Current Gain (hFE) (Min) = 5000 @ Ic=7A ,Vce = 5 , transition f= 5MHz cost =$2.83(1)

while . Vce(sat) = 2.5 V max at Ic =7A, Ib =7mA 25'C or Ic/Ib=1000
it drops Vce(sat)= 1.0V Vmax at Ic= 5A Ic/Ib=1000

While TIP41 is rated at Vce(sat)= 1.5V with only Ic/Ib= 10

So as a switch the Sanken is far superior. Then you don't need the extra transistor in front.

While the BD3xx has more bandwidth, it has less gain and max current but lower Vce(sat)

V CE (sat)=0.5 Collector-Emitter Saturation Voltage I C = 500 mA, I B = 50 mA
IC/Ib=10

Meanwhile Diodes Inc are historically excellent for low Vce(sat)
I just picked out this one. **broken link removed**
hFE=500 min @2A, 5V and 2,000 to 100,000 @1A, 5V
Vce(sat) = 1.5 @1A, 1V@0.25A

These are good in other respects. GBW= 150MHz

So when using a Darlington, although they are excellent due to low input capacitance vs MOSFETs, pay attention to Ic/Ib ratio, Ic current and Vce(sat) .

I tend to calculating the Rce or effective series resistance (ESR) or the slope near max current for Vce(sat) vs Ic as this determines the voltage rise and makes the distinction in efficient power switching and are very cheap and small (SMT) at $0.25 (1k pc)

In single transistors, Diodes Inc transistors are comparable in Rce to MOSFET RdsOn and thus Vce(sat) but at a price premium like MOSFETs but lower input capacitance.

 

[atferrari]

A TIP41 transistor is very slow, almost as slow as a 2N3055. Try a BD13x transistor that has a gain-bandwidth that is about 38 times higher.

I completely obviated GBW, AG. Thanks for that. I should have one or two BD transistors but not sure.

 

[Tony Stewart]

If you have one good output NPN transistor and 2 drivers from 3 to 5V you can achieve lower losses even assuming a current gain of 10
The concept described by Motorola in the 70's was Beta overdrive factor or similar to using 10% of hFE.

To drive this concept home. I simulated the design using a current gain of 10.
Using a gain of 10 with 3MHz BW, you get 300kHz BW.
Using 3V to drive the input stages means only 100mA of power @3V
Then any Load supply such as 24V @1A with only 0.5V Vce(sat) using Ic/Ib=10

The result is only 0.5W is wasted in Q3 driving a 24W load @ 1A

For MOSFETs a similar design is needed in current ratios.
Since Ciss increases with RdsOn , depending on slew rate requirements the effective peak current gain of each stage can be as much as 50 or as low as 10 for max slew rate, which for MOSFETs is only a pulse current during Vgs threshold.

Falstad SIM

Notice the resistor ratios and Beta is only 10 used in each stage, so current levels from input to output are; 1mA , 10mA ,100mA , 1000mA

Then if you have GBW of 100MHz you can expect same results with 10MHz BW.

You can then use lower power transistors.

PLease note that in good MOSFET switch designs to any huge level of current requires multiple stages with the same current gain concept as BJT's except the current is just a pulse during Vgs transition due to input cap. Ciss, which increases for smaller RdsOn devices so for a given family, the Ciss*RdsOn product is a constant Figure Of Merit. YOu will find good designs use a ratio of 10 to 50 in RdsOn ratios for cascading driver stages, depending on slew rate requirements.

This similar to the ratio used for BJT's depending on if it is a ultralow Vce(sat).
For very low Vce(Sat) Ic/Ib =50 ratings are given, or 20 or the default standard of 10.
This translates into an effective ESR or Rce as it is called by Diodes Inc and can be competitive with MOSFETs ( but $)