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Darlington transistor as a switch

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I have some ic which called sn754410
datasheet: https://www.ti.com/lit/ds/symlink/sn754410.pdf

This ic have two h-bridges with darlington transistors.

My question is why there is a need in darlington transistors to create these h-bridges? there is no option to use regular transistors? maybe the high-current is the reason? if yes why darlington allow to switch high currents?

Thanks.
 

Hi,

darlingtons have high current gain (emitter / base).

A darlingtom may have a gain of 1000, a usual bjt maybe has 25.
If you want to drive an 1A load, then:
--> with the darlington you need 1mA of base current
--> with the bjt you need 40mA of base current (wich the 754410 might no be able to drive)

(And for switching applications you have to "overdive" the base. Drive it with 3..10 x the calculated current)

Klaus
 

Hi,

darlingtons have high current gain (emitter / base).

A darlingtom may have a gain of 1000, a usual bjt maybe has 25.
If you want to drive an 1A load, then:
--> with the darlington you need 1mA of base current
--> with the bjt you need 40mA of base current (wich the 754410 might no be able to drive)

(And for switching applications you have to "overdive" the base. Drive it with 3..10 x the calculated current)

Klaus
But how it connected to switching? in h-bridge i don't need the advantage of large amplify.
i assume that the answer is in your last line but i didn't understand what "overdive the base" means.
 

My question is why there is a need in darlington transistors to create these h-bridges?
.....................................
if yes why darlington allow to switch high currents?
To reduce input current.
Ibase=Icollector/Beta.
So if Icollector is large, Ibase is also large.
Darlington makes Beta large.

A darlingtom may have a gain of 1000, a usual bjt maybe has 25.
It is odd.
If beta of single transistor is 25, beta of darlington connection wil be less than 25**2+2*25=675 at most.
https://en.wikipedia.org/wiki/Darlington_transistor

Beta of conventional single transistor is 30~100 in IC.
So beta of darlington connection wil be less than 960~10200.
 
Last edited:

Simply look at the datasheets. The first transistor in a darlington operates with low current so its hFE is high. The output transistor in a darlington operates with high current so its hFE is low.

The hFE of a 2N3904 little transistor at 10mA is about 200. The hFE of a 2N3055 power transistor at 3A is 46 but its saturation voltage is rated when its base current is 1/10th for lots of overdrive.
The hFE of a TIP120 power darlington transistor at 3A is about 4000 but its saturation voltage is rated when its base current is 1/250th for lots of overdrive.
 

Hi,

It is odd.
If beta of single transistor is 25, beta of darlington connection wil be less than 25**2+2*25=675 at most.
https://en.wikipedia.org/wiki/Darlington_transistor

Beta of conventional single transistor is 30~100 in IC.
So beta of darlington connection wil be less than 960~10200.

What is odd?
* the 25? I gave just an example. There is a wide range. The worst i have on stock is BUL1102. datasheets says 12..20 @ 25°C, 2A, 5V
* the 1000? I gave just an example. There is a wide range. You say 960..10200. so 1000 is within your given range.

And it depends a lot on V_CE. With switching applications you have low V_CE. Resulting in very low gain.

Klaus
 

What is odd?
Again obviously odd.
It can not match equation for beta of darlington connection with your values 25 and 1000.

There is a wide range.
You say 960..10200. so 1000 is within your given range.
It is no more than a forced interpretation.
 

Hi,



What is odd?
* the 25? I gave just an example. There is a wide range. The worst i have on stock is BUL1102. datasheets says 12..20 @ 25°C, 2A, 5V
* the 1000? I gave just an example. There is a wide range. You say 960..10200. so 1000 is within your given range.

And it depends a lot on V_CE. With switching applications you have low V_CE. Resulting in very low gain.

Klaus

BJT transistor cannot switch high-current? i still don't understand why darlington is able to switch high-currents and BJT for example is not...
 

Hi,

ahh,

you are complaining that the gain of 25 does not meet the gain of 1000.

So mathematically you are correct with the 675.
But in reality they won´t connect two bjts with exactely the same gain of 25.
Usually the power bjt (with external emitter connection) is much bigger than the other bjt, because it has to drive a lot more current.
and even if they are fo same size they both have totally different operating points, because the inner bjt (with external base connection) drives less current.

Therefore i expext the power bjt maybe has a gain of 30, while the other has a gain of 100, giving a gain of about 3000. (just raw estimations!)

Audioguru is correct: To compare a simple bjt with a darlington we need to know the exact types and compare their datasheets.

Klaus
 

Hi,

ahh,

you are complaining that the gain of 25 does not meet the gain of 1000.

So mathematically you are correct with the 675.
But in reality they won´t connect two bjts with exactely the same gain of 25.
Usually the power bjt (with external emitter connection) is much bigger than the other bjt, because it has to drive a lot more current.
and even if they are fo same size they both have totally different operating points, because the inner bjt (with external base connection) drives less current.

Therefore i expext the power bjt maybe has a gain of 30, while the other has a gain of 100, giving a gain of about 3000. (just raw estimations!)

Audioguru is correct: To compare a simple bjt with a darlington we need to know the exact types and compare their datasheets.

Klaus
In conclusion, darlington is better than regular transistor in switching high-current because of the small base current that needed to saturate the transistor?
 

In conclusion, darlington is better than regular transistor in switching high-current because of the small base current that needed to saturate the transistor?
It's better, if:
- the alternative is only between regular BJT and darlington (true 30 years ago when sn754410 was designed, but not these days)
- you don't have a means to provide a high base current
- switching frequency is low
 

Why are you arguing about beta that is used when a transistor or darlington is linear? A switching device is a saturated switch when beta cannot be used because the gain is much less.
My example showed the 2N3055 transistor with a beta of 46 but saturated its gain is 10 and a TIP120 darlington showed a beta of 4000 but saturated its gain is 250.
 

It's better, if:
- the alternative is only between regular BJT and darlington (true 30 years ago when sn754410 was designed, but not these days)
- you don't have a means to provide a high base current
- switching frequency is low
Darlington needs small base current because of the large beta of it?

if i want the transistor to be in saturate i need:
beta*Ib>Ic

so if beta of the darlington is higher Ib is smaller, Am I right?
 


Beta is used when the transistor is a linear amplifier with plenty of collector to emitter voltage, it is not used as a saturated switch. Look at my post #12 where a TIP120 darlington has a beta of 4000 but its current gain is only 250 when it is a saturated switch.
 

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