Technical clarification of Beta and Collector Current

Hello Everyone!
I have gone through TIP120 transistor data Sheet and suddenly I got confusions with Ic and Beta relationship formula. I knew this may be very simple and I am taking things wrong .I would like to know how to relate all the things very accurately!

As per the datasheet,
Beta (hfe) value is 1000 and Base Current(Ib) is 120mA. Collector current is(Ic) 5A and Peak current is 8A.
Formula to calculate Collector Current:

Ic=Beta X Base Current

Ic = 1000 X 120mA will resulting the value of 120A but specification says Ic is maximum of 8Adc. What are all the other factures need to be considered??.

Hi,

the current gain also depends on the collector current. Have a look at figure 9 in the datasheet [1].

[1] https://www.onsemi.com/pdf/datasheet/tip120-d.pdf

BR

You didn't read the datasheet thoroughly. 120 mA is absolute maximum rating, it has nothing to do with Ib/Ic ratio. Also current gain is determined in active region with sufficient collector emitter voltage.

Each value in a data sheet has a very specific meaning.
You can't mix and match the values.
Look very carefully at the conditions for which the values are given, as those values are only valid for those specific conditions.
So, for example, you can't mix a max current with a nominal gain.

And, as FvM note, the Beta current gain, is for operation in the transistor active region with several volts of Vce voltage (as specified in the data sheet).
It is not used when the transistor is in the saturated mode (fully on as a switch).

There is a term "forced beta" that is used to approximate calculation of base
current need to saturate a transistor.

Its generally used as 10. that is force 1/10 the collector current anticipated into the
base to force it into saturation.


Here is a typ curve showing sat behavior at Ic/Ib = 10




Regards, Dana.

Hello Thanks for everyone's reply! I think I need to study datasheet thoroughly and need to understand each and every Technical name very in a better way!! It will take long time. Better We can start understanding thing with some applications

Herewith I have attached my application Circuit with TIP120 . I have designed this circuit based on ready circuits available in internet!

Solenoid is working at 24V and it will consume just 600-800mA Current! I need to switch ON/OFF as per my application!
Kindly give your suggestion on this circuit! If anything need to Change sure I will do the same!!

stenzer said:

Hi,

the current gain also depends on the collector current. Have a look at figure 9 in the datasheet [1].

[1] https://www.onsemi.com/pdf/datasheet/tip120-d.pdf

BR

Click to expand...

Hello,
As per the datasheet you have attached, DC current gain is nearly 8000 at 25 Degree Centigrade for 1 Amps( For all the Transistors-TIP120, TIP121, TIP122).

To calculate the base resistor Hfe is very important.

Ib= Ic/hfe(min)

Please advice on this

FvM said:

You didn't read the datasheet thoroughly. 120 mA is absolute maximum rating, it has nothing to do with Ib/Ic ratio. Also current gain is determined in active region with sufficient collector emitter voltage.

Click to expand...

Yes you are right! For me getting things directly from data sheet is little bit difficult since it is having many many technical details secretly . Anyhow I am making some steps to knowing the unknown!

To find the minimum Base current required to turn the transistor “fully-ON” (saturated) for a load that requires 800mA of current when the input voltage is 24VDC.
Beta is mandatory! But in my case, What would be my Beta value??

I hope these formulas will give, direct value of Base resistance and base current
Transistor Base current:

Ib=Ic/Beta

Transistor Base resistance:

Rb=Vin-Vbe/Ib

The datasheet shows saturation of a TIP120 occurs when the base current is 1/250th of the collector current.
Then if your collector current is 800mA, the base current should be 800/250= 3.2mA.

Your load will get about 23V since the TIP120 has a 1V loss.

Audioguru said:

The datasheet shows saturation of a TIP120 occurs when the base current is 1/250th of the collector current.

Click to expand...

Where I can collect these information's in datasheet. They dint mention anything explicitly

Hi,

from datasheet:

Collector−Emitter Saturation Voltage
(IC = 3.0 Adc, IB = 12 mAdc)
(IC = 5.0 Adc, IB = 20 mAdc)

Click to expand...

IC = 3.0A = 3000mA;
IB = 12mA
IC/IB = 3000/12 = 250

Klaus