How to interpret the hFE(beta) in a transistor datasheet?

I'm analyzing a ciruit which contains TIP31 NPN transistor. The hFE (current gain) on the datasheet is 25 (Vce = 4V, Ic = 1A), 10 to 50 (Vce = 4V, Ic = 3A). The circuit is about a battery charger and it handles various voltage, i.e. The hFE on the datasheet changes in different voltages or it maintains same range?

I looked at the graphe about hFE but it only shows of 4V test with Ic variation.

My question is how do I estimate the hFE of a transistor which is used different setting compared to the datasheet?

Reply to
nabi
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Nabi, the most direct route is to simply measure hFE at the operating point you are contemplating.

Small signal current gain varies considerably from transistor to transistor. Circuit designers often employ negative feedback to limit a transistor's hFE so that mass-produced circuits will function more-or-less the same despite variability in transistor characteristics.

Chuck

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Reply to
chuck
** Groper alert:

** Find the published curve of Hfe versus Ic.

My data book shows it is over 100 up to 0.5 amp Ic.

Then falls to about 25 at 3 amp Ic.

This is at a fixed Vce of 2 volts.

More Vce will only improve the figure.

Vce sat is about 1.2 volts at 3 amp Ic with 300 mA Ib.

Simple.

........ Phil

Reply to
Phil Allison

The more "sophisticated" way would be to design the circuit such that hFE (aka bets) variations don't really make all that much difference. Like, design for the low value, but arrange the circuit (feedback, etc) so that using a higher-gain transistor will still give the same answer.

In other words, design it out. :-)

Good Luck! Rich

Reply to
Rich Grise

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The HFE changes with voltage and current. I would always allow an overhead of 50% on any design I made to be safe.

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Reply to
Marra

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