[SOLVED] whats hfe during peak current

[Zak28]

A BJT totem pole can be used in variety or circuits however is the current gain of BJTs (hfe) typically lower when Ice and Vce are greater?

This being for sot-89-3 BJTs as the vast variety of packages might have an influence the current gain.

 

[KlausST]

Hi,

If I want to know this ... I´d choose a suitable part, open it´s datasheet, and look at the charts.

Klaus

 

[Audioguru]

The graphs on a datasheet show only "typical" devices. The text in the datasheet says the guaranteed minimum and maximum values.
The package for a device does not affect the hFE, it affects only the amount of allowed heating.
If the transistor is saturated the hFE number is not used. The base current should be 1/10th the collector current for most transistors. An old 2N3055 power transistor datasheet says for a collector current of 10A (its maximum allowed collector current is 15A) then a 3.3A base current allows it to saturate (poorly). It needs more base current for less saturation voltage loss.

 

[Zak28]

The graphs on a datasheet show only "typical" devices. The text in the datasheet says the guaranteed minimum and maximum values.
The package for a device does not affect the hFE, it affects only the amount of allowed heating.
If the transistor is saturated the hFE number is not used. The base current should be 1/10th the collector current for most transistors. An old 2N3055 power transistor datasheet says for a collector current of 10A (its maximum allowed collector current is 15A) then a 3.3A base current allows it to saturate (poorly). It needs more base current for less saturation voltage loss.

Isn't the base 3.3amps much greater than 1/10th 10A going into the collector? Even if 1:10 base - collector current is only rule of thumb its very much disproportional for the mentioned 2n3055.

 

[KlausST]

Hi,

Isn't the base 3.3amps much greater than 1/10th 10A going into the collector? Even if 1:10 base - collector current is only rule of thumb its very much disproportional for the mentioned 2n3055.

All your assumptions are true.
This means: It all depends on the situation (your specification/requiremens about voltages and currents and maybe frequencies) and the used transistor --> specifications in the datasheet.

"Saturation" is not a single point, it is a wide area.
* You may have a saturation situation with 10uA collector current .... or 10A collector current.
* You may be satisfied with 10V of saturation voltage or 1mV of saturation voltage.

Klaus

 

[Audioguru]

Isn't the base 3.3amps much greater than 1/10th 10A going into the collector? Even if 1:10 base - collector current is only rule of thumb its very much disproportional for the mentioned 2n3055.

The maximum allowed collector current of an old 2N3055 transistor is 15A. They guarantee that it saturates not too bad with a collector current of 4A when its base current is 400mA (1/10th). They show that some of them saturate poorly with 10A/3.3A.
A little 2N3904 transistor has a maximum allowed collector current of 200mA. They guarantee that it saturates well when its collector current is 50mA and its base current is 5mA (1/10th).