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instead of long story - i understand mosfet is switched with voltage, with maximum of Vgs mentioned in datasheet. Only little current flows while charging gate.
How is it with bipolar transistors?
datasheet of BD649 says>
EB voltage 5V continous base current - 300mA
Does this mean that to switch it fully on, i need 5V@300mA? That is 1,5W of energy just to turn it on! Can i put more voltage to base, lets say 15v? What datasheet value talks about maximum gate (base) voltage, or current ? What types are good for switching operation- normal, darlingtons?
I am talking only about switching mode, not linear operation
Any simple explanation would be highly appreciated.
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837
No, those are the maximum ratings ... they are limits you should not exceed, they are not meant as guidelines for use :)
At 8 ampere collector current the Vbe saturation voltage is around 2 volt and current gain around a thousand. So it would take a minimum of ~ 2*8/1000 = 16 mW to keep it fully turned on (ie. saturated) at max collector current.
Generally with a BJT (single or darlington) you use a base resistor to limit base current when you're switching with some type of logic level input. So for instance for 5 volt something like 250 Ohm would make sure it will be saturated at max collector current. Of course your logic circuit needs to be able to source 12 mA continuously in that case (with a temporary peak of 20 mA at turn on).
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Driving bipolar transistors
