Hey guys, I have been learning power electronics recently. Specifically switching power supplies, inverters, and etc. One thing which I seen in all books and everywhere online has been bothering me. I don't understand how this works. Please see the picture below.
This is a linear power supply diagram. My confusion is this. You see that the transistor is in the "emitter follower" configuration. I think the transistor in the diagram is an IGBT but it should be similar to an NPN BJT. So I will talk about it as if though its a BJT.
In the emitter follower, the voltage at the emitter is equal to the voltage at the base minus 0.7V Vbe voltage drop. If this is true, in the diagram above, the voltage at the emitter and hence at the load is equal to the controller voltage voltage minus 0.7V.
But the video where I got this diagram, and all books I seen on this subject treat this transistor like a variable resistor. Meaning that the voltage at the load can vary from Vdc to 0. This would be true if the voltage the controller is outputting is equal to Vdc. But that is not usually the case.
Usually the voltage from the controller is 5V from a uC, and we are trying to drive 15V or higher for example at the collector for example to power a motor. So Vdc =15V. Vbase = 5V. And somehow they are telling me I can end up with 15V at the emitter. This would be true if the transistor is a literal switch. But its not. Do you get what I am saying?
Can someone please clear up the confusion for me. Thanks.
-Gene
Edit: In fact the transistor WOULD be a literal switch if the load was connected between Vdc and the collector. But the load is connected at the emitter.The voltage at the load HAS to be Vbase -0.7V. Or V_controller - 0.7V.
This is a linear power supply diagram. My confusion is this. You see that the transistor is in the "emitter follower" configuration. I think the transistor in the diagram is an IGBT but it should be similar to an NPN BJT. So I will talk about it as if though its a BJT.
In the emitter follower, the voltage at the emitter is equal to the voltage at the base minus 0.7V Vbe voltage drop. If this is true, in the diagram above, the voltage at the emitter and hence at the load is equal to the controller voltage voltage minus 0.7V.
But the video where I got this diagram, and all books I seen on this subject treat this transistor like a variable resistor. Meaning that the voltage at the load can vary from Vdc to 0. This would be true if the voltage the controller is outputting is equal to Vdc. But that is not usually the case.
Usually the voltage from the controller is 5V from a uC, and we are trying to drive 15V or higher for example at the collector for example to power a motor. So Vdc =15V. Vbase = 5V. And somehow they are telling me I can end up with 15V at the emitter. This would be true if the transistor is a literal switch. But its not. Do you get what I am saying?
Can someone please clear up the confusion for me. Thanks.
-Gene
Edit: In fact the transistor WOULD be a literal switch if the load was connected between Vdc and the collector. But the load is connected at the emitter.The voltage at the load HAS to be Vbase -0.7V. Or V_controller - 0.7V.
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