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Old 19th May 2014, 09:19 AM   #1
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Default mosfet bjt advantages siries pass reg?

Hello all. Can someone explain the advantages/disadvantages of using mosfets/bjt's as series pass regulators please. I am putting together a simulation (multisim full edition) of a 300 volt 200MA series regulator. I will actually be trying three methods. The first is a basic discrete topology. Zener Vref - error amp- to bjt driving the base/gate of a series pass component. the second will be a floating regulator of some sort but so far I am unsure of what ic to use. And last is the lr8 ic with a pass. So I wonder if someone could tell me a little about the right type of transistor to use in applications such as these and why please? At present I have much knowledge of little, and a little knowledge of much, so if there is an basic to intermediate level link, or explanation someone could give? Thanks reading
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Old 19th May 2014, 08:19 PM   #2
udok is offline udok  Austria
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Application note http://www.ti.com/lit/an/slva072/slva072.pdf
has some basics.
Basically use a darlington npn for least troubles.
Regards,
Udo
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Old 20th May 2014, 08:15 AM   #3
Elvee is offline Elvee  Belgium
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In the region of 300V, the SOAR of all bjt's shrinks to useless levels.
A MOS is a better choice, but in the "easy" source follower configuration, it will need to waste a minimum of several volts. Which is generally not too much of a problem at this voltage level.
I have described here and there a number of HV regulators, some discrete, some with an opamp, in source or drain follower configuration, etc.
Use the "search" of the forum
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Old 20th May 2014, 02:53 PM   #4
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Thanks, I will look now. I have another 2 questions. I have been studying regulator circuits as the one below, if an op amp is used will that make the calculation for zener resistor as simple as (Vin-Vz)/Iz. As in the schematic below It is slightly more complicated due to Q2 and Q1 resistor paralleling Rz. Also how do I calculated the resistor for Q1 base, I guess it is basic ohms law but I am not sure what voltage drop and current I am aiming for and based on what specification. Please capuld someone explain? Click the image to open in full size.
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Old 20th May 2014, 04:11 PM   #5
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by jimmiegin View Post
I have been studying regulator circuits as the one below, if an op amp is used will that make the calculation for zener resistor as simple as (Vin-Vz)/Iz.
Yes
Quote:
As in the schematic below It is slightly more complicated due to Q2 and Q1 resistor paralleling Rz.
In principle yes, but since you need to make the current through the zener much larger than what the transistors need, it makes no difference
Quote:
Also how do I calculated the resistor for Q1 base, I guess it is basic ohms law but I am not sure what voltage drop and current I am aiming for and based on what specification. Please capuld someone explain?
This resistor is required for thermal stability, in which case it has to be low enough to drain the maximum equivalent base leakage at the maximum temperature.
Or you use some easy, round value like 100R , 1K or 10K depending on the power level.
This resistor can also serve to improve performances by making the current through the control transistor almost independent from the load current.
Current is simply Vbe/Rb
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Old 20th May 2014, 07:42 PM   #6
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Cool! that has been baffling me for a while as I couldn't find anything that explained its purpose. Thanks man!!
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