Need reference material/tutorials/help for designing with MOSFETS as HV linear pass element

Discussion in 'General Electronics Chat' started by G.Beard, Jun 22, 2015.

  1. G.Beard

    Thread Starter New Member

    Jun 22, 2015
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    Hi.
    I would like to design a simple linear HV regulator using an LR8 IC with IRF840 MOSFETS as the pass transistors.
    I will also be adding current limiting before the MOSFET input. I think to start I will design a few low voltage PSUs and work my way up.
    I understand the basic principles of operation but when I come to look at the data sheets I have no firm grasp of the graphical representations of MOSFET characteristics. When I search for something like "Designing with power MOSFETS" all that returns are results for switching power supplies or MOSFET amplifiers.

    If anyone knows of a good book, e-book, PDF, web site etc, that I can study from it would be greatly appreciated if you would post your suggestions please.

    Thank you, Ginger Beard
     
  2. Bordodynov

    Active Member

    May 20, 2015
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    I'm afraid that you can fail with an LR8 IC with MOSFETS. An LR8 IC very different from conventional linear regulators. I'm not saying that this is not possible, but may require more effort. I wish you success.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    One way to view a linear power supply is as a badly designed audio amplifier. For example, if it has a 2.5 V voltage reference and an output range of 30 V, that is an overall gain of 12. It might be able to source 1 or 5 or 10 amps, but a traditional design can't sink any current so it has a wildly asymmetrical output impedance. But at their core, linear supplies are just weird linear amplifiers with great DC accuracy, lotsa output current capability, and medium frequency response. So understanding how MOSFET amplifiers work is a necessary starting point.

    ak
     
    #12 likes this.
  4. G.Beard

    Thread Starter New Member

    Jun 22, 2015
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    Ok, thank you for your replies.
    If you would not mind, I have some more questions.

    For HV application 300V 200mA, what would be better, IGBT, MOSFET, FET or BJT... and why?

    @AnalogKid
    I will look over on ESP for MOSFET amplifier design, or something similar.

    @Bordodynov
    Looking at the LR8 data sheet block diagram, it looks to me to be the same as any other regulator IC..... What makes it different?
    http://pdf1.alldatasheet.com/datasheet-pdf/view/25892/SUTEX/LR8.html

    I assumed that since MOSFETs do not draw gate current, it would simply be a case of applying a voltage to the gate and sticking within SOA.
    Is this not correct? Why would it fail?
     
  5. AnalogKid

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    Aug 1, 2013
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    It's max output current is only 20 mA, and this decreases as the input-output difference voltage increases. What is the peak input voltage?

    ak
     
  6. G.Beard

    Thread Starter New Member

    Jun 22, 2015
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    Max input voltage 310VDC. I would not be expecting to get 300V regulated out.
     
  7. ronv

    AAC Fanatic!

    Nov 12, 2008
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  8. AnalogKid

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    Aug 1, 2013
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    In post #4 you said you wanted a 300 V output, now you say you don't expect that. How about telling us what you do expect. What is this for, what is the input voltage range, what is the design point for the output voltage and current, how do plan to cool the device? etc.....

    A standard LM317 has a max differential voltage of 43 V. So, it the input is 310 and the regulated output is 290 (for example), that is only a 20 V difference and you can use a standard 317 part and get the full 200 mA output. Power dissipation would be 4 W so you would need a heatsink. And note - the heatsink will be at the high voltage potential unless you insulate the 317 from it.

    ak
     
  9. crutschow

    Expert

    Mar 14, 2008
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    But note that you will need to add some sort of protection so that the voltage across the LM317 never exceeds 45V from transients, such as during startup, or output short circuits.
     
  10. G.Beard

    Thread Starter New Member

    Jun 22, 2015
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    Hi, right.

    It will be used for powering instrumental tube amps projects on my bench.
    Mainly just a couple of dual triodes so most of the time very low current, 10mA.
    For the time I want to power a single ended or push pull output stage I will require a little more current, only small low power practice amps but if I am to take the time to build it, I want a fare amount of head room just because.
    I understand that the supply does not need to be regulated for a valve amp but it is just my preference.
    I would like it to be variable.
    I have a schematic drawn up for a discrete regulator that meets my power requirements but I wanted an off the shelf solution that has been tweaked and stabilised already which is why I was thinking of the LR8. I am not comfortable with floating an LM317, I would rather use a device that is rated at least close to what i need. Are there an better off the shelf solutions?

    What about using a BJT pass with an LR8, then I would only need a very small amount of current on the base curet?
     
  11. #12

    Expert

    Nov 30, 2010
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    Peek at this.
     
  12. crutschow

    Expert

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    You could use a high voltage PNP such as this, to augment the current of the LR8. The circuit can be similar to that used to boost an LM317 output current.
    Note that you also need some method to limit the current. This could be an additional high voltage PNP in a simple current limit circuit such as this, at the regulator input.
     
  13. ian field

    Distinguished Member

    Oct 27, 2012
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    Apparently - its possible to get high voltage depletion mode power FETs, they behave pretty much like thermionic triodes.

    One of the magazines (probably Elektor) did a high voltage linear regulator project, there was an option in the project to go either tube or solid state. That was where I saw the devices.
     
  14. AnalogKid

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    Aug 1, 2013
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    And just to be clear, an LR8 is "floating" on a resistor just like a 317 would. The difference is that the LR8 has a much higher differential voltage rating.

    ak
     
  15. G.Beard

    Thread Starter New Member

    Jun 22, 2015
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    Wow... This has to be built.
    Thank you.

    Yes, this is exactly what I was thinking of. Is there any reason why this would be difficult to implement?

    I had read through other threads and had seen people advising the use of MOSFETs instead of BJTs and after reading a little on MOSFET operation, had assumed the benefit was something to do with no gate current, the gate being insulated, and breakdown voltage.

    Would someone please explain to me why the LR8 with an IRF840 would not be a good idea?
    Ok so the LR8 output current decreases as the voltage between input and output increases but how how does this impact on the stability of the circuit if the pass device draws no current from the output of the LR8?

    Your time is very much appreciated and thank you all for your input
     
  16. Bordodynov

    Active Member

    May 20, 2015
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    Here is StabHV.png the build version of the high-voltage stabilizer.
     
  17. crutschow

    Expert

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    That circuit won't work since the input voltage to ground maximum voltage rating of the LT1129 will be exceeded when trying to go above 30V output.
     
  18. Bordodynov

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    May 20, 2015
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    Last edited: Jun 23, 2015
  19. G.Beard

    Thread Starter New Member

    Jun 22, 2015
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    So, would we then reference the adj pin to a fixed rail x amount more positive WRT ground or use resistor between adj and 0v to elevate the output to a desired voltage, or inject a voltage to the adj pin?

    How would one adjust the output over a range greater than 48V? Could this be done maybe by having a pre-regulated variable voltage and..... not sure of the correct terminology... float the LM317 on the varied output of the adjustable voltage?
    How would one connect an LM317 (at high voltages) to a circuit without the 48V rating being exceeded?

    What voltage is the zener diode?
    I can see you are using depletion mode MOSFETs so I should think that when the circuit is initially powered the M1 is fully on, R1 and R2 make up a voltage divider and R4 or R1 set the current flowing through the zener which provides a reference voltage which then raises the gate voltage to M1 and the circuit would stabilise at 120V. I this correct? Would you give a brief explanation of your schematic and correct me where needed please?


    Here is a NON-regulated circuit from rod elliott's site that I modified slightly for higher current and lower ripple.
    The zener does no appear to be conduction in the zener breakdown region, the zener device was not specified, nor was there any explanation as to what this zener is doing in the circuit. Any input would be greatly appreciated.

    Again, thank you all for taking the time to reply Screen Shot 2015-06-23 at 12.35.10.png
     
  20. Bordodynov

    Active Member

    May 20, 2015
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    Note that I replaced the scheme.
    Now I am applying input 310Volt and output is 305 volts.The value of the reference voltage equal to 2.5volts.
    Equivalent:

    Equvalent.png
     
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