A 275V 50mA Fixed Linear Power Supply

I tried this one for a tube project when I had no other option available and I had no time. It should provide 275V at 50mA, have meters for voltage and current and be short circuit proof. Internally, it looks like this:

HV_PSU_275V50mAPictureSmall

Click here for a higher resolution picture …

The top left has an 100VA isolation transformer, below there is the power switch and the power inlet with a line filter. At the bottom of the picture you can see the analog panel meters for voltage and current. The PCB is a standard regulator (see power supply pages for schematics). On the right we have an ouitput on/off switch and a neon for power and active output.

The PCB is like this:

HV_PSU_275V50mA_PCB

It is a one sided (mirrored) PCB. An IRF450 was used as a pass transistor. Its ratings (500V/12A/150W) is enough for maximum dissipation in case of a short circuit (330V/50mA/17W). During normal operation we dissipate just a few watts.

The circuit worked as designed, with a few mV of full load ripple.

The schematics is show below:

HVPSU300V50mASchematics

Click here for a higher Resolution Picture

 

5 thoughts on “A 275V 50mA Fixed Linear Power Supply”

  3. Hi,

    Can you please help me to understand this circuit. How the voltage regulation, current control etc working on this circuit. I am planning to make it for 600v o/p. I simulated this circuit in LTSPICE and its o/p is close to 275v. Where and what do i need to change to make it for 600V 300mA supply. I tried to contact you over email, but don’t know how, the mail bounced, last time when I mailed it worked.

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    1. During my tube RF amp design, I am taking your work as reference. Currently I am working on the power supply and in my case the output is 600V. I was looking at your “275V 50mA Fixed Linear Power Supply”. I can use some help from you to understand how the Current control and Voltage regulation working so that I can adapt this to my 600V PSU. I have highlighted three sections of the circuit in yellow box that I could not clearly understand.

      In my understanding

      Left box is a voltage divider that supplies gate voltage to mosfet. Q1 is saturated and Q2 is cutoff. Current flowing through Q1 and Q2 will produce a voltage at the gate that will close to 275V. Here the MOSFET is working as a source follower so the source voltage or the O/P voltage will be at 275V. Infact the LTspice simulation also maintains O/P voltage close to 275V, superb, but I cannot understand how this voltage divider actually works, all I understood is the level of voltage and current on it but not its principle of operation.

      WG: Not quite. The MPSA94 upper transistor is a constant current source. The current is set by R1. The current flowing out of the collector of the MPSA94 flows (in case we are not in current limiting)
      down the collector of Q2. The base of Q2 sits at 10V via the Zener diode, so the collector current of this transistor is adjusted by the emitter voltage, and therefore by the current running thru the
      TL431 regulator.

      Upper right box is the constant current circuit, but could not understand how the constant current is maintained.

      WG: Its correct, that is the constant current part. Assume that the output current is small enough so that the voltage across R47 is less than 2.5V, the TL431 reference voltage. The TL431 will not conduct any
      current then, allowing a sufficient GS voltage across the IRFP150 to exist and to let current flow thru IRFP450. In case the current becomes large enough to drive the TL431 into conduction, the GS source voltage of the IRPF450 will be clipped and the current reduced.

      Lower right box is the feedback circuit for voltage regulation. The TL431 is drawing a current of approx 1.3mA from the emitter of Q2. This current causing a drop of 275V at the collector of Q2, but I could not understand how this TL431 is controlled by the R14, R6, R5 network to draw 1.3mA current. I tried to the usual equations of TL431 from datasheet, but the results were not matching with this circuit values.

      WG: Also correct. The control of the lower TL431 works as follows: The output of the regulator (nominally 275V) is divided and put into the feedback terminal of the TL431. If it is less than the 2.5V internal reference voltage of the TL431, the lower TL431 will not draw any current from the emitter of Q2. The current thru Q2 now is only (10-0.7)/47K, which is much less that the current defined by the Q1 current source. In this case, the current out of Q1 will flow thru D4, cause a 10V drop and drive the pass transistor into conduction, now raising output voltage. If the voltage now went up sufficiently, the S3 terminal of the lower TL431 reaches 2.5V, causing the TL431 to draw current from Q2s emitter and collector, lowering the gate voltage and thereby reducing current.

      This is my humble request to you to kindly help me to understand how the voltage divider, current limiter and the voltage regulation of this circuit is working. I can certainly use some math if required.

      WG: No problem. Play safe, high voltages are dangerous !!

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