High Voltage Power Supply for Nixie Tubes

My nixie clock power supply died. It was one of these cheaply built Chinese modules so it doesn’t come off too much as a surprise. Looking at how to replace it, I realize there is a dire need for a better high voltage design out there. Most the kits seem to be based of a traditional boost converter based on a MC34063 switching controller. This has several issues:

  • The 34063 was never meant for high voltage
  • It’s a design from the mid 70s with everything that it implies -no thermal protection, not a current mode controller, crappy feedback sensitivity

It’s astonishing that there are so many nixie tube enthusiasts out there, yet there is no good solution for the most critical part of a nixie circuit: the high voltage supply. With that in mind, I decided to design my own PSU with one requierement in mind: build it like a tank.

 

Schematics

High Voltage PSU based on the LT3757

 

The resulting design is based off two key components:

  • The LT3757 switching regulator controller from Analog / Linear Tech.
  • The DA2032 3A, 1:10 flyback transformer from Coilcraft.

The LT3757 is a good choice because it can specifically be used for high voltage, as demonstrated in the datasheet with a boost example to 300V. The downside is that it’s extremely expensive. At the time of writing this, I paid US$4.49 for one ($2.55 for 100); which is a ridiculous amount of money for a 3x3mm MSOP-10 package. $4.49 buys you a fairly nice middle of the range ARM micro-controller.

The DA2032 is coupled inductor with a 1:10 turn ratio (a “flyback transformer” is a glorified coupled inductor – see below for further reading). The primary coil is rated for 3A. Because we need such a high boost ratio, we definitely need a high turn ratio on the transformer. On the wallet damage side of things, count US$4.50 for one of these.

We already have $9 worth of components and we haven’t even started building this converter! Then again, the goal of this prototype is not to be cheap, but to build a very reliable power supply capable of lasting years and years without failing. The only way to achieve this goal is to use quality components well within their electrical characteristics. Overstressing parts is the main reason why a circuit may work, but only for a short amount of time.

I simulated the design in LTSpice and everything looked good; so there was only one thing left to do: building it!

 

PCB and initial tests

PCB was manufactured by JLCPCB -once again they did a good job. It’s my first time prototyping with very tiny 0603 components so I ordered a stencil with it which made my life a lot easier and the soldering look very professional. Unfortunately the MOSFET I picked is a D2PAK (TO-263) package and not the DPAK (TO-252) footprint I used on the PCB. Oops. Well it still works but the drain of the MOSFET isn’t completely soldered on.

High Voltage Nixie PSU - 2 EUR coin and American quarter for scale
High Voltage Nixie PSU – 2 EUR coin and American quarter for scale

 

To be noted in this design: the transformer shares a large copper pour with the drain of the mosfet which should increase efficiency. The diode is a tiny SOT-23 dual diode; which as recommended by Linear but in hindsight there are much better components out there. Another noteworthy feature is the current sense resistor which is set incredibly low on purpose: 0.02R. This pretty much prevents the chip from going into shutdown; at the risk of damaging it. A prototype should be pushed to his limits after all.

 

Success!
Success!

 

And so what now?

I want to see how much further I can push optimization, different switching frequencies, different diodes; etc. etc. To that extent I have just submitted a 3-1 protoboard to JLCPCB that will test:

  • A similar design than above but with Texas Instruments’ LM3488.
  • A traditional boost converter approach with a very beefy inductor and diode.
  • Another flyback but with a different diode and a bigger transformer.

Once I am happy with the results, I think I’ll launch a kickstarter to have a few made and release the final board as open source. I can’t front the thousands of dollars that this will cost to make and assemble but if there’s enough community interest I think it’s worth it! Stay tuned for the next episode.

 

References

Texas Instruments – Flyback transformer design considerations for efficiency and EMI

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