Part 4: More Prototypes!

A project log for High Voltage Nixie Power Supply

A modern DC/DC converter design capable of delivering current in excess of 40mA at 170V

TonyTony 03/16/2019 at 04:390 Comments

Probing other ways to generate high voltage

Seeing the first prototype work, it got me thinking. Do I really need to use this expensive LT3757 switching regulator? Is a flyback really better than a traditional boost converter if I use good parts for it? Should I test my theory that the dual diode is absolutely useless and a drag on performances?

Before taking this project further, I had to answer these questions.

So I decided to go ahead and launch a new set of prototypes, based on Texas Instruments’ LM3488. According to TI, the LM3488 is a “versatile low-side N-FET high-performance controller for switching regulators. This device is suitable for use in topologies requiring low-side FET, such as boost, flyback, or SEPIC.”; with a maximum output voltage of 500V.

On the top right, a version of the power supply using the LM3488.

It’s also a lot cheaper, and has simplified input/outputs with only 8 pins versus 11 for the LT3757. I thought to myself that maybe, just maybe, I was overengineering this power supply.

For this prototype I also included some jumpers that allows me to select different switching frequencies to see the effect it would have on efficiency.

A classic boost converter. Notice it requires a beefy MOSFET and diode

The last prototype on the board is the same version as the first one, except it uses another transformer, the DA2033. It’s a bigger version of its little brother. Since I had space left on the PCB, I just wanted to experiment with this other transformer.

None of these designs gave me good results. To this date I don’t know why. Did I do something wrong? Are TI chips a bit more stringent on what they can accept? I do not know. However, it comforted me in my idea that the LT3757 was pretty darn nice in comparison.

Revision B of the prototype

After this brief interlude I worked on the revision B of the power supply.

Revision B of the Power Supply

A few improvements on this version:

When looking for a change of diode, I needed something FAST; with very little leakage. With a recovery time of 35ns and 1uA leakage @ 600V, this diode is a killer so I thought I’d give it a try.

Revision B in the flesh!

On the picture above, don’t pay attention to the botched up job on the diode at the top, it was just an experiment I was running to see what was the impact of the dual diode versus the newly selected fast recovery diode. The answer to this question is: actually not so much difference. I still don’t understand why Linear’s engineers chose this more than average part in their reference design. There are so many good diodes out there, why this one? I doubt I’ll get an answer to this question.