This Flyback Converter Design Really Steps Up to the Plate

At some point or another, nearly all of us will have used some form of voltage converter in our projects.

Most of us will have used a "boost" topology DC/DC converter in some of our battery powered projects. Tasked with taking the typical 3.7V output of a lithium-ion cell, and cranking it up to an electrifying 5.0V (as is a typical use case), these converters can be designed and built in a number of ways, using simple components.

If you wanted to get jiggy with switching boost regulators, a great project to start out with is known as the Joule Thief, a regulator designed to squeeze absolutely every single last scrap of potential energy out of a nearly depleted 1.5V cell. In doing so, it is able to generate the voltage required to allow it to drive a blue or white LED — something even a full 1.5V cell can't do.

James Wilson has some voltage requirements that are a bit beyond the meagre output of the simple joule thief however, with some of the Nixie tubes used in his projects requiring up to 170VDC!

Despite the fact Wilson is using this converter to light up his Nixie tubes, we must impress that even so, a potential of 170VDC is nothing to be made light of!

Voltages in this range are approaching the potential that could result in some nasty shocks to the user, so proper design is a must here, and... well, with his project looking like very much as if it were a manufacturer produced reference board, we'd say Wilson has nailed that aspect of the project in style.

It's interesting to note the topology here. Yes, we've said it is a DC/DC converter, yet, the astute of you will notice that there's a transformer plainly visible in the board shot above. What's going on there?!

In a nut shell, a flyback converter is a buck/boost converter topology, but with the inductor that is normally used to develop the converted voltage being replaced by a transformer, with a ratio split between the primary and secondary windings. This transformer, and the ratio of it's windings, allows some far higher voltages to be generated. Perfect for HV outputs.

Based around an LM5155 switching regulator IC, the circuit as implemented is pretty straightforward.

The LM5155 takes care of generating and modulating a PWM signal into the primary side of T1.

In doing so, a magnetic field is set up, which, when the PWM signal goes low, collapses, resulting in a flow of electricity being imparted into the secondary winding of T1.

By adjusting the ratio of windings between the primary and secondary coils, the voltage generated by the collapsing magnetic field can be adjusted, and further more by adjustment of the PWM duty cycle generated by the LM5155.

The HV AC output from the secondary winding is then half-wave rectified by the D1, to be smoothed by the bulk capacitance located after it. It is this voltage that is sampled, by way of a resistor divider network, back into the FB pin of the LM5155, enabling it to keep track of the output voltage.

It's important to note that the direct connection between the HV and LV sides of the circuit, through the feedback network, obviously makes this a non-isolated supply, and as such, it must be set up and operated with the appropriate amount of safety and caution! We're glad to see Wilson making proper use of shrouded test leads and clips throughout this project.

You don't want a loose wire flying about at that voltage!

Wilson has detailed his work on the project page with a wonderful write-up that goes into lot more detail than we have space for here, so be sure to take a read through his notes if you are curious for more information!