2. Designing a New Psu

full disclosure; i started capturing this information here while awaiting for the shipment of the second design/revision of the prototype PCB to arrive, so please forgive me if there are details missing. i ultimately started this site because i wanted a nice way to document my projects and more easily share them with friends if they were interested and wanted to read about them.

anyway with that tangent out of the way, let’s talk about my design approaches to this project.

Design focus #

my original approach to designing a new PSU circuit was intended to be very similar to the original design. after all, these did last quite a few years with their current design. i aimed to split this project into three separate parts - these are;

1. -8V power rail capable of delivering 400mA.
2. alternating voltage signal with the following attributes;
   • 0V DC offset (i.e voltage centred around 0).
   • ~44kHz frequency.
   • ~7V peak-peak.
3. center tapped isolation transformer.

here are the spoilers; so far i’ve been unable to successfully generate an alternating voltage signal centered around 0V - i tried simulating various different circuit topologies using the falstad circuit simulator, but ultimately couldn’t settle on something reliable. after some consideration, i began conceptualising some other ideas, and now i’m attempting to design a circuit to the following block diagram (don’t judge me for using MS paint, i put this together in like 5 minutes).

note that the supply and load switches are only in place to assist with the prototype phase; if this design is functional and successful, they will be removed in the final revision.

ultimately, the design utilises a h-bridge topology to drive the VFD filament wires. the h-bridge “switches” are in the form of n-channel and p-channel MOSFETs, and electrical isolation is provided using optocouplers between the inverted power rails and the h-bridge controller. the design of the h-bridge is ultimately identical to the 2-wire input design. the EV Engineer’s video on the h-bridge circuit theory was very helpful in the design of this circuit.

so the design approach i’m currently undertaking for this project is as per the block diagram above. ultimately, i’m trying to produce a square wave output at the h-bridge output pins with the following attributes.

• ~7V peak-peak voltage.
• -8V DC offset (i.e. square wave centered around -8V).
• ~44kHz frequency
• 50% duty cycle

i am now attempting to split design into 3(.5) separate elements. with that in mind, i will separate each of the design elements into their own sub-pages of this page, and this page onwards will outline individual PCB design revisions. the design elements are;

1. inverted power rails;
   • 5V inverted power supply rail.
   • 11V inverted power supply rail.
2. h-bridge controller.
3. h-bridge.

PCB Design #

Revision A #

revision A only consists of attempting to design an inverted voltage power rail. therefore, i think it makes more sense to capture details about that in the inverted power rail page. read about it here.

Revision B #

revision B is my first attempt at designing a PCB that has all of the design elements of the block diagram. i’ll write a little bit more about this in the coming days, but have some very cool pictures of it for the time being (spoilers, it doesn’t work (；′⌒`) )

sadly, i absolutely cooked the footprint for the TL494CDR IC - you can see it just above the U4 footprint (note that the TL494 is meant to be U4!). so i’ve had to redesign this PCB regardless. unfortunately, i did not have any success with the inverted voltage rails, so i’m moving on for now with the inverted rail design.

i haven’t fully determined the cause of the non-functioning inverted voltage rails, but for now i’ve decided to take a break on these. instead, i have ordered another board design which is purely based on the H-Bridge and its controller.

Revision C #

as mentioned above, revision C consists of the H-Bridge and the H-Bridge controller (now with a correctly shaped TL494CDR footprint!)

the idea behind this board is i will use external supplies to provide the inverted voltage rails. i’m using very cheap inverted buck converter boards from aliexpress, hopefully these should be adequate enough for the application. for now, we wait until they arrive.