VFD filament power using LM4871 replacement for obsolete LM9022

[floobydust]

What are you guys doing biasing the filament +ve?

You bias the filament at a negative voltage wrt to the anode supply, to lessen the segment driver's voltage requirement.

[Rolo]

We are biasing the (virtual) center tap of the filament. By doing this the AC filament voltage shifts up in the real GND-HV range.
This theory comes from this guide https://www.noritake-elec.com/technology/general-technical-information/vfd-operation

No I did not try the darlington circuit, can't find any PNP transistors (I know...everyone should have some). Waiting for some parts, then I will get the breadboard out.

[Zero999]

Thank you.  I had read this thread.  Yes - shoot through was the term.  In an excellent introduction to h bridges I saw on the net, the author cautioned about shoot through generally speaking in h bridge design.  Perhaps with a VFD filament supply it is not as critical.  I mentioned that I may have to drop some voltage - back to back diodes would provide a 0.6V drop, wouldn't they?  In the pulse transformer case, I can adjust the number of turns.

I appreciate this alternative.  I'm hoping to hear about some others as well, and if someone has used the LM9022 or LM4871.  I am planning to breadboard the pulse transformer and 4871 at this point, and will consider picking up the transistors for your solution to give that a go.

Shoot-through occurs when either two of the left and right transistors is on simultaneously. It doesn't happen for long and is worse at higher frequencies.

This one doesn't suffer from shoot-through and only uses NPN transistors. The voltage loss is high, with the output being 3.7V, at 5V in. the Schottky diodes can be replaced with ordinary silicon diodes, but the output voltage will drop further.

[Rolo]

I did some experiments with the special H bridge timer functions on a STM32 series controller, you can specify the "dead time", this is the time the two sections (A and B) are both off. It worked, choosing a longer dead time's even controlled the total output power of the filament. So you have a software controlled filament driver. I have added the AC coupler and virtual center tap in this schematic, but you get the idea. By using fets's the efficiency of this circuit is high, but the down side is you have to have the dead time. Use a scope to observe the behavior of the brigde, not having the timing right can lead to strange effect and high power consumption.
The selection of the fet's are just what I had laying arround in my parts drawer. Other types could do the job also.

[Zero999]

I did some experiments with the special H bridge timer functions on a STM32 series controller, you can specify the "dead time", this is the time the two sections (A and B) are both off. It worked, choosing a longer dead time's even controlled the total output power of the filament. So you have a software controlled filament driver. I have added the AC coupler and virtual center tap in this schematic, but you get the idea. By using fets's the efficiency of this circuit is high, but the down side is you have to have the dead time. Use a scope to observe the behavior of the brigde, not having the timing right can lead to strange effect and high power consumption.
The selection of the fet's are just what I had laying arround in my parts drawer. Other types could do the job also.

The downside to that circuit is it has a a lot of shoot-through and large current spikes when both transistors turn on. Fortunately the BSS138 and BSS84 have a fairly high on resistance: 6Ω and 8Ω respectively, at 5V, which will limit the spike current to just under 360mA.

What does D1, C2,  R2 and R3 do except waste power?

[Rolo]

What does D1, C2,  R2 and R3 do except waste power?

They provide the Filament Bias Voltage (Ek), discussed earlier in this topic. You are right about the shoot trough and current spikes. That's why I choose the LM4781, this gives a much smoother output and is self oscillating, this gives a wider choice of controllers.
 

[Ian.M]

Of course as 5V of cathode bias is suitable for many VFDs, one can often get away without D1 and C2 and simply return the cathode center tap resistors to the +5V rail, saving the power wasted in D1.

[Zero999]

How about a transformer and suitable driver IC, such as the SN6501?
http://www.ti.com/lit/ds/symlink/sn6501.pdf

The cathode can be biased with a single resistor, saving the power wasted in a potential divider. It's also possible to use multiple windings to get other voltages.

[Rolo]

Update on this project, I stopped working on the "universal" vfd psu for a while, want to do more programming now using my original vdf psu.
https://www.eevblog.com/forum/projects/showing-my-vfd-psu/

[leonababy]

Yes, Vin is the supply voltage on the LM4871. Rload stands for the filament resistance. The filament Vrms is the AC voltage on the filament outputs, measured parallel on the load resistor.

{snip}

I apologize for not being back earlier.  I tested my circuit around this time, and discovered while making measurements that your readings are twice what I expected because I didn't realize a bridged output effectively doubles the output.  Because I only was expecting about 2.3Vrms, I simply drive from one (the positive) output to ground, with a series resistance.  The power wasted in the series resistance is less than 1/2 of what the elements use (I am running about 1.3 to 1.4Vrms in the VFDs I have tested so far).  The LM4871 itself uses about 450mW to drive 6 elements.  I am extremely happy with the result, and the overall power consumption of the design is great for my purposes.  I see this topic took legs and am interested in the other replies and designs.  You guys are on a whole other level.  Thank you for the bandwidth - much appreciated.