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.

[oops]

I don't understand how the Zener diode functions as a center tap, can someone explain the theory in general terms?  Or is it that the resistor divider forms the virtual ground, and the Zener just keeps the voltage from exceeding what the filaments can tolerate?

How do I calculate what voltage Zener I need?  And what values for the resistor dividers?  The display I'm working with has a filament voltage of 3V AC.

Thanks for the help! 

[Ian.M]

Unfortunately Rolo's old hosting has gone away, and with it, all the schematics he posted to this thread.  Usually they can be found on the Wayback Machine at the Internet Archive, at: https://web.archive.org/web/*/http://members.casema.nl/r.kamp/*
but unfortunately the Internet Archive is currently down due to a massive DDOS attack + data breach.

Without those schematics, this thread doesn't make a lot of sense, so you may need to wait for the Internet Archive to harden their systems and come back up, but if you have queries about any specific VFD filament driver schematic that is currently viewable online (and tell us where to find it), or can attach the schematic you are asking about, we'll try to help.

As I remember - the Zener is nothing to do with the RMS voltage across the filament, but is to set the negative grid (and anode) cutoff bias voltage, by operating the filament at a net DC voltage above the 0V that the grid/anode drivers can pull down to.

[oops]

Thanks for the reply.  I think his filament supply schematic is here:
https://github.com/esynr3z/vfd-ps/blob/master/img/sch.png

But I don't understand how the Zener functions to bias the voltage.  Here's what I think I understand. 

So the LM4871 takes a square wave input and alternately raises either side of the filament to +5V, while holding the other side at 0V, which allows current to flow through the filament resistor in alternating directions.  This prevents luminance slant which would occur if you only raised one side to +5V.

Connecting each of the amp outputs to the high voltage rail through a resistor pulls both sides of the filament positive by some amount vs. the virtual grounds inside the amplfier.  Which prevents ghosting.

So what does the Zener diode do?  And, how do I calculate the proper values for the resistors and Zener for a 3V filament voltage?

[Ian.M]

Ah, that's a slightly different setup.  My usual setup is with positive HT of about +30V to feed a high-side driver for the grids and anodes, so the filament needs a DC bias of a few volts positive so Vgk goes sufficiently negative for good cutoff even with the AC filament drive swinging the filament ends up and down a bit relative to the DC bias.  Often one can simply return the filament drive center tap to the +5V rail (or use diodes to clamp the filament ends down to +5V as the drive swing for a typical VFD filament is usually less than 5V pk-pk.

esynr3z's vfd-ps GitHub uses a negative HT supply -Vee (in the range -15v to -30V, generated by an inverting buck converter) for the filament/cathode.  I presume this is to allow  simple P-MOSFETs (or PNPs with base resistor) hung off the +5V rail for his segment anode and digit grid drivers, without any level shifting between the drivers and the 5V logic outputs. They will thus swing to +5V when on, and with pulldown resistors, down to -Vee  when off.

Assuming JP1 is connected, the Zener clamps the filament at approximately the zener voltage above -Vee to provide negative Vgk (i.e. grid is negative with respect to cathode) for the deselected digits for proper cutoff.

[oops]

Sorry, I see now that that schematic isn't super helpful here since I'm just trying to understand the more typical circuit where the supply voltage is positive.  Okay, so how about this one from the Noritake guide:


So the amp is outputting about a 5V swing for about 3V RMS.  The Zener is connected to a virtual center tap resistor divider between the two filaments, so it prevents the center point of the resistor divider from going more than 4.7V above ground.  Okay...?  But aren't we trying to move the entire waveform above Ek?  So that it's always positive relative to the anode/grid?

Sorry I'm not getting it, but I appreciate the help.


 

[pqass]

The objective is to make sure all parts of the floating filament AC are above GND. That way, when you switch a grid or anode to GND it will be completely OFF.

If there wasn't any zener and that center point of the divider was attached to GND, then parts of the AC would be below GND (the ends) and although you've switched a grid or anode to GND, it may still glow.

See attached and simulation here.  I've used a 5.6V zener where the 3Vrms AC never goes below 2.9V above GND.

VFD chips (like the HV518) use a HV PFET to Vpp and NFET to GND on their HV outputs (see below). But to save on using HV PFETs, you can instead just use an ordinary PFET/PNP to +5V controlling each grid and anode with a 10K pull-down resistor to -Vee (lowest potential, say -25V below GND). And a zener will keep the AC above -Vee letting the pull-down shutoff the grid or anode when the ordinary PFET/PNP is turned off.

[Ian.M]

Yes, as Pqass explained.  The one key factor you are forgetting is the electron current through the tube, emitted from the filament surface, accelerated by the active digit's positive grid, (which also intercepts a small proportion of it), and terminating  at the active segments, which are positive anodes.   For analysis consider this as an equal and opposite conventional current flowing from +HT through the high side of the active segments' and grid's drivers, through the tube to the filament cathode, then through the center tap resistors and through the Zener to ground, keeping it biased at its breakdown voltage.  This current is limited by the tube characteristics, so for analysis of the filament drive and bias circuit can be considered as a DC current source feeding the Zener cathode.

[oops]

For analysis consider this as an equal and opposite conventional current flowing from +HT through the high side of the active segments' and grid's drivers, through the tube to the filament cathode, then through the center tap resistors and through the Zener to ground, keeping it biased at its breakdown voltage.

Thanks also to pqass for the simulation, I played around with it and between the two, I think I understand now. 

Without the Zener diode, the virtual ground at the center of the resistor divider would float towards the +HT voltage.  I wasn't seeing that the +HT was in some sense connected to the virtual ground.

I really appreciate both of you jumping in on a 6 year old thread to help me understand.  I'm going to order parts.

Wait though, since you have a 5V supply available why wouldn't you just tie your 5V supply to the center of the resistor divider?
https://tinyurl.com/yu4534ng
I suppose that only works if the voltage swing of the circuit centered at +5V won't cause the voltage to dip below Ek?

[pqass]

Without the Zener diode, the virtual ground at the center of the resistor divider would float towards the +HT voltage.

I'm not sure about the validity of this statement (could be an artifact of measuring two unconnected circuits; floating filament WRT +HV). I'm not an EE but with a heated filament (via floating AC power supply), the electrons emitted will just fall-back like a vacuum tube heater warming an unconnected cathode.  No movement of the emitted electrons will happen. To move the electrons they have to be part of a circuit. The filament (cathode) must be tied to GND (or lowest potential) or slightly higher DC bias(Ek) with respect to the +HT on the grids and anodes; electrons move in a vacuum when between potentials. 

Wait though, since you have a 5V supply available why wouldn't you just tie your 5V supply to the center of the resistor divider?
I suppose that only works if the voltage swing of the circuit centered at +5V won't cause the voltage to dip below Ek?

As the Noritake Guide explains, it doesn't have to be the center-tap (it could be one end of the filament) but there are reasons for longevity and light gradient why AC+center-tap+DC bias(Ek) is preferred.  To generate the DC bias(Ek), it doesn't have to be a zener. It's just a convenient way to derive a small DC voltage (with common GND) using the +HV power supply.   You could also add the usual resistor between zener and +HV to bias it properly but since the electron flow in the vacuum is enough to bias the zener, the resistor is omitted. And it just takes one grid+anode to bias the zener. If all are off, it won't matter anyway.

If you've already got (say) a +5V rail, you can use that instead of the zener. Both +5V and +HV supplies must have their GNDs connected (where +HV >> +5V).

That same +5V rail can be used to generate (via DC-AC converter) the filament supply but the AC output MUST be floating (galvanically isolated via transformer or capacitors) otherwise it won't be able to ride atop the +5V Ek (and all AC parts stay above GND).  As I've said earlier, if some parts of the AC dip below GND (the ends) a GNDed grid or anode (ie OFF) may still glow.