Anode / Grid Power Supply

[dredwinphd1]

I have tried to design a step-up power supply using the Maxim Max1771CPA. Would someone like to trade a critique of this design for a Noritake 4 line x 20 character display?

[c4757p]

A few thoughts immediately come to mind. First, why would you connect the MAX1771 after a 5V linear regulator? You specify 12V input, and it's happy to work up to 16V (17V absolute maximum)... Unless I've missed something, that linear regulator is completely unnecessary. Second, why have you specified 1% precision resistors in the feedback network when you have a trimmer there? Third, you claim to get at least 500mA output, but you've used an output capacitor rated for only 400mA. Did you actually compute (or simulate) the ripple it will see? It's going to see at least 500mA, and really you should allow a generous excess.

[dredwinphd1]

First, I am not an EE.

I tried to design this using the datasheet from the MAX1771.

I was not aware that the 1711 would operate from 12 volts directly. So I will drop the 78L05 regulator. Thanks!
What I needed was 25 volts dc and the example showed that the series of feed back resistors have a pot. I see where you are coming from though. What values would you suggest to drop the pot and provide me with the 25 volts @ 500 mA I require to drive these tubes?

Third, you claim to get at least 500mA output, but you've used an output capacitor rated for only 400mA. Did you actually compute (or simulate) the ripple it will see? It's going to see at least 500mA, and really you should allow a generous excess. I was not aware of this discrepancy. What would you suggest? I don't know how to simulate or calculate the ripple. I have based my design from the datasheet.

Suggestions, assistance and more comments are welcomed.

[T3sl4co1l]

12-25V?  Seems a little big for that... and.. anode/grid?  Oh, VFD, gotcha.

Well hell, you hardly need any current at all; a charge pump might even be sufficient?  (e.g., 555 followed by a diode-capacitor multiplying ladder?)

Offhand, not having checked datasheets:
- Using a 5V converter on a 12V supply doesn't make any sense (12V capable converters are all over the place, even one as old as UC3843)
- The MOSFETs shown are pretty awkward in size and ratings (more volts and amps than necessary, NOT logic level!).  But if you simply have them on hand, yeah, they'll work, more or less.  They should be logic level though (rated for Rds(on) at 4.5Vgs), which the parts listed are not, I think.
- I'd recommend against a schottky above 10 or 20V, just because you're not looking at much difference between that, and something even more pedestrian, like a UF4002 (or if we're talking junkbox parts here -- anything in the FR10n, UF400n, 1N493n, RG1x, etc. range), or even 1N914/1N4148 since we're talking low current anyway.  Really, as long as the schottky is rated for the voltage (plus a useful safety margin, say 50%), it's still not a problem using that.  If you have it on hand as well, no problem.
- Likewise, the inductor is huge, which would be fine for the ratings, along with everything else being big -- they're just well beyond the range needed for your application.

Tim

[dredwinphd1]

Thanks everyone for their inputs so far!

I forgot to mention that I am trying to use the through hole components since I have no way to do SMT at this time.

I ordered some samples of the UF1504S to replace the 1N5817 diode.

Can someone point me to a Fairchild Semi MOSFET which would be proper for what I am trying to do? I can get free samples of the Fairchild MOSFETS.

I also removed the pot from the feedback sense circuit. I tried to calculate a set of fixed 1% resistors to give me the 25 volts I need. I came up with R2 being 330K or so and R1 being 20K. This is probably wrong so some assistance would be helpful??

C4757P Posted: "Third, you claim to get at least 500mA output, but you've used an output capacitor rated for only 400mA." I do not claim to have built anything as of yet. From the feedback I have received thus far my design would not have worked anyway? I found some low ESR capacitors though I don't know if the capacitance is large enough. they are 56 uF / 50 volt Panasonic EEU-FR1H560B.

Finally, two questions are the sense resistor of 50 milliohms and the 22 uH inductor. Will these work or am I also wrong with these selections too?

Can you two help me to correct my errant design attempt so I can build and test this design? If either of you are into Microchip I could trade you some used development tools for your assistance or the previously mentioned display, if interested.

[dredwinphd1]

I was rereading the 1771 datasheet and ran across something interesting on page 8 concerning the use of a pot in the sense circuit. It states that: "Use 1% external feedback resistors when operating in adjustable-output mode (Figures 2b, 2c) to achieve an overall an overall output voltage accuracy of +/- 5%." So you could use a Cermet Linear Taper 10% pot for adjusting the output voltage. I have attached a copy of the 1771 datasheet for those wishing to assist me.

[dredwinphd1]

Here is the datasheet for the output capacitors I found at Panasonic. I was planning on using two in parallel. I was also going to use a 0.1uF MLCC ceramic rated at 50 volts also.

The datasheet for the Panasonic EEU-FR1H560B caps is attached.

[dredwinphd1]

Just a note on this open item. It is now closed. I was able to get with Maxim Tech Support and the two attached files are for the proven power supply and complete parts list. So any one working with the russian IV-11 VFD displays (and others) can utilize this design if they need a 12 volt to 25.2 power supply to power the Anodes and Grids. I also looked at the IV-6 and some others and this should work for any of them.