feedback and stability

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Michael Robinson 16 years ago

I built a voltage regulator for an old truck I own. The circuit uses a voltage controlled oscillator with a square wave output to drive the gate of a mosfet that controls the alternator's field. The voltage regulation feedback loop uses a voltage divider from the alternator output/vehicle battery, connected to the reference pin of a TL431. The TL431 controls the vco.

Now, for a bit of a digression. Years ago I built a boost converter with a similar feedback loop, the major difference being that it used a bjt instead of a TL431. But it worked on the same principle, with a voltage divider from the output to the base of the bjt, which controlled a blocking oscillator. The output voltage wandered up and down and was very unstable, so I put a capacitor from the collector to the base of the bjt. That fixed the problem and made the output voltage stable.

So, having had this experience, I put a cap from the cathode to the reference of the TL431 in my truck regulator. When I installed it in the truck, the alternator output regulation was unstable; the voltage jumped up and down like crazy. After much head scratching, on a hunch I took wire clippers and removed that cap from the circuit board. Then the regulator worked GREAT. System voltage perfectly stable, good load regulation.

Perhaps I could be forgiven for thinking the boost converter and the vehicle voltage regulator would operate on the same principle and that a cap would have the same effect, in view of the fact that in both circuits the cap provides negative feedback in the controlling element of the feedback loop. But the two circuits have opposite behavior, and adding a cap to the controlling element in the feedback loop has exactly the opposite effect in each circuit. The boost regulator was unstable without a capacitor, and stable with one. Contrarily, the alternator regulator was stable without a capacitor, and unstable with one.

At best I have some intuitive sense that the presence of a big mechanical device that drives the load and resides within the loop changes things dramatically. But I'm lacking in knowledge of theory and could use elucidation. Any pointers?

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Tim Wescott 16 years ago

A schematic would help - either an ASCII art one, or put one up on a website somewhere so folks could look.

Yes, the dynamics of your alternator are likely to be different from the dynamics of your boost converter output stage, so that could certainly have an effect on stability. Without knowing more about what you did (and probably more about the alternator's dynamics) I can't say much.

www.wescottdesign.com

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Michael Robinson 16 years ago

"Tim Wescott" wrote in message news:K56dnfGL45LFBLPWnZ2dnUVZ snipped-for-privacy@web-ster.com...

I don't think I can find the diagram for the circuit, but the following should help: Here's a spice file for a vco with the topology of the one I used. I used a different comparator, and component values may be different, but I didn't change the basic design. This spice file was actually posted by John Popelish a few years ago, and I based my circuit on it.

Version 4 SHEET 1 880 680 WIRE -16 -80 -128 -80 WIRE 304 -80 64 -80 WIRE -64 16 -80 16 WIRE 32 16 16 16 WIRE 304 48 304 -80 WIRE 336 48 336 16 WIRE 336 48 304 48 WIRE 336 80 336 48 WIRE 288 160 224 160 WIRE 224 176 224 160 WIRE -240 240 -304 240 WIRE -128 240 -128 -80 WIRE -128 240 -160 240 WIRE -16 240 -128 240 WIRE 96 256 96 16 WIRE 96 256 48 256 WIRE 128 256 96 256 WIRE 176 256 128 256 WIRE -80 272 -80 16 WIRE -48 272 -48 176 WIRE -48 272 -80 272 WIRE -16 272 -48 272 WIRE -48 304 -48 272 WIRE -128 320 -128 240 FLAG 432 288 0 FLAG 432 208 p12 FLAG 336 176 0 FLAG 224 272 0 FLAG 16 288 0 FLAG -48 384 0 FLAG 336 -64 p12 FLAG -48 96 p12 FLAG 16 224 p12 FLAG 128 176 p12 FLAG 224 80 p12 FLAG -304 320 0 FLAG -128 384 0 FLAG 336 48 Out SYMBOL voltage 432 192 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 12 SYMBOL Comparators\\LT1017 16 256 R0 SYMATTR InstName U1 SYMBOL nmos 176 176 R0 SYMATTR InstName M1 SYMATTR Value BSS145 SYMBOL nmos 288 80 R0 SYMATTR InstName M2 SYMATTR Value FDS6680A SYMBOL res 208 64 R0 SYMATTR InstName R1 SYMATTR Value 1k SYMBOL res 320 -80 R0 SYMATTR InstName R2 SYMATTR Value 1 SYMBOL res -64 288 R0 SYMATTR InstName R3 SYMATTR Value 100k SYMBOL res -64 80 R0 SYMATTR InstName R4 SYMATTR Value 100k SYMBOL res 112 160 R0 SYMATTR InstName R5 SYMATTR Value 10k SYMBOL cap -144 320 R0 SYMATTR InstName C1 SYMATTR Value 220n SYMBOL cap 96 0 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C3 SYMATTR Value 10n SYMBOL res 32 0 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R6 SYMATTR Value 100k SYMBOL res 80 -96 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R7 SYMATTR Value 100k SYMBOL res -144 224 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R8 SYMATTR Value 100k SYMBOL voltage -304 224 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value PULSE(0 12 0 1) TEXT 184 360 Left 0 !.tran 1

At the input to the vco there's a TL431 with a pullup resistor on the cathode, and the cathode is connected to the vco input. For what it's worth, the truck is a 1994 Dodge 2500 Ram diesel.

Putting a cap across the TL431 cathode/reference terminals ought to slow the circuit's response, which could be a problem. From the way the voltage was jumping around, it was something more than just loose regulation -- it's like the cap introduced some kind of nonlinear effect in the feedback loop's dynamics. It wasn't just drifting, it was bouncing around.

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Hammy 16 years ago

I don't see a TL431 in your schematic but here is a screen shot of the TL431 showing capacitor values which may cause the device to oscillate.

Page 24 of the data sheet.

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Michael Robinson 16 years ago

This won't get anywhere until I provide some visuals. Here's an abstract of the system, a flow chart if you will. It's a loop. View in fixed font.

,----------------------

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fitlike min 16 years ago

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Put a resistor in series with the capacitor.

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ehsjr 16 years ago

Drawing it out will make it clear. The first schematic below is a "sanity check", just to make sure that the circuit you described is something like it:

----+----[PASS]------------+-----------------> field | | | +-----[VCO]---gnd | | | | +--[R]--+------+ | | | | __|__/ [C] [R] / \ | | /___\----+-------+ | | | [R] | | Gnd ----------+--------------+

Assuming the above captures the essence of what you have, read on. You have something like the above, but for the discussion it can be simplified to:

------------+--------------+ | | [Rload] | | | +------+ | | | | __|__/ [C] [R] / / \ | | /___\----+-------+ | | | [R] | | Gnd ----------+--------------+

Note that the output of the 431 is connected by C to the input of the 431.

A 431 is essentially an op amp with an output transistor and one leg of the op amp input tied to an internal 2.5 volt reference. Perhaps the effect of the cap is more easily seen if you draw the TL431 in circuit like this:

V+ ----+-----------------------+ | | | [Rload] | | | +-------[C]------+ [R] | | | | |\ |

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Jim Thompson 16 years ago

[snip]

That's what a TL431 looks like, but it doesn't address that you need a TC like this...

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...Jim Thompson

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ehsjr 16 years ago

True, and a good point. But his problem wasn't caused by temperature. His _next_ problem with it will be, without temperature compensation. :-(

Ed

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Michael Robinson 16 years ago

No, the circuit looks like this: