TL431 High Voltage Regulator - How to get it thermally stable? - Page 2 - diyAudio
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Old 14th March 2014, 08:13 AM   #11
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Originally Posted by franzm View Post
Q1 and R5 are useless and incorrect. Both, Q5 and U1 are inverting amplifying devices. The result will be an oscillator. Connect TL431 cathode directly to the gate of IRFP240 or insert a thermally stable Zener diode between them (level shifte, for higher output voltage).
Franz, thank you!
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Old 14th March 2014, 09:53 AM   #12
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Another way to increase the voltage capability ist to use a common base level shifter between the cathode of the TL431 and the gate of the IRFP240 ("cascode" the TL).
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Old 14th March 2014, 11:19 AM   #13
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I don't see how Q1 functions as an error amplifier. The TL431 should be in the emitter circuit for Q1, not the base circuit. The voltage divider on the output goes to the base, not the TL431.
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Old 14th March 2014, 12:17 PM   #14
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Originally Posted by franzm View Post
Another way to increase the voltage capability ist to use a common base level shifter between the cathode of the TL431 and the gate of the IRFP240 ("cascode" the TL).
Hi Franz,

Could you be more specific, or give an example, etc.?
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Old 14th March 2014, 12:32 PM   #15
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Originally Posted by dirkwright View Post
I don't see how Q1 functions as an error amplifier. The TL431 should be in the emitter circuit for Q1, not the base circuit. The voltage divider on the output goes to the base, not the TL431.
Hi dirkwright,

Here is the LTSpice file. Could you illustrate your circuit?

MostfetVoltageRegulatorWithDriverBJTBrief2.asc
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Old 14th March 2014, 03:28 PM   #16
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Sorry, it was late and I locked in on the 431 as the control of a shunt regulator. M1 was then just a problem that needed to be dealt with in the morning.
Here are two circuits from the data sheet that illustrate basic approaches.
Attached Images
File Type: jpg Series Pass 431.jpg (18.7 KB, 83 views)
File Type: jpg 431 Shunt Reg.jpg (15.9 KB, 85 views)
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Old 14th March 2014, 04:39 PM   #17
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Originally Posted by jazzclassics View Post
Hi dirkwright,

Here is the LTSpice file. Could you illustrate your circuit?

Attachment 405832
I don't use LTSpice, sorry.

I have a simple regulator, but it's for 300VDC. Basically, the TL431 is in the emitter circuit (as a stable voltage reference, not as a shunt regulator) with the supply for the TL431 coming from the raw DC side of the series pass device. The base of the error BJT is driven by a voltage divider on the output side of the series pass device. The collector of the error amplifier is connected to the base of the series pass device and also, via a resistive device, to the raw DC side of the series pass device. Hope this helps.
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Old 14th March 2014, 05:29 PM   #18
wayne is offline wayne  United States
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This will work. The left NPN base is held at 2.5 V in the diff pair and so will be the right adjust R114 to set.
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File Type: pdf TL431Reg.pdf (7.3 KB, 31 views)
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Old 14th March 2014, 05:52 PM   #19
ilimzn is offline ilimzn  Croatia
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The correct answer here, and no offense, is stop using the simulator and start reading datasheets and basic literature. You will not learn electronics from a simulator - it is a 'garbage in -> garbage out' device. Most of the components used in it are represented by models which are in most cases (I'd say 99%) valid only for the expected use of the device. If it's not connected correctly, the results will be nonsense, and rarely represent what will happen in a real circuit. This is why you need to understand the basics of a circuit before you even attempt simulating it, otherwise you will simply not know how to interpret the results of the simulation, not what to look for if they are unexpected. This has been said over and over on this forum, and apparently it always needs to be said again.

In your circuit, there are basic errors.
In the original one, the TL431 has no supply. It is a a shunt regulator, which is really a sort of zener diode replacement (albeit adjustable). There is a reason whu it is drawn as a zener diode! So, it needs to have a supply of current through it.
Secondly, there is no need for an added NPN transistor. Although in principle the schepatic could work with it, it can't work woth a TL431 simply because a TL431 can only go as low as 2.5V, and the B-E treshold of the transistor is ~0.7V. So, it will be permanently turned on to some extent, while the TL431 will essentially do nothing. The fact that the lowers voltage the TL431 can regulate is 2.5V (the value of it's internal reference) is well and clearly stated in it's datasheet (with examples of use!) and the datasheet is available from literally hundreds of places on the web. This clearly shows you didn't bother to read it.

Since the TL431 can go up to some 36V, but of course you don't want to use it at it's limit. the simplest way to extend it's voltage handling capability for your case is to add a zener diode in series. It should be for a voltage less than your intended output voltage, and 2.5V plus it's value is the minimum you can get with that circuit, 36V plus it's value is the maximum (again this is for the highest voltage on the TL431). There is no need to do anything special with thermal compensation of the zener - the TL431 ia an active device and it's regulation input is actually a feedback node. It will correct for any thermal drift of the zener.

Again - using a simulator is no shortcut to designing circuits for people who can't design them without one. Especially a simple one like yours - it's design can be done on paper in 5 minutes or less.
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Old 15th March 2014, 12:02 AM   #20
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Originally Posted by wayne View Post
This will work. The left NPN base is held at 2.5 V in the diff pair and so will be the right adjust R114 to set.
thank you very much wayne. this is illuminating..
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