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Old 21st April 2005, 06:12 AM   #1
Tubist is offline Tubist  Singapore
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Default Need Help on Calculations for Negative TL431 with Pass Transistors!!

Dear All.

I have come across this schematic for a +/- TL431 with pass transistor implementation.

The positive cycle is easy to understand and calculate, as the calculations is in the various data sheets

However, I have problems in the negative cycle and have mainly 2 questions.

1. Is the output voltage of the TL431 in the negative cycle dependent on the input voltage?

2. What is the calculation for the negative output voltage with the setting resistors R8 and R9?

Thanks for any help

Have a nice day
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Old 21st April 2005, 01:22 PM   #2
dhaen is offline dhaen  Europe
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Default Not a direct answer but...

TL431 is just a shunt regulator. It's being used to supply a fixed voltage to the base of the series-pass transistor.
Use the TI design utility here to make the TL431 work in isolation ie without the pass transistor, then determine the maximum base current the pass transistor can take (bearing in mind the emitter current and hfe) and tweak the calculation accordingly.
There will be a slight difference between the +ve and -ve implementation because of the control current. I notice that these TL's are being run very "cold" which makes the control current more significant than normal. I normally run them near to max ratings where they are quite warm to the touch.
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Old 21st April 2005, 02:45 PM   #3
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I know I've seen that schematic somewhere before
http://www.tech-diy.com/hp465a_ampli...ernization.htm
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Old 21st April 2005, 02:48 PM   #4
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Default Re: Not a direct answer but...

Quote:
Originally posted by dhaen
There will be a slight difference between the +ve and -ve implementation because of the control current. I notice that these TL's are being run very "cold" which makes the control current more significant than normal. I normally run them near to max ratings where they are quite warm to the touch.
you can get TO-92 heat sinks, you can also use the SO8 version of the TL431 and spread the heat onto copper.
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Old 21st April 2005, 04:30 PM   #5
Eva is offline Eva  Spain
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The positive regulator topology is correct, however the negative one is wrong and it isn't easy to fix. I suggest to use a transformer with two independent secondaries and independent rectifiers. This allows to use two positive regulators each floating relative to the other and symmetric output is then obtained by connecting the negative output of one of them with the positive output of the other, effectively stacking them

Unstability problems may arise due to parasitistic capacitance connected to the output of the lower regulator so frequency compensation or filter inductors may be required
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Old 21st April 2005, 08:35 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi Eva,
should R9 be connected to 15031 base? thus ruining the post reg sensor location!
How do split supplies solve this?
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Old 21st April 2005, 09:32 PM   #7
johnnyx is offline johnnyx  United Kingdom
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The internal voltage reference of the TL431 is connected to the "anode" terminal, so for the negative regulator, the reference is in series with the pass transistor Vbe. This is not so good, hence Eva's suggestion to use two series connected positive regulators.
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Old 22nd April 2005, 05:48 AM   #8
Tubist is offline Tubist  Singapore
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Thanks for the great help here everyone

I would then use Eva's suggestion to construct the regulators

Regards
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Old 22nd April 2005, 02:55 PM   #9
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Default unerstanding the TL431

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Old 23rd April 2005, 08:52 PM   #10
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Eva's suggestion is a good one - one I used with LT317s in the Eidetic (Australian) preamp outboard power supply in the early '90s.

But I love TL431s and this one (15V shown) simulates well.
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