# Need Help on Calculations for Negative TL431 with Pass Transistors!!

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.

#### Tubist

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

#### Attachments

• negative tl431 implementation!!.gif
15.7 KB · Views: 619

#### dhaen

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.

#### jackinnj

Re: Not a direct answer but...

dhaen said:
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.

#### Eva

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

#### AndrewT

Hi Eva,
should R9 be connected to 15031 base? thus ruining the post reg sensor location!
How do split supplies solve this?

#### johnnyx

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.

#### Tubist

Thanks for the great help here everyone

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

Regards

#### amplifierguru

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.

#### Attachments

• tl431reg.png
1.5 KB · Views: 469

#### hermanv

TL431 stability

I've had trouble with the stability of the TL431. The ON Semi data sheet warns about minimum current and pretty much assumes a large capacitor anode to cathode. I had some luck with a smaller capacitor cathode to feedback pin but be careful. There is a negative resistance state as the power is first applied, some circuits hang and won't pass this point without the large byass cap. It took a lot of work to find an unconditionally stable solution.

No it wasn't this amp it was in an isolated power supply feedback loop. So my difficulty or results may not apply.

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.