Tl431 as voltage regulator with complimentary components

Thread Starter

rahul411

Joined Feb 19, 2018
251
5381.boost (1).jpg
This is a boost converter made with uc3843. Input is 24vdc and it goes to the circuit made with tl431 as a voltage regulator ( i don't know what to call this). I run this part of circuit in ltspice and found that with varying the voltage on the input side i get steady 10.77v on output (wire connecting to the vcc pin of 3843).
Question is, why not use a linear voltage regulator instead of this. I connected a 100ohm load to the output and the voltage dropped to something 6volts. This is not even capable of supplying 50mA then why to use such setup? Lm7810 or lm7812 are not good for this?

Please correct me if this is something other than voltage regulator, i really don't know, it is just a assumption i made after simulation.
 

Thread Starter

rahul411

Joined Feb 19, 2018
251
It's the voltage the regulator uses to give an accurate/stable output voltage.
Datasheet of uc3843 says max voltage upto 30 volts and it also has an internal 34v zener. So why do we need to have something such precise to power the chip. Also from datasheet, the maximum current requirement of chip is 30mA and when I connect 300ohm resistor on output it draws 30.6mA but the voltage drops down to 9.2v. also the same supply is used to power the opamp meaning additional current.

Switching regulators typically have much higher efficiency than a linear regulator (and they can't boost voltage).
Sorry if I was unable to give my point first time but i want to know why not use a linear voltage regulator like lm7810 or 7812 here instead of this tl431 setup.
 

crutschow

Joined Mar 14, 2008
25,446
300ohm resistor on output it draws 30.6mA but the voltage drops down to 9.2v.
Then there is something apparently wrong with the circuit design.
why not use a linear voltage regulator like lm7810 or 7812 here instead of this tl431 setup.
Why would you want to?
The TL431 is more stable and accurate.
The 7810/7812 are series regulators, designed for relatively high power voltage regulation.
The TL431 is a shunt regulator, designed to provide a precise, relatively low current voltage.
 

Thread Starter

rahul411

Joined Feb 19, 2018
251
I chang
Then there is something apparently wrong with the circuit design.
Why would you want to?
The TL431 is more stable and accurate.
The 7810/7812 are series regulators, designed for relatively high power voltage regulation.
The TL431 is a shunt regulator, designed to provide a precise, relatively low current voltage.
I changed the value of base resistor of Q2 from 10k to 1k and got stable output of 10.7v at 35.5mA. is this alternation okay? I checked the power dissipation of transistors and base resistors and they are under 20mW. Will this hold in real life?
 

ronsimpson

Joined Oct 7, 2019
940
You could use a 5.1V Zener to drop the 24V into the ICs. R?=0 or something small. 22ohms? or just use some LEDs as Zener diodes.
C8 could be smaller to help with charge up current. I see that seems to be a problem.
1592756146421.png
 

MrChips

Joined Oct 2, 2009
21,890
TL431 should not be considered a regulator such as 78xx voltage regulator.

TL431 is a precision voltage reference used to provide an accurate voltage output. You should not take more than 100mA from this reference.
 

ronsimpson

Joined Oct 7, 2019
940
You should not take more than 100mA from this reference.
Any current "from" the TL431 must come from the resistor above it. R4 in this case. The TL431 is a shunt regulator so it can not supply any current. It acts like a Zener.

In this circuit Q2 amplifies the current. So current is mostly limited by Q2, Q2 beta and R4 current.

TL431s are in many power supply circuits. I have used is as a audio amplifier. Inside there is a 2.5V reference and a op-amp with OC output. (it only pulls down)
 

MrChips

Joined Oct 2, 2009
21,890
True, the TL431 acts like a zener diode.
What current you take comes from the series resistor.
The TL431 has to sink what current you do not take. Therefore select the value of the series resistor carefully.
 
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