TL494 half bridge smps

Thread Starter

aksnis

Joined Oct 6, 2020
11
Hello everyone,
I was studying a tl494 based half bridge smps
I came across the current sense section opamp pin 15 & pin16
Now pin16 is connected directly to the ground
But there is some arrangment made to pin 15

I have drawn a part of it
Please can anyone help me what is a purpose of “103 trimpot” and why does the transistor “ 2n2222a” is also used which Is connected to microcontroller pic18f452
 

Attachments

Thread Starter

aksnis

Joined Oct 6, 2020
11
I was studying a tl494 based half bridge smps
I came across the current sense section opamp pin 15 & pin16
Now pin16 is connected directly to the ground
But there is some arrangment made to pin 15

I have drawn a part of it
Please can anyone suggest why a “103 trimpot” is used and why does the transistor “ 2n2222a” is also used which gets base signal from pic18f452
 

Dodgydave

Joined Jun 22, 2012
9,685
Pins 15,16 and 1,2 form part of two internal op amps that are used to sense Current and Voltage and help regulate the feedback on pin 3, they are controlled by the Ref pin 14.

Can you post the full diagram..


pin-layout-for-TL494-IC.png
 

ronsimpson

Joined Oct 7, 2019
1,020
The amplifier is comparing pin 15 to ground. Is pin-15 at ground? Current sense pulls down on the left side while MCU lifts on the right side. (10k pot + 8.2k) on the left side 330k on the right side. This is like a balance beam.
If the MCU pulls up a small amount (0.5V) it takes very little current to pull in 15 to ground and shut off the TL494. If the MCU pulls up hard (4V) it takes much more load current in the power supply to get pin 15 below ground.
1603630590674.png
The "gain" is the ratio of (pot+8.2k)/330k. The pot is used to set the current range.
Example. MCU controls the output current from 0 to1A or 0 to 2A depending on how the pot is set.
 

Thread Starter

aksnis

Joined Oct 6, 2020
11
And sir when the smps is on 100v pin number 15 has 0.24v on it
And the voltage on that pin reduces as the power goes down from 100———>0
 

Dodgydave

Joined Jun 22, 2012
9,685
And sir when the smps is on 100v pin number 15 has 0.24v on it
And the voltage on that pin reduces as the power goes down from 100———>0
That's controlling the Voltage , pin 1 is fed from the ref 5V pin , and is compared to pin2 to maintain the output, it's the same with pins 15, 16 , when the current sense through the 0E1 resistors creates a voltage across them and when it's the same as pin 16 it shut s the output down.

Your drawing is wrong ,pin 15 should be at reference and pin 16 is the sense pin.
 

Dodgydave

Joined Jun 22, 2012
9,685
Have a look here at several ATX circuit diagrams using the TL494 , as you can see pin 1 is the sense pin and pin 2 is at reference , it will give you an idea how it works. It's the same for pin 15 at reference , and pin 16 sense pin.

http://danyk.cz/s_atx_en.html

To maintain a steady output, the reference comparator output from pin 3 , is normally low until it reaches same voltage on its sense pins , then it goes high and shuts the pwm down, and maintains a steady output.
 
Last edited:

ronsimpson

Joined Oct 7, 2019
1,020
The circuit looks like current limit. It does not look like voltage regulation.
Voltage regulation usually looks at the output voltage (divided down by resistors) There are often small capacitors around the amp.
Current limit usually looks at the current passing through very low value resistor like the two 0.1 ohm resistors. There should not be small capacitors.
1603652677595.png
 
Last edited:

Thread Starter

aksnis

Joined Oct 6, 2020
11
The circuit looks like current limit. It does not look like voltage regulation.
Voltage regulation usually looks at the output voltage (divided down by resistors) There are often small capacitors around the amp.
Current limit usually looks at the current passing through very low value resistor like the two 0.1 ohm resistors. There should not be small capacitors.
View attachment 220624
And one more thing how they will be callibrating this system
Will they be using a certain load and at full smps output and tune the pot till the desired current is acheived
Am i right?????
 
Top