How do you add soft start to a TL494? I hasve seen applications that have it but how do they do it?
Also how do you figure out the required resistors for dead time?
From what I can understand, they have a resistor from Vref to Dead time pin and then a resister from dead time to ground. how do I fiqure out the the values for a given % of dead time?
Thanks a lot
Dave.
Also how do you figure out the required resistors for dead time?
From what I can understand, they have a resistor from Vref to Dead time pin and then a resister from dead time to ground. how do I fiqure out the the values for a given % of dead time?
Thanks a lot
Dave.
I made a soft start for a buck regulator, by soldering a capacitor between DTC and Vref legs, an a resistor was between the GND and the DTC.
When I turn on the circuit, the capacitor is empty => It's a short circuit, and it's pulling DTC to Vref, so the dead time is 100%, there's no PWM signal. As the capacitor charges through the resistor, the PWM rises from 0% duty-cycle to the actual duty-cycle.
The datasheet says, that maximum DT is, when you put 3V to the DTC pin, and minimum DT (so the maximum duty cycle) when you connect DTC to GND. I don't know, that the dead-time can be set linearly between 0V-3V. Try athat, with a trim.pot!
When I turn on the circuit, the capacitor is empty => It's a short circuit, and it's pulling DTC to Vref, so the dead time is 100%, there's no PWM signal. As the capacitor charges through the resistor, the PWM rises from 0% duty-cycle to the actual duty-cycle.
The datasheet says, that maximum DT is, when you put 3V to the DTC pin, and minimum DT (so the maximum duty cycle) when you connect DTC to GND. I don't know, that the dead-time can be set linearly between 0V-3V. Try athat, with a trim.pot!
Danko, I need to set the dead time and the soft start for my design.
Where's the best place to put the trimpot? The upper leg of the divider or the lower leg? (I'm guessing the lower leg.)
Anyone else have any knowledge that they would like to share?
Thanks
Dave.
Where's the best place to put the trimpot? The upper leg of the divider or the lower leg? (I'm guessing the lower leg.)
Anyone else have any knowledge that they would like to share?
Thanks
Dave.
I would also put it in the lower leg. If accindentally the pot becomes an open circuit, then the divider's upper resistor pulls DTC pin to Vref, and the circuit stops.
Bump....
Come on people the TL494 chip is extremly popular chip, someone has to know how to do softstart and custom dead time with theses other than Danko.
Please, anything will be helpfull. I'M starting to get frustrated trying to figure it out.
Come on people the TL494 chip is extremly popular chip, someone has to know how to do softstart and custom dead time with theses other than Danko.
Please, anything will be helpfull. I'M starting to get frustrated trying to figure it out.
I found this thread on google, go figure LOL 😀
Anyway, I'll help you with the adjustable deadtime. I don't have specific values, I used a POT to make it adjustable.
I used a 22K resistor in series with a 68K ohm trimmer POT to GND. The 22K is so the top of the POT has about 3.75V so the full range to the POT can be used. You can also use 33K and 100K POT as substitites.
I connected the middle pin to the DT pin. For stability, I used a 1uf cap from DT to GND because it was very sensitive to noise and PS fluctuations without the cap. With this, the DT is fully adjustable.
TO change it to a variable DT with softstart, add a 1K ohm resistor from the bottom leg of POT in series to GND. Then connect a 10uf cap from Vref pin to top of 1K resistor.
*On startup, the bottom of the POT connected to the 1K gets pulled up to Vref by the 10uf then as the cap charges, it goes slowly back to GND, so no matter what adjustment the POT has, the softstart works.
Have fun, the TL494 and 594 are great fun once you learn how to use them to their potential.
Anyway, I'll help you with the adjustable deadtime. I don't have specific values, I used a POT to make it adjustable.
I used a 22K resistor in series with a 68K ohm trimmer POT to GND. The 22K is so the top of the POT has about 3.75V so the full range to the POT can be used. You can also use 33K and 100K POT as substitites.
I connected the middle pin to the DT pin. For stability, I used a 1uf cap from DT to GND because it was very sensitive to noise and PS fluctuations without the cap. With this, the DT is fully adjustable.
TO change it to a variable DT with softstart, add a 1K ohm resistor from the bottom leg of POT in series to GND. Then connect a 10uf cap from Vref pin to top of 1K resistor.
*On startup, the bottom of the POT connected to the 1K gets pulled up to Vref by the 10uf then as the cap charges, it goes slowly back to GND, so no matter what adjustment the POT has, the softstart works.
Have fun, the TL494 and 594 are great fun once you learn how to use them to their potential.
ifrythings said:
the way danko explained it is the way to do it.
The application notes for the tl494 clearly explain the ranges of the DTC pin and how to use if for soft-start.
Hello there,
The other intresting way of softstart is limitation of current througt
MOSFETs by applaying low initial value of Vgs. The advantage - true limitation of current, because MOSFETs are not able to be fully opened.
Example:
Very popular IRF3205 MOSFET with applayed Vds 10V will be able to conduct:
At 4,5Vgs - <10A
5Vgs - 30A
5.5Vgs -65A
6Vgs - 100A
The transistor above rated for Continuous Drain Current about 110A, or short pulse of 390A. So can be clearly seen that by redusing Vgs you will be able to start with absolutley safe value of current , limited by MOSFETS internal resistance .
You`ll say OK, so why I didn`t see this realized in a lot of projects ?
The answer: increased number of components, plus advenced design of voltage source - it should to have very small output resistance after start phase.
The next question can be - so in witch cases using of such topology is resonable ?
If you have to charge very large filtering capacitors bank.
And last: Whats wrong with conventional soft start ?
It mainly limits lenght of current pulse, but not able to control current amplitude directly.
I have tl494 appnote, with details about conventional soft start.
Its too large to attach it here, but I can to email it.
Hope this helps 🙂
The other intresting way of softstart is limitation of current througt
MOSFETs by applaying low initial value of Vgs. The advantage - true limitation of current, because MOSFETs are not able to be fully opened.
Example:
Very popular IRF3205 MOSFET with applayed Vds 10V will be able to conduct:
At 4,5Vgs - <10A
5Vgs - 30A
5.5Vgs -65A
6Vgs - 100A
The transistor above rated for Continuous Drain Current about 110A, or short pulse of 390A. So can be clearly seen that by redusing Vgs you will be able to start with absolutley safe value of current , limited by MOSFETS internal resistance .
You`ll say OK, so why I didn`t see this realized in a lot of projects ?
The answer: increased number of components, plus advenced design of voltage source - it should to have very small output resistance after start phase.
The next question can be - so in witch cases using of such topology is resonable ?
If you have to charge very large filtering capacitors bank.
And last: Whats wrong with conventional soft start ?
It mainly limits lenght of current pulse, but not able to control current amplitude directly.
I have tl494 appnote, with details about conventional soft start.
Its too large to attach it here, but I can to email it.
Hope this helps 🙂
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