TL494 DCDC converter wide range input issues

Z

Zac1

Member level 1
Joined
Mar 17, 2024
Messages
36
Helped
0
Reputation
0
Reaction score
1
Trophy points
8
Activity points
380
With a help of few people I managed to design fairly wide range DCDC converter using TL494

I confirmed it works in range of 30-190V DC
Has fairly stable 12V output with ~40mV peak to peak noise
Can deliver up to 3A.

However it has one fatal flaw that haunts me in my dreams. It kills mosfet Q5 and diode D2 if you suddenly apply 160V+ (high dV/dt)
It does work properly if slowly raising voltage across 40-190V. Also works up until 140V (sudden turn on).

I have managed to capture the problematic moment on thermal camera

(http://imgur.com/CHKR4Ew)
Basically there is a full short on Q5 and D2 for a moment, then fuse pops and both components have dead short. I have tried to remedy this situation with help of PNP clamp (Q1) but it doesn't do much. I am out of ideas how to fix this... perhaps the driver is at fault? Or maybe diode is too slow?


 

Solution
Z
Easy peasy said:
it needs to be speeded up / higher gain - until it becomes unstable - then back off a bit
Click to expand...
I think this can be done after I order PCB, right now i need to order asap to have something to play with.

both soft start and/or dc gain can be adjusted with just cap/resistor values.


FvM said:
Why is the switcher driving high current into D3 even in steady state although the nominal output voltage is said to be 12V? There seems to be a problem with voltage feedback operation point.
Click to expand...
I am sorry, i did not understand question? D3 after startup should see a max of 52micro amps.


FvM said:
As already stated, soft start time constant should be much higher, I'd test with 5 or 10 uF C5.
Click to expand...
Will do, right now i think i have complete PCB that should...
Sort by date Sort by votes
Hi,
Zac1 said:
That is not what is happening. I can't stress that enough but startup circuit built on depletion fet works marvelously.
Click to expand...
The thread is so lenghty one can not read every single post ...

Thus I wrote "IF the gate driver starts delayed..." with the emphasys on "IF" ...

From some of your scope pics it looks like switching does start delayed.
For the problem it does not matter what causes the delay. The delay may cause a problem in combination with the integrator part of your regulation loop.
If (again: "if") the soft start causes this delay, then one needs either to adjust the soft start or the regulation loop.

So to find out: Did you already post scope pictures of the regulation loop signals? If yes, can you just tell me which post#.

Klaus
 
Upvote 1 Downvote
OK - you have a solution that kinda works @ 190V in. If you ever re-spin the board, give some thought to using the other opamp in the TL494 to stop the pwm if the Vout goes higher than 12.5V

All the best going forward.
--- Updated ---

Oh, just out of interest could you set the blue trace Vin, to 100nS / div and apply start up ? it will be instructive to see what the Vin goes up to on the pcb.

trigger at 150V say on the blue trace
 

Upvote 0 Downvote
Easy peasy said:
OK - you have a solution that kinda works @ 190V in. If you ever re-spin the board, give some thought to using the other opamp in the TL494 to stop the pwm if the Vout goes higher than 12.5V
Click to expand...
I have been extensively testing the board recently and as you say, under the load whole thing collapses, although it can output over 3A with ease I think you are right and so the revised schematic was born:



Long story short, new elements marked with arrows:
R28 R9 that forms a divider that should output 1.016V (current limit)
R27, 10mOhm shunt
U2 INA213 current amp with 50V/V amplification (2A * 0.01ohm * 50 = ~1V)
CFB and CFB- are connections to internal TL494 opamp.

As for the schematic i did put INA close to TL494 routed with (pseudo) differential pairs from resistor.

I am not sure however with placement of shunt, it makes most sense to me to measure current after the coil before voltage feedback. Or should I move it?
Are there perhaps any obvious issues?
 

Upvote 0 Downvote
You must log in or register to reply here.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…