Overcurrent, overheat and overvoltage protection for TL494 - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 9th December 2007, 01:25 AM   #11
gootee is offline gootee  United States
diyAudio Member
 
Join Date: Nov 2006
Location: Indiana
Blog Entries: 1
I am not sure if this is what you need. But here is what I did, to slow the turn-on speed:

I have an SMPS that is powered from a transformer output plus bridge rectifier and smoothing capacitance (like unregulated linear supply).

I made the SMPS voltage come up more-slowly by inserting a big MOSFET in parallel with a small resistance, between the rectifier bridge and the smoothing caps. The mosfet is turned on after a delay. So the parallel resistance carries almost all of the initial current. By varying the parallel resistance, and the turn-on time of the mosfet, I can set the speed of the power-up. After start-up, the resistor is basically out of the circuit, since the mosfet's Rdson is so small.

I have built quite a few of these power supplies and they seem to work well.

I also use this "soft start" circuit for a regulated linear supply that has 3x 4700 uF smoothing caps and a "large" (47uF) bypass cap on an LD1084 adjustable 3-terminal regulator's adjust pin, in order to prevent the regulator's input voltage from rising so fast, compared to its output voltage, that the maximum in-out differential voltage rating is exceeded.

(Edit: You can actually use either PMOS or NMOS, for the mosfet, with proper configuration, as noted farther below.) For a positive DC voltage system (25VAC-->30VDC), I used a P-channel mosfet, STP80PF55 (like IRF4905, but cheaper; you could use others, instead, if needed), with a 1-Ohm 5W resistor in parallel (i.e. from S to D). I placed a 33K 1/4W resistor from mosfet gate to ground, and 22uF cap in parallel with a 15v Zener from the mosfet's gate to source (zener's cathode and cap's + to mosfet source). Mosfet source was connected to bridge rectifier's + output, and drain was connected to + of smoothing caps.

I also have a very similar version for a negative power rail, which uses an nmosfet. And at the site below, I show a slightly different version, which uses N-channel mosfets for both the positive and negative rails (with STP80NF55-06 n-mosfets).

See the schematics at

http://www.fullnet.com/~tomg/gooteesp.htm ,

which has a couple of linear regulated power supply schematics that include soft-start circuits that are similar to what I described above.

You can also download the LTspice circuits, there, so you can simulate them, and can copy and paste parts of them into other circuits, etc.

The easiest way to set the turn-on speed/characteristics is probably by playing with the values in the simulator (i.e. the 1 Ohm, 33k, and 22uF), after the soft-start portion is pasted into something similar to your circuit. The resistor parallel to the mosfet sets the initial turn-on slope, depending on the caps that follow, and the voltage. And the 33k and 22uF control the speed of the change of the mosfet's Vgs (and it turns on when its threshold is reached).

My values, with my circuit, give about 0.5 second turn-on time, although the actual added delay is less than 0.15 second (since that's all I needed). But the delay can easily be increased.

Note that the 1 Ohm resistors that I use (parallel to mosfet) are only 5-Watt components. That 5W is exceeded, by a very large margin (800W), for a very short time. But it's OK, in my case. Note that some resistor datasheets actually specify those types of allowable short-time overloads. If not, I guess you could "try it and see". But with your longer start-up time, you might have to worry a little more about it.

In my case, the mosfet only dissipates about 0.74 watt, and barely needs a heatsink.
__________________
The electrolytic capacitors ARE the signal path: http://www.fullnet.com/~tomg/zoom3a_33kuF.jpg
  Reply With Quote
Old 9th December 2007, 03:58 PM   #12
Janez is offline Janez  Slovenia
diyAudio Member
 
Join Date: Nov 2007
Thaks, it's a nice idea, I'm going to try it, but last night I was thinking about using second error amplifier in TL494 for slow start with capacitor and resistor.

Before I try this, I will connect potentiometer to DTC and try to find out what happens if I change voltage to this pin.
Attached Images
File Type: jpg amp.jpg (8.4 KB, 373 views)
  Reply With Quote
Old 9th December 2007, 03:58 PM   #13
Janez is offline Janez  Slovenia
diyAudio Member
 
Join Date: Nov 2007
This is my prototype with my previous protection circuit that worked great in theory, but in practice not at all At output is NE555 and restarts switcher 10 seconds after error condition. I found shematic somewhere in internet.
Attached Images
File Type: jpg switcher2.jpg (84.9 KB, 428 views)
  Reply With Quote
Old 9th December 2007, 08:16 PM   #14
djolejr is offline djolejr  Serbia
diyAudio Member
 
Join Date: Nov 2007
voltage betwen 0V-3.5V (0%-9x%<100%) at DTC or pin 4. directly control pulse widith, and outputs of 2 error amplifiers are connected to DTC so they just amplifier difference betwen + and - and change voltage at DTC. I have tried to use similar overcurrent protection with current transformer in PC PSU ( as car battery charger), it had worked very fine when i tried to shortcircuit it (for several hundreds times), but when i have tried to charge big battery (1,2V) with about 10 Amps (10Amps was overcurrent protection) overcurrent protection did not work as i planed, and simply it was dead.Than i tried to create some kind of current regulation and to use second error amplifier from TL494, and i could not make it to work. I connected Voltage from current transformer to noninvert. (after rectification and after filter ) input and to inv. input i connected Vref (Vref is changeable by pot). But it did not work good.For example i load it with light bulb, and i tried to control current. so i start from 0A and slowly rise it up until light bulb had full brightnes(potentiometer was at middle and Vref=2.5V), but when i tried to reduce current i needed to rotate potentiometer to much (pot was at quarter, Vref=1.2V).
i limit current for some load, but when i connect smaller load ( but big enough to draw more current that i limit) it did not limit current, load consumption of current was more that i limited.
  Reply With Quote
Old 9th December 2007, 09:38 PM   #15
Janez is offline Janez  Slovenia
diyAudio Member
 
Join Date: Nov 2007
djolejr; I've just take a look at block diagram of TL494 and you are absolutely right about DTC and error amplifier. I don't know, maybe i made a mistake on protoboard and 220uF was not connected correctly.

I have similar power supply like you made, but is capable of 25A at 14,4V. When you take a look at primary transistors and transformer you won't belive me . I'm using it almost every day for several years now and I didn't change anything yet, it's pretty undestroyable . It's quite amazing that while I was developing it, I didn't destroy any transistor. With prototype I'm developing now I blew about 6 pairs of transistors, mostly because of stupid mistakes like overheating (I had a phone call during testing ), bad isulator, screw under board etc. Diode bridge and fuse survived all of those mistakes.

With switch I select between 12V and 14.4V output. It's able to start with load of 4 bulbs 12V 45W in paralel, but if I connect a bulb while it's working it turns of.
Attached Images
File Type: jpg 12vab2.jpg (78.1 KB, 384 views)
  Reply With Quote
Old 9th December 2007, 09:44 PM   #16
Janez is offline Janez  Slovenia
diyAudio Member
 
Join Date: Nov 2007
Another photo:

(I won't be able to read this forum or repy till next weekend - school)
Attached Images
File Type: jpg 12vab1.jpg (67.7 KB, 370 views)
  Reply With Quote
Old 18th December 2007, 05:42 AM   #17
Serge66 is offline Serge66  Switzerland
diyAudio Member
 
Serge66's Avatar
 
Join Date: Nov 2007
Location: Canton of Jura
Hi,

Why not use a photo-thyristor? It latches and provides isolation between primary and secondary side of your PSU.

Serge
__________________
I simulate, therefore I am.
  Reply With Quote
Old 21st December 2007, 02:36 PM   #18
diyAudio Member
 
chriscam's Avatar
 
Join Date: Jul 2003
Location: Amsterdam
Quote:
Originally posted by Janez
Thaks, it's a nice idea, I'm going to try it, but last night I was thinking about using second error amplifier in TL494 for slow start with capacitor and resistor.

Before I try this, I will connect potentiometer to DTC and try to find out what happens if I change voltage to this pin.
there is a better way to do slow start, goto http://onsemi.com and search for the tl494 datasheet. Open it up and on page 9 figure 16 you'll find a softstart circuit using the dtc pin.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Making car amplifier SMPS with tl494 + DC Protection Risto80 Car Audio 750 18th May 2014 07:59 AM
Overvoltage protection with filter Igla Chip Amps 4 27th February 2009 12:11 AM
lossless overcurrent protection in UcD scheme zilog Class D 11 17th December 2006 03:55 PM


New To Site? Need Help?

All times are GMT. The time now is 07:06 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2