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Janez 7th December 2007 02:07 PM

Overcurrent, overheat and overvoltage protection for TL494
2 Attachment(s)

I draw simple protection circuit for my new switcher. I need your opinion if this thing should work.

At right is a comparator used as flip-flop. I never used it that way but I think it shoud work ;).

Overcurrent sensing is made with current transformer, diode bridge and capacitor that are not drawn in shematic.

Electrolit capacitor 47uF is used for soft start, as described in design guide for TL494.


Janez 7th December 2007 02:21 PM

2 Attachment(s)
I don't know why in first message I couldn't attach shematic. I hope now it will work.

gootee 7th December 2007 03:55 PM

You could easily simulate it, with the free LTspice software, from . (The software is also known as SwitcherCADIII, but is a great, full-featured circuit simulator.)

Janez 8th December 2007 03:30 PM

Thanks, this simulator is realy good, but somehow I can't get my circuit working. I'm interested only in output comparator used as flip-flop and transistor.

Today I will try my circuit at test board. I'm mostly afraid that during startup some comparator will triger and shut down SMPS. I haven't been working a lot with comparators in practice, so I'm not sure what happens when power is applied.

In my workshop is freezing, as I don't have heating there, othervise I would test it already :D. Also instruments don't work corectly :xeye:


sawreyrw 8th December 2007 03:34 PM

The output comparator is not a latch. Instead it is a comparator with positive feedback and thus hysteresis. When the fault condition is removed, the comparator will switch back to the no fault state. You are also right that the circuit may not work correctly on start up. Finally, how fast do you want the circuit to be. The caps seem large.

djolejr 8th December 2007 06:57 PM

yor circuitry will not work for sure.
1. 339 is open collector so you need pull-up resistor for each output.
2. because of same reason as in previous, you will need diode for each output to separate outputs one from another.You will need to connect diode after pull-up resistor
3. And i asume that it is much better to use logic 'And' circuit with 3 inputs instead of last comparator, because it will be much faster and you will not need to think about feedback for each comparator.
Gain of comparator with open loop is very bad choice for pwm, as you intend to use. You need to use any compensation techniques for closed loop fedback stability but it is not necessary.

djolejr 8th December 2007 07:48 PM

I drew something. For more stuff you should search net.

djolejr 8th December 2007 07:49 PM

Sorry, i have not noticed where you drew junctions, so you don't need scheme that i drew.
But for overcurrent protection you must use any kind of stability control, or your flip-flop will always be 'On' when you start your circuit under load, or you will need to tune treshold current much more than you should expect, or to use bigger caps at input, but this will slowdown protection.

Janez 8th December 2007 11:34 PM

Belive or not, circuit works great :D

I had some problems when power was applied, but I solved it by adding 47nF capacitor between outputs of comparators and +5V, so when power is applied, + input of output comparator is much higher than reference voltage and it doesn't trigger.

Outputs of comparators are active low, so when they are triggered, they are connected to ground. So when any of them is triggered, all outputs drop to zero. Pull-up resistor is 10k. and it serves also for flip-flop as voltage divider with resistor 3k3.

Let me explain how flip flop works. When comparator is not triggered than voltage on + input is 5V and on - input is always 2.5V. When one of input comparators gets triggered, voltage on + input drops to 0V, that triggers comparator and sets output to 0. If fault condition disappears, + input remains lower than reference voltage, because resistors 10k and 3k3 act like voltage divider with voltage of 1,3V.

Capacitors at input of every comparator are 100nF, i think they are not too big, as voltage diferences are quite low. If you have any idea how to improve my circuit (different elements, etc) please tell me, I would be very grateful.

I have no idea how fast protection circuit reacts, but it sure prevents input transistors of SMPS from blowing up. I know because I tried wery hard to destroy them by shorting output, but I didn't succed :D. I didn't test overheat and overvoltage protection, but I'm sure they work.

I made short video and few pictures, I will post them, as soon as I compress them.

OK, protection circuit works great, but I have another problem - soft start. I have 10.000uF capacitance at output and SMPS should be able to turn on light bulbs, motors etc. I just can't make smps to turn-on slowly, lets say 2 or 3 seconds until full output voltage. I used even 220uF capacitor at DTC, but it seems there is no effect at all. OK I know that without capacitor transistors just blow (That was my discovery after 2 blown transistors several years ago :D). What should i do?

Thanks for your help,

Janez 8th December 2007 11:34 PM

djolejr: Sorry I didn't see your last message. I had a phone call during I was writing. You are right about triggering protection during start-up, I have already asked for help with this problem in my last message. I think (very) slow start would solve this.

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