Simulation shows that breakdown of the output transistors in the '494 at around 41V would prevent the FET from turning off if it has a low Vgs(thr).

Aye! -- Seems this thread's real 'poser' lies in the fact that the circuit functions with the IRF9540?!

Best regards
HP

 

Aye! -- Seems this thread's real 'poser' lies in the fact that the circuit functions with the IRF9540?!

Best regards
HP

Is it possible the the pin out on the mosfets are different? Not all brands or models of mosfet/transistors are pinned in the same matter. (I didn't bother to look at the datasheets.)

 

Is it possible the the pin out on the mosfets are different? Not all brands or models of mosfet/transistors are pinned in the same matter. (I didn't bother to look at the datasheets.)

I also speculated back in #2 that the answer lies in the datasheets. The two linked by @HP in #3 show the same pinouts but I don't think we don't know if the TS has those parts from those manufacturers.

 

Is it possible the the pin out on the mosfets are different? Not all brands or models of mosfet/transistors are pinned in the same matter. (I didn't bother to look at the datasheets.)

I also speculated back in #2 that the answer lies in the datasheets. The two linked by @HP in #3 show the same pinouts but I don't think we don't know if the TS has those parts from those manufacturers.

While I cannot imagine disparity in pin-out across manufactures (paint me naive) - I suppose it's possible @gumattos is 'mixing' package styles?...

Best regards
HP

 

This is not a FET problem! This is a basic design problem in that the specs of the FET are being exceeded. If the circuit worked at all with the other FET it was only because some of these FETs will (for a short time) take more gate voltage than specified without blowing.
Your FETs are probably blown. Connect the gate and source together with a wire and measure between the source and drain with a meter. It should read infinity, or very high megohms. Readings below a megohm reveal a punched gate. The transistor will turn on on its own when you apply drain voltage. It is useless.
Change the circuit to apply a maximum of 12 volts to the gate with respect to the source. You can do this with a zener diode in series with R13.

 

TL494 Vcc may be lower than the power stage voltage.
The gating signal goes through complementary emitter follower i.e. has slightly lower swing.
If the PFET source sits at higher voltage it may not shut.

 

OK guys, sorry for the delay, thanks to everybody that is helping me here. So, when I tried to test the circuit with the oscilloscope it was not working anymore, with the same problem like the other MOSFET, so I decided to make the circuit from the beggining but with the scope. Now something more curious is happening, I set the TL494 to oscillate without feedback from the output with 24V.

THE CIRCUIT SHOWS A TL493 BECAUSE THERE IS NO TL494 ON ORCAD!!



Ch 1 - Gate / Ch 2 - Drain


Ch 1 - Gate / Ch 2 - TL494 collector


Ch 1 - Gate / Ch 2 - Source


Ch 1 - Gate / Ch 2 - TL494 oscillator


Ch 1 - Gate / Ch 2 - Trigger set on the turn on of the circuit


So, I thought It could be the MOSFET, so I replaced it but got the same problem, what am I missing here? And I have tried two 100 ohms resistors for the voltage divider to have more current, but no changes. I changed the circuit for this simpler version just to have less components to give me a problem so I can analyse it better, but I still think it could be the MOSFET, any suggestions?

 

The problem with the MOSFET is solved, a lot of time of thinking for a bad wire, I will now continue on making the changes until I get back to the original circuit.

 

...a lot of time of thinking for a bad wire...

Not to rub salt in the wound, but are you saying the problem all along was mis-wiring? Don't feel bad if it was. A large portion of problems reported here turn out to be "stupid" stuff. That's just the way nature is. First-timers get mad at us when we ask the stupid questions; is it plugged in? Are the components installed backwards? That sort of stuff. But it's so often where the problems hide – in plain sight.

 

Not to rub salt in the wound, but are you saying the problem all along was mis-wiring? Don't feel bad if it was. A large portion of problems reported here turn out to be "stupid" stuff. That's just the way nature is. First-timers get mad at us when we ask the stupid questions; is it plugged in? Are the components installed backwards? That sort of stuff. But it's so often where the problems hide – in plain sight.

Turns out this didn't solved the IRFR5305 problem, but I soldered the circuit in a board for testing, and it is working again with the IRF9540, and now that I have the scope I can show some results of how the circuit behaves with the IRF9540. I couldn't notice any reasons for the IRFR5305 not to work, but there are two things bothering me, I know it's a bit off topic but are these oscillations between the switching OK? There are some oscillations on the Gate that makes the MOSFET oscillate too.

The second thing is that in every place I searched I couldn't find the pinout of the IRFR5305, I assumed this:


All this problem could be just some mistake on the connection (and I really should have asked if my pinout is correct a long time ago).

As for the scope, here are the images:

In yellow is the Gate of the MOSFET (with the strange oscillation), in blue the Drain:


In yellow is the collector of the TL494 and in blue the Drain of the MOSFET (with the strange oscillations):


In yellow the oscillator capacitor and in blue the Drain:


Here in blue is the Gate of the MOSFET with the oscillation and in yellow is the feedback pin after the voltage divider:


Here I tried to trigger any voltage on the Gate that could damage the MOSFET, but results showed nothing higher than 15V.


Schematic (the results showed in the scope are with a 1K load on the output, as no load generates some strange results):


As for the IRFR5305 not working, still no clues. As for the oscillations, if that is not normal and it is a problem ( I think it is beacuse of the MOSFET turning on with low voltages applied to Gate several times) I will try to make the 1K resistor on the Collector of the TL494 smaller, something around 300 Omhs maybe and I will try to put a capacitor on the feedback to make the signal more stable, but I am open for suggestions.

 

Ok, after some reading I got it, it is because the buck regulator is entering discontinuous mode and oscillating when in OFF state for low current, I will put a snubber on the circuit and see if it improves the results.

 

Ok, after some reading I got it, it is because the buck regulator is entering discontinuous mode and oscillating when in OFF state for low current, I will put a snubber on the circuit and see if it improves the results.

Maybe the regulator needs a minimum amount of current for it to work properly?
If that's the case, maybe you could connect a resistor to ground at its output. Say, 100K for instance.

 

As for the oscillations, if that is not normal and it is a problem ( I think it is because of the MOSFET turning on with low voltages applied to Gate several times) I will try to make the 1K resistor on the Collector of the TL494 smaller

An LTspice simulation of your circuit suggests that the oscillations are due to the gate-source and gate-drain capacitances of the FET resonating with the choke (L1) inductance. Changing the FET and/or the inductance gives a corresponding change in oscillation frequency.