Changing MOSFET makes circuit stop working - Page 1
Forum section: electronics / beginners

author: gumattos
author_rank: Newbie
timestamp: January 16, 2017, 02:06:56 pm
content: Hello everyone, I am new to the forum and I don't know if I am posting this is the right place. I am facing a very interesting problem here. I already worked with MOSFETs before, but never seen something like that. I have a circuit using TL494 oscillating at ~80KHz and it works fine, I am driving the MOSFET normally, and it is a IRF9540. However, if I replace the IRF9540 for a IRFR5305, the output goes to Vcc and not the regulated output, any ideas? I am using a SB360 schottky diode with an input of 24V (the same for 48V input). Anybody have any idea why the circuit works with one MOSFET and not the other?The original circuit is 48V and it works great with the IRF9540 but the same problem happens with the IRFR5305.I couldn't find the model for TL494 on TINA so I just build this schematic for representation.I tested the IRFR5305 with the multimeter and it seems to be working correctly, I have 5 of them, all new, one of them I haven't used yet in case I am doing something very wrong so I have a brand new for testing, the other four have the same problem. Sorry for the bad english.

author: capt bullshot
author_rank: Super Contributor
timestamp: January 16, 2017, 02:39:01 pm
content: First guess: your gate driver circuit exceeds the absolute maximum ratings for Vgs of the Mosfets. Both have a max. of +/-20 Volts Gate to Source voltage, your circuit appears to drive up to -24V (somewhat less indeed). There must always be a good reserve between the abs. max. rating and the used driver voltage. I'd recommend to drive the MOSFET with no more than 12 or 15V.Next guess: The IRF5305 has 55V Drain to Source abs. max., this is not recommended for a 48V supply rail (not enough safety margin). As you say, the MOSFET fails at 24V, first guess is more likely to be your problem.The survival of the IRF9540 is pure luck in that case, not proper circuit design.

author: orolo
author_rank: Frequent Contributor
timestamp: January 16, 2017, 04:13:02 pm
content: Maybe the R1/R2 divider does not turn T2 on?

author: gumattos
author_rank: Newbie
timestamp: January 16, 2017, 04:20:53 pm
content: Quote from: capt bullshot on January 16, 2017, 02:39:01 pmEDIT: Sorry, the schematic is wrong, the voltage divider is 100 Ohm and 220 Ohm!I don't think the design exceeds the maximum ratings, as it is a P Channel MOSFET it works with 48V and (48*220)/(100+220) = 33, that is 15V difference so it isn't exceeding the absolute maximum ratings with 5V of tolerance. And I agree with the 48V too close to 55V, that's why I tested with 24V and got the same results.With 24V the voltage divider creates a 8V difference which is more than enough too.

author: Andy Watson
author_rank: Super Contributor
timestamp: January 16, 2017, 05:02:56 pm
content: What supply connects to the TL494 ? The datasheet says maximum collector voltage is 41V. I would not be surprised if the collector voltage is limited to the Vcc of the chip.

author: gumattos
author_rank: Newbie
timestamp: January 16, 2017, 05:20:18 pm
content: Quote from: Andy Watson on January 16, 2017, 05:02:56 pmHello, I have a regulator for the TL494, the value of Vcc in this case doesn't matter in this case, since de emitter is connected to ground and the TL494 will only activate or not the voltage divider. And if it was the problem, when I tested with 24V it should have worked. I am starting to think that as the SB360 is 60V and the MOSFET is 55V, the surges of the inductor could be affecting the MOSFET, but not sure.

author: capt bullshot
author_rank: Super Contributor
timestamp: January 16, 2017, 05:24:02 pm
content: Quote from: gumattos on January 16, 2017, 04:20:53 pmSorry, didn't see the voltage divider at my first reply. Must clean my glasses ;-)Next guess: usually you want to add a gate resistor in between the emitter followers and the MOSFET (in the range of 10 Ohms). Missing the resistor may cause parasitic oscillations. Sometimes it is better to have somewhat slower switching edges. I suggest starting with a 33 Ohms gate resistor. Do you have an oscilloscope to view the gate and switching node waveforms?

author: Andy Watson
author_rank: Super Contributor
timestamp: January 16, 2017, 05:32:02 pm
content: Quote from: gumattos on January 16, 2017, 05:20:18 pmIt matters if the collector voltage cannot rise above Vcc. The datasheet does not give details (not that I can see ?) but it does indicate that there is more than just a transistor collector connected to the "C1" pin.QuoteWas that the whole circuit that was operated at 24V, or was it 24V on top of Vcc?

author: gumattos
author_rank: Newbie
timestamp: January 16, 2017, 06:00:02 pm
content: Quote from: Andy Watson on January 16, 2017, 05:32:02 pmWith the 24V I removed the regulator so everything was running with 24V. As for the TL494, I set the confuration for the collector to connect to GND and as it is an NPN transistor with emitter connected to GND I don't know how it could change something. And if that was the reason, I don't see why it would work with IRF9540 and not with IRFR5305. Of course I could be wrong, but I don't see how the voltage on the base of a NPN transistor with emitter connected to GND change from 30V to 48V would affect the circuit.

author: Andy Watson
author_rank: Super Contributor
timestamp: January 16, 2017, 06:13:29 pm
content: Quote from: gumattos on January 16, 2017, 06:00:02 pmIt is not the GND that's the problem. When the TL494 transistors turn-off you need the collector voltage to rise all the way to the reference voltage of the mosfet (minus the threshold and R1, R2 etc.) - the mosfet is referenced to the +48V. If your collector voltages are limited to (say) 30V you still have 16/2V driving the mosfet gate - which is enough to keep the mosfet on and the output at full voltage - which I think is what you originally described as the problem. Note, I do not know for sure that the collectors are limited to Vcc - perhaps you could disable the TL494 and measure the voltage?

author: gumattos
author_rank: Newbie
timestamp: January 16, 2017, 06:25:56 pm
content: Quote from: Andy Watson on January 16, 2017, 06:13:29 pmHmmm, OK, I think I get it, I turned the TL494 off and indeed the voltage goes trough and I get 48V at the output, but the IRF9540 at the same conditions, TL494 turned off, the IRF9540 stays off and 0V at the output. And the real problem is, in both cases (IRFR5305 and IRF9540) the voltage at the gate is 48V, the same as Vcc so the MOSFET should be off in both cases and I am starting to think I damaged the IRFR5305 somehow, because Source at 48V and gate at 48V and the MOSFET is turned on, comething is not right here.