How did I blow up my voltage level converter???

Thread Starter

SpiderSpartanju

Joined Apr 10, 2009
80
I have a BIGTREETECH SKR 2 3D printer control board I'm trying to use to control a 24V, 100mA fan. The microcontroller has a pwm output at 3.3V that connects to a 74LVC1G125 voltage level converter. I copied a mosfet circuit already built into the board for powering 3 other fans. This circuit should take the 5V pwm output from the voltage converter to trigger an AO3400A mosfet gate, allowing 24V across the fan.
Full PWM Circuit.JPG
When I first plugged this circuit into the control board the fan ran for 5-10 seconds and then cut out. I cycled power to the controller, but the fan never came back on. I hooked my circuit back up to my benchtop supply and checked that my the current being drawn from the level converter (through PE6) is 50 microamps, well below the 74LVC1G125 continuous output current rating of 100mA. I have a new voltage level converter ( MC74VHC1GT125DT1) on the way, but I don't want to just plug back in and fry another one.

I'd appreciate any ideas on what might have gone wrong here.

Here's some pictures of the circuit I built for the external mosfet.
 

Alec_t

Joined Sep 17, 2013
12,808
The 74LVC1G125 datasheet (which describes the device as a buffer/line-driver rather than a voltage-converter) says the logic-high voltage is 0.7 x Vcc, i.e. >= 3.5V in your case. Your PWM input is only 3.3V. Maybe you were lucky when you first powered up your circuit and the logic high threshold was exceeded.
Per that datasheet, the logic-high output voltage may be as low as 3.8V with a 4.5V supply, so likely not much more with Vcc=5V. That is not enough to turn your specified MOSFET fully on. It needs about 10V for that.
The MOSFET has a rated Vds of only 30V. That doesn't leave much headroom for switching a 24V fan.
Your schematic shows no spike-suppression components. Perhaps fan switch-off generated a spike which killed the 74LVC1G125.
If the MOSFET didn't turn on fully it would be running hotter than it should. Are you sure it's the 74LVC1G125 which has failed and not the MOSFET?
 

Irving

Joined Jan 30, 2016
2,668
That is not enough to turn your specified MOSFET fully on. It needs about 10V for that.
Not so, the AO3400 is characterised at 48mOhm max Rds(on) at Vgs=2.7v and Id=3A so well within spec for this (though I agree I'd go for a 40v device at 24v VDD).

Arguably the buffer isn't needed. At the 1Khz pwm rate (another thread by OP) there will be negligible heating in the AO3400; At a typical 3.3v gpio output of 4mA the turn on rise time of the MOSFET is around 500nS, which gives the switching losses at 24v/1A as 24mW approx and Rds(on) losses at 48mW = 72mW total giving a junction temp worst case of 18 degC above ambient, nothing to worry about.

Its possible that spikes from the fan exceeded the MOSFET Vds rating of 30v but thats unlikely to have a repercussion on the buffer. A diode across the fan wouldn't hurt (bar end to +24v).

Not sure where the OP got output current of buffer as 100mA from, its 25mA absolute max, 8mA typical at 5v, but there's nothing in this circuit that would suggest a problem with using that buffer.
 

Thread Starter

SpiderSpartanju

Joined Apr 10, 2009
80
The 74LVC1G125 datasheet (which describes the device as a buffer/line-driver rather than a voltage-converter) says the logic-high voltage is 0.7 x Vcc, i.e. >= 3.5V in your case. Your PWM input is only 3.3V. Maybe you were lucky when you first powered up your circuit and the logic high threshold was exceeded.
Per that datasheet, the logic-high output voltage may be as low as 3.8V with a 4.5V supply, so likely not much more with Vcc=5V. That is not enough to turn your specified MOSFET fully on. It needs about 10V for that.
The MOSFET has a rated Vds of only 30V. That doesn't leave much headroom for switching a 24V fan.
Your schematic shows no spike-suppression components. Perhaps fan switch-off generated a spike which killed the 74LVC1G125.
If the MOSFET didn't turn on fully it would be running hotter than it should. Are you sure it's the 74LVC1G125 which has failed and not the MOSFET?
Thanks for the response. That's an interesting about the 74LVC1G125 output voltage. The other pwm fan ports are fed by a 74HCT365, which looks like it outputs around 4V for a high signal as well, if I'm reading the datasheet correctly. This port is designed to feed a neopixel LED strip. It has 5V(which I'm not using), the pwm signal, and GND. I'll have to hook up and check the pwm signal voltage better. It's in a pretty tight spot on the board and maybe I wasn't making a good connection with my meter.

I don't think it was an issue with the mosfet failing, because when I hooked it up to my benchtop supply with ground connected and 12V on the LED the LED turned on and off when I applied 5V to the gate. Any recommendation on a simple spike-suppression? Small zener in the opposite direction of the LED maybe?
 

Thread Starter

SpiderSpartanju

Joined Apr 10, 2009
80
Not so, the AO3400 is characterised at 48mOhm max Rds(on) at Vgs=2.7v and Id=3A so well within spec for this (though I agree I'd go for a 40v device at 24v VDD).

Arguably the buffer isn't needed. At the 1Khz pwm rate (another thread by OP) there will be negligible heating in the AO3400; At a typical 3.3v gpio output of 4mA the turn on rise time of the MOSFET is around 500nS, which gives the switching losses at 24v/1A as 24mW approx and Rds(on) losses at 48mW = 72mW total giving a junction temp worst case of 18 degC above ambient, nothing to worry about.

Its possible that spikes from the fan exceeded the MOSFET Vds rating of 30v but thats unlikely to have a repercussion on the buffer. A diode across the fan wouldn't hurt (bar end to +24v).

Not sure where the OP got output current of buffer as 100mA from, its 25mA absolute max, 8mA typical at 5v, but there's nothing in this circuit that would suggest a problem with using that buffer.
Thank you for the detailed response. I just checked again and it looks like the buffer is fine and this may be a controller config issue. On the local HMI I was able to get the board output to turn on outputting 5V which triggered my circuit and turned on the fan. I am not able to get the fan working through the web interface, but I'll have to dig through code to work that out.

As for the surge-suppression diode, I shouldn't need anything more than the 100mA fan rating, right? Should I be using a zener, schottkey, switching, or just simple silicone diodes? See any issues with a 1N5818 Schottky diode? It has a forward voltage drop of 0.5V and a forward current rating of 1A with a reverse voltage rating of 30V. Thank you again.
 
Last edited:
Top