Switching p-ch mosfet with 5V

Hey,

my goal is to achieve an on/off switchable 12V voltage source meant for supplying some power mosfet gates out of 60V, controlled by a TTL output pin of a Attiny4, 85 or something like that.
This is my first approach, it’s a n- and a p-channel mosfet switching a 12V zener diode.
Because the voltage is 60V I limited Vgs of the p-channel mosfet with a 12V zener diode but because of this only 12V out of the 60V drop at R8 and therefore a high current will flow through the n-channel mosfet, how is the right way to fix that? Should I just put an resistor in there?

Thanks!

You can get buck converters with an input voltage range of 18V to 72V and various output voltages
including 12V - these are industry standard components.

Any attempt to use a dissipative power supply that drops 60V to 12V is likely to be a massive
heat source that means large heat sinks and fans, all more expensive than a little buck converter!

For instance a quick search found this: DC Less than 10 V Industrial Power Supplies for sale | eBay

At that sort of price its worth the risk that it might be poor quality - a fuse and a TVS diode might be worth
adding to the 12V rail to protect against buck converter failure.

Is the output of this to be applied TO a set of MOSFET gates, or is it meant to be a 12V supply for something else that will be switching those gates? The reason I ask is because of how you worded it: "+12V supply...".

When you say "supply" I think you mean a power source. I would call something that is feed to the gate(s) of a MOSFET, a "signal".

This makes a difference, because if it's a signal, then I would want to know things like: How fast does it need to switch, and how often, which would lead me to recommending a capacitor be placed across the 100k resistor. If it's a "power source", then I would recommend a capacitor across the Zener, and perhaps a lower resistance than 100k. And, in fact, if switching is frequent, that 100k might be too high in the "signal" case, as well.

Otherwise, the circuit is good.

In either case, the only real current demand is charging the gate capacitance on those MOSFETS.

Thank you for your input and time. It is a 12V signal, there will be only two mosfet gates connected (low side switch) so I think a buck converter is overkill. There will be no switching frequency, it will be turned on and stay on for about 30 to 60 minutes and then switched off.

However I'm concerned when the PWM1 Port (it will be used as DigitalPin) gets HIGH. The n-channel mosfet will get conductive pulling the gate of the p-channel mosfet to ground potential. Then 12V will drop at R8 but what happens with the other 48V? From my understanding they will try to drop at the n-channel mosfet which will cause a failure? Or not?

Why do you think you need 12volt gate drive to switch n-channel mosfets low-side.
Can't you use logic level mosfets?
Leo..

ArduRobotBuilder:
However I’m concerned when the PWM1 Port (it will be used as DigitalPin) gets HIGH. The n-channel mosfet will get conductive pulling the gate of the p-channel mosfet to ground potential. Then 12V will drop at R8 but what happens with the other 48V? From my understanding they will try to drop at the n-channel mosfet which will cause a failure? Or not?

Holy I didn’t notice that. Yup, a resistor from P-Channel Gate to the N-Channel Drain will take care of that. Another 100k should do it.

But, yeah… like Wawa said…why not just a Logic Level MOSFET, or are the MOSFET gates you are trying to drive, some kind of infrastructure that you can’t change, and they have a higher voltage threshold?

Even so, why not do it like this:

SwP-ChMOSFET__Figure01.png

Use a Logic Level MOSFET that can handle, say 100V. I set the Drain Resistor to 10k so when the MOSFET is off, close to 5ma will flow in the Zener – 5ma is a typical nominal current for achieving the target reverse voltage in a small wattage zener (like this will probably be) – but change as needed – in fact, you can probably get away with a lower current because the goal here, is not to produce a “regulated” 12V, but just to limit the max voltage to 12V, yet achieve a high enough voltage to turn the mystery MOSFETS on. Typically, the Zener knee is at something like 1ma, so around 2ma should work, too – unless those mystery MOSFETs have a very high Gate Threshold Voltage – or it’s important to get the channel resistance down as low as possible to limit power loss/mitigate heating.

When the Logic Level MOSFET is on [i.e. presenting a low impedance path to ground at the Drain], the Drain current will be around 6ma. So, this doesn’t have to be a beefy MOSFET. A ZVN2110A should do the trick.

I note that in the circuit diagram an PWM pin is being used to drive the output - if you are using
PWM you cannot use any kind of resistive supply to the gate, you need a good strong gate drive
capable of 50mA or more at 12V.

I'd suggest buck converter to provide the 12V, then a low-side mosfet gate driver chip to drive the
gate - mosfet drivers accept standard logic signals as input and can handle the large capacitive load
of a MOSFET gate and switch them rapidly (needed for PWM to avoid excessive switching losses).

The gate of a MOSFET is not a "signal", its a power connection (certainly for PWM), as the currents
involved are in the 50mA to several amps range (depending on the size of the MOSFET being driven and the
PWM frequency wanted).

Thanks a lot for all the suggestions!
Their Rdson should be as low as possible, that's why I would like to apply 12V to their gate.
Also the pin from the uC will be a digitalPin, I'm sorry for the irritating schematic.

@ReverseEMF
Thanks for drawing a schematic! I didn't mention that the uC will get supplied by a battery because I didn't think it would be relevant. According to your schematic, a digitalHigh is required at the uC pin to pull the gates of the power mosfets from 12V to ground and a current of 60V/10kOhm will flow. So the uC has to be powered and the 6mA will flow in order to switch the load off and keep it off right?

Hi,
This circuit;

If it is going to a high side P-CH MOSFET gate, it will not let the P-CH MOSFET turn off as the P-CH gate will not go to P-CH Source potential.
It will be held 12V above GND or 60 - 12 = 48V below the P-CH source voltage.

When the N-CH is turned OFF the drain of the N-CH and the P-CH gate need to go to P-CH source level to allow P-CH to turn OFF.

The addition of a resistor between N-CH drain and P-CH gate will fix you problem in your circuit in your original post.

Tom.... :slight_smile:

ArduRobotBuilder:
I didn’t mention that the uC will get supplied by a battery because I didn’t think it would be relevant.

Then I would change that 10k resister, that goes from the Gate [of the Logic Level MOSFET] to GND, to a 1Meg resistor [to reduce the power demand on the battery]. All this resistor does is discharge the gate capacitance, which is quite small.

ArduRobotBuilder:
According to your schematic, a digitalHigh is required at the uC pin to pull the gates of the power mosfets from 12V to ground and a current of 60V/10kOhm will flow. So the uC has to be powered and the 6mA will flow in order to switch the load off and keep it off right?

Not quite sure what your question is, so I’ll take a shot in the dark: When the PWM1 pin is set to HIGH, the [Logic Level] MOSFET will “switch ON” [i.e. the Drain to Source channel resistance will drop to a low value, allowing considerably more current to flow]. This will pull “Out” as low as it can go. This will, also, allow a current equivalent to [essentially, 'cuz we’re going to ignore the MOSFETs channel resistance, since it’s so low in relation to the 10k resistor in the Drain Circuit] 60V/10k = 6ma. When the PWM1 pin goes LOW, the MOSFET will turn off, and restrict current flow. But, current will still flow through the Zener, causing Out to rise to approximately 12V.

So, when PWM1 is HIGH, your mystery MOSFETs will be turned off, because the voltage at their gates will be nearly zero. When the PWM1 pin goes LOW, around 12V will be applied to the mystery gates, thus turning those MOSFETs on. What we have here, folks, is an inverter!

Also, when PWM1 is HIGH, 6ma will be drawn from the 60V source, and when PWM1 goes LOW, more like 4.8ma will be involved. Now, if that’s too much current, then the Drain resistor can, probably be changed to a higher value. The intention was to drive that Zener to near the “Nominal Point” – i.e. where the Zener will produce a solid voltage and be closest to the specified voltage, i.e. “12 volts”, and not waver for nothin’. But, like I said before, the goal here isn’t necessarily to regulate voltage, but to protect the gates on your mystery MOSFETs. So, the Drain resistor can be adjusted to a higher value until the Zener is just producing enough voltage to get those MOSFETs turned on as much as desired – BUT, to really engineer the s**t out of this, you will need to consider environmental conditions, and tolerances and such.

And, like I also said, there are zeners that can do the job at much lower current, but the only ones I could find are hard to work with ['cuz they’re SMD and all – in fact, one of the ones I use, is around the size of a flea!!]

TomGeorge:
Hi,
This circuit;

If it is going to a high side P-CH MOSFET gate, it will not let the P-CH MOSFET turn off as the P-CH gate will not go to P-CH Source potential.

If you look at ArduRobotBuilder's original schematic, it is, clearly intended to drive an N-Channel device, since the voltage will never go higher than 12V. But, you're correct. I should have asked if the mystery MOSFETs are N-ch or P-ch or some kind of combination of both, etc. ... @ArduRobotBuilder ?

Update: And it occurs to me that I assumed the Title of this topic, "Switching p-ch mosfet with 5V" was referring to the P-ch MOSFET in ArduRobotBuilder's schematic. So if those mystery MOSFETs actually are P-channel devices, that's a driver of a different color! And, it's important to verify that the Source on those two mystery MOSFETs are connected to that 60V supply. I'm sure they are, but I just want to be certain.

But, heck, I already drew it, so if those mystery MOSFETs are, indeed N-channel devices, then here’s another possibility:

SwP-ChMOSFET__Figure02.png

BUT!!!

This will make those mystery MOSFETs switch much more slowly, and that could be a problem if they are switching a lot of current, because they will be dissipating a great deal of power as they switch, and if they heat up too much, they could lose their “magic smoke” – [in other words, BURN UP!] [or, if they don’t BURN UP right away, their life span could be shortened, due to thermal stress]. Also, if that 60V supply can’t deliver the extra current needed during switching, it will either fail, or the 60V line will sag. A hefty capacitor on the 60V line can solve, or at least lessen the sag issue [but, coming up with the correct value will be a problem, unless you know how to do the math – and look up the specs]. The fact that they will be switching infrequently, makes this solution more feasible, but still, some engineering will be necessary. BTW: the values I selected came out of my a$$, which is why I supplied the formulas. To use them, pick one of the resistor values and then use the appropriate formula to find the other value.

So, it’s a tradeoff between switching speed and power demand. If that 60V supply is not from batteries, and if that 60V supply can easily deliver a few extra milliamps, then no problem – use the Zener with the 10k resistor. And change that 10k gate resistor to 1M [like I show in the above schematic]. Because, the part of the circuit being supplied by the 60V, will not draw power from the 5V circuit, and thus the batteries supplying that 5V will not be impacted.

Update: I re-read this post, and realized that it might not be clear. R1 is the only component drawing significant current from the MCU (e.g. the “Arduino”) [there is minute leakage current on the Q1 gate, but it is so small as to be negligible], and thus is the only component drawing power from the battery (that powers the MCU). R2, R3 and the Q1 Drain/Source path only draw current from the 60V supply, and thus, do not impact the battery that runs the MCU.

ArduRobotBuilder:
Their Rdson should be as low as possible, that's why I would like to apply 12V to their gate.

"as low as possible" is meaningless. What exactly are you switching.
Getting 12volt from 60volt is just silly without a valid reason.
There are plenty of logic level mosfets with very low Rds(on).
Leo..

Wawa:
“as low as possible” is meaningless.

Actually, it’s not meaningless. The higher the gate voltage, the lower the channel resistance (until, of course, saturation, or the Gate fails). We can’t know for sure if the MOSFETs he/she’s talking about saturates at a VGS of 12V. And, when switching large currents, every bit of channel resistance reduction is significant.

Have a look at a typical MOSFET datasheet. Typically there are graphs that show the VGS to RDSON relationship, either directly, or by the slope of the IDS vs VDS lines (each line being a different VGS).

Wawa:
Getting 12volt from 60volt is just silly without a valid reason.

I’m getting the impression this person is dealing with existing “equipment”, thus, the “valid reason” is the infrastructure.

Wawa:
There are plenty of logic level mosfets with very low Rds(on).
Leo…

If I understand this persons requirements, correctly, it’s these two MOSFETs (I call them the “mystery MOSFETs”), NOT depicted in his diagram, that we’re talking about. And apparently, these are NOT Logic Level MOSFETs – the thing up for discussion, currently, is whether this person can use a Logic Level MOSFET to drive these two NON-Logic Level MOSFETs [that apparently already exist, in some piece of equipment, that is powered by 60VDC].

The reason I think this, is because on this person’s diagram, it shows an “Output” labeled “+12v supply for mosfet gates”. And on at least a couple of occasions, this person directly referred to the “two MOSFETs”.

Another thought. Are you sure 12V is well enough below the VGS Absolute Max Voltage [i.e. the breakdown voltage]

Another thought. Are you sure 12V is well enough below the VGS Absolute Max Voltage [i.e. the *breakdown voltage*]? Do you know what the Absolute Max VGS is? And, in fact, do you know what the part number is on these mystery MOSFETs [should have asked that a long time ago].

A big thanks to everybody and especially @ReverseEMF!
I've figured everything out to go on.
The powermosfets are indeed n channel (I mentioned low side switch somewhere), the partnumber is FDBL0150N80 and they have a max. Vgs of +-20V, so I see no problem there.

By the way, if you don't mind a quick question regarding something else.. if I generate a voltage with a zener diode e.g. 5V (for signal), does it make sense to implement a TVS diode next to the zener diode to protect sensitive devices wired after it? Or does the zener diode ensure or kinda act as TVS diode anyway? I am familiar with the parameters and values of TVS diodes and how to choose one, I'm just not sure if it would make sense.

Thanks!

ArduRobotBuilder:
A big thanks to everybody and especially @ReverseEMF!
I've figured everything out to go on.
The powermosfets are indeed n channel (I mentioned low side switch somewhere), the partnumber is FDBL0150N80 and they have a max. Vgs of +-20V, so I see no problem there.

By the way, if you don't mind a quick question regarding something else.. if I generate a voltage with a zener diode e.g. 5V (for signal), does it make sense to implement a TVS diode next to the zener diode to protect sensitive devices wired after it? Or does the zener diode ensure or kinda act as TVS diode anyway? I am familiar with the parameters and values of TVS diodes and how to choose one, I'm just not sure if it would make sense.

Thanks!

Cool -- glad we were of help!

One thing, though -- it would be great if you asked your supplemental question in a new thread -- that way, others will have an easier time finding it [should they, also, be seeking info on this topic] :wink: