This is my first time designing a High voltage switch, as come up with some interesting thing that I want to ask about. My application needs to short out a signal line to HIGH, with a 3v3 signal from the MCU. Very little current needs to pass through to I thought well, a MOSFET will do! and designed a circuit with 2 N-FET. But the problem was that there was some voltage drop across the line which was big enough(4V for 52V supply) to keep the signal to be realized as HIGH. enter image description here

This is the first circuit.

So after some research I designed a circuit with a N-FET, NPN which work like a charm on PSPICE. enter image description here

enter image description here

The 3V3 signal from the NPN was enough to drive a 2n7000, which got me to build the circuit and test it. But, the output from the actual circuit is when 3.3V square wave goes in, I get 4.5V when the high side voltage is 56V.

So I moved to TINA-TI for another simulation, which gave me a totally different result from PSPICE and my experiment. Here is the schematic and the sim result.

enter image description here

enter image description here

You can see that the FET doesn't even switch at all.

Now I realize that the part i have used(RK7002BM) whichis a 60A, 250mA small signal mosfet might not be getting enough Vgs for a Vds of 56. P-channel is not an option, since I need to make this work some how within a few days, and I don't have any lying around at the moment.

I have considered using relays, but all I have around are rated for 30V so no option here too.

So in conclusion the question is 1. Why would my simulation results be different between themselves and the experiment. Would it be simply due to the different parts? 2. Any help on how to get this thing working? The application, embedded layer is almost done and I really need to get this circuit done fast.

Thank you for baring with such a long question.

Thank you for the comments. I don't know what I was thinking when I wrote this, I was overwhelmed by the fact that I am still working on a Sunday 4pm from Monday, never got a chance to get home....

To clarify, I am trying to build a high voltage, low-side FET switch.

The high voltage is the 50V/56V, that I wish to toggle on/off. I was trying to achieve it through drain of the FET. However, OptoFETs like this seems to do the job. I have tried with PowerFETs, but the resistor got burnt. This is the circuit with IRF830

This is the circuit with IRF830

I am kinda clueless to what is going on......

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    \$\begingroup\$ I don't see any high side switching here \$\endgroup\$ – Mike Feb 3 at 5:44
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    \$\begingroup\$ Your text talks about "high side" switching, but all your schematics show the MOSFET in the low side position --- with source connected to ground. You talk about a "signal line" but it's not clear in your schematics which node this refers to. Please edit your question to clarify these points. \$\endgroup\$ – The Photon Feb 3 at 5:49
  • \$\begingroup\$ I get that you do NOT have any PFETs. But do you have any PNP BJTs capable of easily standing off 60 V? (MPSA92 or MPSA93?) \$\endgroup\$ – jonk Feb 3 at 7:15
  • \$\begingroup\$ My sorry stock of bench supplies is down to NPN/NFET. Big reason I am struggling is it is holiday time in my country, will not get any supplies till the end of next week, BUT I have to somehow get this build into a report. Thank you all for the comments, I have made it more clear. \$\endgroup\$ – Jung Haemin Feb 3 at 7:19
  • \$\begingroup\$ @jonk the last picture is my desperate attempt. The problem here is that I CAN pull up the signal to the supply voltage with a little bit of drop, but when the system turns on the signal line is pulled low, requiring a low impedance connection with the supply. I am thinking of getting an optoFET.... \$\endgroup\$ – Jung Haemin Feb 3 at 7:23

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