Anti-series MOSFETs as AC switch in LTSpice

Thread Starter

toby1234321

Joined Jun 23, 2022
9
I am trying to use two MOSFETs in anti-series as an AC switch. I have some problems turning the switches on and off, most likely because of the different source potentials and the fact that V_ds becomes negative. Any to suggestions for how I can fix this in LTSpice? I have tried to find similar projects, however, there seems to be a lack of projects containing this kind of switch.

The goal is to block current through the left branch for the first 0.2s and then through the right branch from 0.2-0.8s. The final circuit will contain even more switches, but I have to figure out how to turn the switches on/off first.
 

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Papabravo

Joined Feb 24, 2006
19,578
Q: What is the purpose of R1 & R2 at a value of 99 MegΩ?

The reason I ask is because of the following VERY explicit statement in the datasheet

GND (Pin 4):
Driver Ground. Connect to a low impedance ground. The VCC bypass capacitor should connect directly to this pin. The source of the external MOSFET should also connect directly to the ground pin. This minimizes the AC current path and improves signal integrity.
 

ericgibbs

Joined Jan 29, 2010
16,790
hi,
I assume you have allowed for the internal Reverse Voltage diode for Drain/Source.?
Are you try to switch the full sine wave.?
E
 

Papabravo

Joined Feb 24, 2006
19,578
Even if there was a reason for R1 & R2 of 99 MegΩ in your schematic they render the device completely incapable of driving a load which you can see from the following plots
1669137385813.png
This is not even to mention that swapping the polarity of the drain and the source will turn on the N-channel device when the source drops below the gate voltage by more the Vth(gs). You should review basic NMOS operation as a minimum.
 

crutschow

Joined Mar 14, 2008
31,123
I have some problems turning the switches on and off, most likely because of the different source potentials and the fact that Vds becomes negative.
Yes, it's a little tricky to control the MOSFETS because of the plus and minus AC voltage, since, to keep them off, the gate voltage for the MOSFETs must follows the AC voltage to always keep Vgs at 0V.

Below is the LTspice simulation of one way to do that without using an opto isolator (assuming the 60Vac is isolated by a transformer from the mains):
It uses a high-voltage (must tolerate the peak AC voltage), PNP grounded-base transistor to switch the control signal.

When V(in) is low and Q1 is off, its collector does not conduct for negative collector voltages.
The gates are connected to the source through R2 so the gate tracks the negative AC voltage (blue trace) that comes through M1's substrate diode (M1 biased off), keeping the gate-source voltage at 0V (yellow trace) for the negative portion of the AC voltage.
The positive AC voltage is blocked by M1, with R2 connected to ground by M2's substrate diode, keeping the Vgs again at zero for the positive portion of the AC voltage.
The MOSFETs thus stay off, independent of the AC voltage value or polarity.

When V(in) goes high and Q1 is turned on, the input voltage is applied to the gates [Vgs = V(in)] turning them on.
This works, of course, since a MOSFET conducts equally well in either direction when turned on.

Note The one limitation is that, for this circuit to work, it must have a floating load with the MOSFETs connected to ground.
I assume that's not a problem to drive the motors.

1669162867437.png
 
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Thread Starter

toby1234321

Joined Jun 23, 2022
9
This is not even to mention that swapping the polarity of the drain and the source will turn on the N-channel device when the source drops below the gate voltage by more the Vth(gs). You should review basic NMOS operation as a minimum.
I know.. That´s what I am trying to solve. I have tried a lot of different circuits now, the one I sent with 99Meg resistors was my latest try.
 

Thread Starter

toby1234321

Joined Jun 23, 2022
9
Yes, it's a little tricky to control the MOSFETS because of the plus and minus AC voltage, since, to keep them off, the gate voltage for the MOSFETs must follows the AC voltage to always keep Vgs at 0V.

Below is the LTspice simulation of one way to do that without using an opto isolator (assuming the 60Vac is isolated by a transformer from the mains):
It uses a high-voltage (must tolerate the peak AC voltage), PNP grounded-base transistor to switch the control signal.

When V(in) is low and Q1 is off, its collector does not conduct for negative collector voltages.
The gates are connected to the source through R2 so the gate tracks the negative AC voltage (blue trace) that comes through M1's substrate diode (M1 biased off), keeping the gate-source voltage at 0V (yellow trace) for the negative portion of the AC voltage.
The positive AC voltage is blocked by M1, with R2 connected to ground by M2's substrate diode, keeping the Vgs again at zero for the positive portion of the AC voltage.
The MOSFETs thus stay off, independent of the AC voltage value or polarity.

When V(in) goes high and Q1 is turned on, the input voltage is applied to the gates [Vgs = V(in)] turning them on.
This works, of course, since a MOSFET conducts equally well in either direction when turned on.

Note The one limitation is that, for this circuit to work, it must have a floating load with the MOSFETs connected to ground.
I assume that's not a problem to drive the motors.

View attachment 281240
Thank you for a very good solution. I tried it for two switches supplied by the same voltage, and it worked perfectly. However, in the circuit I am working on there is unfortunately not possible to connect all MOSFETs to ground. I have attached the circuit with your solution for turning on/off the switches. I know why this isn't working properly as of now, but I don't know how to fix the problem that the MOSFETs blocks for half a cycle...

The purpose of the circuit is to switch between series and parallel. So for the first half, I want the middle switch to conduct and the bottom and upper switch to conduct for the second half.
 

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crutschow

Joined Mar 14, 2008
31,123
I want the middle switch to conduct and the bottom and upper switch to conduct for the second half.
Well, that's a new requirement that was not originally stated.
Always post all your requirements, not just part of them, to avoid us going off on a tangent.
If I had know that ahead of time, I won't have bothered to post my circuit. :(

Easiest way is likely just to use three SSRs (solid state relays).
 
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