# Back-to-back MOSFET (source connected) as a overvoltage protection

#### sunney

Joined Sep 3, 2015
23
Hello,

I am planning to use below circuit as voltage clamping for voltages > 12V up to 28V.
And it does as expected (at least in simulation) however I am not understanding its working properly:

Here are my doubts:
1. As per below configuration, Vg = 14V
Vgs (M1/M2)= ~2.1V,
VTH (M1/M2) = 1.65V (from the datasheet of RSQ015N06, attached Vth vs temp graph below)
VDS(M1) = 16V
VDS(M2) = -67mV

For Cutoff:
Vgs<Vth (which is not the case, so both M1 & M2 are not in cutoff)

For Linear:
Vgs > Vth [true for both M1 and M2]
Vds < Vgs - Vth [False for M1 , true for M2?? (I am referring simulation results)]

For Saturation:
Vgs > Vth [true for both M1 and M2]
Vds > Vgs - Vth [True for M1 & False for M2]

From above conditions;
M2 is in linear mode and M1 is in saturation?? This doesn't look right, I am not sure what I am missing.

However as per functionality, M1 should be ON, M2 should be OFF and body diode of M2 is ON in order to complete the circuit.
And M1 should be in Linear mode for dropping the voltage?

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#### Ian0

Joined Aug 7, 2020
3,760
M1 will act as a source follower. The voltage at the source will be the voltage at the gate minus Vgs(th).
M2 will have the same enhancement voltage as M1, so it will be partially turned on, but it has a diode in inverse parallel.
Together they will behave much the same as if M2 were replaced by a piece of wire.

#### LowQCab

Joined Nov 6, 2012
1,341
A better explanation of what You are trying to achieve would get more responses.
Are You trying to create a Voltage Regulator ?
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#### sunney

Joined Sep 3, 2015
23
A better explanation of what You are trying to achieve would get more responses.
Are You trying to create a Voltage Regulator ?
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Hi LowQCab,

I am planning to use this circuit on a signal line which has to protected against STB up to 28V.
Now I had tried other methods too, like using series resistor and Schottky diode or zener diode, but they are not suitable as I cannot put more than few ohms (i think even 50ohm might create accuracy issues) of series resistor as it may create problem on accuracy. This signal line belongs to O2 sensor.

#### LowQCab

Joined Nov 6, 2012
1,341
What is "STB" ?
Oxygen Sensors can be modeled as a very poor Battery-Cell,
with a lot of Internal-Resistance.

How You treat its Output will also depend on whether it is a
"Narrow-Band" , or a "Wide-Band" Oxygen-Sensor.
For a Wide-Band-O2-Sensor you should use ONLY a commercially available,
completely self contained,
Sensor and Interface-Unit which has a self-calibrating feature,
with both Digital, and Analog-Outputs.

It sounds like what You are trying to achieve is Input-Protection for an Amplifier,
in that case, there are easier ways of accomplishing that end.
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#### sunney

Joined Sep 3, 2015
23
STB is Short to Battery.
Yes, it is for input protection for an amplifier, its a wideband sensor and it has its dedicated interface ASIC.

#### LowQCab

Joined Nov 6, 2012
1,341
What are You doing for Heater-Regulation and Calibration of the Output ?
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#### sunney

Joined Sep 3, 2015
23
What are You doing for Heater-Regulation and Calibration of the Output ?
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The ASIC CJ125 has calibration of output feature and heater is being driven from its own dedicated heater control circuitry (different from ASIC) however ASIC has diagnostic features for the heater. there is one dedicated output from the ASIC which can be related to sensor temperature and then that information is used to regulate the heater by controlling control PWM signal.

#### LowQCab

Joined Nov 6, 2012
1,341
It sounds like You've got things figured out.
I would use a string of Diodes for Input Protection,
but on the other hand,
I would never Trust my Engine to a questionable Wide-Band.
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