I just took a standard N-MOSFET part in the simulator tool by just clicking the "N" key. But in any case, could you please explain me how the circuit should behave and what voltages should appear at the nodes (Zener Cathode & Zener Anode) in normal working conditions?Apparently the transistor models used in your simulation have quite different parameters than specified in the linked datasheets. Which MOSFET parameters did you set?
What I mean by Normal working condition - Both the MOSFETs are turned ON.What is normal working condition? The simulation results are correct for the used small signal MOSFET models. Review Falstad explanation of MOSFET parameter, for a power transistor you'll use a beta value several magnitudes larger than default 20m, e.g. 10 to 80.
It may be easier to use a full featured circuit simulator like LTspice, that has various built-in power MOSFET models.
SirI have the below circuit in which I am trying to perform simulations.
Simulation Tool used - Falstad Simulator - Falstad
12V Zener Diode - Datasheet
Top MOSFET - Datasheet
Bottom MOSFET - Datasheet
Question 1 :
The Zener diode is 12V Rated.
I want to simulate a current of 12A through both the MOSFETs
So, for my 16V input voltage, I calculated my Resistance to be 1.33Ohms to get a current of 12A. But when both the MOSFETs are ON, my drain current through them is only 91mA. Can someone tell me what is the problem and why am I not getting a drain current of 12A? And I tried to change the Rds(on) MOSFET parameter in the tool, but not able to find and change it.
Next Question :
When I apply a negative voltage of -14V to the gate of the bottom MOSFET as shown below, I get a voltage at the Zener cathode as 2.195 and a voltage between the MOSFET nodes as 2.7V with 6.4mA of current?
Can someone tell me how this 2.195V and 2.7V is appearing? Just want to understand the circuit behaviour in these conditions. Please help.
Can you tell me how am I getting the 2.7V at the drain of the bottom MOSFET?Hi,
It looks to me like the top transistor doesn't turn on enough, so the junction of both has a low voltage and current. Or top not very on and bottom fully on.
Try the +16V on the gate of the top NMOS, possibly both need >10V to fully turn on. Might need e.g. 100 Ohm resistors on the gates rather than 1k and 10k. And top NMOS may need bootstrapping.
Why a negative voltage on the lower NMOS gate? It's presumably an enhancement mode device in the simulator.
Agree that simulators with models of devices with datasheets make learning and making easier. You can put voltmeters and ammeters everywhere in some simulation tools, run transients, and interpret the results at your ease.
Thank you for the answer. I just want to understand, when I apply a negative voltage of -14V at the Gate of the bottom MOSFET through the 10k series resistor, how is the voltage at the zener (12V Zener diode) cathode and zener anode and MOSFET gate is determined? Could you please explain that analysis.Hi,
For a raw estimation:
See the drain-source as resistance.
Then two resistors are in series, firming a voltage divider.
The resistance value depends on V_GS. So different V_GS (for each FET) result in different resistance values.
This results in unequal voltage across the FETs (drain-source)
In detail the upper resistor (1.3) and the load current through the resistor (82k) will have influence on the resulting voltage.
(Schematics without clear part names makes it difficult to discuss)
Klaus
Can you tell me how am I getting the 2.7V at the drain of the bottom MOSFET?
when I apply a negative voltage of -14V at the Gate of the bottom MOSFET through the 10k series resistor, how is the voltage at the zener (12V Zener diode) cathode and zener anode and MOSFET gate is determined?
Thank you for the answer. Could you tell me how did deduced that the MOSFET is a small signal MOSFET with a low saturation current? Please tell me how you arrived at this decision? (Just want to learn and understand how you figured it out)Reviewing the previous posts, I'm a bit disappointed that nobody thinks about MOSFET parameters. The top MOSFET won't fully turn even with 16V gate voltage because it's a small signal MOSFET with rather low saturation current.
Two aspects of the same problem. Z-diode is in forward bias, 0.6V voltage drop. 5V at upper MOSFET gate - 0.6V is equally shared between both transistors, Vgs of 2.2V gives the observed 6 mA Id. That's it.
The OP already asked how to adjust R_ds_on parameter in Falstad (post#1). So I thought he is aware of that the simulation parameter does not match datasheet specification. (Now after post#14 I'm not sure about this)I'm a bit disappointed that nobody thinks about MOSFET parameters.
See post #4 above and learn about MOSFET parameters in Falstad.Could you tell me how did deduced that the MOSFET is a small signal MOSFET with a low saturation current? Please tell me how you arrived at this decision? (Just want to learn and understand how you figured it out)
Although limited to a very simple MOSFET model, it can.In the end ... if you can't adjust Falstad for useful R_ds_on it simply can't simulate your circuit.
Maybe try LTspice...
Thank you for the answer.Hi,
Let me try and analyze the circuit with -14V connected to its low-side MOSFET gate. Analyses of the other circuit can be deduced from this one.
This analysis is made with an assumption that the MOSFET models used in the simulator is made to replicate the characteristics of the ones you presented datasheets. If not, if the gate-voltage thresholds are close to those on the datasheet, the analysis could still be valid.
The Low-side MOSFET is OFF with the -14V connected like that. Although the forward biased zener is supplying charges to its gate, the voltage at that gate, with reference to its gate threshold voltage, is just starting to turn the MOSFET ON. The voltage at the gate, which is the same as the zener diode cathode voltage is 2.195V. the voltage at the drain, which is the same as the zener diode anode voltage is 2.7V. That means that the diode forward voltage is 2.7V-2.195V=0.505V.
The gate threshold of the high-side MOSFET is 2V. Since we are having 2.7V at its source and approx 5V at its gate, then it's Vgs is 5V-2.7V=2.3V, barely 0.3V above the threshold, and barely turns the MOSFET ON, leaving it in the triode region. In the triode region, Rds is high. So since we have approx 16V at its drain, the Vds is 16V-2.7V=13.3V.
The 6.4mA current that you have is a combination of the currents conducted through the high-side MOSFET biasing circuit and its drain current. For the gate bias circuit, the current is (5V-2.7V)/101kohm = 22.77uA. Its drain current is then 6.4mA-22.77uA = 6.377mA. This means that this biasing arrangement leaves the high-side MOSFET with an Rds of the 13.3V/6.377mA = 2085ohm.
The current flowing through the Low-side MOSFET gate resistor is (2.195V-(-14V))/10kohm = 1.62mA. This means that the current flowing through its drain is 6.4mA-1.62mA = 4.78mA. Thus, t
Rds of the Low-side MOSFET due to this bias is 2.7V/4.78mA = 565ohm
This is just my basic analysis and is subject to confirmation.
--- Updated ---
L
I will try and do that later.Thank you for the answer.
I just have 2 doubts with your answer.
1. In the simulation with the negative -14V at the gate of the Low side MOSFET, how is the voltage 2.7V arrived at the Source of the High Side MOSFET (or the drain of the Low Side MOSFET)? Could you please provide a KVL with the circuit if possible? (It would really help me in the understanding of the concept)
One important point to note here is that all voltages in the circuit are DC. Please bear this in mind as we proceed.2. The 1.62mA you have calculated, does this current flow into the gate of the Low side MOSFET or does it flow to the -14V? Just want to understand how the current splits from the Zener cathode to the -14V and the gate of the Low side MOSFET.
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