My aim is to boost a 3v DC input voltage to an output DC voltage of around 12 to 15v. Based on the schematic I have designed this should work theoretically with the components I have chosen. Right now it seems as though the circuit isn’t doing much at all, I believe the issue lies with the MOSFET I have chosen – perhaps the voltage at the gate is too low? currently I am feeding a square wave input at 1MHz, 5v amplitude through to the gate. Can anyone spot the issue based on the schematic?

One of my MOSFET source-drain shorted after I connect them in parallel?

So I was making an ARC speaker yesterday, and I used one MOSFET ( irf540), and it got really hot, but it did not blow up, so I decided to put another MOSFET connected in parallel, and before I connected them I have tested both MOSFET and both of them worked normally, but when I connected them in parallel, the whole setup shutdown immediately due to overloading, and I found that one of my MOSFET was source-drain shorted that’s why the power supply shut down to protect itself.

How do you connect the MOSFETs?
Just normal parallel circuit gate to gate, drain to drain, source to source, .connected with a piece of wire

PWM controller?
TL494CN, powered by a battery, completely isolated from the MOSFET side

MOSFETs?
IRF540 from IR

As you can see in this video there are two MOSFETs on the heat sink but the lead was disconnected because it was shorted.
By the way this video is not specially made to show the setup so it would be more focused on the fly-back.

Here is the full schematic and the PWM board.

I am trying to build the v4.3 circuit described here: Will this schematic work?

I am building just one branch out of the decoder IC to test on a breadboard. Component info is as follows:

1. The optocoupler is PC817 i.s.o TLP621;
2. The MOSFET is IRF630B;
3. The flyback diode is 6A4;
4. The resistor between MOSFET gate and ground is 5.2K;
5. The peak power rating of the pump is 2.7A @12V.

Now when power is on, RasPi is up and the input to the decoder is set correctly, the pump does not work.

I tried the following to troubleshoot,

1. Connected the pump directly to 12V input, it works. Connected a multimeter in series and found the current draw to be approx 3A to start and then dropping down to 1.8A;
2. Connected an LED via a suitable resistor across the output side of the optocoupler (no mosfet/pump) and it turns on and off depending upon logic input to the decoder;
3. Removed LED and measured the voltage across the output side of the opto-coupler to be ~12V;
4. Added the mosfet and measured the voltage across the drain and source with correct logic input to decoder. This measured ~11-12V;
5. Added pump (w/o flyback diode). Now with correct logic input to decoder, it does not work;
6. Connected a multimeter (in current reading mode) in series between the cathode of the pump and drain. It measures only 0.63A;
7. Replace the pump with LED and proper resistor and enable the decoder, the LED lights up.

It appears to me that connecting the pump via the MOSFET somehow impedes the current flow to a great extent even when it is biased according to specs.

I thought that the MOSFET in question is a voltage controlled device. So when a proper $V_{gs}$ is applied, it should simply conduct the current required by the load.

Why does it conduct only 0.63A when the starting current requirement is around 3A? Am I missing anything?

I need to run 12v motor with using Raspberry Pi. I made a circuit which works but then I noticed a “little” problem. Pi’s software PWM signal is only 3 volts so BUZ11 doesn’t open enough.

I found this schematic:

Is it possible to somehow crank my PWM signal up? Ex. Using that schematic given with inverted pwm? Or can I just use transistor before FET to raise pulse voltage?

I am using an Allegro Microsystems A4935 3-phase MOSFET driver in a brushless speed controller. I recently got the motor spinning, but when I began increasing the voltage, my commutation started to become jerky, for reasons I suspect to be in software.

What I’m confused about is that, when this happens, both my MOSFETs and MOSFET driver heat up. At one point I released the magic smoke in the driver, and eventually the VBB trace (high voltage, supply for gate driver circuitry) going into the MOSFET driver even burned and broke! I understand that whenever the motor gets stuck, it causes large amounts of current through the MOSFETs, but why does this cause the MOSFET driver to heat up also?

The resistors I have at the gates of my IRFS7530 MOSFETs are 4.7Ω, and I’m driving the PWM at 25kHz. The driver only gets a little warm at no load driving the capacitance of the MOSFETs. My question is, what causes this stress on the driver when there is a load? Why does sudden high current flowing through the MOSFETs cause the MOSFET driver to need lots of current? Isn’t the only load on the driver due to charging the gates, which is independent of the current through the MOSFETs?

Here is the schematic for the relevant part of the circuit:

How would you choose Vb and bias voltage for M1 to make M1 and M2 operate in saturation mode for the configuration below:
M1 saturation: Vout – Vb < Vt1
M2 saturation: Vin – Vout < Vt2
How to calculate voltage swing of the circuit?

I’m salvaging parts from old computers, read the datasheet of those parts and try to understand the important things build stuff. While i now understand some crucial things about mosfets there is alot that i need to learn. Anyway i’m playing with the tlc5940 (16ch 12bit led driver) and i wanted to create a “animated” illumination for my room. I already created some led drivers with transistors , but also mosfets. Not logic level mosfets. To do so i needed to build a complex “Saturation” circuit involving other 2 transistors per Mosfet.

In my local electronics shop they on ly have higher voltage mosfets. Also on the websites i normally buy parts.

The salvaged mosfets are all N-type Logic level … so perfect both for the raspberry and the Arduino.I can saturate them correctly, there is almost no voltage drop, they are really fast and they sink alot.

My question is:

Looking at the datasheet i could not find that big difference regarding each different mosfet. But they all have another number.I tought the amperage but some lower number mosfets have more amps.

05N03L  = 80 Ampere
07N03L  = 30 Ampere
15N03L  = 42 Ampere
32N03L  = 50 Ampere
46N03L
55N03L
60N03L

So what are those 2 numbers in front of the code?

N means N-type, L means logic level … 03?

And most important question:

If i use different types of those mosfets with the tlc5940 could there be a SERIOUS problem or would that work nicely?

note: pullup 10k each ch, tlc with 6,8k resistor to let just sink enough to activate the mosfet, keep all channels under 3A.

Sample circuit using arduino pins

^{simulate this circuit – Schematic created using CircuitLab}

And here using TLC5940NT

^{simulate this circuit}

Here is a sentence from Design of Analog CMOS Integrated Circuit that I don’t quite understand:

“Note that in contrast to bipolar transistors, a MOS device may be even on if it carries no current.”
Could you explain this?
Ron = Vds/Id
If there is no current then Ron is infinity.
How is it possible for the device to be on?

I’m having trouble with TC4421A MOSFET drivers in a new PWM DC Heater design blowing. The 12V power supply used during testing is capable of 25A. While doing my initial testing, I started off with higher value load resistors with a 5 kHz PWM duty cycle of 25%. I successfully tested the design at up to 4A. That’s when the TC4421A driver failed and literally caught fire. The MOSFET was not damaged. The PWM is generated by a PIC with a Vdd of 5V. My test setup had approx 2 feet of wire connecting the dc supply. Not the most ideal setup, but I didn’t expect the driver to fail. The P-Chan MOSFET is rated at 80A and the driver is rated at 9A peak. Also, the 100 ohm gate resistor was not damaged nor was the PIC. I’m hesitating with any further testing until I at least make some attempt to keep the driver from blowing up. My ultimate goal is to be able to deliver up to 100W into a resistive load by varying the PWM duty cycle.

Thanks in advance for any advice.

Suppose one wishes to increase maximum drain current of a MOSFET. What factors need to change, and will the area of the MOSFET necessarily increase by this decision?

The common base configuration has been well documented for the BJT .Small signal analysis shows better high frequency capability then common emmitter for the same transistor.I have seen common gate used on JFETs at VHF where the low input impedence would be a better match to 50 ohm. Would a common gate circuit arrangement which would be more complex on an orthodox SMPS give lower switching losses ? Is there a definative statement about how much lower they would be ? I did this on a small ZVs royer where the circuit complexity penalty for common gate was minimal but the actual switching losses of ZVs are so low anyway when input volts are high and input currents are low .Is it worthwhile to look at common gate ?

I have designed this circuit to turn off the mosfet as soon as I apply 1.2A of current for 2ms.
Also I want to know the status through the LED whether the mosfet is turned off or not.When I simulated this circuit,LED starts glowing before the current reaches to zero.Could you please suggest me whether this circuit serves the purpose.

Simulation profile:

I am using FDC5614P from Fairchild semiconductor.Here is my simulation snapshot of drain current Vs Voltage across the load

I have two MOSFET circuits; one is working and the other fails and I’m not sure why. I just want to turn on/off the LED, nothing special, no fast switching.

Below is the working circuit (series gate resistor):

^{simulate this circuit – Schematic created using CircuitLab}

Below is the non-working circuit. By non-working I mean when I apply a ’0′ to the gate, there is 2V sitting on the gate and the LED dimly comes on. Why is this happening?

^{simulate this circuit}

I understand that a MOSFET has high gate impedance and a gate resistor is not a necessity but I wouldn’t expect the MOSFET to completely not work right. Am I missing something?

I’m trying to build a subwoofer with integrated LEDs which will pulse to the beat. I’ve seen a few projects for this around the net, but everybody seems to be ‘winging it’ and not necessarily creating a circuit that will last.

So, if it’s possible I’d like to do this the ‘right’ way. The extra twist I’ve got going on is I’m using RGBW strip lights with a controller.

The intent is to have the strip color (and initial brightness) controlled by the LED controller. For this portion the controller has a dedicated 12v DC input. The complicated part that I’m trying to achieve is to have the strip intensity driven by the amplifier output, such that the LEDs would ‘pulse’ with the subwoofer output. What I presume is that the output from the controller needs to be regulated, and the most promising solution so far seems to be a transistor, controlling the power sent to the leds based on the input from the audio amplifier. In my limited electronics experience the common wire in the 4-conductor output from the LED controller was the candidate for this regulation.

The amp would be putting out ~0-18v AC for this application.

I plan to use this controller.

And these LEDs.

A helpful user on another forum submitted this sketch for a possible circuit, with amp power coming from the left and the LED controller on the right.

Does this look like something that will do what I’m looking for? Any suggestions or ideas for improvement?

I have an application where i need to be able to switch from a 20V supply to a 40-50V (variable) supply in a very small amount of time. Ideally this changeover needs to be prompted with a signal pulse in the region 5-12V. It also needs to occur as fast as possible – the microsecond to nanosecond regime.

I have the power supplies ready and varying the voltage is also taken care of, however I’ve yet to find a suitable way to switch between the two at these speed.

Edit: Some more information about the application below.

I’m driving a CCD; an imaging device that converts light into electrical signal that is stored within pixels. The signal is read out and used to recontruct the original image. Within the pixels are seperate gates that have biases applied to transfer charge between one pixel to the next. This transfer process occurs in the us to ns regime depending on operation speed.

The typical gate voltage is around 20V for this device. I need to be able to increase this gate voltage from 20V to 40V and then reduce it to 20V again in a time comparable to the speed at which charge is transferred from one pixel to the next. The gate voltage is currently controlled by a power supply that contols the low level, the plan was to add another that controls the high level and to switch between the two quite fast. I’m trying to design a ciruit that interfaces the power supplys and supplies the voltage needed to the gate of the CCD.

Ideally I’d like a switch where when its in the “off” state, the 20V is being supplied and when its in the “on” state the high voltage is supplied. There needs to be no time where no voltage whatsoever is supplied (I may have about 1-5ns of grace, but not much room for error).

I’d previously done this by hooking up a couple of relays, but they are too slow. Is there a faster alternative out there that i can get hold of in a single IC package?

In terms of requirements the max voltage will be about 50V DC. the max current draw will be about 200mA.

Forgive me if the answer is very simple, however I’ve searched for answers with not much luck.

I need to discharge a power rail from 3.8V to 0V within 200ms. The control signal is active low (i.e. start discharging when control becomes low). I think the easiest implementation probably is to use NFET and invert the control signal to feed the gate. That means I need an inverter + NFET. Any single IC that can achieve this function?

In detail, the control signal comes from a watchdog timer, which is powered by a different power supply. The signal will hold low ~200ms and disable 3.8V regulator meanwhile, and then go back high and enable the regulator again. I plan to use this signal to discharge the 3.8V rail at the same time. I can’t use PFET because it will stop discharge (~0.7V) when Vgs<-Vt. My concern is if I don’t discharge the rail to GND, I am not sure if the logic circuits can power up to the right states every time the power is cycled. Can these residue voltages inside the ICs cause, say, latch up or other unintentional shorts?
Thanks.

Many books (Chenming-Hu Ch.6 Page 15, Neamen Page 413) use the term ‘channel voltage’ $V(y)$ to mean ‘the potential in the inverted channel at a point $y$ distance away from the source along the channel length’.

First part:
What do they mean by ‘potential in the channel’? Is it the same as the potential at a point inside a resistor (because the channel in a MOSFET is similar to a resistor)?

If so, is this potential constant along the direction perpendicular to the current flow, as in the resistor?

If it is constant, then how is ‘potential in the channel’ any different from the surface potential (i.e., the potential at the semiconductor-oxide interface $psi_{s}(x)$, also a function of $x$?

Second part:
When a drain voltage $V_{ds}$ is applied (the source and the body are grounded and a channel has been formed), the band diagram of the MOSFET along the channel is expected to show Fermi level splitting because the system is out of equilibrium.

Relevant image :

What I don’t understand is the band diagram in the $x$-direction at a point $y$ distance away from the source and in the channel…

The x-axis runs from left to right in the above image.

The image shows that the Fermi levels split inside the body of the semiconductor and the amount of the splitting is $eV_{ds}$ at the drain, and by extension, $eV(y)$ inside the channel. Why does this happen? Is there any connection between the ‘potential in the channel’ interpretation of $V(y)$ and this splitting?

Legend for the band diagrams :

• Black dotted lines – intrinsic level
• Red dotted line – Quasi Fermi level for holes
• Red solid line – Quasi Fermi level for electrons
• Black solid lines – Bottom of conduction and top of valence bands

There’s a few things that I don’t understand with these questions. First, thing I don’t understand is why they are connecting Vdd to the source terminals on Q2 and Q3. I understand what all values except for the Va_NMOS/Va_PMOS values mean. In my studies of BJTs Va stood for early effect. I’m not sure if that is what they mean here for MOSFETs. I also need help setting all this up (because I don’t know where to start) to answer the three questions below. If anyone could help that would be awesome.

I am using the following MOSFET Bridge to harvest AC bursts that coming off from a electromagnetic transducer.

Infront of this circuit is a Super capasitor I have placed.

I read the following on a webpage that talks about this design.

One caveat of the FET bridge circuit: do not use it as the rectifier in front of a capacitor-input power supply! In a conventional rectifier bridge, the diodes prevent the backflow of current from the power supply input capacitor as the applied voltage drops below the voltage on the capacitor. With this design, the MOSFETs act like switches rather than one-way valves for current flow. They don’t care which way current flows, hence the input capacitor of the power supply will be discharged to near zero volts with each half-cycle of the applied AC power! This limits the power supply applications for this circuit to inductive- or resistive-input designs.

I would like know a successful method I can use to store the charge coming off the electromagnetic harvester into a super capacitor using this FET based rectifier. This temporarily charge in the capacitor is used by an energy harvesting IC to store into a LiPo battery. The bottom line is that I want to overcome this problem of losing charge in the capacitor in the reverse direction.