Hi, the mosfet source is at 12 volts dc. This is what I find confusing. Does the mosfet Vgs consider 12 volts as ground. If this true than the gate needs 10 v plus 12 volts to turn on with respect ground? The nand gate oscillator and voltage multiplier does produce 24 volts, so I am searching for a circuit that will oscillate from 12 to 22 volts so the mosfet will turn on and off.

 

Hi, the mosfet source is at 12 volts dc. This is what I find confusing. Does the mosfet Vgs consider 12 volts as ground. If this true than the gate needs 10 v plus 12 volts to turn on with respect ground? The nand gate oscillator and voltage multiplier does produce 24 volts, so I am searching for a circuit that will oscillate from 12 to 22 volts so the mosfet will turn on and off.

An N-Channel mosfet will turn on when the difference between Vg and Vs is greater or equal to the minimum turn-on voltage of the device, which you can see in the part datasheet. Ideally, you're well above this voltage for fully-on conditions.

I understand what you're trying to do with your circuit.

The wind turbine generates some voltage higher than 12v, the middle section of logic forms an oscillator circuit that pulses the gate of the mosfet (and runs off the 12v source), the mosfet, diode, and inductor form the buck converter to charge your 12v battery.

It is beautifully simple!

First, I would like to suggest a gate resistor from the mosfet gate to ground. Perhaps 1k to 10k. This will ensure that the mosfet is either on OR off at all times. Next, assuming your oscillator does oscillate, and it does so between zero and 12v, feed that into the gate of the mosfet.

You may need to buffer the output of your oscillator because of the input capacitance of the mosfet. You can scope the gate and observe an RC-looking effects. The goal is to have a nice, clean, sharp square wave. Any RC-looking curves implies that the mosfet is "sort of on" as opposed to "completely on" or "completely off", which causes power dissipation, which you don't want.

You might also need to experiment with the duty-cycle of the oscillator, but the stuff above, first.

Let me know how that works out.

 

Hi, thank you. It will be a few days before I can test the circuit. I will post results when available.
Thanks again.

 

Hi, thank you. It will be a few days before I can test the circuit. I will post results when available.
Thanks again.

I look forward to seeing how this turns out. I would also like to contribute to this worthwhile project if you need any assistance.

Best,
Paul

 

Hi, update of basic buck converter circuit.

When voltage at point C is greater than the battery voltage
point A is enabled
point X is 24 volts
The top nand gate oscillator and inverter toggle the transistor switches
turning the MOSFET on and off.

I am ordering some gate driver IC's.

I will post test results when available. Comments welcome.

 

Hi, update of basic buck converter circuit.

When voltage at point C is greater than the battery voltage
point A is enabled
point X is 24 volts
The top nand gate oscillator and inverter toggle the transistor switches
turning the MOSFET on and off.

I am ordering some gate driver IC's.

I will post test results when available. Comments welcome.

Can you explain the BJTs and voltage divider scheme at point X? What was your reasoning behind doing this in that way?

 

Hi, this is a first attempt at this type of circuit. The nand gate oscillator and voltage multiplier will double the battery voltage. The top nand gate oscillator and transistor switch circuit should produce a square wave to turn the MOSFET on and off. The Inverting comparator is used to enable the circuit when the voltage at point C is greater than the battery voltage.

I am having a lot of confusion trying to determine what voltage is required to turn on the MOSFET.

I am using a IRF510. The data sheet info says Vgs is +- 20 volts and the gate(thershold) is 2 to 4 volts.

If the source is connected to +12 volts, what voltage value is needed at the gate to turn on the MOSFET?

I am trying to start with just a 5 amp max circuit, once it operational it can be expanded.

 

drawing update:

 

Hi, waveform verifying function of nand oscillator and npn transistors switching circuit.
circuit update: all the parts of the control circuit are working, switching frequency @ 4 k.
Next I will try to piece them together and try to trigger the MOSFET.

 

Hi, this is a first attempt at this type of circuit. The nand gate oscillator and voltage multiplier will double the battery voltage. The top nand gate oscillator and transistor switch circuit should produce a square wave to turn the MOSFET on and off. The Inverting comparator is used to enable the circuit when the voltage at point C is greater than the battery voltage.

I am having a lot of confusion trying to determine what voltage is required to turn on the MOSFET.

I am using a IRF510. The data sheet info says Vgs is +- 20 volts and the gate(thershold) is 2 to 4 volts.

If the source is connected to +12 volts, what voltage value is needed at the gate to turn on the MOSFET?

I am trying to start with just a 5 amp max circuit, once it operational it can be expanded.

If the gate threshold voltage is +2v to +4v, then that means the gate voltage must be +2v to +4v MORE than the voltage at the source. That means +16v, but no more than the max of +20v (if that was what you meant by that figure).

 

The nand gate oscillator and voltage multiplier will double the battery voltage.

Please explain what you're talking about here...

The top nand gate oscillator and transistor switch circuit should produce a square wave to turn the MOSFET on and off. The Inverting comparator is used to enable the circuit when the voltage at point C is greater than the battery voltage.

So, the comparator sees the generator voltage exceed +12v and kicks on the oscillator to drive the MOSFET, right?

I'm not sure I understand why you have those transistors there. I would hook up an NPN transistor:

-collector to +V out of the generator
-base to output of oscillator (which, when OFF, generates ZERO volts, as dictated by the comparator)
-emitter to GATE of MOSFET, which also has resistor to ground.

The chain of resistors and the other BJT - I would take those out...so basically, you'd have something like the image below:

There are issues, though.

This buck converter will work fine if we know the voltage off the generator...but we don't always know. Just because it's greater than +12v does not mean it'll be the voltage that, when buck'd, will equal +12v.

I understand that you will be fine-tuning, but I figured I'd still point that out.

If it were me, I'd use a 12F683 pic microcontroller. It can read in voltage, think about it, and output a pulse-width modulation wave to the mosfet gate to allow you complete control.

For now, I see that you're trying to do this the old-fashioned way, which is very cool and very simple, in theory...

 

Hi, I redrew the circuit hoping it will be easier for me to explain.

Starting at the bottom: the PMA charges to input capacitor. I have tested the PMA just charging a capacitor. The capacitor charges up very fast and can reach over 50 volts in a very short period of time. Because I am just charging a battery I do not believe a capacitor is necessary at the output before the battery, but it can be added if required.

The inverting comparator at the top is needed to enable the system and will disable the system when Vin falls below Vref. Fan out of the comparator should be able to drive two gates.

When point A is high the nand gate oscillator and the voltage multiplier will generate 24 volts across the voltage divider and the second nand gate oscillator will toggle the transistors and create the Vgs signal to turn the MOSFET on and off. ( This is the part of the circuit that needs improvement)

The buck converter will continue to function until Vin is below Vref.

I have tested all the parts of the circuit separately and they do function. I am currently constructing the two nand oscillators on the proto board to try to determine if they will generate Vgs. Once I have the Vgs signal then I can add the MOSFET and test with the PMA.

Thank you for the suggestion, it is good to have options. A micro controller can be added after I have a basic circuit working, it will make the algorithm easier to develop once I am able to observe the affect that the buck converter has on the PMA.

 

Hi, I redrew the circuit hoping it will be easier for me to explain.

Starting at the bottom: the PMA charges to input capacitor. I have tested the PMA just charging a capacitor. The capacitor charges up very fast and can reach over 50 volts in a very short period of time. Because I am just charging a battery I do not believe a capacitor is necessary at the output before the battery, but it can be added if required.

The inverting comparator at the top is needed to enable the system and will disable the system when Vin falls below Vref. Fan out of the comparator should be able to drive two gates.

When point A is high the nand gate oscillator and the voltage multiplier will generate 24 volts across the voltage divider and the second nand gate oscillator will toggle the transistors and create the Vgs signal to turn the MOSFET on and off. ( This is the part of the circuit that needs improvement)

The buck converter will continue to function until Vin is below Vref.

I have tested all the parts of the circuit separately and they do function. I am currently constructing the two nand oscillators on the proto board to try to determine if they will generate Vgs. Once I have the Vgs signal then I can add the MOSFET and test with the PMA.

Thank you for the suggestion, it is good to have options. A micro controller can be added after I have a basic circuit working, it will make the algorithm easier to develop once I am able to observe the affect that the buck converter has on the PMA.

Excellent! I just now realized that you have a voltage multiplier there! Very cool.

The only thing that I would change is the bottom NPN transistor. I'm not sure why that needs to be there. With just the top one pulsing, the gate of the MOSFET gets +24v or 0v (the pull-down resistor) which would turn it on and off nicely.

Why the bottom BJT?

Sorry for my confusion

 

Hi, this is a first attempt at this type of circuit. I do not have a lot of experience, thank you for pointing out that the bottom transistor is not needed. I will remove the bottom transistor and I will post the results.

I did find a paper that does cover charger design.

http://www.ti.com/lit/ml/slyp089/slyp089.pdf

At the beginning it does explain why a p channel mosfet is preferred for this type of circuit.

Thank you for the help it is really appreciated.

 

If the gate threshold voltage is +2v to +4v, then that means the gate voltage must be +2v to +4v MORE than the voltage at the source. That means +16v, but no more than the max of +20v (if that was what you meant by that figure).

Not meaning to be contrary here, but when turning on a gate voltage at the threshold voltage your looking at burning up a mosfet. Threshold voltage is of more use in audio type circuits. The mosfet in his circuit is being used in a high side configuration, it needs to be turned on at ~ 10V higher than the source voltage. Not ~10V more than common but ~10V more than source. Its still only ~10V, not as in this example 22V with 12V at the source. A high side driver would make this easier, no need to double the voltage to 24V.

 

Hi, I am starting to understand the circuit requirements , I am slowly filling in the unknowns, thank you.

I removed the bottom transistor from the load and connected the emitter to a LED.
Vgs triggers now as desired, the LED gives a visual indication that part of the circuit is working.

After reading the .PDF on battery charger design it does indicate that using a p channel mosfet may solve a lot of problems. I plan to continue testing with the n channel and will post results when available.

 

Hi, it has been suggested that for my circuit to work it will require a gate drive pulse transformer.

http://omapalvelin.homedns.org/tesla/SSTC/general-sstc-notes-gatedrv.htm

 

Often, looking at existing designs for these things helps you better understand the ways that you can build yours. P channel MOSFETs are good, but you need a negaative voltage (greater than or equal) on the gate to turn it on and off, as opposed to your setup.

Very cool!

 

Is there a reason for not using an existing buck regulator IC in your project? It would really make your life easier. All of the things your wanting to do are already built in to the IC and would actually end up being cheaper to implement.

Another idea would be to use a "buck - boost" circuit to get the most energy out of the PMA.

 

http://www.usna.edu/EE/ee320/Supplements/dcdc5_driver.pdf


Hi, I found the above .PDF and I am using it as a design guide, basically I am trying to copy the work they have done.

Fig. 5.3 shows a buck chopper with boot strap high side gate driver.

I have ordered some IR2117 gate drivers. This should get me started in the correct direction.

I have spent the last few days reviewing as many articles as I could find on the subject, most of them are about motor control. I have not been able to any that address interfacing with PMA. But I am starting to understand more of the problems. I have decided to stay with the n channel mosfet mainly because I have reference material and a small quantity n channel MOSFET’s.

Ultimately a micro will be integrated into the project, but I am intrigued with the idea of possibly using a voltage controlled oscillator controlling a pulse width modulator to operate the gate drive. But I first have to have a operational buck converter that works with a pma.

I do have real concerns, I am not sure what will happen if the input voltage to the buck converter is to high, I do not know how the gate driver will react or if it will be damaged, I do not know.

The order parts should be here within the week. I will post results when available.
Thank you for time and efforts, it is appreciated. Enjoy the day.