Author Topic: Buck converter for small wind turbine project  (Read 20839 times)

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GoVertical

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Buck converter for small wind turbine project
« on: January 12, 2012, 05:10:56 AM »
 Hi, I am trying to piece together how the circuit works.  I am manly interested in how it affects the load on the turbine. If the stator internal resistance is high, it will produce a high voltage with low current. The MOSFET and inductor isolates the input from the output load. When the MOSFET is on, energy is stored in the inductor.  When the MOSFET is off the inductor and diode allows the stored energy to be transferred to the load.  The output voltage will always be fixed to the voltage level of the battery bank value.  The switching frequency of the MOSFET should be high enough not to allow the inductor to saturate.  The diode should be rated higher than the maximum current being delivered to the load.


I started a project for a very basic 5 amp buck converter. The Nand gate oscillator is used to allow micro control if needed. A hand crank will be used to manually spin the PMA so the current input and output can be measured and the affect of the load on the PMA can be observed with and with out the circuit. This is a first attempt at this type of project.
Comments welcome.  

 
« Last Edit: January 12, 2012, 05:12:59 AM by GoVertical »
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Flux

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Re: Buck converter for small wind turbine project
« Reply #1 on: January 12, 2012, 06:39:59 AM »
Are you trying to understand how a buck converter works ( in which case you are on the right lines) or are you trying to understand how mppt works for wind power. If you are trying to understand the mppt concept then you are starting from a strange and confusing starting point. Stator Resistance is inevitable but the lower it is the better.

If you connect a normal machine to a battery via a rectifier, the thing starts to produce current as soon as your input exceeds battery volts ( cut in). A high efficiency alternator and a circuit with minimal resistance will mean that the current rises very rapidly with speed and if there was no resistance the current would rise so rapidly that you couldn't increase the speed. This is not good for the prop, which wants to increase speed with wind speed and you go into stall.

To make the thing work reasonably well you need enough circuit resistance to let the prop speed rise enough to keep it working fairly well.The result is a compromise between loosing electrical efficiency and loosing prop efficiency.

Now if you could imagine a situation where your battery volts rose with wind speed then you could keep the high alternator and circuit efficiency and also keep the prop happy on the peak of its power curve.

This is what your buck converter should attempt to do. It does it by storing energy ,at a voltage that rises with wind speed, in the inductor and recovering the energy at higher current and lower voltage into the battery ( just regard it as a dc transformer that lets the alternator voltage rise but transforms it to a fixed voltage at the battery.

As in the perfect case the alternator volts will rise directly with wind speed, the converter input volts will also follow wind speed. If your wind speed reaches 4 times cut in then the converter will be running at 4 times battery volts and at 1/4 of the charging current.

The stator resistance is a menace and would be better avoided but you can see that if you are running at 1/4 the current in high wind ( for the same alternator input power) your I^2 R losses will be greatly reduced.

The first and foremost benefit comes from allowing the prop to run at constant tsr, that is a real big gain over stall operation. The next virtue is that you can now use a much more efficient alternator without running into the stall problem so you could potentially double the electrical efficiency.

The third factor is that if you are stuck with a useless high resistance and inefficient alternator then the losses you incur will not be so bad as with direct coupling ,but it not an excuse to use a low efficiency ( or too small) an alternator. You will not get the best results from mppt unless you keep the alternator efficiency as high as possible.

It does mean that an alternator chosen for direct connection will work at higher efficiency with a well designed mppt converter, but it will mainly let you gain the aerodynamic improvement without the full benefit of the electrical improvement you could achieve.

Think of internal resistance as a secondary problem to solve and you will understand it better. If you have too much resistance and your prop runs above its peak power point with direct connection, no form of buck converter will help you and if you have one it will never phase back.

Flux

GoVertical

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Re: Buck converter for small wind turbine project
« Reply #2 on: January 12, 2012, 08:32:13 AM »
Hi, the concept of MPPT is new for me. I found some basic circuits for MPPT and they used a micro controlled buck converter. They used different algorithms.  The purposed
experiment is just a attempt to fill in the blanks, but they all used a buck circuit.

I recently fabricated a working prototype VAWT. Max output is 9 amps at 40 MPH, but during a road test I did see the output jump to over 20 amps during a high cross wind.
I fabricated the PMA to allow the stators and rotors to be stacked vertically to address the internal resistance stator issue. Basically as I stack more stators and rotors the required cut in RPM decreased by 100 and output was increased for a given RPM. I could continue to stack rotors and stators but weight becomes a real problem.

The PMA has a geared belt drive and I can fabricate larger blades. I chose the VAWT because of the materials and fabrication method is easiest for me and I am not able to fabricate a HAWT. My VAWT is slow but has high torque and I do not foresee over spin becoming a problem.

The buck circuit experiment is to determine if it will improve my projects output at lower wind speeds and provide me with a better understanding of another aspect of a wind turbine system.  Although my project is working I do not advocate the design to others, it is just a experiment.

Thank you for the information, I am slowly filling in the blanks.






           
« Last Edit: January 12, 2012, 08:40:39 AM by GoVertical »
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Flux

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Re: Buck converter for small wind turbine project
« Reply #3 on: January 12, 2012, 11:53:10 AM »
Whether you gain anything from trying mppt depends on what is happening to your machine now.

I get the impression that you are struggling to get enough volts, due to the low rotational speed of your wind rotor.

You certainly haven't helped the situation by stacking machines in series rather than putting all your magnets on a pair of larger diameter discs. This will be costing you a fair bit in terms of stator resistance.

I suspect that more mechanical speed increase would have helped, as anything you can do to increase alternator speed is a good thing.
If you have cut in in the low winds and you are stalling the blades in high wind then you will gain. If you are not cutting in slow enough to get something in light winds then a buck converter won't help.

Similarly if your cut in is ok but the stator resistance is holding back current production in high winds without stalling the blades then it won't help.

I am not sure how beneficial it will be for a drag type machine anyway as these don't suffer so badly if you drag the speed down.

Flux

GoVertical

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Re: Buck converter for small wind turbine project
« Reply #4 on: January 12, 2012, 12:25:37 PM »
Hi, the PMA stators were made with input and output connections so it can be wired many different ways. A ten inch diameter stator is the largest I can fabricate. The magnets have a small surface area and the stator coils were made using two in hand. I can fabricate larger blades, make new stators with one in hand wire for the coils, and alter the gear ratio to better match the PMA output to the blades. For now it is just something I can use to determine if adding a buck converter will help.

 I am currently constructing the gate oscillator to determine if it will operate the MOSfet. Then I have to determine what frequency  to use to avoid inductor saturation.  I am using this article as a guide.

http://my.ece.ucsb.edu/Bobsclass/194/References/NonIsolated/Buck/Buck%20Converter%20Design%20Demystified%20606PET25.pdf

The equations assume ideal conditions and produce results for minimum values.  

Because I am starting with a fixed switching frequency I am fabricating the board using binding posts for the inductor and input capacitor so different value can be tested. I will post results when available. Thank you for the help.  


« Last Edit: January 12, 2012, 01:05:15 PM by GoVertical »
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joestue

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Re: Buck converter for small wind turbine project
« Reply #5 on: January 12, 2012, 06:18:59 PM »
I don't recall if i posted this before, but this seems like a good thread to post it in.



This is a photo of a crudely thrown together synchronous buck. the mosfets are irfz44.
The board is the heatsink, and at 50 khz each half bridge is good for 5 amps average current flowing through the inductor.
5 amps at 50 volts input, 24 out, 50% duty cycle, about 5 watts loss.
it would be good for more current but the fets took 100ns to turn on because i was driving the transformer though a cheap signal generator.
no load loss is ~ 1/2 watt, depending on the time of day and frequency.

The 200 volt 470uF capacitors on the input (top left) have an internal resistance high enough such that 10 amps of inductor current at 50% duty cycle they lose about one half watt each. They aren't new however, so ymmv. The input capacitor losses for single phase buck are not negligible. its a tradeoff between cost, lifetime and losses.

the switches are turned on through ~20 ohms, off through 10 ohms.

the two toroidal inductors are not necessary. the ferrite core in the center is 20 turns of 14 awg with a 1.2 mm gap. it starts to saturate at 12 amps. but even if it does the mosfets don't care, so it does not need soft starting. its losses are very low, it could probably handle 500 watts of power without overheating at a much higher frequency.
(note that the inductor requirements change with duty cycle, 500 watts would be best case of 50% duty cycle)




Don't use binding posts for the capacitors, the 6 inches of wire between the capacitor and the circuit means your turn off losses will still be excessive. it needs to be a direct connection. just use double sided board and cut the traces out with a dremel for prototyping.
As you probably know the output capacitor only carries the ripple current, which is typically on the order of 10-15% designed full load current.
also, since at full load 80-90% of the inductor current is continuous, there's no reason not to use 10 awg solid wire :)

Also, if you insist on using cmos for switching, use something like a 74act14 schmitt trigger and wire up all 6 inverters in parallel, then drive the fet though the proper resistor.

GoVertical

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Re: Buck converter for small wind turbine project
« Reply #6 on: January 12, 2012, 09:11:19 PM »
Hi, the CMOS 4011 are left over’s from another project. Did you notice a improvement of turbine performance? Thank you for the information. 
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boB

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Re: Buck converter for small wind turbine project
« Reply #7 on: January 13, 2012, 12:03:24 AM »
I don't recall if i posted this before, but this seems like a good thread to post it in.

the switches are turned on through ~20 ohms, off through 10 ohms.

the two toroidal inductors are not necessary. the ferrite core in the center is 20 turns of 14 awg with a 1.2 mm gap. it starts to saturate at 12 amps. but even if it does the mosfets don't care, so it does not need soft starting. its losses are very low, it could probably handle 500 watts of power without overheating at a much higher frequency.
(note that the inductor requirements change with duty cycle, 500 watts would be best case of 50% duty cycle)

Don't use binding posts for the capacitors, the 6 inches of wire between the capacitor and the circuit means your turn off losses will still be excessive. it needs to be a direct connection. just use double sided board and cut the traces out with a dremel for prototyping.
As you probably know the output capacitor only carries the ripple current, which is typically on the order of 10-15% designed full load current.
also, since at full load 80-90% of the inductor current is continuous, there's no reason not to use 10 awg solid wire :)

Also, if you insist on using cmos for switching, use something like a 74act14 schmitt trigger and wire up all 6 inverters in parallel, then drive the fet though the proper resistor.

Love your prototyping style !!   MANY of my past protos were built on copper clad as well as my very first buck converter.

What are you using for gate drive ?  Is that a gate drive transformer I see on the left side ?

boB




GoVertical

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Re: Buck converter for small wind turbine project
« Reply #8 on: January 13, 2012, 02:22:59 PM »
  Hi, I received a recommendation to add a voltage comparator that would turn off the MOSFET when the input voltage is lower than the battery voltage. This will be the first circuit to determine what else will needed for buck converter. I should be able measure input current and compare it the output current. Because the controller circuit will be on a bread board it will be easy to make improvements after I have a working buck converter.

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Thank you for posting photos of your project. I will probably use a similar method to mount the MOSFET’s.
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boB

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Re: Buck converter for small wind turbine project
« Reply #9 on: January 13, 2012, 02:40:41 PM »
Govertical, you realize that this circuit will only be good up to about 35 Volts input, maximum with 12V battery?
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GoVertical

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Re: Buck converter for small wind turbine project
« Reply #10 on: January 14, 2012, 06:18:14 AM »
Hi boB, so far all I really know is how much I do not know.  Often what I think should happen is wrong.

What I think I know about the circuit:

Output voltage will be the battery voltage level.
I should try to avoid inductor coil saturation because it will cause the MOSFET to over heat.
The higher the voltage level at the MOSFET gate the high the output current will be up to a limit.
The value of the diode should  be rated higher than I max out. 
The switching frequency should be high enough to prevent the output inductor coil from saturation.
The input capacitor voltage rating should be higher than the PMA DC voltage at max RPM. I do not know how the determine PMA DC max voltage. Should it be load or no load voltage?

After the basic test circuit is fabricated I will try to determine if the circuit will negate the affects that the PMA stator internal resistance has on the system. I can test with and without circuit to make this determination.

Fabrication of test circuit will be done in stages:
Make sure the Nand gate oscillator is working.
Add voltage multiplier and determine that a increase in voltage occurs.
Add MOSFET and determine that it is switching.
Add Inverting comparator and determine if it functions as intended.
Test circuit with PMA, bridge rectifier and 12 voltage battery using a hand crank to determine if circuit will product a higher output at a lower RPM.

Comments welcome.
   
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Flux

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Re: Buck converter for small wind turbine project
« Reply #11 on: January 14, 2012, 07:45:22 AM »
Your circuit should be good enough to test the basic idea with your alternator but I think the failing will be the gate drive to the mosfet.

"The higher the voltage level at the MOSFET gate the high the output current will be up to a limit."

This virtually implies that you are running the mosfet in linear mode.  The gate drive voltage must be above the minimum to saturate the device but not high enough to break down the gate insulation. For normal  devices that implies 10 to 15v. For logic level devices it requires 5v.

The gate needs to be driven on and off from a low impedance source. Your turn on looks questionable and the turn off is dreadful. You may be ok at quite low current and that may be enough to prove your point.

I think your other assumptions are ok

Flux

GoVertical

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Re: Buck converter for small wind turbine project
« Reply #12 on: January 14, 2012, 10:46:42 AM »
Hi, delivering packets of energy to charge a battery is a new concept for me. Is the objective to achieve resonance at different RPM’s?   Comments welcome.


http://www.youtube.com/watch?v=ZYgFuUl9_Vs
« Last Edit: January 14, 2012, 11:08:23 AM by GoVertical »
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GoVertical

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Re: Buck converter for small wind turbine project
« Reply #13 on: January 14, 2012, 01:15:32 PM »
Greetings, I tried a very basic experiment by hooking a 7200 MFD 50 VDC to the bridge rectifier and used the PMA to charge the capacitor. I was surprised how fast and how little effort was required to charge the capacitor to 50 VDC. I then discharged the capacitor using a piece of metal. This resulted in a very large spark and snap sound and the piece of metal had pitted scorch marks. I now have a new respect for the components I will be working with. Safety is a new priority for the project.  

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Re: Buck converter for small wind turbine project
« Reply #14 on: January 14, 2012, 01:43:17 PM »
"  Hi, delivering packets of energy to charge a battery is a new concept for me. Is the objective to achieve resonance at different RPM’s?   Comments welcome. "

No, you are confusing yourself by watching that video and trying to apply it to your buck converter.
The buck converter is just a means of energy transfer where you take short pulses of current at a higher voltage and convert them to a higher current at lower voltage.

You store energy in the inductor during the time the mosfet is on and recover the energy via the flywheel diode when the mosfet is switched off.

At cut in you need the fet full on, but as the alternator speed increases your emf is a lot higher than the battery and you have a large current trying to flow in the internal resistance of the windings. This gives a lot of loss and the battery decides the output volts, the difference between emf and terminal voltage is pushing current through the internal resistance.  With the fet turned on for a short time the alternator sees the mean inductor current as a much smaller one and thinks it is connected to a much higher load Resistance and it develops a terminal voltage much nearer the emf with much lower losses. The energy is increased as the load is a better match and the alternator efficiency rises.  The energy is then recovered into the battery at the lower voltage but at higher current. It is exactly analogous to a dc transformer.

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joestue

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Re: Buck converter for small wind turbine project
« Reply #15 on: January 14, 2012, 04:34:55 PM »
Quote
I should try to avoid inductor coil saturation because it will cause the MOSFET to over heat.

the inductor needs to be big enough to reduce the ripple current to reasonable values, when it saturates the inductance falls, and the ripple current rises, the core might overheat.
using air cores for experimentation is totally appropriate.

Quote
The higher the voltage level at the MOSFET gate the high the output current will be up to a limit.
the gate voltage needs to be at least 3 volts higher than the source for the fet to start to turn on, it needs to be as high as is practical (10-15 volts) for the bulk of the "on" time.

Quote
The value of the diode should  be rated higher than I max out.
it needs to be big enough so the silicon inside stays below a reasonable temperature (say, 100C)

Quote
The switching frequency should be high enough to prevent the output inductor coil from saturation.

No, the switching frequency dictates the minimum inductance needed, fewer turns = less inductance, and less resistance, meaning you can push more power though the core if you both raise the frequency and decrease the turns.
Quote
The input capacitor voltage rating should be higher than the PMA DC voltage at max RPM. I do not know how the determine PMA DC max voltage. Should it be load or no load voltage?

if you intend to let the turbine free spin in high winds, then everything has to be rated to the highest unloaded voltage you can expect.
Quote
After the basic test circuit is fabricated I will try to determine if the circuit will negate the affects that the PMA stator internal resistance has on the system. I can test with and without circuit to make this determination.
all the circuit does is change the voltage, enabling you to control the amount of current that flows into the battery. the reduced current allows the turbine to spin faster, increaseing the voltage, which increases the amount of power  you can extract.. etc.
you don't need to build a buck regulator, you could use a three phase variable transformer, or you could use a variable resistor.
It really doesn't matter what you use because the challenge is discovering what the correct duty cycle to send the buck or boost converter, the position of the variable transformer, or the (God forbid) salt water rheostat.
If you do use a variable resistor, the energy it dissipates would be sent into the battery when the resistor is replaced with a buck regulator, it really doesn't make any difference.

Quote
Fabrication of test circuit will be done in stages:
Make sure the Nand gate oscillator is working.
Add voltage multiplier and determine that a increase in voltage occurs.
Add MOSFET and determine that it is switching.
Add Inverting comparator and determine if it functions as intended.
Test circuit with PMA, bridge rectifier and 12 voltage battery using a hand crank to determine if circuit will product a higher output at a lower RPM.

i can't say strongly enough don't waste your time with that circuit. buy a 2$ half bridge driver instead, use it to drive a synchronous buck regulator, this will enable you to better understand what is going on.

Do have an oscilloscope? you need one that has at least 20mhz bandwidth, otherwise this whole exercise will be a waste of time.

GoVertical

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Re: Buck converter for small wind turbine project
« Reply #16 on: January 16, 2012, 04:15:49 AM »
Greetings, I started on the controller circuit. After decades of use, the dinosaur, aka “oscilloscope” is finally extinct. I order a used replacement. I am hope it works when it arrives. I have wired the nand gates and I am able to control the output state using a switch. The voltage multiplier is not working and I waiting for the oscilloscope so I can complete to control circuit. You suggested using a gate drive, can you recommend a suitable device? 

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joestue

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Re: Buck converter for small wind turbine project
« Reply #17 on: January 16, 2012, 01:50:21 PM »
that looks like a Tek scope, what is the number? it looks like the vertical and horizontal amps are plugins.

as far as a half bridge driver, try this one.
http://uk.farnell.com/international-rectifier/irs21844pbf/ic-half-bridge-driver-dip14/dp/1271815
you can breadboard it to an extent, but only below say 50khz because the boot strap capacitor needs a low inductance path to charge, and that's just impossible to do on a breadboard.

i did find a half bridge driver that had its own boost converter, enabling you to drive the high side switch at 100% duty cycle, but its maximum voltage is 60 volts, i think you need to build this for at least 100 volts. On that note, at 50khz the maximum duty cycle is probably going to be 90-95% with a good layout.
I doubt that will affect anything though.

i'm not sure how you plan on building the control mechanism to set the duty cycle but one thing you need to be aware of is that synchronous buck regulators work backwards too, and make boost converters.
So when you're prototyping the control mechanism you may want to disconnect the lower mosfet's gate and connect it to ground, or replace it with a diode. that will prevent anything blowing up if you accidentally set the duty cycle too low.

GoVertical

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Re: Buck converter for small wind turbine project
« Reply #18 on: January 19, 2012, 08:07:00 PM »
Hi, the new used oscilloscope arrived. It displays the calibration wave form on each channel. What connections are needed to view the AC output of the PMA?   
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Re: Buck converter for small wind turbine project
« Reply #19 on: January 19, 2012, 08:19:18 PM »
I've got the Tek 2445 - to monitor the AC connections is straightforward, but before I say anything about connections, I must warn you that the channels GROUND are common across all channels, and there are very few connections of a 3-phase machine where you can put the ground from two or more channels on both the AC side AND the DC side of your circuit, without letting out magic smoke.

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GoVertical

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Re: Buck converter for small wind turbine project
« Reply #20 on: January 22, 2012, 04:06:15 AM »
Hi, project update:
I was able to measure the output of the PMA and confirm a sine wave output.
The nand gate oscillator is oscillating and the voltage multiplier is working, 10 volts in and 20 volts out. The next step is to add the MOSFET and voltage comparator. I will post results when available.   I am researching gate driver IC’s. Thank you for the help I have been receiving. 


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GoVertical

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Re: Buck converter for small wind turbine project
« Reply #21 on: January 22, 2012, 07:54:17 AM »
Hi,  The RC timing constant at the voltage multiplier output is not allowing a pulsed output. The output of the voltage multiplier has 1.5 volt ripple riding at 20 volts and is not discharging to zero, so the MOSFET will never turn off when point A is enabled. I am searching for alternatives. Ordering a gate driver is the obvious solution. Comments welcome.   
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Re: Buck converter for small wind turbine project
« Reply #22 on: January 22, 2012, 08:30:40 AM »
Ordering a gate driver is the obvious solution. Comments welcome. 

I've had good results in the past with the TC4420 gatedrivers, though I still prefer discrete ones, even though it results in a higher component count - but that's not an issue for hobby projects, only if you want to develop a commercial product that has to be manufactured at the lowest possible cost.

For an induction heater I used discrete totem-pole drivers - worked fine. Heavily overbuilt, but overbuilding is good.

As you know no doubt, the switching FET has to be switched hard and fast for minimum losses (heat) in the FET. Most FETs have a gate capacitance close to one nF, or even more (the ones I was playing with had about 4.5 nF gate capacitance, IIRC).

So instead of waiting for the order to arrive, why not try a discrete gate driver using components you have likely laying about the place already? Here's a link to one I used with good results, at 100 kHz:

https://picasaweb.google.com/motorconversion/Induction_heater#5429594036506882178

Note that one is heavily overbuilt, as the driver transistors don't get even warm.
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boB

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Re: Buck converter for small wind turbine project
« Reply #23 on: January 22, 2012, 02:35:57 PM »
Ordering a gate driver is the obvious solution. Comments welcome. 

I've had good results in the past with the TC4420 gatedrivers, though I still prefer discrete ones, even though it results in a higher component count - but that's not an issue for hobby projects, only if you want to develop a commercial product that has to be manufactured at the lowest possible cost.

For an induction heater I used discrete totem-pole drivers - worked fine. Heavily overbuilt, but overbuilding is good.

As you know no doubt, the switching FET has to be switched hard and fast for minimum losses (heat) in the FET. Most FETs have a gate capacitance close to one nF, or even more (the ones I was playing with had about 4.5 nF gate capacitance, IIRC).

So instead of waiting for the order to arrive, why not try a discrete gate driver using components you have likely laying about the place already? Here's a link to one I used with good results, at 100 kHz:

https://picasaweb.google.com/motorconversion/Induction_heater#5429594036506882178

Note that one is heavily overbuilt, as the driver transistors don't get even warm.


Absolutely AWESOME projects, Peter !

boB



GoVertical

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Re: Buck converter for small wind turbine project
« Reply #24 on: January 24, 2012, 06:39:23 AM »
Hi, would a PNP help discharge the cap?

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Re: Buck converter for small wind turbine project
« Reply #25 on: January 24, 2012, 07:53:44 AM »
To be perfectly honest, at best that circuit would only be suitable for a low volt disconnect where the mosfet is switched very infrequently. You will never get it to switch fast enough for a buck converter.

To me the circuit is crazy and is full of defects. I am not sure it is Worth persuing. Check that when you stop the oscillator the output at pin 4 is zero, if it high then you will have dc dropped by 3 diode drops and not zero. If it is zero then you have a wiring or component fault. You don't say whether you have a normal mosfet or a logic gate variety. If it is a normal mosfet it should be off if the gate is below 4v. If it is a logic type it will have to drop a lot lower.

When you get the mosfet to switch you have a new problem in that your point A is not ground referenced and you need an isolated drive. Get a proper high side driver and forget this thing.

Flux

Flux

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Re: Buck converter for small wind turbine project
« Reply #26 on: January 24, 2012, 08:18:52 AM »
That last comment made me look at the circuit again and I see a major issue. For a buck converter you need to drive the mosfet between gate and source ( and it needs high side driver). I see your circuit it driving the gate relative to ground and this is not going to work once the mosfet turns on and the source moves away from ground.

Are you sure you haven't killed the mosfet and it is permanently on, what you measure on the gate may be coming out of a damaged gate.

Flux