I've decided to keep it as simple as possible without trying to achieve any kind of regenerative braking scheme. I was reading an article that stated it was better for light EVs like skateboards and bikes to just 'freewheel' when no power is being used. The returns are negligable plus it adds to cogging down the motor. Useful on motorcycles and cars. You're right Wookie that I can start the project with what I need and later if I can I will see about making it super efficient. Plus I never started with the intention of making an H bridge. I just need a one-way controller.
Ive spent the last couple of days looking for a gate driver and I appreciate the one you suggest, I see its rated for up to 30 volts and my rfg70n06 mosfet needs 20 volts to drive it. A novice question here but what happens when my battery bank falls at or below 20 volts? If I have a 36 volt system then I only get 16 volts worth of drive time until the transistors become useless? The motor is from a 24 volt DC craftsman lawnmower its probably rated at a horse and a half and 50 Amps .I want to run it at 36 v dc and 30 amps. Its a permanent magnet brushed dc about 5 inches in diameter and 8 inches long. Ive built a 555 pwm from a diagram I found but in studying how to build this thing Ive come across some posts that say for this type of power switching its inadequate. I cant remember why exactly maybe something to do with the switching frequency? So a basic block diagram for my project will be a :

power supply-power regulator-pwm-gate driver-mosfets-motor

Im not sure how Im going to do the throttle other than it will be a tether cord maybe with a switch or a pot . The batteries will probably be NiMh since they can be run down alot. My cordless drill is NiMh. LiPolys cause too many fires in my opinion. This motor and controller will fit on a custom skateboard deck with one tire in the back and two in the front. The wheel in the back will be fixed and the two in front will be attactched to a mountainboard truck for the width. I'm going offroad with this puppy. My wheels are salvaged wheelchair wheels the real nice inflateable ones with profile rims lol. My drive system will consist of the motor-bicycle hub from a 3 speed-back wheel. From the motor to the hub will be chain driven, from the hub to the back tire will be belt driven. This scheme will allow me to align the motor with the back tire. Plus Ill get 3 speeds out of the deal. I only need to turn it on very little then I can freewheel. Im just concerned about a big enough battery bank to keep them mosfets working. Also, should I have a separate battery bank for the PWM side and another for the motor side? At first I was going to use the same batteries and a regulator. Thanks for taking the time to look up parts for me Wookie that was a really big help. I'm really anxious to have this thing ready by spring, I have some real nice trails I can go on where I live and also to test the max distance . For safety reasons I will incorporate a kill switch and a manual brake . Mongoose makes a real nice one I might be able to adjust to fit the back wheel. So the brake cord and the throttle will be tethered together. Thanks for all of the feedback.

 

I kept my mouth shut earlier, but I used to be a skater. The thought of the wheels slowing with force scared me. If you have ever hit a tiny pebble on a skateboard, you come to a quick stop and meet the ground quickly. If the board tried a regen braking technique, it would slow your feet but not your upper body, thusly throwing you off the skateboard. (Not a scooter, but a hands-free skateboard.) Im glad you abandoned the brake. Also, you should use a clutch system to allow the wheels to freespin after acceleration torque is done. (If not, The motor would still cause drag resulting in being on the asphalt)

[ed]
And for the throttle, use a pistol grip, pulling a trigger for kill switch. thumb for throttle. It will keep people from dropping it. but if they did, the released trigger would stop the motor. (and again NO BRAKES! - people WILL get hurt! If you were carving quickly and accidentally hit the brake, you are very hurt.) Skateboard brakes kill. The board slows and the body does not.

 

It will definately have the freespin ability. Thats why Im using an old 3 speed bicycle hub I had all of the spokes taken out. Theres a hill where I live thats really steep and I was thinking a bicycle brake would be a good investment and Ill put it on the back tire. Ill just have to adjust the pressure so they wont be too sticky. The only thing that I'm worried about is using such high power mosfets at 20 V to drive the gate what happens when the batteries fall below that? Kaput?lol

Also are there any ideas out there for an IC chip to build a PWM around that will drive the HCPL-3120 optocoupler driver? I think I would like to do a beefy PWM rated for higher frequency than a 555. If this will lead to heating issues I do have a lot of salvaged heatsinks from old computer processors, some with fams still on them that I could use too if the cooling fins aren't enough. DO i have this right when I say that the higher the frequency the faster the motor will spin but the mosfets will get hotter and at a low frequency pwm the motor will go slower but the mosfet will run cooler? Can I just build a PWM for a tiny frequency range if I know what it will be or should be? My mosfets are rated at 125 ns turn on time and 125 ns turn off time and the turn off delay time is 40 ns. So what frequency range should I use for the PWM? As far as the PWM is concerned should I use an IC or SMPS transformer? Whats the most efficient way?

 

I'm wondering where you got the idea that the gates were supposed to be driven to 20v?

The absolute maximum for Vgs is +/-20v.

Rds(on) is specified when Vgs=10v. Increasing Vgs beyond 10v will have no real benefit; but it will increase the energy used in charging/discharging the gate.

The PWM frequency does not really matter so much; as long as it is over 100Hz, you probably wouldn't be able to tell the difference, except for the sound(s) it makes. It'll be audible until the frequency goes above perhaps 25kHz.

What makes the difference as far as motor speed (power) goes, is the ratio of ON to OFF time.

Something that I completely overlooked before is that the HCPL-3120 opto-driver requires 15v to 30v as a supply; if it is not within that range, it will go into shutdown. I modeled a circuit using it in LTSpice that seemed to work just fine at 12.6v, but would not work at all at 12v. Apparently, the SPICE model does not mimic the real component as well as it should.

You have not mentioned what your power supply will be, other than 36v. Is this one single battery, or several 12v batteries?

 

The batteries I'm not too sure about at this point. First let me say thanks for the correction on those mosfet gate drive voltages. The gate driver itself will shut down around 12.6 though I see? So I get 24 volts to play with I guess.

I will need batteries meant for heavy usage that are meant to be run all the way down. NiMH Seems the way to go. I have a feeling I will need 30 amps for an hours ride. So lets say I get five 12 volt batteries rated at 7.2 AH each and hook them up series /parallel to equal 36 volts and 21.6 amps, its not quite the 30 amps Im looking for but is close enough. I'm not exactly sure yet Wookie until I do a little more research on the power requirements for the circuit. Would you use Lithium Poly? Is there a safe version yet?

 

Keep in mind that in general, batteries are rated for AH based on a 20-hour rate of discharge. If you are planning on just riding for an hour, you will get far less than the 20-hour discharge rate.

I don't know how you planned on connecting five 12v batteries rated at 7.2AH to get 36V and 21.6A; that will be a neat trick. You'd need nine batteries; 3 strings of 3.

Batteries in series adds voltage. 3x12v=36v.
Batteries in parallel adds current.

I am not well versed on the current LiPo offerings. Those who are deeply involved in R/C boats and airplanes would be a good source of info. I do know that LiPo's tend to fail rather spectacularly.

 

http://www.all-battery.com/36V10Ahlifepo4lithiumironphosphaterechargeablebattery.aspx These LiFe batteries sure would be nice. As far as my math on those batteries in my previous post I was thinking 3 in series was 36 volts but that still gives me only 7.2 AH so I thought if I added 2 more in parallel I would get 2x 7.2 more AH . SO it would be a series & parallel. Did I get that wrong? Remember Im just a novice but I'm learning alot already doing the research on this controller. Right now I'm looking up some PWM designs for the optocoupler.

 

Yes.
You forgot that the additional two batteries you want to parallel with the 1st string of three batteries would only be 12v batteries.

I find it interesting that you want to utilize salvaged MOSFETs to save on costs, yet are looking at $500 battery packs.

 

This was relative to my earlier question about efficiency and what is available just thought I'd share. Nice to know industry standards get better by the month these days.







A New Resonant Gate-Drive Circuit With Efficient Energy Recovery and Low Conduction Loss
Eberle, W.; Yan-Fei Liu; Sen, P.C.
Industrial Electronics, IEEE Transactions on
Volume 55, Issue 5, May 2008 Page(s):2213 - 2221
Digital Object Identifier 10.1109/TIE.2008.918636
Summary:In this paper, a new resonant gate-drive circuit is proposed to recover a portion of the power-MOSFET-gate energy that is typically dissipated in high-frequency converters. The proposed circuit consists of four control switches and a small resonant inductance. The current through the resonant inductance is discontinuous in order to minimize circulating-current conduction loss that is present in other methods. The proposed circuit also achieves quick turn-on and turn-off transition times to reduce switching and conduction losses in power MOSFETs. An analysis, a design procedure, and experimental results are presented for the proposed circuit. Experimental results demonstrate that the proposed driver can recover 51% of the gate energy at 5-V gate-drive voltage.

 

I was just dreaming about those LiFe's they are a really nice item though. I was wondering is the SG3525A a good PWM IC to drive the HCPL-3120 which in turn will power the gate of a rfg70n06 mosfet?

 

Ive been looking at PWM ICs and am having a little trouble deciphering the codespeak tech language used in the IC descriptions. In other words I need a fast switching PWM IC able to drive a Gate Driver IC (HCPL-3120 ) which powers my mosfet(rfg70n06).

At this website, http://www.nteinc.com/specs/1700to1799/pdf/nte1729.pdf, is a PWM and what it seems to say is that this IC runs at a 50% duty cycle? Why on earth would anyone only want to run at 50%? Another thing was its operating frequency 10kHZ. Why would they make a PWM for that specific frequency, what a waste? Or am I just reading the stats wrong.

I'm trying to get the right PWM IC the first time and am trying to be careful what I invest in because I don't want to burn anything up. I was also wondering if I should use a different optocoupler since this one needs 15-30 v dc and my mosfet only needs 10 vdc to turn the gate on?

If someone knows of a fast switching PWM able to drive my rfg70n06 mosfet let me know, or let me know what PWM IC is the best frequency for me. Theres a lot of information I need to build this so bear with me while I sort it all out. I've been looking over this websites forums for info on PWM but so far no one is trying to build what I'm building. Mostly just H bridges and what not. Thanks for all of the help so far.

 

Try to stay away from NTE parts.

NTE doesn't make their own parts. They will buy a run from a manufacturer with their own labels on them. Their prices are generally much higher than the real manufacturer's items - and they make a killing in the hard-to-get or obsolete parts category.

Price out an LM3914 at Avnet Express.
Then price out an NTE1508; the cross-reference part for an LM3914.

Note that the cross-reference only goes one way.

However, an NTE1729 is a replacement for the following ICs:
ECG1729
MB3759
SK9912
TL494
UPC494

Now if you want to read some application notes on the TL494, they are available on TI's page:
http://focus.ti.com/docs/prod/folders/print/tl494.html

 

I looked at the tl494 and I see it looks like a good pwm ic for me. One thing I don't understand though about the duty cycle . It states the maximum duty cycle is 45% at each output. Theres two outputs so does this mean I get a maximum 90% duty cycle overall from my PWM IC? Also who is a good online supplier of electronics? I ask because the local store here only deals with, you guessed it, NTE, lol.

 

I looked at the tl494 and I see it looks like a good pwm ic for me. One thing I don't understand though about the duty cycle . It states the maximum duty cycle is 45% at each output. Theres two outputs so does this mean I get a maximum 90% duty cycle overall from my PWM IC? Also who is a good online supplier of electronics? I ask because the local store here only deals with, you guessed it, NTE, lol.

i order mainly with
Newark
Digi-Key
Allied
Mouser

there are a few others i use for a few specific items, but for the most part the above distributors work well for me.

 

I looked at the tl494 and I see it looks like a good pwm ic for me. One thing I don't understand though about the duty cycle . It states the maximum duty cycle is 45% at each output. Theres two outputs so does this mean I get a maximum 90% duty cycle overall from my PWM IC? Also who is a good online supplier of electronics? I ask because the local store here only deals with, you guessed it, NTE, lol.

NO, you will not get a duty cycle of 90 percent. You may review the following:

http://en.wikipedia.org/wiki/Duty_cycle

You may consider and update to the tl598...

 

If you parallel the two outputs it will be up to 90%

You can go higher if you pull the dead time pin below ground.

 

If you parallel the two outputs it will be up to 90%

You can go higher if you pull the dead time pin below ground.

Can you expand to the two parts of your response? How is this possible......

 

Do you think it is a coincidence that the .12 volt bias is 5% of 2.5 volts?

Take a look at the deadtime vs voltage graph, fig 6 in the datasheet I downloaded from alldatasheet.

90% because each floating transistor is 90% out of phase with the other, you can parallel them.

 

I got the two mosfets(RFG70N06 and the lower sided RFG45N06) for my project from an electric lawnmower, a brushed DC motor type. I still had the old circuitboard and I noticed that the driver for them both is a IR2101S, a high and low sided half bridge driver, http://www.irf.com/product-info/datasheets/data/ir2101.pdf.

There is a partial circuit there on the website for a typical connection but I did not understand the High and Low inputs because most PWM's I see have only one output. So if I use the two mosfets like a high/lo halfbridge this will change the whole architecture of the circuit to that of a half-bridge controller? For overall efficiency is it better to have a high/lo half bridge for fast and slow or is it better to have just one mosfet for this one-way controller? Thanks for all of the responses.

 

A half bridge implies a buck regulator, and for that you need a synchronous rectification buck regulator driver, or design your own from scratch.

how efficient is the motor? is losing .5 volts in the diode too much?