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Old Apr 04, 2012, 12:53 AM
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Even with logic level FETs you will still need approx 10 volts to fully turn them on, this is from experience, also how is your Vcc voltage, you should use a standard regulator or low dropout type so you can have more current to work with.
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Old Apr 04, 2012, 02:12 AM
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I would have to disagree that LL FETs need 10v gate drive to turn on fully. Many are fully rated at 4.5V drive and display little improvement at 10v drive. There's even a growing number of FETs rated to 1.8V drive.
The attached is a device I use in a brushed PWM motor controller and is driven off a PIC micro running from 5V. The difference in Rds On at 4.5V and 10V is almost academic.

Cheers,
David
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Old Apr 04, 2012, 03:15 AM
"Simplify, then add lightness"
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Quote:
Only problem: my high-side V-gate-to-source is only reaching ~3.8V. My mosfets need 4.5+ to turn on (they are logic type).
I just re-read your original post and it says you are using IRF540S. Those are not logic level mosfets and are only spec'd for 10v gate drive. Just because the threshold voltage is below 5v does not make them logic level mosfets. Most logic level mosfets have thresholds of around 2v. You will need logic level mosfets for use with that 5v driver.

PS There is an IRL540 that has the same 100V rating as the IRF540 but is a logic level device.
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Old Apr 04, 2012, 07:49 AM
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Quote:
Originally Posted by jeffs555 View Post
I just re-read your original post and it says you are using IRF540S. Those are not logic level mosfets and are only spec'd for 10v gate drive. Just because the threshold voltage is below 5v does not make them logic level mosfets. Most logic level mosfets have thresholds of around 2v. You will need logic level mosfets for use with that 5v driver.

PS There is an IRL540 that has the same 100V rating as the IRF540 but is a logic level device.
I switched Mosfets for the second go around because I couldn't solder the D2Pak worth a damn.
These are PSMN3R-430-127
As Kiwi says, their on-resistance between 10V and 4.5V gate is almost the same.

I just can't explain why I get so much voltage droop on the boot capacitor even at very small duty cycles.
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Old Apr 04, 2012, 10:52 AM
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Keep listening to the other people in this post, and you will not fix your trouble, I think you should find a Novak Reversing Speed control, and see how Bob Novak does it! He Knows!
Enough said!
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Old Apr 04, 2012, 01:23 PM
"Simplify, then add lightness"
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Quote:
I switched Mosfets for the second go around because I couldn't solder the D2Pak worth a damn.
These are PSMN3R-430-127
As Kiwi says, their on-resistance between 10V and 4.5V gate is almost the same.

I just can't explain why I get so much voltage droop on the boot capacitor even at very small duty cycles.
With those mosfets you should be ok with 3.8v Vgs drive. On resistance will be slightly higher than with 4.5v drive, but less than the difference between 4.5v and 10v. (see fig8 on the data sheet)

I know it is a puzzle why you are not getting the full 5v gate drive. One end of the bootstrap cap is being pulled low by the low side mosfet and the other is pulled to supply voltage thru internal schottky diode, so I would expect the bootstrap caps are actually charging to about 4.5v(assuming 5v supply). Have you measured across them? Rest of the drop is most likely in the drivers but it does seem like a lot. Could they possibly be bad?

Also, how are you measuring the voltages? Are you moving the ground clip on the probe to locations other than ground. If device is powered from a battery shouldn't make that much difference, but would still be better to use two probes in differential mode.
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Old Apr 04, 2012, 06:07 PM
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Quote:
Originally Posted by jeffs555 View Post
With those mosfets you should be ok with 3.8v Vgs drive.
I rehooked my motor, and you are correct. 3.8V must be enough, because the motor IS running.

Quote:
I know it is a puzzle why you are not getting the full 5v gate drive. One end of the bootstrap cap is being pulled low by the low side mosfet and the other is pulled to supply voltage thru internal schottky diode, so I would expect the bootstrap caps are actually charging to about 4.5v(assuming 5v supply). Have you measured across them?
Well, they DO go to 4.5V with the high side mosfets off. I measure across with the probe and it's ground lead directly touching each end of the capacitor. I don't think my scope supports differential input probes.

I'm close to calling this "working well enough for a DIY thing slapped together in a couple days".

edit: I take back what I said about it working well enough. I think these mosfets might just be too damn big for these drivers. Here's a shot of Vgs in action with the duty cycle continuously varied back and forth.
H-Bridge Test Run.mp4 (0 min 36 sec)

The voltage is 3.8 at best. Then sags down at high duty cycle (kind of expected that, no big deal). Oddly enough, it *does* jump to 5V at the start of each new cycle. Weird.
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Old Apr 05, 2012, 12:40 PM
"Simplify, then add lightness"
Raleigh,NC
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I don't think my scope supports differential input probes.
Don't have any more insight into the gate drive levels, but FYI you can do differential measurements with that TEK scope you showed in your pictures. You use two probes and set channel 1 and 2 to ADD and then invert channel 2. The display will be the differential voltage between the two probes.
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Old Apr 05, 2012, 03:00 PM
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Originally Posted by jeffs555 View Post
Don't have any more insight into the gate drive levels, but FYI you can do differential measurements with that TEK scope you showed in your pictures. You use two probes and set channel 1 and 2 to ADD and then invert channel 2. The display will be the differential voltage between the two probes.
Yes, I can subtract channels, but is that really the same as a differential probe? I thought they had fancified amplifiers built in for common-mode-rejection reasons.

I have more probes coming, so I will try some things anyways.
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Old Apr 05, 2012, 04:01 PM
"Simplify, then add lightness"
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Unless the common mode voltage is a lot higher than the differential voltage, the two probe add invert method works essentially the same as a differential probe.

The only reason I asked about differential is that connecting the probe ground clip to points other than ground can affect the circuit. Not usually a problem if the circuit is low frequency, battery powered and totally isolated from ground and you only need to measure one signal at a time. That seems to be the case here, but since I didn't know exacty where how you were measuring, had to ask. Always have to be careful that the measuring equipment is not changing the operation of the circuit.
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Old Apr 09, 2012, 09:57 AM
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Reliability ?

I use a older Novak " Reversing Super Rooster ESC " in a 8' WW II PTB with a 41 thrust trolling motor.

Car battery, 50 amp circuit breaker. I was tired 1 night and hooked the 12 volt car battery BACKWARDS 2 times in a row in a about a minute.

0 damage to the Novak ESC. There is a lot to be considered when doing ESCs.
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Old Oct 09, 2012, 09:05 AM
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CombatWombat, I am wondering how your project turned out. Did you get it working ok? With what you have learned, would you do it differently now?
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Old Oct 09, 2012, 09:39 AM
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hi,driver fet bridge h have brake ? i want use break for car .
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Old Oct 09, 2012, 05:49 PM
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Quote:
Originally Posted by jdraughn View Post
CombatWombat, I am wondering how your project turned out. Did you get it working ok? With what you have learned, would you do it differently now?
The final project can be seen in action here:
BalanceBot (0 min 36 sec)


Eventually I found the high-side drivers I was using simply could not handle the gate capacitance of the Mosfets. I wound up with with P-highsides for simplicity sake. I would consider putting the a low-end microchip on the h-bridge board itself, in the future, to make the logic of safely driving the mosfets self contained.


Quote:
hi,driver fet bridge h have brake ? i want use break for car .
Please don't break your car on purpose.
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Old Oct 10, 2012, 06:43 AM
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Wow, that thing is rock solid. Do you have a link detailing what components you used, and how you built your drive train? I am in the process of building my drive train using two 3" plane wheels, a large 90t gear to mount on the axle and a 15t gear for my motor. I am planning on using threaded rod to mount the gear and wheels, and I am also planning on mounting two bearings that are for skateboards or roller skates.

I had a long bolt and I got one wheel, a bearing, and the gear all mounted up nicely, I just need to replace the bolt with a rod so I could add another bearing and the
other wheel. Then I just need to figure out how to mount the assembly to some kind of chassis.

While I have been playing around with the mechanical aspects I have been trying to research how I should build the h-bridge. From the experiments I have conducted with random assortment of mosfets, the p-channel ones always get waaay hot, so I was going to try to go with a boot strapping high side driver like the MCP14700, but from your experience and results, maybe I need to buy GOOD mosfets. Which mosfets did you end up using, and what motor are you driving with them?
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