OK, you can't do it.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Why not set R1 to a bit higher value (when the switch is OFF, it dissipates near half a watt)? I'd prefer a good bipolar switch to replace the 2N7000, just connect the base of an NPN to
+3.3, and drive into the emitter with a current-limiting resistor.... an NPN and a resistor is still cheaper than a MOSFET, and has better known thresholds
The output MOSFET has to be definitely off when it goes to 2.8V on the gate, though: I can't make out the part number on the diagram.
A 2N7000 is cheap, just a couple of cents. But an actual 2N7000 has a max gate threshold voltage spec of +3 at 25C, which is flakey when the gate drive is nominally 3.3. Simulation production.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
2N7000 was for illustration since most of you won't have a model for the DMG1012T used by customer.
And the speed will suck.
Si4666DY
Get glasses or a PDF reader that can zoom ;-) ...Jim Thompson
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| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
0.5*L*I^2 = Qg*Vg (Qg = Total Gate Charge, from data sheet, at gate voltage, Vg)
Not "L = R^2 * C"
"Close to it" (pretty close actually) would be...
0.5*L*I^2 = 0.5*C*V^2It's all about energy transfer from magnetic storage to capacitive storage... and boost converters rely on it. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
[neglecting losses in diode and resistor, I'll include that RSN, otherwise someone will have another hissy fit :]
...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Good grief! Do the algebra!
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Thank You! I had the same problem. After reading your post, I saved it then opened it. Bingo! Now it's readable.
Ed
That's of no consequence if the switching speed is too slow. I don't know what the poster was thinking, but if the driver switched very slowly, the inductor's energy would all be lost.
In life there's no danger of that--the transistor is more than fast enough.
That's reasonable.
It all depends. Sounds reasonable if you're set on one steady part. A zener might make sense when using or substituting various output FETs, etc.
Agreed that it works and saves in your situation.
Here's a gate driver I played with recently:
+12v -+- | | V D1 --- | C1 +---------||-----------. | | .-. | R1 | | +12v +12v | | | -+- -+- | '-' | | | | |/ | | +------+--| Q2 | | | | |>. | | ||--' | | |/ | ||
Luckily, all of Jim's pdf's open using my old version of Adobe 5.0 so is easy here.
I did NOT try Foxit, which I have for those pesky new format pdf's. Did you try zooming in for detail?
Could be the way the information is presented. Both readers have to reduce the data to show on smaller screens. Adobe knows to preference towards black, since Adobe thinks they're showing a printed character or line. BUT! Foxit is thinking the original is an image, so Foxit averages the adjacent pixels. The end result for Adobe is that Adobe THICKENs a line, but Foxit is THINNING the line, or worse, decimating, so just can't see them. ...Just a guess on my part.
I've used a similar scheme, but it only boosts base drive, the output cannot exceed rail, which was my need. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
[snip]
That's because, while I compose in Acrobat v7, I do a "filesize reduction" to Adobe v4 when I'm finished... for two reasons... everyone can read v4, and v4 is an easier tool to do text tweaks... PSpice leaves extraneous text on the plots, like and (A) that annoy me, so I strip them out easily with v4. Also, if I merge several simulation runs using PostScript, then PDF the result, v4 can remove the excess overlaid text... getting rid of that blurry look. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
I think you should replace that GS diode part of the BAV with a 20V zener or something, it's not going to put a dent in the already slow switching speed, and buys you some margin in overdriving the gate charge.
Two of the existing diodes seem to have no function. But a zener makes sense. It would allow super-enhancing a cheap mosfet without blowing out the gate.
-- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
I've already indicated D1 buys so little improvement in energy transfer as to be almost immeasurable, so I eliminated it.
Which two diodes "seem to have no function"? (The real load actually has some capacitance ~80nF, and must be dumped quickly.
Possibly. As I indicated earlier, the spread in Qg is such that the Vg spread will be +/-20%, so why bother, it isn't going to be overdriven. (The Si4666DY data sheet doesn't specify an absolute max VGS, but shows curves for 4.5V and 10V, so design for 6.5V and take the +/-20% spread with no problems.) ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
If the load is 1.5 ohms, a 2N7000 isn't going to help pull it down enough to matter. With 3.3 volts on its gate, it might not turn on at all.
80nF * 1.5 ohms = 120 ns.See? An Equation!
-- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
In the actual application, the pull-down device is a DMG1012T, designed for 3.3V logic gate, and mucho amps capability.
I substituted the 2N7000 for demo purposes, figuring everyone has a model in LTspice for it.
That's not an "equation", that's an "evaluation".
The real load is an LED array (pulling the 8 Amps from a 3.8V supply) and thus won't discharge the voltage low enough fast enough... thus the DMG1012T.
And the BAV diodes handle the surge just fine. A version (not my design), with all kinds of auxiliary supplies (no resonant charging) to drive the devices, has been running continuously at that ~2kHz rate for more than a year. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Let me help you out, I believe that FET has a Von(Thr) of ~ 2.4 so.
That equates to 3.3 * 0.63 = ~ 2.0, so with that FET, it won't turn on! At least not in that time frame! :) Jamie
Fairchild says 3 volts at 1 mA, 25C. It would be lower cold.
Apparently he's not actually using the parts or the voltages that he posted. We're not allowed to know the actual voltages, or parts, or the application, all of which we now know.
-- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
I don't see how you're getting that, the Vgs will /increase/ for a component Qg /less/ than typicals and they don't show a minimum. I have to review the total charge models but shouldn't Qg =Qgd +Qgs? The datasheet isn't even close.
This datasheet shows abs max of +/-12V for VGS:
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