HH2 -- first cut

[mts]

No, I'm not using a darlington PNP. I'm using common switching transistors. That would probably account for the delay. I didn't think the schematic called for a darlington transistor on the push-pull driver?

[Carl-NC]

The only real difference I saw was that the BJT push-pull circuit doesn't turn off immediately. There is about a 6us delay before they actually switch the MOSFET off.

Are you using a Darlington PNP? I get <1us delay.

[wam]

Schematic

Sorry I have forgotten schematic for comments above.

[wam]

Synchro from pulse.

When it comes to delay, I like the idea of having interrupt taken from decaying signal. It is shown on this example. The other good idea from this circuit is digitally adjustable level for first gain amplifier. Honestly, presented concept is not my desire.
The electronic approach is not modern. I believe that it is available different circuit with greater ability to detect low level electronic signal pulse changes. The other concept of mine (…or some one else) is to recover pulse from separate high inductance coil then compare signal against original TX signal then extract phase difference between and signal strength at the same time with two build in lines of logarithmic electronically adjustable amplifiers and all this in one smart not expensive IC which operates in new generation electronics. I do not want to go to details but may be some one knows what it is about.
Regards

[mts]

Thanks Wam,

As a follow up I ordered a couple of sample MOSFET drivers from Microchip. They were similar in nature to the MAX4427. Both of the chips I received can be purchased online for less than $1.50. I tested these drivers against Carl's push-pull BJT design and the results were very similar. I basically couldn't tell a difference in terms of turn off time, flyback voltage, and detector depth.

The only real difference I saw was that the BJT push-pull circuit doesn't turn off immediately. There is about a 6us delay before they actually switch the MOSFET off. This does not appear to hurt depth at all. You just need to start your sampling delay 6us later.

So overall I would definitely recommend using one of these MOSFET driver IC's instead of a bunch of discrete components. Carl's BJT circuit is fairly cheap and is made of off the shelf components. So it is a good alternative if you don't want to order special IC's. But buying discrete JFET's to create your own circuit is really not worth the money or time in my opinion. This is a solved problem so it seems right to use accepted solutions. These chips were made exclusively for turning off MOSFETS as quickly as possible.

[wam]

MOSFET Driver

I am using MAX4427 in my GoldPic3 - it keeps transistor cool.
I am using it for several years in professional applications as well - never got any problems. Priory to this we have experimented with variety circuits and suffered with large number of MOSFET’s damaged.
Regards.

[mts]

The following doc is pretty good at describing the process of matching MOSFET drivers with MOSFETS. Based on the formulas in the doc and the IRF740 specs the total gate capacitance at our operating conditions is probably more like 4000pf (compared to the value specified it the data sheet for Ciss which is only 1400pf). Therefore, using the TC1411 would probably result in rise/fall times in the 100ns range. That's still pretty good but would not be as good as the MCP1407 which would likely be able switch off the MOSFET in 50ns or less.

http://ww1.microchip.com/downloads/en/AppNotes/00799b.pdf

Can anyone comment on the desired rise/fall times for our particular application? Is 100ns too long? How would the BJT/Push-Pull and FET versions compare with this?

One thing to keep in mind is that these MOSFET drivers are designed for a different application. They are typically used for switching power supplies to try and preserve as much power as possible by switching quickly. Given that we want very fast switching as well these drivers seem well suited to the task. However, we are certainly not nearly as worried about power efficiencies so the simple push-pull BJT circuit is likely to work just fine in our application.

Thoughts? I apologize for the multiple posts in succession. I'm learning as I go so the thoughts flow rather freely at times.

[mts]

Another example may be the MCP1407. It costs $1.06 from Digikey and can handle up to 6A peak switching currents. It is rated at 40ns for a 2500pf capacitive load. I can't imagine that the IRF740 is anywhere near that but I could be very wrong. For lower capacitance loads it is rated at nominal rise/fall time of 20ns.

This seems like it would fit the bill but I may be way off base.

Thanks again.

[mts]

Again, I'm not an expert. That's why I'm asking the question.

But if I go out to Digikey and search for mosfet driver chips from Microchip, STMicroelectronics, and Maxium I find all kinds of hits. And most of them are less than $2. An example may be the Microchip TC1411. It's less than $1.50. If you price out the discrete components on the FET design it comes out to over $3.00. Now I have no idea if this particular chip is an appropriate driver for the IRF740. But surely this is not a new problem and there should be reasonable solutions available. I can't find the post now but I remember someone out here comparing the results of these discrete component designs with those of a mosfet driver IC. Perhaps they can shed some light on what they were comparing to and what the cost would have been.

It seems to me that it is a solved problem and we are reinventing the wheel. I'm just trying to figure out if that is truly the case. I have no problem admitting that I am wrong or that I don't know what I'm talking about.

A couple of final points and then I'll turn it back over to the experts.

First off I have to say that Carl's original BJT design seems to work very well in my opinion. So if I were going to go with a discrete component design I'd be inclined to go with that one. It's made of less than 50 cents worth of parts that I can pick up at my local Radio Shack. Now I'm not saying that spending more money/time on a better design might not be justified. Given the constraints Carl has put on this project I think his design fits the bill nicely. But those of you who are trying to wring out better performance are certainly justified in seeking to improve upon the design.

Second, I'm an IC guy by nature. Back in my EE days I had a phobia of discrete components to a certain extent. Many of the "old timers" I've met over the years tended to be the opposite. They would shy away from IC's just because they "felt wrong". I have to wonder how much each of those mindsets are impacting the decision here. I'm not saying that discrete designs are bad. I'm simply saying that most of us fall back on what we are familiar with regardless of whether or not there is a better design available. I've definitely been known to plug a bunch of IC's together when in reality a few discrete transistors would have done the job.

So again, I'm not an expert. I'm just curious. I have no experience with mosfet driver IC's so I'm hoping you folks can enlighten me.

Thanks for your patience.

[Rov]

What equivelent chips are you refering to!Im interested to know? I get my regular updates for Linear Tech. new generation of chips etc. an some from Dallas ,Max etc. but all seem very cost prohibitive for us home developers/builders.Thanks.

[mts]

I'm fairly new to all of this. But it sure seems like we are spending a lot of time designing a mosfet driver circuit when there are evidently chips that are readily available for this. Other than cost, what reason would we have for designing our own mosfet driver over using a standard driver IC? Given that the cost of these standard IC's appear to be around $1, why would you want to rely on so many discrete components instead? It seems like I could easily spend just as much on all of these individual components after shipping and counting the time spent tracking them all down.

What am I missing here? I know that reducing cost is important but if this is a critical component of the system then spending a few extra cents doesn't sound like a bad idea to me. Keep in mind I'm a complete newbie so I'm looking to the real experts here for guidance.

Thanks!

[rinos]

pi driver

Nice work Aziz,

I made some improvement on your slight improved design.

Hope it will work nice.

RINOS

[Carl-NC]

Carl, can you post a final pcb layout (2 pictures, one bottom-side, one top-side) and a coil data schematic for building coils?
And the C programm for the pic ?

I've just started a new thread "HH2 -- Take two".

- Carl

[quink]

Carl, can you post a final pcb layout (2 pictures, one bottom-side, one top-side) and a coil data schematic for building coils?
And the C programm for the pic ?

inverting gone

Forgot to mention: Pay attention to the pulse logic level! It is changed with the low cost variant (inverting totem pole gone). So the micro should give right logic levels.

Aziz

inverting gone?? Does it mean we give +5 pulse and the mofset turns on ???

what about the older HH version1 Toff times,are they in n seconds , whats the exact value using 2n2906, 07 transistors in Version 1.