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Faster switching speed from mosfets in existing circuit/PCB
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blazini36:

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The remote is an open drain output from the camera, which basically sinks to ground. The pull-up on the board sets the potential. Whether I’ve done that correctly or not I’m not 100% sure of. I have a scope but I’m no scope wizard. I take it this would be adjusted with the pull-up resistor on the gate
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According to this: https://www.infineon.com/dgdl/mosfet.pdf?fileId=5546d462533600a4015357444e913f4f , on the section called "GATE CHARGE", the way to calculate the turn on time, is based on the gate charge. Take notice of the following paragraph:

The advantage of using gate charge is that the designer can easily calculate the amount of current required from the drive circuit to switch the device on in a desired length of time because Q = CV and I = C dv/dt, the Q = Time x current.  For example, a device with a gate charge of
20nC can be turned on in 20 μsec if 1ma is supplied to the gate or it can turn on in 20nsec if the gate current is increased to 1A.  These simple calculations would not have been possible with input capacitance values.

In this datasheet, they specify 223 pC of "Total Gate Charge", so let's suppose that you want to switch this in 10 us, then current = Q/time = 223 pC / 10 us = 22.3 uA . Doesn't seems difficult to turn on at all.

A simulation seems to agree in that you should have short times to turn the state of the mosfet (see attachment). Of course,

The fact that the LDH-45 has analog and a PWM dimming could mean that the PWM input is just turned into a voltage internally, as others have already mentioned. Also take notice of the voltage limits on the PWM and Analog dim inputs, that seem to be exactly the same, so that further suggests that they're related. And they don't specify the time to turn on. Also, take notice that there is a graph that shows "output current vs PWM dim input", so, it isn't turning the leds on and off with the PWM duty cycle, but just modifying the output current according to it.

It seems quite possible that the LED drivers themselves are just slower to turn on. You can check that with an oscilloscope. Tie one channel to the PWM dim input, and the other to Vout. That way you can check how much time it takes the output to rise after you enable the driver. You should be able to do that with a digital scope with the trigger in manual mode, using the PWM dim input as trigger. (VOUT- is the same than GND? otherwise you could have trouble trying to measure the output, see Dave's video on "how to not blow your scope").

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That's very informative and thanks for verifying the validity of the passives, I didn't quite get a chance to wrap my head around Spice yet. I haven't had a chance to play with the scope yet, I really only use it for verifying the switching of proximity sensors on tooth wheels and encoders and checking for noise, but I will get into it as you suggested if I continue on this route. This board as is seems more feasible now that I've worked the lighting a little better to accommodate the lower voltage output of the LDH compared to the AC-DC driver. I'm down to about 460us which is probably acceptable, my only small gripe is that there is a very slight change in light output between camera frames. Again it's something that wasn't seen on the AC setup but I'm sure it has to do with slight inconsistencies in the LDH driver while using it in this way. Not quite a dealbreaker yet though.

I did breadboard an SMD SSR to test the LDH for switching the output and this looks like a pretty big fail, which is probably why the PWM, On/off function exists in the first place. Unilike the AC-DC drivers the LDH drivers do not recover the output well at all while being switched so that's not an option. I did consider ealex's comment a little more but it still seems it would be an issue having to pull 48v DC out of something.......Then it dawned on me that I've been a bit narrowminded again because of all the different turns I've had to take with this whole project. I use 12 LEDs wired in series on a PCB I had fabbed based on Testing with the previous methods and that works well. Not sure Why I didn't figure sooner that I can achieve the same thing running them in a series/parallel combination at 24v. I modified one of the LED PCBs for this. I still like trying to switch them with an Opto for isolation so I'm going to play around with that and see how that turns out.
fsr:
You're welcome. I hope you find a way to make the circuit work as expected.
Of course, if you get the led board voltage under you PSU's voltage, you don't need any DC-DC converter at all.
If the voltage sags with the rapid change of the load, you can always add some capacitance close to the switching element to act as reservoir.
If you use two or more led strips in parallel, remember to use one resistor per string.
blazini36:

--- Quote from: fsr on December 23, 2018, 10:23:26 pm ---You're welcome. I hope you find a way to make the circuit work as expected.
Of course, if you get the led board voltage under you PSU's voltage, you don't need any DC-DC converter at all.
If the voltage sags with the rapid change of the load, you can always add some capacitance close to the switching element to act as reservoir.
If you use two or more led strips in parallel, remember to use one resistor per string.

--- End quote ---

I bread boarded 1 "channel" of the schematic with the opto,  and it seems to work well with the modified LED Board. Spent most of the day routing new boards, There's plenty of space on the PCBs for extra caps if necessary.

spec:
UPDATE #1 2018_12_25

Hi blazini36

There needs to be 56R gate stopper resistors connected directly to the gate terminals of any MOSFETs. This is to eliminate parasitic oscillations (especially with the long lead from the camera strobe output connected directly to the gates of the MOSFETs).

In general, you also need some decoupling using 100nF X7R dialectic ceramic capacitors (not surface mount, but the larger through hole type)

I take it that stobe 1 and strobe 2 inputs are from separate cameras and that a single camera strobe line is not feeding both NMOSFET gates?

Can you provide information on the camera strobe output: current sinking capability, resistance to 0V/max voltage when sinking current?

blazini36:

--- Quote from: spec on December 25, 2018, 10:38:31 am ---Hi blazini36

I haven't looked at your circuit in detail, but the first thing that stands out is that R2/R5 and R6/R7 have very high resistances (10k) and will slug the operation of the NMOSFET (2N7002), which has relatively high parasitic capacitances. A BJT, would be much better in this position.  The 2N7002 wide gate threshold range of 1V to 2V5 also complicates the issue. Depending on the current capability of the camera strobe switch, I would suggest lowering the values of R2/R5 to 1k or less if possible. Also, I would suggest removing the LED from across R2/R5 for the initial investigative stage at least.  And R6/R7 should be reduced to 1K. These modification should speed the switching on and off of the of the NMOSFET by 10 times.

There also needs to be 56R gate stopper resistors connected directly to the gate terminal of the NMOSFETs (between the junction of the camera strobe input and R2/R5 and the NMOSFET gates). This is to eliminate parasitic oscillations (especially with the long lead from the camera strobe output connected directly to the gates of the NMOSFETs) which may be another cause for effective delay in the NMOSFET switching.

In general you also need some decoupling using 100nF X7R dialectic ceramic capacitors (not surface mount, but the larger through hole type)

I take it that stobe 1 and strobe 2 inputs are from separate cameras and that a single camera strobe line is not feeding both NMOSFET gates?

I am not saying that speeding up the switching of the NMOSFETs will solve your problem but, if you are interested in pursuing this approach, please give details of the camera strobe switch, especially its current sinking capabilities and forward voltage/resistance.

https://www.diodes.com/assets/Datasheets/2N7002.pdf

--- End quote ---

Hi spec,

You're referring to the original schematic I posted right? The first iteration of this which I had someone help me with used an MMBT3904 rather than an N-fet. This only seemed to work with a diode and resistor between the remote and the base of the BJT. There were a bunch of other problems with that board so I just scrapped it and came up with the schematic I posted. The N-fet was pretty much because that's what I had laying around in a package I could bread board and it just worked.

Strobe 1 and 2 are completely separate for handling the 2 camera's individually. I'm attaching a page from the datasheet for the I/O on the camera's. Every one of these types of cameras I've seen use the same type of strobe output (what I call remote). Unfortunately they all seem to have lousy documentation on it but it states they can handle 200ma. Pin 10 is the output and through testing it seems that pin 12 is the reference.  I leave pin 12 connected to the common of the 24v power supply and my FPGA board sends 24v+ to pin 11 to trigger the camera. The camera internally handles its output on pin 10 as it can be adjusted through software. It's confusing (to me at least) the way it's listed but this is how I find it to work.

The adjustments you mentioned are pretty easy to implement, especially the 1K resistor as I can just swap that out on the current board. As I said a couple of posts ago it it seems I've been a bit narrow sighted using the Meanwell drivers rather than just rearranging the LEDs from 12 in series to 6x6 series parallel and just feeding them with 24v and sticking some high current resistors on the LED board. That approach just lets me use a high current switching device (an opto-mosfet in this case) and makes the boards much smaller. I've got the opto boards out for fab to test. If you wanna take a look at the last schematic I posted maybe you'll have some suggestions. Hopefully they'll only be component values as I'd have to fab new boards (again lol). The plus side to ditching the Meanwell drivers is that I can keep the boards under 100mm x 100mm and they're super cheap to get made.
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