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| Faster switching speed from mosfets in existing circuit/PCB |
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| blazini36:
--- Quote from: fsr on December 20, 2018, 12:40:11 pm --- --- Quote from: blazini36 on December 20, 2018, 12:40:11 am --- --- Quote from: fsr on December 19, 2018, 09:38:12 pm ---It seems like the 2n7002 are controlled by what seems to be a long wire. Could the slowness to turn be caused by something like wire inductance? --- End quote --- I don't think so. The cable that the remote wire is in is about 7 or 8 feet long, it's a 22 or 24awg 4 wire shielded cable. The cable that runs the VOUT to the strobe is just as long going back to the same place (2 wire 18AWG). The thing is that they are the exact same cables that ran the remote and VOUT with the AC-DC Driver and the External SSR. Unless the length of the wires is affecting the Mosfet differently than it did the SSR, I'd say it's probably not the cables. I will say that I don't expect the exposure to be as fast as it originally was at 30-100us depending on the amount of light diffusion used. The power output of the DC-DC drivers is lower so the exposure time will be longer due to less available light. I would say I'd expect it to be ~400us max. ~900us implies a delay, and the exposure trigger is slightly out of sync with the strobes flash. I had to cut this clip short but it illustrates the necessity for the quick strobe switching, This is on the original LED Driver setup. https://www.dropbox.com/s/66sjfddnhj5u0mg/Video%20Sep%2007%2C%209%2014%2016%20AM_0.mov?dl=0 --- End quote --- Well, the SSR could react differently, because they're optocoupled, so that any driving circuit is internal to the SSR. With a mosfet directly connected, you depend on the driving circuit to charge/discharge the gate fast enough. --- End quote --- True, hence the reason for the post as I'm not sure my pullup values and such were optimal to operate the mosfet of that this mosfet was the best choice. Though I tried to do my research my limited component level electronics knowledge makes me unsure of my component selection. I have many other considerations for the whole design of this thing so it's best I try to make the best of the PCB as relocating the drivers themselves to shorten the wiring is not as practical or easy as it seems. |
| blazini36:
--- Quote from: ealex on December 20, 2018, 02:40:44 pm ---i don't think the LED drivers you are using are intended for what you want. I can't find in their datasheet anything related to timing, etc. the PWM dimming input can have a RC filter inside the module, and that adds delays, and then you have the control loop delays / internal soft-start / etc - you have a "black box" with an unspecified behavior. if you only use the leds as a strobe light, why don't you have a constant voltage power supply, use some properly sized resistors for current limiting and a N-FET + proper driver for a low side switch ? --- End quote --- Well they're not exactly intended for this purpose, there is nothing on the common market that is. A LED driver/power supply manufacturer (Autec) suggested the Constant Current route to operate the LEDs in "burst" mode. It has actually worked flawlessly in the previous implementation, so much so that it had to be bumped down a bit. The built in on/off-PWM thing was a feature I had to try for simplicity and I just wanted to make sure my circuit was giving it the proper chance. To be apples to apples I have to use a switching device on the output of the LDH, as you said it could have a soft start of sorts but that's not an issue if the output is switched instead. A CV power supply would have to hit at least 48v to compare to the LDH driver. That would add another AC-DC supply to the mix unless I did a boost supply on the board from 24v but I don't know how that would react to 2 separate loads loads. Besides that I have no idea how to use a gate driver IC. It took me long enough to figure out how to use the Mosfet in the current design.The opto will be easy to implement if I can't get the exposure times down in the current design. If that doesn't work I'll probably make a post asking how to use a gate driver lol. |
| fsr:
--- Quote from: blazini36 on December 20, 2018, 11:31:28 pm --- --- Quote from: fsr on December 20, 2018, 12:40:11 pm --- --- Quote from: blazini36 on December 20, 2018, 12:40:11 am --- --- Quote from: fsr on December 19, 2018, 09:38:12 pm ---It seems like the 2n7002 are controlled by what seems to be a long wire. Could the slowness to turn be caused by something like wire inductance? --- End quote --- I don't think so. The cable that the remote wire is in is about 7 or 8 feet long, it's a 22 or 24awg 4 wire shielded cable. The cable that runs the VOUT to the strobe is just as long going back to the same place (2 wire 18AWG). The thing is that they are the exact same cables that ran the remote and VOUT with the AC-DC Driver and the External SSR. Unless the length of the wires is affecting the Mosfet differently than it did the SSR, I'd say it's probably not the cables. I will say that I don't expect the exposure to be as fast as it originally was at 30-100us depending on the amount of light diffusion used. The power output of the DC-DC drivers is lower so the exposure time will be longer due to less available light. I would say I'd expect it to be ~400us max. ~900us implies a delay, and the exposure trigger is slightly out of sync with the strobes flash. I had to cut this clip short but it illustrates the necessity for the quick strobe switching, This is on the original LED Driver setup. https://www.dropbox.com/s/66sjfddnhj5u0mg/Video%20Sep%2007%2C%209%2014%2016%20AM_0.mov?dl=0 --- End quote --- Well, the SSR could react differently, because they're optocoupled, so that any driving circuit is internal to the SSR. With a mosfet directly connected, you depend on the driving circuit to charge/discharge the gate fast enough. --- End quote --- True, hence the reason for the post as I'm not sure my pullup values and such were optimal to operate the mosfet of that this mosfet was the best choice. Though I tried to do my research my limited component level electronics knowledge makes me unsure of my component selection. I have many other considerations for the whole design of this thing so it's best I try to make the best of the PCB as relocating the drivers themselves to shorten the wiring is not as practical or easy as it seems. --- End quote --- But in this case, whatever is driving the mosfets is over the remote1/2 connections. That part isn't on the schematic. I'm no expert by any means, but i think that you need some kind of buffer that can supply enough current to charge the gate capacitance as fast as you need. Well, or at least you need to test if you need it. A scope on the gate should reveal how much does it take to rise. |
| blazini36:
--- Quote from: fsr on December 21, 2018, 04:17:01 pm --- --- Quote from: blazini36 on December 20, 2018, 11:31:28 pm --- --- Quote from: fsr on December 20, 2018, 12:40:11 pm --- --- Quote from: blazini36 on December 20, 2018, 12:40:11 am --- --- Quote from: fsr on December 19, 2018, 09:38:12 pm ---It seems like the 2n7002 are controlled by what seems to be a long wire. Could the slowness to turn be caused by something like wire inductance? --- End quote --- I don't think so. The cable that the remote wire is in is about 7 or 8 feet long, it's a 22 or 24awg 4 wire shielded cable. The cable that runs the VOUT to the strobe is just as long going back to the same place (2 wire 18AWG). The thing is that they are the exact same cables that ran the remote and VOUT with the AC-DC Driver and the External SSR. Unless the length of the wires is affecting the Mosfet differently than it did the SSR, I'd say it's probably not the cables. I will say that I don't expect the exposure to be as fast as it originally was at 30-100us depending on the amount of light diffusion used. The power output of the DC-DC drivers is lower so the exposure time will be longer due to less available light. I would say I'd expect it to be ~400us max. ~900us implies a delay, and the exposure trigger is slightly out of sync with the strobes flash. I had to cut this clip short but it illustrates the necessity for the quick strobe switching, This is on the original LED Driver setup. https://www.dropbox.com/s/66sjfddnhj5u0mg/Video%20Sep%2007%2C%209%2014%2016%20AM_0.mov?dl=0 --- End quote --- Well, the SSR could react differently, because they're optocoupled, so that any driving circuit is internal to the SSR. With a mosfet directly connected, you depend on the driving circuit to charge/discharge the gate fast enough. --- End quote --- True, hence the reason for the post as I'm not sure my pullup values and such were optimal to operate the mosfet of that this mosfet was the best choice. Though I tried to do my research my limited component level electronics knowledge makes me unsure of my component selection. I have many other considerations for the whole design of this thing so it's best I try to make the best of the PCB as relocating the drivers themselves to shorten the wiring is not as practical or easy as it seems. --- End quote --- But in this case, whatever is driving the mosfets is over the remote1/2 connections. That part isn't on the schematic. I'm no expert by any means, but i think that you need some kind of buffer that can supply enough current to charge the gate capacitance as fast as you need. Well, or at least you need to test if you need it. A scope on the gate should reveal how much does it take to rise. --- End quote --- 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 |
| fsr:
--- Quote from: blazini36 on December 21, 2018, 04:36:13 pm --- --- Quote from: fsr on December 21, 2018, 04:17:01 pm --- --- Quote from: blazini36 on December 20, 2018, 11:31:28 pm --- --- Quote from: fsr on December 20, 2018, 12:40:11 pm --- --- Quote from: blazini36 on December 20, 2018, 12:40:11 am --- --- Quote from: fsr on December 19, 2018, 09:38:12 pm ---It seems like the 2n7002 are controlled by what seems to be a long wire. Could the slowness to turn be caused by something like wire inductance? --- End quote --- I don't think so. The cable that the remote wire is in is about 7 or 8 feet long, it's a 22 or 24awg 4 wire shielded cable. The cable that runs the VOUT to the strobe is just as long going back to the same place (2 wire 18AWG). The thing is that they are the exact same cables that ran the remote and VOUT with the AC-DC Driver and the External SSR. Unless the length of the wires is affecting the Mosfet differently than it did the SSR, I'd say it's probably not the cables. I will say that I don't expect the exposure to be as fast as it originally was at 30-100us depending on the amount of light diffusion used. The power output of the DC-DC drivers is lower so the exposure time will be longer due to less available light. I would say I'd expect it to be ~400us max. ~900us implies a delay, and the exposure trigger is slightly out of sync with the strobes flash. I had to cut this clip short but it illustrates the necessity for the quick strobe switching, This is on the original LED Driver setup. https://www.dropbox.com/s/66sjfddnhj5u0mg/Video%20Sep%2007%2C%209%2014%2016%20AM_0.mov?dl=0 --- End quote --- Well, the SSR could react differently, because they're optocoupled, so that any driving circuit is internal to the SSR. With a mosfet directly connected, you depend on the driving circuit to charge/discharge the gate fast enough. --- End quote --- True, hence the reason for the post as I'm not sure my pullup values and such were optimal to operate the mosfet of that this mosfet was the best choice. Though I tried to do my research my limited component level electronics knowledge makes me unsure of my component selection. I have many other considerations for the whole design of this thing so it's best I try to make the best of the PCB as relocating the drivers themselves to shorten the wiring is not as practical or easy as it seems. --- End quote --- But in this case, whatever is driving the mosfets is over the remote1/2 connections. That part isn't on the schematic. I'm no expert by any means, but i think that you need some kind of buffer that can supply enough current to charge the gate capacitance as fast as you need. Well, or at least you need to test if you need it. A scope on the gate should reveal how much does it take to rise. --- End quote --- 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 --- End quote --- 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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