| Electronics > Projects, Designs, and Technical Stuff |
| Faster switching speed from mosfets in existing circuit/PCB |
| (1/8) > >> |
| blazini36:
This is a project that I've tried to get some help from a couple of electronics guys with that ultimately wound up with me wasting money on 2 bad board fabs. I decided to handle it myself and with a little help in the beginners forum was able to get a good circuit going. I designed some boards in Kicad and had them fabbed up and it's functionally working well. My only issue is that the switching speed is a little slow. The whole purpose of the board is to hold 2 Meanwell DC LED drivers (LDH-45) and invert the logic to trigger them. The "PWM-DIM" pin of the driver acts as a remote off pin when it is sunk to ground. The inverted logic is necessary because the LED drivers are used for 2 machine vision cameras that output an "open drain" which in practice sinks to ground for strobe on-time. Without the inversion the drivers would be normally on and the cameras would effectively turn them off which is backwards of the intended function. I've only gotten this to work using an N-channel mosfet used as the schematic below. The fellas I had help me used BJTs and it just didn't work right. I've incorporated a couple of switching regulators to handle some of the on-board components as well as some minor external power outputs. My main problem with this as it is is that it's slower than the original setup I had going but I'm not sure how much of it is attributed to the drivers themselves vs my "winging it" selection of component values. My original setup used 2 AC-DC Constant current LED drivers with the cameras sinking the V- control side of a DC SSR to switch the LED driver's outputs. I wanted to tidy things up by using these DC-DC drivers and get rid of a bunch of AC driven power supplies hence this PCB. The AC Drivers/SSRs could switch fast enough that the exposure time of the camera's could be set at 100us. Using this design my exposure time has to be set to 800us which implies that there is a switching delay between the camera's output and the strobe turning on. I'd like to get this switching as fast as possible. I'm not sure my choice of mosfet and component values was really optimal, but it otherwise works well. Hopefully this can be improved just by swapping out components of the same footprint as I'd like to use my existing boards. |
| blazini36:
120 views, no suggestions? |
| Circlotron:
My guess is that the LED drivers are some sort of switching regulator so they would have an inductor in series with the load. The larger this inductor the slower the turn-on speed of the load. There is also unknown circuitry between the control input and the load... What is the LED current? Might be more straightforward to just use a bigger mosfet + resistor to drive the LEDs directly. That may not be what you wanted to hear though. |
| nick_d:
Yes. I checked the datasheets etc but forgot to write the actual response. From memory, it gives a maximum PWM frequency in the datasheet for the LED driver, which sort of implies that the switching should be faster than that frequency (whose period is much less than 100us). But the LED is not necessarily switched synchronously with the PWM. They could simply be averaging your PWM and using the analog voltage to determine a voltage or current or duty cycle for the real LED drive. Since it is a module, you do not really know what they might be doing. The datasheet does not give a minimum turn-on time, suggesting that even the observed behaviour can't necessarily be relied upon. It would be more sensible to have a suitable LED drive voltage available, as I think you did in the previous version of the project, and instantaneously switch this onto the LED when you get your signal. You can use something like an LM317 adjustable voltage regulator as a current regulator by adding a sense resistor, it doesn't have a shutdown pin though, so you'd need a transistor or a better regulator (or a dedicated LED driver without its own voltage generation). Another option to consider is pulsed LED drive directly from a flyback inductor, which may well be what your module does, but if so you need to do it more quickly. It would be a cool circuit to build but possibly requires some research. cheers, Nick |
| David Hess:
The LDH-45 responds too slowly. Since the LDH-45 is a constant current driver, I was going to suggest using a transistor to short the output however the datasheet has a warning not to do this. What might work is switching a zener diode in parallel with the output to lower the output voltage lower than the LED forward voltage. I would get rid of the LDH-45 and use something else more suitable instead of trying to get it to work in a way it was not intended. |
| Navigation |
| Message Index |
| Next page |