| Electronics > Projects, Designs, and Technical Stuff |
| Faster switching speed from mosfets in existing circuit/PCB |
| << < (8/8) |
| nick_d:
Yes, if you have 5V available use that, however bear in mind that the switching regulator (DC-DC converter module IIRC) is complete overkill if all it is supplying is your LED switching circuit. For low average current jobs as here, the 7805 is preferred as it is simple and cheap. It works by simply converting the unwanted volts to heat. So, say your power supply is 25V and you need 5V for your gate driver chip (or p-type small signal MOSFET). When your circuit draws 1mA at 5V, the regulator adjusts itself to act as a 20K ohm resistor so that it develops 20V across it when the current is 1mA, leaving 5V for your circuit. Thus the 7805 consumes 20mW and your circuit 5mW in this example (always in a 4:1 ratio as long as your supply remains 25V giving a 4:1 ratio with your desired 5V). The switching regulator would consume more than 5mW due to switching losses, leakage and parasitics, but would pay off at higher currents such as 100mA. Also, about MOSFET selection, if you have 5V available use logic level power MOSFETs, if you have 10V or 12V use ordinary power MOSFETs. Avoid having multiple supplies whereever possible! If you have a choice, go with what's cheapest. A 7812 is about the same price as a 7805, maybe slightly more. Logic level MOSFETs were traditionally more expensive but I think that may have changed now. Logic level MOSFETs are popular and getting better all the time (used in phones, PCs etc). cheers, Nick |
| blazini36:
--- Quote from: nick_d on January 03, 2019, 12:46:53 am ---Yes, if you have 5V available use that, however bear in mind that the switching regulator (DC-DC converter module IIRC) is complete overkill if all it is supplying is your LED switching circuit. For low average current jobs as here, the 7805 is preferred as it is simple and cheap. It works by simply converting the unwanted volts to heat. So, say your power supply is 25V and you need 5V for your gate driver chip (or p-type small signal MOSFET). When your circuit draws 1mA at 5V, the regulator adjusts itself to act as a 20K ohm resistor so that it develops 20V across it when the current is 1mA, leaving 5V for your circuit. Thus the 7805 consumes 20mW and your circuit 5mW in this example (always in a 4:1 ratio as long as your supply remains 25V giving a 4:1 ratio with your desired 5V). The switching regulator would consume more than 5mW due to switching losses, leakage and parasitics, but would pay off at higher currents such as 100mA. Also, about MOSFET selection, if you have 5V available use logic level power MOSFETs, if you have 10V or 12V use ordinary power MOSFETs. Avoid having multiple supplies whereever possible! If you have a choice, go with what's cheapest. A 7812 is about the same price as a 7805, maybe slightly more. Logic level MOSFETs were traditionally more expensive but I think that may have changed now. Logic level MOSFETs are popular and getting better all the time (used in phones, PCs etc). cheers, Nick --- End quote --- If you look at the original schematic, you'll see that this board serves another purpose as well. It takes 24v input and handles this strobe business, but it also uses a 12v and 5v DC-DC converter to supply an output connector. So I have 5 and 12v available for another purpose already but the 5v is used for on board logic, the 12v could also be used if needed. Why do you say to avoid having multiple supplies? Or did that not concern the 5v and 12v DC converters I just mentioned? Since now I'm just using 24v to power the LED arrays I tried wiring back up the external DC-DC solid state relays. The speed is much faster as it was with the original setup.This proves the opto-mosfet probably was just not a good choice. I whipped up a quick schematic with a simple gate driver that I think will work with the chosen mosfet. Its an inverting MIC5015. The idea is that the input is pulled up to 5v then the open drain of the camera will pull it down, since it's inverting it should operate as expected. I think I did this right lol. According to the datasheet I can also use 24v to pull up the input and the camera should be capable of pulling it down at 0.2A , 5v seems preferable since it's handy. The only thing I see that really effects switching speed is the voltage on the V+ pin, using logic level would slow it down. Thoughts? The other option that exists is that Opto22 makes a version of the external DC-DC SSR I used (DC60S-5) in a SIP (DC60-MP) and it has exactly the same switching specs. They're expensive and not quite as elegant as the gate driver and mosfet here but it is almost guaranteed to work and I already know it's very fast. |
| blazini36:
I read over the datasheet for the MIC5015 and it seems that low side switching is much faster than high side unless you were to "bootstrap" the gate driver with additional components. I reworked the schematic for low side switching. I've tried to imagine some downsides to low side switching the LEDs and it doesn't seem too bad. The board is fused modestly at 2amps for the V+ to the LEDs, The LED board itself is powered through about 7ft of wire but it's not connected to chassis ground externally. The LED board contains a thermal switch so if the low side were shorted to ground and the LEDs were to stay lit they would not overheat. Another safety measure I could use is to stuff another wire in the cable and since V+ is a constant connection unless the fuse blows or the thermoswitch opens, I can use the 3rd wire to feed back 24v+ from the board to an input on the FPGA board to which could throw a fault on an open circuit. Any thoughts on this approach? |
| fsr:
The only things i see that need attention are: You have one arrow to 24v at every side of the first fuse. The fuses for the leds are tied directly to the 24v input, or are below the main fuse? What kind of PSU do you have? It's enough with the two 100 uF caps? That seems to be all. Should work. |
| 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 --- Why didn't I listen to this guy lol. I saw the mention about the gate driver but I initially wrote it off because I really didn't know what it was. I didn't notice that you had mentioned "low side switch", thought I came up with that myself after the previous post. Anyway that last schematic was a winner. I got those boards back and whipped one up. Camera's expose at about 250us while pulsed through the board, but the fastest exposure I currently get if the strobes are on continuously is the same 250us. That means that I'm at rock bottom tor the amount of light available, the FETs are adding zero discernible overhead. I may see some FET overhead if I get the light output higher and the camera's are not limited by available light anymore, but I'm happy where it's at, I still have some breathing room. |
| Navigation |
| Message Index |
| Previous page |