Faster switching speed from mosfets in existing circuit/PCB

[spec]

Hi spec,

You're referring to the original schematic I posted right?

Whoops, your reply #22, with the revised circuit was not showing when I posted reply #23. Apologies. I have modified reply #23 accordingly.
Thanks for the additional information though.

[spec]

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.

Hi blazini36,
just looked at your new circuit in reply #22. The architecture is much improved, but the SSR would have a turn-on delay of over 1ms at the currents required to drive the LED banks in series parallel.

https://standexelectronics.com/viewer/pdfjs/web/viewer.php?file=https%3A%2F%2Fstandexelectronics.com%2Fwp-content%2Fuploads%2F2018%2F09%2FPhoto-MOSFET-Relay-SMP-36_V01.pdf

Do you actually need isolation between the 5V line and the 24V line?

Also, am i correct in saying that you do not care how slow the LEDs turn off (within reason)?

Is it your intention to have 500mA flowing through each LED string, giving a total LED current of 1A?

[blazini36]

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.

Hi blazini36,
just looked at your new circuit in reply #22. The architecture is much improved, but the SSR would have a turn-on delay of over 1ms at the currents required to drive the LED banks in series parallel.

https://standexelectronics.com/viewer/pdfjs/web/viewer.php?file=https%3A%2F%2Fstandexelectronics.com%2Fwp-content%2Fuploads%2F2018%2F09%2FPhoto-MOSFET-Relay-SMP-36_V01.pdf

Do you actually need isolation between the 5V line and the 24V line?

Also, am i correct in saying that you do not care how slow the LEDs turn off (within reason)?

Is it your intention to have 500mA flowing through each LED string, giving a total LED current of 1A?

Bah, why did I not notice the on time on that opto, maybe I got confused about ms vs us or something.  There does actually seem to be an overlap of what the actual camera's behavior vs the strobe outputs is. Worse case scenario is that I have to rework the board for something like this https://www.opto22.com/products/dc60mp, which is a SIP version of the external SSR I originally used with the AC driver with the same specs. this switches on in 100us, yet I was able to set camera exposure at less than that. I'll have to test and see what happens, worse case is that I have to rework the board for  a different switching device.

Isolation is more of a concern between the camera's GPIO and the strobe business I have going on. Plus I find the optos a bit easier to work with (speed may wind up being a concern though). It might not be a concern but the camera's are far more expensive than what the PCB components cost. Honestly all I really have to do now with the drivers out of the way is safely switch 2-3amps quickly

Turn off time isn't super important. The only reason it's switched at all is the LEDs will get super hot and waste alot of power.

The LED's can handle quite a bit more if heat is managed but 500ma per string is actually conservative for what I'm trying to do, I may actually drop the resistors down or break them into 3-4 strings to increase the voltage per string.

[blazini36]

Hi blazini36,
just looked at your new circuit in reply #22. The architecture is much improved, but the SSR would have a turn-on delay of over 1ms at the currents required to drive the LED banks in series parallel.

https://standexelectronics.com/viewer/pdfjs/web/viewer.php?file=https%3A%2F%2Fstandexelectronics.com%2Fwp-content%2Fuploads%2F2018%2F09%2FPhoto-MOSFET-Relay-SMP-36_V01.pdf

Do you actually need isolation between the 5V line and the 24V line?

Also, am i correct in saying that you do not care how slow the LEDs turn off (within reason)?

Is it your intention to have 500mA flowing through each LED string, giving a total LED current of 1A?

Actually now that I look at it again it looks like the biggest mistake I've made regarding the turn on time is the  forward current of the photomosfet's LED. I chose the resistor based on the typical current listed in the datasheet. Looking at the graph it seems that if I just switch the 4.7k resistor with a 150ohm I can speed the opto up. I didn't see anything in the datasheet regarding the load current vs turn on time, can you point that out? It seems at a minimum on time of .3ms I chose one of the faster  optos I could have in this package.

[spec]

  Mistakes, boobies, cock-ups are all part of development. Big errors don't matter, it is the nasty pernicious little errors, that are not obvious, that bite you in the arse and cause the most problems. My favorite errors are the ones that only show at customer demonstrations.

Thanks for info.

There is just one thing though. It is not possible to isolate the camera strobe output from the 5V/24V DC supply because it is a switch to 0V, but I do understand about the need to protect expensive cameras (I have one  ).

I have sketched a design for driving the LEDs using a power PMOSFET and four strings of LEDS. When I have put it into EAGLE I will post it for discussion. The 5V supply is not used, only the 24V supply.

[spec]

As mentioned in reply #29, attached below is a schematic for a notional circuit for your camera strobe light.

If you favor this approach just say and I will do a schematic suitable for prototyping.

Or if you have any questions or would like any modifications also let me know

[blazini36]

Mistakes, boobies, cock-ups are all part of development. Big errors don't matter, it is the nasty pernicious little errors, that are not obvious, that bite you in the arse and cause the most problems. My favorite errors are the ones that only show at customer demonstrations.

Thanks for info.

There is just one thing though. It is not possible to isolate the camera strobe output from the 5V/24V DC supply because it is a switch to 0V, but I do understand about the need to protect expensive cameras (I have one  ).

I have sketched a design for driving the LEDs using a power PMOSFET and four strings of LEDS. When I have put it into EAGLE I will post it for discussion. The 5V supply is not used, only the 24V supply.

Maybe isolation is the wrong word, seems to me that optos offer better protection from shorting the signal side on the load side if components fail.

I have the components and boards for my opto setup coming in so I'm going to test that and see how it works out. If it's too slow I'll take ya up on the offer to help me out with a power mosfet schematic.

thanks bud

[spec]

Maybe isolation is the wrong word, seems to me that optos offer better protection from shorting the signal side on the load side if components fail.

If you feel happier with an optocoupler that is fine. But I would not say that an opto is any safer, per se, than a non opto with built in protection. Opto couplers can fail too.   A high speed optocoupler could be added to the circuit of reply #30 though.

I have the components and boards for my opto setup coming in so I'm going to test that and see how it works out. If it's too slow I'll take ya up on the offer to help me out with a power MOSFET schematic.

thanks bud

No sweat. Be interesting to find out how well the opto relay works.

[nick_d]

How about using an N type power MOSFET which is cheaper, and driving the gate with a P type small signal MOSFET. This will reduce the gate capacitance shown to the camera down to picafarads and you can arrange things so that the gate of the power MOSFET gets pulled up quickly but pulled down by a resistor such as 330 ohms. The only drawback to this is the power MOSFET would heat slightly during turnoff, as it would become resistive during the time to discharge the gate capacitance (about 1nF) through the 330 ohms (about 330 ns). A fast opto can be inserted too, they have open drain outputs so would behave pretty much like direct connection to the camera. This arrangement would need the 5V supply though. Probably a 7805 is the cheapest way.
cheers, Nick

[blazini36]

How about using an N type power MOSFET which is cheaper, and driving the gate with a P type small signal MOSFET. This will reduce the gate capacitance shown to the camera down to picafarads and you can arrange things so that the gate of the power MOSFET gets pulled up quickly but pulled down by a resistor such as 330 ohms. The only drawback to this is the power MOSFET would heat slightly during turnoff, as it would become resistive during the time to discharge the gate capacitance (about 1nF) through the 330 ohms (about 330 ns). A fast opto can be inserted too, they have open drain outputs so would behave pretty much like direct connection to the camera. This arrangement would need the 5V supply though. Probably a 7805 is the cheapest way.
cheers, Nick

The opto-mosfet(SSR) is turning out to be too slow. It requires too much on-time to drive the output. I've been reading u on mosfet characleristics and I've pretty much come up with what you mention here, minus the opto. The LED array is drawing a little under 1.2a so I'm thinking I can use one of those 6pin N/P mosfet arrays for convenience. I'm looking into gate drivers as as was mentioned before as well. The board has a 5v switching regulator so it shouldnt need the 7805 right?

[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]

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

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]

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 ?

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.