Complete noob stacked MOSFET question

Alec_t

Joined Sep 17, 2013
14,280
Keep bottom half the same except switch the MOSFET to the low side...
I'm unclear what you're trying to do. Since LED1 is in series with LED2 then the MOSFET would not be on the low side of LED2.
If you are trying to PWM both LED arrays individually, then your CC driver is going to see a combined PWM load which will be 'on' at some instants and 'off' at others. This may well conflict with its own PWMing trying to maintain a constant load current. It's anybody's guess how it will react to that. Just bite the bullet and buy a second CC driver :rolleyes:.
 

shortbus

Joined Sep 30, 2009
10,045
Hey, looking into 555 timers for a pulse generator.
I don't know if I'd go that route. The 555 is a good learning circuit and was the thing when it came out but there are better ways of doing a PWM. Have a look at the SG3525 IC or one of the variants. You can even drive the mosfets directly. A Google of the part number will give quite a few proven circuits for using it in PWM.
https://www.st.com/en/power-management/sg3525.html
 

Thread Starter

Chiefrunningphist

Joined Feb 22, 2019
37
I don't know if I'd go that route. The 555 is a good learning circuit and was the thing when it came out but there are better ways of doing a PWM. Have a look at the SG3525 IC or one of the variants. You can even drive the mosfets directly. A Google of the part number will give quite a few proven circuits for using it in PWM.
https://www.st.com/en/power-management/sg3525.html
Great Scott! Haha thanks man ill give it a look up. I gave the schematic a glance but it'll take me quite some time to decipher what's going on lol but that's just the way it goes when you're new, I think seeing it in action will help, thanks again!

3,2,1, Google!
 

shortbus

Joined Sep 30, 2009
10,045
Have you used one of these before?
Not lately. You would need two circuits though if you want to PWM directly to 2 panels, like your showing in your sketch. To do the same with a 555, you would also need 2 circuits.

The big advantage of the SG3525 over the 555 is it's output amperage value. While a mosfet gate is voltage dependent it is still like a capacitor. And a capacitor charges faster with higher amperage, so the SG3525 will turn the mosfets on and off faster and cause less heat in the mosfet. The main reason for over heating a mosfet is slow turn on and off times.

I'm still having a hard time trying to follow what you're trying to do. Are you wanting two different LED panels at different brightness, on at different times, but over the same area? Area like a grow room? If this is the case why not make one big panel and just switch off part of the LEDs when you want the lower light level?
 

Alec_t

Joined Sep 17, 2013
14,280
The maximum allowable supply voltage for the SG3525 is 40V, so it won't survive in your circuit if the CC driver is supplying 60V.:(
If the LED1 array has a forward voltage ~5V, then Z1 and R5 will be wasting a lot of power.:(
 

Alec_t

Joined Sep 17, 2013
14,280
With the LED1 and LED2 arrays in series then, even if the PWM duty cycle for LED1 is different from that of LED2, the average current in LED1 will be the same as in LED2, i.e the dimming will not be independent.
 

shortbus

Joined Sep 30, 2009
10,045
The maximum allowable supply voltage for the SG3525 is 40V, so it won't survive in your circuit if the CC driver is supplying 60V
The SG3525 is only a timing and switch signal generator, to generate the gate signal. The mosfet D/S you chose is what would handle the 60V. He is/was using a low side mosfet, but that is subject to change.

A 555 isn't able to use 60V either.
 
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shortbus

Joined Sep 30, 2009
10,045
I want different LEDs populated on a PCB to be dimmed up or down independent of each other. Not an entire grow area, just the light itself.
That is what I was saying earlier but didn't explain it good enough I guess. Instead of panel 1 and 2, I should have said row 1 and 2. Shutting off every other (or how ever you arrange them) will be the same as dimming a whole area. When every row of LEDs is on it's bright, but when certain rows, on the same panel, are off it isn't as bright. This could be done with either mosfets or even a relay.
 

Alec_t

Joined Sep 17, 2013
14,280
Not having them in series.
I await your solution. If the LED1 and LED2 arrays are effectively in parallel rather than in series then, because the forward voltage of LED1 is much lower than that of LED2, LED1 pulls down the voltage of the CC source below the point where LED2 can operate, unless some energy-wasting voltage dropper is inserted in series with LED1.
 

Alec_t

Joined Sep 17, 2013
14,280
Have you any info on what I'm thinking would be an issue if going the series route?
As I said in post #120, "With the LED1 and LED2 arrays in series then, even if the PWM duty cycle for LED1 is different from that of LED2, the average current in LED1 will be the same as in LED2, i.e the dimming will not be independent." IMO the same goes for however you try dimming multiple arrays/channels arranged in series ..... you get the same current in each array/channel.
 

Alec_t

Joined Sep 17, 2013
14,280
So you're saying that the supp channel MOSFETs, when operating at a lower duty cycle than the main channel MOSFET, will not act as a variable resistors reducing averaged current flow though the supp array?
If the supp channels are all in series with the main channel then all channels pass the same current. Where else can the current go?
Here's a sim for LED1 and LED2 channels in series. LED1 is pulsed with a 75% duty cycle 12u period and LED2 is pulsed with a 30% duty cycle 10u period. Compare the current waveforms of the two channels.
Series+PWM.PNG
 
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Alec_t

Joined Sep 17, 2013
14,280
Parallel channels 2-5 now face the problem I mentioned in post #126, unless they all have the same Vf. Also, if the parallel combination of channels 2-5 is in series wih the Main channel, the 'same current' effect illustrated in post #132 will come into play. Of course we still don't know if the CC driver will play nicely with all this switching and PWM-ing of its load.
 
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