Need Help with LED Driver design

Thread Starter

adrumsolo4u

Joined Aug 9, 2019
10
-- in simple case the PWM frequency won't need to be too high - just about the eye won't notice it or the interference of (incase the multi channel/-color)
My wife is a photographer and takes issue with most LED's. Christmas lights, LED strips, etc. With LED strips, the problem occurs when you attempt to dim certain channels in order to attain a specific color. At full brightness, and close to full brightness, the LED flicker is not noticable at all, even to her. Which makes sense given the PWM.

If you compare stage-production LED lighting such as Chauvet Professional COLORdash Par-Quad 18, you can see in their specs that they use a 600 Hz PWM Frequency to achieve "flicker free" lighting.

Here's a relevant resource.
 

Thread Starter

adrumsolo4u

Joined Aug 9, 2019
10
I can't find the actual specs for the LED strip but am I wrong to assume that they are meant to be driven with Constant Voltage at 24V? Wouldn't I need an LED strip specifically designed to be run CC? I believe SIRS-E makes such a strip.

Will the following setup work?

I tried to calculate the resistor for the TLC5971, which has 65 mA current capability, does this look right?

Rmax = (Vcc-Vih)/Iih
(5-2)/0.065
3/0.065 = 46.1
46 Ohm Resistor minimum. I used a 1K to be well within parameters.

which makes This Schematic. Will that work? Will the MOSFET need a heatsink? It looks like it intermittantly draws 1W of power, but this is at 24kHz. Should I still consider a second MOSFET instead of the BJT?
 

Audioguru again

Joined Oct 21, 2019
1,770
Your Mosfet has no part number. Many of the, work poorly when their gate to source voltage is only 5V and are designed to use 10V. You need a "logic level" Mosfet.
The Mosfet does not use 1W, instead the LEDs use 1W. A Logic level Mosfet with a current of 42mA and an on-resistance of 0.016 ohms uses a power of only 42mA x 0.016 ohms= 670 micro Watts.
We do not know if your unknown LED strips need current limiting or not.
 

MisterBill2

Joined Jan 23, 2018
6,745
The power dissipated as heat in the mosfet while it is switched into conduction will not be much because the voltage drop is quite low. If the drain to source voltage is 100 millivolts than the dissipation will be 100 milliwatts, but most mosfets have a much lower saturation voltage. But you will need to provide adequate gate drive voltage, meaning that the transistor may neeg to have a collector voltage greater than 5 volts. And that may be a problem with assuring cutoff.
And if the LED strip is designed to be run from a 24 volt source, then a current regulator is not needed, but your source does need to not exceed24 volts.
Because that latest circuit has an internal pull-up, the driver will only need to be able to pull downenough current to allow the 2N3906 to switch on. I do wonder if that 2 nanovolt off-state gate voltage is realistic. It seems unreasonably small.
 
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Thread Starter

adrumsolo4u

Joined Aug 9, 2019
10
Your Mosfet has no part number. Many of the, work poorly when their gate to source voltage is only 5V and are designed to use 10V. You need a "logic level" Mosfet.
This is a copy schematic of post 4, but using the simulator that @ci139 used to simulate the power loss of the MOSFET.

The Mosfet does not use 1W, instead the LEDs use 1W. A Logic level Mosfet with a current of 42mA and an on-resistance of 0.016 ohms uses a power of only 42mA x 0.016 ohms= 670 micro Watts.
We do not know if your unknown LED strips need current limiting or not.
As I said in another comment, the LED strip is 96 Watts at 24V. This is a four channel (color) LED strip. When I put my shop power supply (which is the only piece of electronic test equipment I own, besides a multimeter) on the LED strip, on one channel at 24V C.V., It read about 1 Amp. So each LED strip channel should consume ~24 Watts.

Given the schematic, how much power is consumed by the MOSFET? In other words, when the LED strip is controlled at full brightness, so essentially a 100% duty cycle, will these MOSFETS need to have heatsinks installed in order to avoid a fire or the release of "magic smoke"?

My second question was, would it be worth modifying the schematic; changing the BJT to a MOSFET. Another pertinent question I have about the MOSFET, is does it need any reverse current protection in order to handle the constant on/off? IE, a reverse diode, and capacitor?

Edit:
By the way, I want to thank you all so much for your help with this. I have had some basic training in electronics principals when I was in the military... I think I didn't learn anything at all in that course, because my parents gave me an electronics lab kit when I was 10. Those combined with aircraft avionics experience is the extent of my knowledge ... so actually designing circuits is far beyond what I've been trained to do.
 
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eetech00

Joined Jun 8, 2013
1,965
I can't find the actual specs for the LED strip but am I wrong to assume that they are meant to be driven with Constant Voltage at 24V? Wouldn't I need an LED strip specifically designed to be run CC? I believe SIRS-E makes such a strip.
I'll try to explain.
Constant current drivers are intended for: 1) a single LED, 2) a series connected string of LEDs.
A series connected string will draw a constant amount of current from the driver but requires a higher voltage through the string. A CC driver will maintain the same current thru the string since the series LED string presents itself as a single load to the driver.
LEDs are not perfectly matched, there are manufacturing tolerances. That said, when you parallel connect LEDs, each one will draw a slightly different amount of current. This type of connection doesn't lend itself well to a CC driver since each LED is a load, and should be avoided.
Hope that makes sense...

which makes This Schematic. Will that work? Will the MOSFET need a heatsink? It looks like it intermittantly draws 1W of power, but this is at 24kHz. Should I still consider a second MOSFET instead of the BJT?
Electronic devices consume current provided by a voltage source (power supply). The consumed current produces heat.
The devices don't "draw power". They dissipate heat that is called "Power Dissipation" and is described in terms of Watts.

eT
 

MisterBill2

Joined Jan 23, 2018
6,745
It is correct to assume that the LED strips are intended for a constant voltage, since a voltage specification is given. There is undoubtedly something in the LED strip to keep the current limited to some intended value, which may be as simple as a series resistor, or possibly more complex than that. You will also find that you can dim them by reducing the voltage, but the dimming will be a bit tricky because it is quite non-linear.
When you get the project completed it will be interesting if you can post a picture of it in operation. Completed projects are always interesting to see.
 

Thread Starter

adrumsolo4u

Joined Aug 9, 2019
10
For 24khz switching then the 10k resistor will not turn the Mosfet off quick enough. Here is a Mosfet driver circuit that produces fast switching:
It took me a bit to understand what was happening here with the complementary BJT's. If I understand correctly: when the top (NPN) transistor is open, after being pulled low, the bottom (PNP) transistor has the capability to become closed. I don't understand how/why this works, because VCE is rated at 40V max. Or can it work at a much much lower voltage as well?
 

Audioguru again

Joined Oct 21, 2019
1,770
Drummer-guy, the complementary output transistors are emitter followers that have no voltage gain but use a low input current and produce a high output current to quickly charge and discharge the high input capacitance of a Mosfet for high frequency switching.
The NPN pushes its output up to about +4.1V and the PNP pulls its output down to about +0.8V which is fine for driving the gate of a logic-level Mosfet.

The maximum allowed Vce of 40V for these transistors is not exceeded in this circuit that has only a 5V supply voltage
for them.

Please talk about windows and doors that open or close but transistors turn on or they turn off.
 

MisterBill2

Joined Jan 23, 2018
6,745
It took me a bit to understand what was happening here with the complementary BJT's. If I understand correctly: when the top (NPN) transistor is open, after being pulled low, the bottom (PNP) transistor has the capability to become closed. I don't understand how/why this works, because VCE is rated at 40V max. Or can it work at a much much lower voltage as well?
"Max" ratings are the limits of what the device should be expected to be able to work with. Bipolar transistors will work very well with much less voltage in most cases. Look at the Vce (sat) voltage for a clue as to the minimum voltage, and often that is even less.
 
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