Constant Current Source (MultiChannel)

Thread Starter

Atma888

Joined Nov 24, 2020
4
Greetings!
I´m new in circuit design and I am trying to design a circuit capable of driving some led´s, in a multichannel configuration. From the little I know and from the research that I´ve done, mosfets can do this job, but still a lot is missing from my schematics. I´m trying to build up this circuit with the help of spice simulation and achieve so far what can be seen in the picture.
On the top I have a voltage source from arduino (0 to 5V), connected to mosfet (still trying to understand the datasheets, but found this one: https://pt.mouser.com/datasheet/2/308/NTD3055L104_D-1389361.pdf), with a resistance to set the current for that channel. On the left I have the main power source, a voltage source of 48V. On the bottom the string of led´s. There is 1ch 36v@2400mA; 3ch at 34v@720mA; remaning channels at 350mA with leds in series.
Apologize for the rudeness of my circuit, may you guys tell me what I need to add for circuit protection and efficiency?
Appreciate a lot your help.
All the best regardsoutput.jpg
 

GeBJT

Joined Nov 3, 2020
19
Some suggestions as to what to do next in refining your circuit.

For protection, adjust the source resistors to give a nominal LED current of 20mA. You will find lots of single LEDs which use this de facto standard. LED voltages will vary according to colour.
The LED voltages and currents on your present circuit are unrealistically high.

Check the power dissipation in the MOSFETS (after you have changed the LED currents to 20mA). Your simulator will be able to display something like Idrain*(Vdrain - Vsource), translated into the simulator's exact language.
Your current MOSFET NTD3055L104 will not have any diffculty with the power involved but, for efficiency, you might want to choose a smaller device to experiment with in the future.
 

GeBJT

Joined Nov 3, 2020
19
Hello again!

Thanks for posting links to datasheets for lamps/modules. These are potentially dangerous levels of optical power: be sure to read and act on the Vision Advisory paragraphs to be found in both datasheets. Can you guarantee eye safety for both yourself and those around you?

Looking again at MOSFET power dissipation when driving the lamps/modules, the demands are high, well above the 2W guideline in the NTD3055L104 datasheet. I suggest you check the package's thermal resistance figures and assess whether heatsinking is viable.
[Power MOSFETs are designed for high speed switched circuits where they are either on (and in saturation) or off.]
 

Thread Starter

Atma888

Joined Nov 24, 2020
4
Hello
Thank you for the reply.

So running mosfets under the saturation area will result in a lot of heat?
So, if I choose different mosfets with different saturation currents, ie 1 mosfet with 2.4A saturation, 3 mosfets with saturation on 720ma and the rest with saturation on 350ma, that would improve the situation?
Other question is the surplus of voltage in each channel, does that imply any problem?
Kind regards
 

GeBJT

Joined Nov 3, 2020
19
Other question is the surplus of voltage in each channel, does that imply any problem?
This is a very good question and shows that you are identifying some design issues. I will provide an outline examination of power dissipation and consequences. But before that some words on saturation.

Your configuration is technically Class-A, so the MOSFETs are not in saturation. Saturation related parameters are not relevant for Class-A operation.

Context for examination of power dissipation: The left hand channel 1, with 48V at 2.4A. Handling 115 Watts must be done very carefully. I will break down the context into 3 parts: (Bias) Resistor R3, FET M3, Lamp D1.

Bias resistor R1.
Dissipation is (2.4A)^2 * (1.4 Ohm) = 8 Watts (approximately).
I suggest that you check with your component supplier on cost, size and running temperature of an 8W resistor.
[Hint: a typical 8W power resistor might run at 300 degC, dropping to 125 degC for a larger 30W size with 8W]

MOSFET M3.
Power
Voltage across Lamp (D1) = 36V.
Voltage across R3 = 2.4*1.4 = 3.4V.
Voltage across M3 = 48 - 36 - 3.4 = 8.6V.
Dissipation in M3 = 8.6*2.4 = 21 Watts (approximately).

Thermal
From NTD3055L104 data sheet, the absolute maximum heat handling capacity is 2W with 71.4 degC/W Junction−to−Ambient.
With 21W you will need 71.4*(2/21) = 6.8 degC/W Junction−to−Ambient.
Less Junction−to−Case: 6.8 - 3.13 = 3.67 degC/W Case−to−Ambient.
3.5 degC/W is a suitable starting specification for a heatsink.
I suggest you check with your heatsink supplier on cost, size and feasibility of mounting an IPAK Case 369D.

Lamp D1.
Dissipation in Lamp = 36*2.4 = 86 Watts (approximately).
Check on thermal management of high power lamps, eg Figure 1 in Design Guide https://www.cree.com/led-components/media/documents/CXA_design_guide.pdf and Table 5 in Thermal Management https://www.cree.com/led-components/media/documents/XLampThermalManagement.pdf

Summary: where are we in relation to your original question in post #1 about "... circuit protection and efficiency"?
There are some protection related design investigations to be done: optical safety (post #4), heat dissipation (above).
Class-A operation is inefficient, leading to large quantities of waste power in MOSFET and bias resistor.

Best wishes.
 

Sensacell

Joined Jun 19, 2012
2,838
While the concept you are persuing will work, its doing things the hard way.

High power LED's are almost never driven with linear current regulation, unless the "burden voltage" (supply voltage minus total LED voltage) is very small. The efficiency will be very low, managing the heat produced is difficult and requires bulky and expensive heatsinks, fans etc.

The modern solution is to use an inductor-based switching current regulator.

Google LED Driver ICs - you will find many options, all of which will yield smaller, cheaper, cooler and more efficient solutions.
 

jeffl_2

Joined Sep 17, 2013
45
Actually for efficiency, since you have an Arduino you should be considering "nominal" current since the Arduino will let you use PWM control (switching each gate on at a fractional duty cycle using hardware assistance). You didn't really indicate whether you want these to dim "in concert" or with independent control. I don't believe a single Arduino has ten independent PWM channels but I guess you can get more than one, you might need to add some gates with 5 volt output to "blend" the PWM channel(s) you would have in with individual channel control. In that case you can move the "ballast" resistors to the drain side, now you don't really need the gate resistors at all nor is there any need for the SPICE analysis, you just need to select the ballast resistance and wattage based on a maximum 100% duty cycle, and yes as others have noted the supply voltage you selected is way too high. You've made this all way too complicated but I have to say you did a good job documenting what you were thinking about and how you got there, good luck with this project.
 

jeffl_2

Joined Sep 17, 2013
45
I just remembered, on AliExpress for a couple bucks they have some 16 channel PWM boards based on PCA9685 that can be controlled by the Arduino, you can even daisy-chain multiple boards to control up to 992 outputs, just an option but it's a nice simple way to go.
 

Thread Starter

Atma888

Joined Nov 24, 2020
4
I've read that constant current is better for the LEDs and that is the basis for building this way, but yah didn´t comprehend at all if the difference is substantial for the lifetime of the leds since most drivers does not use constant current. pwm i´ve put aside because is for horticulture and I thought that would impact fotossintese and the general well being of the plant.
I've searched for full-on builted drivers but would come out very expensive so started searching mosfet or others options and did spice to try to comprehend the behavior. the control will be by raspberry, I've been learning python and have a working gui for that. so my idea right now is buying the led drivers for the 60w and the 30w leds and try later the mosfet array for the 3w leds.
 

AnalogKid

Joined Aug 1, 2013
9,262
If you operate the MOSFETs as saturated switches, the power dissipation will be much lower. To do this for each circuit, connect the source directly to GND, and move the resistor over so it is in series with the LED, between the LED cathode and the drain.

ak
 
Top