LED driver basic assembly

Teemu Hakala edited this page Jun 4, 2013 · 8 revisions

PoE LED driver schematics with low-pass filter highlighted

Please note that it may be a good idea to build the circuit gradually, piece-by-piece, and measure the result with an oscilloscope before continuing. Alternatively, you may want to design and build the circuit first, but then disconnect some components and measure only a partial circuit first. However, if you are experienced, just go ahead and assemble and test the full circuit.

The reason why it may be better to assemble first a partial circuit is that 48V is already a fairly large voltage, and it is easy to burn components if your connection is not right. For example, if you connect the Q2 helper N-FET the wrong way, it may either burn or pass continuous current (through its reverse diode), thereby opening the Q1 switching P-FET, in which case the full 48V current may pass continuously through the coil L1 and even the HP-LED or other load, burning one of them.

The circuit can be broken into the following networks, each of which can be one track of a breadboard:

Network name | | | | | |
--------------------------------|--------------|------------|------------------|------|--------------|------------ Positive supply input | PoE supply + | R2 pin 1 | Q1 Source | | |
Switch element gate | R2 pin 2 | R1 pin 1 | Q1 gate | | |
Helper FET drain | R1 pin 2 | Q2 drain | | | |
PWM input | PWM control | Q2 Gate | | | |
Supply input negative | PoE - | Q2 source | D2 anode | C1 - | Rshunt pin 2 | C2 pin 2
Switch FET drain | Q1 drain | D2 cathode | L1 pin 1 | | |
Buck output positive | L1 pin 2 | C1 + | D1 anode LED | | |
Current shunt measurement point | D1 cathode | R3 pin 1 | Rshunt pin 1 | | |
ADC connection point | R3 pin 2 | C2 pin 1 | | | |

IMPORTANT! Remember to add whatever you need to make sure Q2 remains off if its gate is not connected to an Arduino pin, or if the Arduino pin is left floating due to a programming mistake.

Now that we have analysed the circuit into connection networks we can start assembling the components.

Start with placing the switching P-FET Q1 in the bigger TO-220 package and the helper N-FET Q2 in the smaller TO-92 package. After these, add the EMC filter capacitor C1, which is a large blue electrolytic capasitor. Hopefully you have a place for the inductor L1 (black, round) and the shunt resistor Rshunt (white, large). Now that the biggest components are set, add the rest. Try to avoid letting component legs touching anything but the breadboard hole. The legs can be bent and shaped. Placement of components can be refactored with this in mind.

You might want to draw a map of the breadboard section on a graph paper. It may be a good idea to draw a prettier version of the actual design, to cross check between that the actual breadboard and the net list above do match with each other.

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