ledled stripled-drivermosfet-driver
I need to design a constant current driver for a filament LED bulb. The input is 230VAC and the output should be 120VDC, 20mA (constant current) for 4 filaments arranged in series-parallel structure. Can somebody please help me to find a simple transistor-based solution or some small linear circuit with low BOM cost.
please help me
I have some concerns about the current limit circuit (based around the BC337 / R2) and its ability to work with PWM - every time the pulse goes high from U3 (initiating a current flow through the MOSFET), the voltage developed across R2 will activate the BC337 and clamp the drive voltage to the MOSFET. This means the MOSFET will get quite warm because it is operating as a current limit device.
Added to this you are trying to control the LED current via PWM BUT the BC337 circuit is also trying to limit the current. It doesn't look like it should work as you expect.
When you say the LED starts blinking, what is your PWM frequency?
Given the circuit you have I also have concerns about possible high voltages from the LED string being capacitively coupled (through the MOSFET) back to an IO pin on your MCU. Maybe protection diodes would help.
Yet another simple LED driver request and yet another application for the LM317 as constant current source to be a simple (if somewhat inefficient) LED driver, but, being in current source mode, at least a good one for LEDs.
simulate this circuit – Schematic created using CircuitLab
You can get brave and figure a value for the resistor, or you can sneak up on it with adding resistors in parallel as shown at an early stage, given you have sketchy parts.
1.25 volts is maintained between Adj and Out (the principle of the LM317's operation) - so each 10 ohm in parallel will add 125mA to the output in this configuration; or if you think the thing should take 900 ma, you could put together some combination that would get you close to 1.38 Ohms. Note that you'll burn over a watt in that resistor as well as the dissipation in the LM317 itself (which is best regarded as a "smart resistor" at least for power use.) Other driver toplogies can be more efficient - this one is mostly simple, and involves just a few components.
As commented above "2-3A at 12V" would be 24-36 watts. 10 watts at 12V would be 833 mA, 10 watts at 9V would be 1111 mA. It is important to note that LEDs are primarily current driven devices - that is, the current, and especially the maximum current, is what you need to control.
Best Answer
You could consider a circuit like this one (one for each string, plus a bridge/capacitor to make +300VDC):
simulate this circuit – Schematic created using CircuitLab
M1 will have to be rated appropriately and have an appropriate heatsink, perhaps of a type typically used in off-line switchers.
Or see if two resistors (one per string) plus the bridge + capacitor is good enough, because that will be cheaper and more reliable while using the same amount of power.