boostdc/dc converterefficiencyled strippwm
I am stepping up ~4V to 12V to power an LED strip with a boost-regulator.
I don't want to run the LED strips (3 leds in series with a resistor) at full brightness so I need to PWM them.
Is it better to PWM the output of the boost circuit before the LED strip with some sort of FET, or to PWM the enable signal to the switching regulator itself?
I haven't tested this, and its off the top of my head, but based on http://www.stuffandymakes.com/blog/2012/07/08/disney-glow-with-the-show-ears-teardown the LEDs are common anode with the microcontroller sinking current (open drain). Using a optocoupler on each line, with the led amp providing the power switching, you should get exactly what you want. All you need is the optocoupler in between vcc and the rgb led (one per color).
simulate this circuit – Schematic created using CircuitLab
Option B will need a bigger boost converter that can handle 2.5 Amps out (5050 strip current) and a AC-DC adapter of let's say 120% of that in power. So 2.5 A * 12 V = 30 Watts * 120% = 36 Watts / 5 V = 7 + Amps at 5V. Plus the likely minimal current of the microcontroller.
Option A doesn't have those needs. A 12V 3 Amp supply, with a step-down converter of a few hundred milliamps at best for the microcontroller. Let's assume 250mA at 5V and 120% for efficiency. 250 mA * 5V = 1.25 Watts * 120% = 1.5 Watts / 12 Volts = 125 mA at 12V input.
Much smaller regulator is needed, as is the power supply. And that means both will be cheaper and smaller physically, while being easier to find. It's a no brainer. A 7 Amp 5V supply is not a common item, but you could find 2.5 or 3 Amp 12V supplies everywhere.
120% means 80% efficient regulator (20% power loss), and it's more likely to be 90%. My math here is all fuzzy back of the napkin but it's generally correct.
As to your question on how to test, basically just connect everything in ammeter or current sensing mode on a multimeter.
Best Answer
Why use PWM? Since you require a boost converter anyway, the simplest option would be to use a variable output voltage type and set the voltage to the required level to give the brightness you require. e.g., Boost to 9 V for reduced brightness.
A range of 6 V to 12 V would be adequate for most applications and wouldn't require anything too fancy in hardware.