Constant Current Power Supplies for LEDs David Treharne Ford Amateur Radio League May 9, 2019

Why a Constant Current Driver? LEDs are current operated devices. Their brightness depends on the current going through them. An LED needs to be current limited or it will burn out. A resister can be used in series, but this wastes energy in the resistor, and the light is dependent on the supply voltage. A constant current supply has constant light as long as the voltage is high enough for the full current going through!

How Does a Constant Current Supply work? A power supply is connected through an inductor (to store energy) The LED is connected in series with the inductor An IC switches on to let current pass through the inductor When the current reaches a threshold, programmed by a small series resistor, the IC turns the current off A flyback diode keeps the current flowing through the inductor and LED. When the current drops to a threshold, the IC turns the current back on. The switching frequency is a function of the size of the inductor The time to reach full current is a function of the input voltage. The higher the voltage, the shorter the PWM.

The Constant Current Supply Components Flyback Diode Current sensing resistor: R=V/I V=0.1 V. for 100mA, R=1 ohm AC Input Supply IC: SW closes to ground to charge the inductor Filter Capacitor to smooth electrical noise LED Bridge Rectifier Filter Capactor Inductor; Larger values = lower frequency, but smaller is less expensive

The Constant Current Supply in Motion 1. Current from the Power Supply 2. Through the R1 sense resistor 3. Through the LED 4. Through the Inductor 5. Through the IC to ground. Switch Off 1. Current continues through inductor, due to magnetic field 2. Through Zener Diode 3. Through R1 4. Back through LED

An Actual LED Driver Circuit for My Landscape lights

Power Consumption of Constant Current vs Resistor 10V input, 163mA= 1.63 Watts At LED: 3.2V, 500mA = 1.6 Watts How about using a voltage source and resistor? V= 10V, I= 500mA. Vdiode= 3.2V At resistor: 10V-3.2V= 6.8V R= 6.8/0.5A= 13.4 ohms Power thru the resistor= 6.8V x .5A = 3.4 watts. Constant Current consumes 1.6W Resistor consume 5.0W for same light. Constant Current uses 1/3 the power!!!!

Original Constant Current LED Driver Frequency = 77kHz

Duty Cycle Changes with Supply Voltage Top is at 10V, bottom is at 12V Note the bottom the Low time (time charging the inductor) is less than at 10V. The constant current circuit charges the inductor to the required current faster at higher voltage, so less time is needed. The net power drawn is the same, 1.5W here. This is the Femtobuck. It is running at 432kHz.

LED Output of Femtobuck LED Driver The voltage at the LED is 3.2V. In the time domain, the voltage is very steady, meaning the current is also quite steady.

Femtobuck LED Spectrum at LED A spike at ~437kHz, right where expected. There is not any harmonics present here.

Current Profile at bottom of Inductor Note the square edges of the on and off, with some ringing on the edges. This is efficient for the IC, but harmonics are being generated.

Spectrum at Bottom of the Inductor Two spikes seen, plus considerable 3rd, 5th, 7th and beyond harmonics. If the filtering of the power supply is not done properly, the amount of power that could be radiated is significant. The current is about 470mA to the LED at 3.2V.

Commercial Constant Current Drivers for LEDs Wide input voltage range: 90-300V AC. Pick the voltage (how many LEDs in series) and the current. Has protection on the input to prevent noise back into AC line, and output filtering.

Cheap LED Drivers As you could see from the small Femtobuck example, the switching circuit produces a lot of harmonics. An improperly filtered LED driver can radiate these harmonics, getting to quite the high frequency, especially if the inductor is chosen small (high base frequency) We know various LED light suppliers interfere with 2m operation, and some are worse than others. Commercial LED signs (and a certain blue light trim around a building) scream of interference.

Home LED Light Bulb

We Adapt and so do the Manufacturers Look at various manufacturers to pick LED lighting that 3rd parties have measured as less interfering Some have changed because they wipe out FM radio or Over the Air TV broadcasts Try and turn off the lights and see if your 2m radio reception changes. If the light is new, you can take it back for a different brand. The energy savings is quite significant, so LED lights are not going away!