AN3410

Circuit description

Doc ID 018890 Rev 1

5/28

2

Circuit description

2.1

Power components

C7, L2, and L3 provide filtering for conducted EMI. Bridge rectifier BR1 feeds the step-down

(buck) switching regulator. The regulator appears inverted – the flywheel diode, D1, is

connected to the positive rail instead of the negative. Q2 pulls the inductor input negative,

rather than positive. Inductor L1 filters the PWM voltage into a triangle wave of current. C2

removes the high-frequency ripple and attenuates the 120 Hz component in the LED load.

Note that the buck regulator is not capable of supplying power to the load if the load voltage

is greater than the input voltage. There are “flat spots” in the input current waveform around

the input voltage zero crossings. Power factor remains excellent, even with this distortion.

2.2

Power factor controller operation

Startup

The circuit starts up with a trickle of current into C8 through R7. It takes about ¼ second to

charge C8 to U1’s startup voltage. The trickle current adds to LED current, slightly improving

circuit efficiency.

The startup timer in U1 starts the switching cycle by turning on Q2. Current in Q2 and L1

increases from zero to about 1400 mA at the peaks of the input sinewave. This current

appears on R22 which drops about 1 volt max.

L1’s current continues to flow after Q2 turns off, instead flowing in D1. The current ramps

toward zero, at which time D1 turns off. The FET drain voltage then begins to fall.

Quasi-resonant FET turn-on

L1 and stray capacitance then ring the voltage at D1’s anode down to about twice the LED

voltage below the positive rail. When the ringing voltage turns up, U1 senses the end of L1’s

discharge and turns on Q2 very close to the minimum ringing voltage, starting the next

cycle. Current in L1’s upper winding therefore ramps between zero and twice the load

current.

When Q2 turns on, D1 has already turned off, so Q2 never sees D1’s reverse recovery

current.

Bootstrap power

Housekeeping power is supplied by the auxiliary (lower) winding on L1. The winding is

connected through D4 so that the transformed LED voltage (positive) is applied to C9, which

powers U2, and C8 which powers U1. R2 and C9 form a filter to remove ringing spikes due

to leakage inductance.

The auxiliary (lower) winding on L1 has a turns ratio that puts about 15 V on C9 with the AC

line applied. The voltage on C9 is proportional to the LED voltage. This will limit the number

of series LEDs in the load to a relatively narrow range, set by the acceptable Vcc for U1 and

U2.

The auxiliary winding also provides U1 with timing for the zero-current sensing function,

through R5.