AN3256

Circuit description

Doc ID 17790 Rev 1

5/23

2

Circuit description

C1, L1, and C2 provide filtering for conducted EMI. Bridge rectifier BRI feeds the stepdown

switching regulator. The regulator appears inverted - the flywheel diode, D8, is connected to

the positive rail instead of the negative. Q1 pulls the inductor input negative, rather than

positive. Inductor L2 filters the output current, feeding C7 and the LED load.

The circuit starts up with a trickle of current into C3 thru R1. It takes about ¼ second to

charge C1 to U1's startup voltage.

The startup timer in U1 starts the switching cycle by turning on Q1. Current in Q1 and L2

increases from zero to about 800 mA. This current appears on R10 and R11 which drop

about 0.6 V. A filtered portion of the rectified AC line (from R9 and C5, added by R14 and

R13) is added to this voltage, to compensate out the duty cycle shift due to line voltage. U1

turns off Q1 when it sees about 1 V at pin 4. The LED average current would increase with

line voltage if not compensated, due to the decrease in FET ON time. R10 and R11 drop

about 0.6V at the current peak. Line voltage compensation is added to this from divider R9-

R14-R13. C6 filters the line voltage to DC to avoid further distortion of the line current”.

L2's current continues to flow after Q1 turns off, instead flowing in D8. The current ramps

toward zero, at which time D8 turns off. L2 and stray capacitance then ring the voltage at

D8's anode down to about twice the LED voltage below the positive rail. When the ringing

voltage turns up, U1 senses the end of the discharge and turns on Q1. The cycle then

repeats. Current in L2's upper winding therefore ramps between zero and twice the load

current. When Q1 turns on, D8 has already turned off, so Q1 does not see D8's reverse

recovery spike.

Housekeeping power is supplied by the auxiliary (lower) winding on L2. This winding steps

down the peak-to-peak voltage (the bulk input voltage) applied to the upper winding by 5:1.

D1 and D2, with C8, peak detect the transitions on the auxiliary winding. D2 is a Zener

diode, clamping the maximum voltage applied to C3 to slightly over 16 V. The 5:1 turns ratio

is not optimum - about 7:1 would be better, wasting less power in D2 and R12.

Note:

It should be noted that this circuit is not capable of providing output open circuit protection.

Monitoring of the voltage on C3 no longer provides an indication of the output voltage.

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

through R7.

The current waveform at the circuit's AC input is not sinusoidal, but it is clean enough to give

acceptable power factor for residential use in the U.S. Some improvement at the leading and

trailing edges is provided by U1's multiplier input, which is allowed to affect the current limit

only when the instantaneous line voltage is very low.