ST AN3410 APPLICATION NOTE

AN3410

Application note

A 93% efficient LED driver solution for the US market

Introduction

This application note describes an LED driver that meets present requirements for the US market. It utilizes ST’s L6564 power factor controller in an unconventional circuit to regulate the input power to a step-down switching regulator. The circuit also compensates for variations in LED voltage drop, to maintain the average output current in a tight band over a wide range of line voltage and LED characteristics. While the input current waveform is not perfectly sinusoidal, power factor and harmonic content are well within the requirements for the US commercial market. The form factor was designed to fit into the PAR38 envelope - the driver and LEDs can be used to replace 65 W incandescent floodlamps.

■Specifications:

–Output current 350 mA +/-3% over 90 V-138 V line range

–Load: 18 series-connected 1 W LEDs

–Efficiency > 93%

–Power factor > 0.97

–Dimmer safe

–Non-isolated

■ST devices:

–L6564 transition-mode PFC controller

–STD5NM50 FET

–STTH1R04A fast recovery diode

–TS321AILT low power op amp

Figure 1. Physical envelope

!-V

Figure 2. STEVAL-ILL041V1 demonstration board picture

September 2011

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www.st.com

Contents	AN3410

Contents

1	Schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 4
2	Circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		5
	2.1	Power components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
	2.2	Power factor controller operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
	2.3	Controlling the LED average current . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
	2.4	Setting the “DC” operating point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
	2.5	Designing the “DC” control loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9
3	Control loop dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		13
4	Performance with LED loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		14
5	Graphical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		15
6	Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		17
7	Startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		18
8	Component stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		19
9	Thermal stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		20
10	Conducted EMI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		21
11	PC layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		22
12	Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		24
13	References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		26
14	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		27

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AN3410	List of figures

List of figures

Figure 1. Physical envelope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 2. STEVAL-ILL041V1 demonstration board picture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 3. Circuit schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 4. Constant-power V-I curve and linear approximation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 5. Linear approximation error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 6. Input current compensated for 3 levels of output power. . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 7. Error for 3 levels of load power. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 8. LED current vs. line voltage for 18 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 9. Power factor vs. line voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 10. Efficiency vs. line voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 11. Power loss vs. line voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 12. Waveforms at 96 V line. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 13. Waveforms at 108 V line. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 14. Waveforms at 120 V line. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 15. Waveforms at 132 V line. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 16. Startup waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 17. Component electrical stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 18. Conducted EMI, line 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 19. Conducted EMI, line 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 20. Top side foil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Figure 21. Top side placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Figure 22. Bottom side layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Figure 23. Bottom side placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

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AN3410

AN3410	Circuit description

2 Circuit description

2.1Power 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.

2.2Power 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.

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Circuit description	AN3410

2.3Controlling the LED average current

The control circuit works by controlling average input power. For this explanation, it is assumed that the power converter efficiency is constant over the range of line voltage and LED voltage. Therefore, average output power is also controlled.

Linear approximation of input power

Over a narrow range of line voltage, the sum of scaled average line voltage and scaled average line current closely approximates a constant power curve. If the sum is held constant, the input power can be held approximately constant by a feedback circuit.

Figure 4. Constant-power V-I curve and linear approximation

Figure 5 below shows the approximation’s error over a typical line voltage range.

Figure 5. Linear approximation error

This will be explained in more detail later.

Obtaining the line voltage reference

In previous work it was found that the reference waveform for the inverted step-down switching regulator should be taken from the negative output terminal for best power factor.

This point gives a line current waveform that goes to zero around the zero crossings and rises (and falls) more rapidly than the line voltage sine wave. (The converter input current goes to zero when line voltage falls below the DC output voltage.) The resulting line current can be seen in the scope photos. It’s ugly, but the power factor is excellent and THD is acceptable.

Average line voltage (minus the LED voltage) is derived from the peak voltage at the bottom of the LED string by R6, R15, and R20. U1 contains a precision peak detector, which places

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AN3410	Circuit description

the peak from this divider on C6. (Normally this voltage is used internally by the L6564 to adjust its multiplier gain to accommodate a wide line voltage range.)

Obtaining the line current (the controlled variable)

The vast majority of input current flows through R22, the sense resistor for the PFC-flyback converter. The average of the current in R22 and the average of the line voltage (minus the LED voltage) will be used in the power calculation.

Calculating the average power

Scaling and addition of voltage and current is done by R17 and R14. The AC noise present at their junction is removed by C12 - the DC voltage on C12 now represents the input power as calculated by the linear approximation. This voltage will be regulated by the slow PFC feedback loop.

Op amp U2 is wired as a non-inverting amplifier. The feedback loop requires only one inversion, supplied by the op amp in U1.

U2 performs three different functions:

–Derives a reference voltage from U1.

–Provides gain for the relatively low voltage on C12.

–Provides a point in the circuit to compensate for different LED voltages.

A DC reference voltage is derived from U1’s inverting input. This point will always be at 2.5 V if the control loop is in steady-state, because there is no DC current path to any other voltage source. The reference voltage is delivered to U2’s inverting input by divider R18R21. The voltage divider R18-R21 also sets the DC gain for U2.

If this circuit acted alone, the input power would be (approximately) regulated to a fixed value, and the LED current would inversely track the LED voltage.

2.4Setting the “DC” operating point

The control loop is to set the average current through R22 to deliver slightly more than the desired LED current when both the line voltage and LED voltage are at design center.

Deviations of line and LED voltage from this point will then cause smaller deviations of LED current.

The input current required is ILED x VLED / (VLINE x efficiency). The straight-line approximation (of the constant-power curve) should give equal voltage from the average line

voltage and the average input current.

The value of R22 is determined from the usual calculations (see ST’s excellent application note AN1059, reference 1).

The average input current (in R22) can now be calculated from the design center line voltage, output power, and efficiency. At design center line voltage, LED current, and LED voltage, the average voltage appearing across R17 due to current from R14 must match the average voltage on R22.

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Circuit description	AN3410

Stirring in the LED voltage

Compensation for LED voltage changes follows two paths in this design:

–by direct subtraction of the LED voltage from the line voltage (path 1)

–from the transformed LED voltage on C9 through R12 (path 2).

Path 1

An increase of LED voltage will reduce the voltage at U1 pin 5. This reduces current through R14. The feedback loop will then call for more line current to compensate.

Path 2

The voltage compensation obtained from the 18-LED load is not quite sufficient to flatten the LED current over the expected range of LED voltage, so the second path through R12 is also implemented. The LED voltage (multiplied by L1’s turns ratio) is available on D4’s cathode, filtered by C9. Current proportional to this voltage is delivered to U2’s inverting input by R12. The reference for the operating point is thus compensated by the LED voltage.

The figures below show the results of compensating the input current setting for load voltage.

Figure 6. Input current compensated for 3 levels of output power

In Figure 6 the reference has been compensated by adding a current proportional to the LED voltage. Three levels of input power are shown, corresponding to LED voltages of 10% below nominal, nominal, and 10% above nominal. Figure 7 below shows the error between the approximations and the ideal values.

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AN3410	Circuit description

Figure 7. Error for 3 levels of load power

Note that the curves coincide at the nominal line voltage, where LED voltage compensation is perfect. Also, for nominal LED voltage, line voltage compensation is at its best, +/-1.5%.

Over the entire range of line voltage (120 V +10%,-20%) and LED (+/-10%) voltage, the variation of LED current is less than 4.5% (+1.5%, -3%).

2.5Designing the “DC” control loop

Finding values for the resistors in this loop is surprisingly easy.

(For this procedure, diode drops will be ignored for simplicity.)

Let us assume that the average LED current is to be 350 mA, and a string of 18 LEDs drops about 54.6 V. Output power is 18 watts. Input power will be assumed to be 20 W (90% efficiency, actually conservative).

Assume the input current and voltage waveforms are sine waves (power factor is high, so this is a good assumption).

The converter output current waveform is a triangle wave with an (assumed) rectified sinusoidal upper envelope, and a lower envelope at zero. The short-term average of the triangle wave is half its height. The rectified sinusoid has an average current of 63% of its peak current.

At the peak of the input voltage, the peak current delivered to the LEDs is therefore.

Equation 1

We will allow some margin for the relatively slow fall time of the drain voltage due to the zero current detection, so the peak current will be slightly higher. We will design the inductor for 1.4 A peak current in the main winding. The L6564 control chip has an upper voltage limit on its current sense input of 1.08 V for linear operation. So the maximum value of R22 is

Equation 2

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