ST AN3407 Application note

August 2011 Doc ID 018857 Rev 1 1/25

AN3407

Application note

Resonant driver for LED SMPS demonstration board

based on the L6585DE

Introduction

This application note describes the performance of a 100 W LED switched-mode power

supply (SMPS). The L6585DE embeds a high-performance transition mode (TM) power

factor correction (PFC) controller, half-bridge (HB) controller and all the relevant drivers

necessary to build a combo IC. The L6585DE embeds a wide range of features to provide

an energy-saving and cost-effective solution for the LED SMPS demonstration board

(STEVAL-ILL038V1).

Previous dedicated ICs for LED SMPS applications allowed designers to achieve good

driver efficiency. The PFC section has superior performance in terms of harmonic content

mitigation. High power factor (PF) and total harmonic distortion (THD) reduction are

obtained as required by international norms, especially concerning universal input voltage

operations. The TM PFC operation and high-efficiency performance of the half-bridge

topology provide very good overall circuit efficiency.

Film capacitors are one of the most popular types of discrete components. They generally

offer excellent electrical properties and are advantageous in high current and high

temperature conditions. For these reasons, film capacitors are used in LED SMPS

applications. In order to guarantee maintenance-free operation required by these types of

applications during the useful lifetime of the LED, electrolytic capacitors have been replaced

by film capacitors in the STEVAL-ILL038V1 board.

Other features, such as half-bridge overcurrent with frequency increase and PFC

overvoltage, allow designers to build a reliable, flexible solution with a reduced component

count.

Figure 1. STEVAL-ILL038V1 demonstration board

www.st.com

Contents AN3407

2/25 Doc ID 018857 Rev 1

Contents

1 L6585DE combo IC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Main characteristics and circuit description . . . . . . . . . . . . . . . . . . . . . 5

2.1 VCC section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Power factor corrector section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.3 Resonant power section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.4 Output voltage feedback loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3 Efficiency measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4 Input current harmonics measurement . . . . . . . . . . . . . . . . . . . . . . . . . 11

5 Functional check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5.1 PFC circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5.2 Half-bridge resonant LLC circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5.3 Converter startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

6 Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

7 EMI choke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

8 PFC coil specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

9 Transformer specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

10 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

11 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

AN3407 List of figures

Doc ID 018857 Rev 1 3/25

List of figures

Figure 1. STEVAL-ILL038V1 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Figure 2. Application example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Figure 3. Multiplier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 4. Oscillator characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 5. Half-bridge protection thresholds during run mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 6. Electrical schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 7. STEVAL-ILL038V1 demonstration board: efficiency vs. load . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 8. STEVAL-ILL038V1 demonstration board: full-load efficiency vs. VAC . . . . . . . . . . . . . . . . 10

Figure 9. STEVAL-ILL038V1 demonstration board: power factor vs. load . . . . . . . . . . . . . . . . . . . . 10

Figure 10. EN61000-3-2 Class-D standard - 185 VAC/50 Hz, THD=4.86%, PF=0.993 at full load. . . 11

Figure 11. EN61000-3-2 Class-D standard - 230 VAC/50 Hz, THD=5.98%, PF=0.980 at full load. . . 11

Figure 12. Input current waveforms - full load at 115 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 13. Input current waveforms - full load at 230 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 14. PFC stage waveforms at 115 VAC - full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 15. PFC stage waveforms at 230 VAC - full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 16. PFC stage waveforms at 115 VAC - full load - detail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 17. PFC stage waveforms at 230 VAC - full load - detail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 18. Primary side LLC waveforms at 115 VAC - full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 19. Secondary side LLC waveforms at 230 VAC - full load . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 20. High-frequency ripple on output voltage at 115 VAC - 60 Hz - full load . . . . . . . . . . . . . . . 15

Figure 21. Low-frequency ripple on output voltage at 115 VAC - 60 Hz - full load . . . . . . . . . . . . . . . 15

Figure 22. Wake-up at 115 VAC - 60 Hz - full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 23. Wake-up at 230 VAC - 60 Hz - full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 24. PCB: topside and through-hole components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 25. PCB: bottomside and SMD components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 26. PCB: topside placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 27. PCB: bottomside placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 28. EMI: OTC21V-4S vertical type EMI choke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 29. PFC: QP2520V-vertical type for PFC choke. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Figure 30. Transformer: LP2920H - horizontal type for LLC transformer. . . . . . . . . . . . . . . . . . . . . . . 22

L6585DE combo IC AN3407

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1 L6585DE combo IC

The L6585DE embeds both a PFC converter and a half-bridge resonant in a single SO20

package.

● Transition mode PFC converter with overvoltage and overcurrent protection

● Half-bridge controller with high-voltage driver (600 Vdc) and integrated bootstrap diode

● 3% precise, fully programmable oscillator

● Overcurrent protection

● Hard-switching detection

Figure 2. Application example

AN3407 Main characteristics and circuit description

Doc ID 018857 Rev 1 5/25

2 Main characteristics and circuit description

The main features of the SMPS are listed here below:

● Extended input mains range: 90 ~ 265 V

AC

- frequency 50/60 Hz

● Output voltage: 48 V at 2.08 A

● Long-life, electrolytic capacitors are not used

● Mains harmonics: according to EN61000-3-2 Class-D

● Efficiency at full load: better than 90%

● Dimensions: 75 x 135 mm

2.1 VCC section

The L6585DE is supplied by applying voltage between the VCC pin and GND pin. An

undervoltage lockout (UVLO) prevents the IC from operating with supply voltages too low to

guarantee the correct behavior of the internal structures.

An internal voltage clamp limits the voltage to around 17 V and a delivery up to 20 mA. For

this reason it cannot be used directly as a clamp for the charge pump (current peaks usually

reach several hundreds of mA), but can be easily used during startup in order to charge the

VCC capacitor or during save mode in order to keep the IC alive, for example, connecting

VCC to input voltage through a resistor.

The L6585DE is supplied by the startup MOSFET Q4 and R40 charging the capacitor C25.

A charge pump connected to the auxiliary winding of the HB inductor T2 supplies the

controller via a small linear regulator represented by Q7. Once both stages have been

activated, the controllers are supplied also by the auxiliary winding of the resonant

transformer, assuring correct supply voltage during all load conditions. As the voltage on the

VCC pin reaches the turn-on threshold, the chip is enabled, and the half-bridge and the PFC

sections start at the same time.

2.2 Power factor corrector section

The PFC output voltage is controlled by means of a voltage-mode error amplifier and a

precise internal voltage reference. The PFC section achieves current mode control

operating in transition mode, offering a highly linear multiplier including a THD optimizer that

allows for an extremely low THD, even over a large range of input voltages and loading

conditions.

The controller is the L6585DE (U1), working in transition mode and integrating all functions

that are needed to perform the PFC. It delivers a stable 450 Vdc. It is a conventional boost

converter connected to the output of the rectifier bridge. It includes the coil T1, the PFC

transformer by YuJing, the diode D2 (STTH3L06U) and the PFC output capacitors C2, C3

and C4 by film type of 5 µF/800 V. The T1 provides also the information about the PFC coil

core demagnetization to pin#11 (ZCD) of the L6585DE. The T1 auxiliary winding is

connected to pin#11 (ZCD) of the L6585DE through the resistor R10. Its purpose is to

provide the information that T1 has demagnetized which is needed by the internal logic for

triggering a new switching cycle. The boost switch is represented by the power MOSFET

Q2. The T1 secondary winding (pins#8-6) and related circuitry are dedicated to power the

L6585DE during normal operation.

Main characteristics and circuit description AN3407

6/25 Doc ID 018857 Rev 1

The divider R6, R9, R14 and R16 provides to the L6585DE multiplier the information of the

instantaneous mains voltage that is used to modulate the peak current of the boost. In

Figure 3 the characteristic curves of the multiplier are given. The resistors R1, R3, R7 with

R11 and C31 are dedicated to sense the output voltage and feed to the L6585DE the

feedback information necessary to maintain the output voltage regulated. The components

C7, R13 and C8 constitute the error amplifier compensation network necessary to keep the

required loop stability.

The resistors R2, R4, R5 with R8 are dedicated to detecting two different overvoltage

protections: dynamic overvoltage usually due to fast load transition, and static overvoltage

due to an excessive input voltage. The PFC boost peak current is sensed by resistors R23 in

series to the MOSFET source. The signal is fed into pin#12 (PFCS) of the L6585DE. The

protection is not latched, once the PFCCS falls below 1.7 V, the PFC driver restarts.

Figure 3. Multiplier

2.3 Resonant power section

The resonant converter half-bridge topology works in ZVS. The resonant transformer T2,

manufactured by YuJing, uses the integrated magnetic approach. The leakage inductance is

used for resonant operation of the circuit. The T2 doesn't need an external coil for the

resonance. The T2 secondary winding configuration is the typical center tap, using a couple

of type D5 and D7 power Schottky rectifiers. The output capacitors are film type C15 and

C16 (4.7 µF/63 V). L2 and C17 filters have been added on the output, in order to filter the

high-frequency ripple.

The half-bridge driver oscillation is regulated by a current-controlled oscillator. It needs a

capacitor connected to pin#1 (OSC) of the L6585DE and uses the current flowing outside

pin#2 (RF) of the L6585DE as reference. Pin#2 (RF) of the L6585DE has a 2 V precise

voltage reference that lets the designer fix the run mode frequency simply by connecting a

resistor R17 between pin#2 (RF) of the L6585DE and GND. Each curve is related to a value

of the C13 capacitor and is depicted in Figure 4. Pin#3 (EOI) of the L6585DE is driven by

the internal logic in order to set the frequency during the startup.

Pin#4 (Tch) of the L6585DE is connected to the parallel of a resistor R18 and C11 and is

used to define the protection time. Pin#6 (EOL) of the L6585DE is the input of an internal

window comparator that can be triggered by a voltage variation due to a rectifying effect.

The reference of this comparator and the amplitude of the window can be set by connecting

a suitable resistor to pin#5 (EOLP) of the L6585DE. The reference of this comparator can be

set at a fixed voltage or at the same voltage as pin#7 (CTR) of the L6585DE.

AN3407 Main characteristics and circuit description

Doc ID 018857 Rev 1 7/25

Figure 4. Oscillator characteristics

Pin #14 (HBCS) of the L6585DE is equipped with a current sensing and a dedicated

overcurrent management system. When the EOI voltage reaches 1.9 V, the IC enters run

mode and the switching frequency is set only by R17 (RRUN). In Figure 5 the protection

thresholds are shown. They are sensed by the circuit C18, R26, D8, D9, R27, and C19 and

are fed into the L6585DE pin#14 (HBCS).

Figure 5. Half-bridge protection thresholds during run mode

2.4 Output voltage feedback loop

The output voltage is kept stable by means of a feedback loop implementing a typical circuit

using U3 (TS2431) modulating the current in the optocoupler diode. On the primary side,

R34 connecting pin#2 (RF) of the L6585DE to the optocoupler's phototransistor allows

modulating the L6585DE oscillator frequency, thus keeping the output voltage regulated.

R17 connects the same pin to ground and sets the minimum switching frequency.

Main characteristics and circuit description AN3407

8/25 Doc ID 018857 Rev 1

Figure 6. Electrical schematic

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