ST AN3089 APPLICATION NOTE

AN3089

Application note

19 V - 65 W quasi-resonant flyback adapter using

L6566B and TSM1014

Introduction

This application note describes the characteristics and features of a 65 W demonstration
board (EVL6566B-65W-QR), tailored to the specifications of a typical hi-end portable
computer power supply. The peculiarities of this design are the very high average efficiency
of about 90%, without synchronous rectification, and very low no-load consumption of 100
mW at 230 Vac. The result is that this converter is more than compliant with Energy Star
eligibility criteria (EPA rev. 2.0 EPS).

Figure 1. EVL6566B-65W-QR: 65 W adapter demonstration board

®

July 2010 Doc ID 16493 Rev 1 1/25

www.st.com

Contents AN3089

Contents

1 Main characteristics and circuit description . . . . . . . . . . . . . . . . . . . . . 5

1.1 Power stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2 Startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.3 Brown-out protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.4 Output regulation feedback loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.5 L6566B current mode control and voltage feed-forward function . . . . . . . . 7

1.6 L6566B short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.7 Overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.8 Overtemperature protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.9 Burst mode operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2 Efficiency measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3 Functional check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.1 Standby and no-load operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.2 Overcurrent and short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.3 Overvoltage and open-loop protection . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4 Thermal map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5 Conducted emission pre-compliance test . . . . . . . . . . . . . . . . . . . . . . 17

6 Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

7 Transformer specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

8 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

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AN3089 List of tables

List of tables

Table 1. Overall efficiency and no-load consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Table 2. Thermal map reference points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Table 3. EVL6566B-65W-QR demonstration board: bill of material . . . . . . . . . . . . . . . . . . . . . . . . . 18

Table 4. Transformer winding data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Table 5. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Doc ID 16493 Rev 1 3/25

List of figures AN3089

List of figures

Figure 1. EVL6566B-65W-QR: 65 W adapter demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Figure 2. Electrical diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 3. Light load efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 4. Flyback stage waveforms at 115 V -60 Hz – full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 5. Flyback stage waveforms at 230 V-50 Hz – full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 6. Flyback stage waveforms at 115 V -60 Hz – light load . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 7. Flyback stage waveforms at 230 V-50 Hz – light load . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 8. No-load operation at 90 V-50 Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 9. No-load operation at 265 V-50 Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 10. Transition full load to no-load at 265 Vac-50 Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 11. Transition no-load to full load at 265 Vac-50 Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 12. Short-circuit at full load and 230 Vac – 50 Hz. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 13. Short-circuit detail at full load and 230 Vac – 50 Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 14. Flyback open-loop – detail at 230 V - 50 Hz - half load . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 15. Flyback open-loop at 230 V - 50 Hz - half load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 16. Thermal map at 115 Vac – 60 Hz - Full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 17. Thermal map at 230 Vac – 50 Hz - Full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 18. CE average measurement at 115 Vac and full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 19. CE average measurement at 230 Vac and full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 20. Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Figure 21. Transformer winding diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Figure 22. Transformer mechanical diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

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AN3089 Main characteristics and circuit description

1 Main characteristics and circuit description

The main features of the SMPS are listed below:

● Universal input mains range: 90 ÷ 264 Vac - frequency 45 ÷ 65Hz

● Output voltage: 19 V @ 3.42 A continuous operation

● Mains harmonics: Acc. to EN61000-3-2 Class-D or JEITA-MITI Class-D

● Standby mains consumption: <100 mW @ 230 Vac

● Average efficiency: better than 89% without synchronous rectification

● EMI: according to EN55022-Class-B

● Safety: according to EN60950

● Dimensions: 58x121 mm, 25 mm maximum component height

● PCB: single side, 35 µm, CEM-1, Mixed PTH/SMT

1.1 Power stage

The Flyback converter implements the new ST dedicated current mode L6566B (U2)
controller operating in quasi-resonant mode and detecting the transformer demagnetization
through the ZCD (#11) pin.

R23 on the OSC (#13) pin sets the maximum switching frequency at about 165 kHz.

Because the maximum switching frequency is imposed, the converter operates in
discontinuous conduction mode during light load operation. The L6566B valley skipping
function is capable of turning-on the MOSFET in valley switching even in DCM, therefore
reducing switching losses.

The MOSFET is a standard 800 V, STF7NM80, housed in a TO-220FP package, needing
just a small heat sink. The transformer is a layer type, using a standard ferrite size EER28L,
designed according to EN60950 and manufactured by MAGNETICA.

The flyback reflected voltage is ~150 V, providing enough room for the leakage inductance
voltage spike with a still margin for reliability of the MOSFET. The D5 rectifier and the D4
Transil clamp the peak of the leakage inductance voltage spike at MOSFET turn-off. A small
capacitance in parallel with D4 smooth leakage inductance spikes, reducing EMI and Transil
dissipation.

The output rectifiers are two STPS20H100CFP dual centre tap Schottky diodes (D2 and D3)
in parallel, housed in TO-220FP. They have been selected according to the maximum
reverse voltage, forward voltage drop, and power dissipation. The snubber, made up of R5,
R7 and C12, dampens the oscillation produced by the diode capacitance and the leakage
inductance.

A small LC filter has been added on the output, filtering the high frequency ripple and
spikes.

D6, R4, R5, R8, R9, Q2 and Q3 implement an output voltage “fast discharge” circuit, quickly
discharging the output capacitors when the converter is turned off. It has been implemented
to quickly decrease the residual output voltage after the converter is turned off at no-load.

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Main characteristics and circuit description AN3089

1.2 Startup

The L6566B flyback controller pin #1 (HV) is directly connected to the bulk capacitor, and at
startup an internal high voltage current source charges C9 until the L6566B turn-on voltage
threshold is reached, then the high voltage current source is automatically switched off. As
the IC starts switching it is initially supplied by the C9, then the transformer auxiliary winding
(pins 5-6) provides the voltage to power the IC.

Because the L6566B integrated HV startup circuit is turned off, and therefore not dissipative
during normal operation, it gives a significant contribution to power consumption reduction
when the power supply operates at light load.

1.3 Brown-out protection

Brown-out protection prevents the circuit from working with abnormal mains levels. It can be
easily achieved using the AC_OK controller pin (#16).

This feature is typically implemented sensing the bulk voltage through a resistor divider,
however on this board a different solution has been applied. The mains voltage is sensed
before the bridge rectifier. This has two main achievements: it is less dissipative and it
allows faster restart in case of latching, because there is no need to wait for the bulk
capacitor discharge.

If the input voltage is below 90 Vac, the startup of the circuit is inhibited, while the turn-off
voltage has been set at 80 Vac. The internal comparator has in fact a current hysteresis
allowing to set the converter turn-on and turn-off voltage independently. R19 sets the
relation between the comparator hysteresis and the actual voltage hysteresis.

C13, R20 and R21 set the discharging time constant of the AC_OK voltage. This value must
be dimensioned properly, taking two main points into account:

● The voltage must keep up during the mains missing cycle to avoid the converter

shutting down during mains dip.

● In the case of converter switch-off, the voltage must go down promptly to avoid an

operation with improper input voltage.

Basically, the ideal dimensioning would allow C13 to discharge slightly faster than the bulk
capacitor in the case of switch-off at nominal load.

1.4 Output regulation feedback loop

Output regulation is done by means of two control loops, voltage and current, working
alternatively. A dedicated control IC, the TSM1014 (U3), has been used. It integrates two
operational amplifiers (used as error amplifiers) and a precise voltage reference. The output
signal of the error amplifiers drives an SFH617A-4 (U1) optocoupler to achieve the required
insulation of the secondary side and modulate the COMP pin (#9) voltage of the L6566B.

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AN3089 Main characteristics and circuit description

1.5 L6566B current mode control and voltage feed-forward function

R16 senses the flyback MOSFET current and the signal is fed into the CS pin (#7)
connected to the PWM comparator. This signal is compared with the COMP pin (#9) signal,
which comes from the optocoupler.

The maximum power that the converter can deliver is set by a comparator limiting the peak
of the primary current, comparing the CS and an internal threshold (V
signal exceeds the threshold, the comparator limits the MOSFET duty cycle, hence the
output power is limited too.

As the maximum transferable power depends on both the primary peak current and the
input voltage, in order to keep the overload set point almost constant, which changes
according to the flyback input voltage, the L6566B implements a voltage feed-forward
function via a dedicated pin. Therefore, V
(#15) sensing the mains voltage through a resistor divider. A higher voltage causes a
smaller V
voltage.

On this board, VFF is implemented via the same circuit of brown-out, saving components
and reducing consumption at light load.

CS,MAX

so that the maximum power can be kept almost constant at any input

is modulated by the voltage on the VFF pin

CSX

). If the current

CSX

1.6 L6566B short-circuit protection

An internal comparator senses the COMP pin after the soft-start time: in case of short, the
COMP pin goes high, and the said comparator activates a current source that restarts
charging the soft-start capacitor from the initial 2 V level. If the voltage on this pin reaches 5
V, the L6566B stops the operation and enters into the so-called “Hiccup mode”. The L6566B
restarts with a startup sequence when the Vcc voltage drops below the Vcc restart level (5
V). Because of the long time needed by the Vcc capacitor to drop to 5 V, it results in an
increase of the duration of the no-load operation, therefore decreasing the power dissipation
and the stress of the power components. This sequence is repeated until the short is
removed, after that normal operation of the converter is automatically resumed.

A second protection, dedicated to protecting the circuit in the case of MOSFETs or output
diode short or transformer saturation, is implemented by another comparator on the CS pin
(#7). If the voltage on this pin exceeds the 1.5 V threshold, the IC immediately shuts down.
In this way a hiccup mode operation is still obtained, avoiding consequent failures due to the
power components overheating. To prevent spurious activation of the protection in the case
of temporary disturbances, for example during immunity tests, the comparator must be
triggered two consecutive times.

1.7 Overvoltage protection

The ZCD pin (#11) is connected to the auxiliary winding by a resistor divider. It implements
the OVP against feedback network failures. When the ZCD pin voltage exceeds 5 V four
consecutive times, the IC is shut down. This protection can be set as latch or auto-restart by
the user with no additional components. On the board it is set as latched. Therefore the
operations can be resumed after a mains recycling.

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Main characteristics and circuit description AN3089

1.8 Overtemperature protection

The R3 thermistor, connected to the L6566B DIS pin (#8), provides a thermal protection of
the flyback MOSFET (Q1). Therefore, in case of overheating, the flyback converter activity is
latched off. To maintain this state, an internal circuitry of the L6566B monitors the Vcc and
periodically reactivates the HV current source to supply the IC.

1.9 Burst mode operation

The L6566B implements a current mode control, thus it monitors the output power through
the COMP pin, which has a level proportional to the load. Therefore, when the voltage on
the COMP pin falls below an internal threshold, the controller is disabled and its
consumption reduced; normal operation restarts as soon as the COMP voltage rises again.
In this way a low consumption burst mode operation is obtained

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