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
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Doc ID 16493 Rev 1 |
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www.st.com
Contents |
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Contents
1 |
Main characteristics and circuit description . . . . . . . . . . . . . . . . . . . . |
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1.1 |
Power stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
5 |
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1.2 |
Startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
6 |
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1.3 |
Brown-out protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
6 |
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1.4 |
Output regulation feedback loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
6 |
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1.5 |
L6566B current mode control and voltage feed-forward function . . . . . . . . |
7 |
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1.6 |
L6566B short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
7 |
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1.7 |
Overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
7 |
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1.8 |
Overtemperature protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
8 |
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1.9 |
Burst mode operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
8 |
2 |
Efficiency measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
10 |
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3 |
Functional check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
11 |
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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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
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List of tables |
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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
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List of figures |
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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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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
1.1Power 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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1.2Startup
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.
1.3Brown-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.
1.4Output 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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1.5L6566B 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 (VCSX). If the current 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, VCSX is modulated by the voltage on the VFF pin (#15) sensing the mains voltage through a resistor divider. A higher voltage causes a
smaller VCS,MAX so that the maximum power can be kept almost constant at any input voltage.
On this board, VFF is implemented via the same circuit of brown-out, saving components and reducing consumption at light load.
1.6L6566B 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.
1.7Overvoltage 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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1.8Overtemperature 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.
1.9Burst 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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