AN4248
Application note
60 W PSU design details for water purifier system
Harjeet Singh
Introduction
This power supply is based on quasi-resonant mode of operation using ST multi-mode
controller L6566B. The L6566B is an extremely versatile current-mode primary controller IC,
specifically designed for high-performance offline flyback converters. Both fixed-frequency
(FF) and quasi-resonant (QR) operation are supported. The user can pick either of the two,
depending on application needs.
The device features an externally programmable oscillator: it defines the converter's
switching frequency in FF mode and the maximum allowed switching frequency in QR
mode. When FF operation is selected, the IC works like a standard current-mode controller
with a maximum duty cycle limited at 70% min. The oscillator frequency can be modulated
to mitigate EMI emissions. QR operation, when selected, occurs at heavy load and is
achieved through a transformer demagnetization sensing input that triggers MOSFET turn-
on. Under some conditions, ZVS (zero-voltage switching) can be achieved. The converter's
power capability increasing with the mains voltage is compensated by line voltage feed-
forward. At medium and light load, as the QR operating frequency equals the oscillator
frequency, a function (valley skipping) is activated to prevent further frequency rise and
keep the operation as close to ZVS as possible. With either FF or QR operation, at very light
load the IC enters a controlled burst-mode operation, to help keep consumption from the
mains low and meet energy saving recommendations.
The protection functions included in this device are: not-latched input undervoltage
(brownout), output OVP (auto-restart or latch-mode selectable), a first-level OCP with
delayed shutdown to protect the system during overload or short-circuit conditions (auto
restart or latch-mode selectable) and a second-level OCP that is invoked when the
transformer saturates or the secondary diode fails short. A dedicated pin to latch the
controller in any abnormal condition is also provided.
The main features of the controller are listed here:
•
Selectable multi-mode operation: fixed frequency or quasi-resonant
•
On-board 700 V high-voltage startup
•
Advanced light load management
•
Low quiescent current (< 3 mA)
•
Adaptive UVLO
•
Line feed-forward for constant power capability vs. mains voltage
•
Pulse-by-pulse OCP, shutdown on overload (latched or auto restart)
•
Transformer saturation detection
•
Programmable frequency modulation for EMI reduction
•
Latched or auto restart OVP
•
Brownout protection
May 2014
DocID024214 Rev 1
1/46
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Contents
AN4248
Contents
1
2
3
4
5
6
7
8
9
10
Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Electrical specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
AC_OK setting for brownout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Mains feed-forward setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Mains overvoltage shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Overload delay and shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Overtemperature shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Output overvoltage protection (ZCD setting) . . . . . . . . . . . . . . . . . . . . 13
Basic power stage calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Transformer specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1
10.2
10.3
10.4
General description and characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Winding details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Winding construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
11
12
13
14
15
16
Control loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Test results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Functional check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Holdup test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
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18
19
20
21
22
Soft start test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Short-circuit test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Burst mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Temperature rise test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
DocID024214 Rev 1
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List of figures
AN4248
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Figure 30.
Figure 31.
Figure 32.
Figure 33.
Figure 34.
Figure 35.
Figure 36.
Figure 37.
Figure 38.
Figure 39.
L6566B block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Brownout protection: internal block diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
L6566B mains feed-forward section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Power capability change vs. input voltage in QR flyback converters . . . . . . . . . . . . . . . . . . 8
Mains overvoltage shutdown implementation using AC_OK pin . . . . . . . . . . . . . . . . . . . . 10
Overload delay circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Power discrete thermal shutdown using the DIS pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Output overvoltage setting using Z the CD pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Transformer windings & termination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Transformer winding construction method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Magnitude and phase plot at 230 V
ac
, full load 60 W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Magnitude and phase plot at 230 V
ac
, light load 6 W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Magnitude and phase plot at 115 V
ac
, full load 60 W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Magnitude and phase plot at 115 V
ac
, light load 6 W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Electrical schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Condition 1: V
ac
= 100 V; I
out
= 0.6 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Condition 2: V
ac
= 100 V; I
out
= 1.2 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Condition 3: V
ac
= 100 V; I
out
= 1.8 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Condition 4: V
ac
= 100 V; I
out
= 2.4 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Condition 5: V
ac
= 180 V; I
out
= 0.6 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Condition 6: V
ac
= 180 V; I
out
= 1.2 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Condition 7: V
ac
= 180 V; I
out
= 1.8 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Condition 8: V
ac
= 180 V; I
out
= 2.4 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Condition 9: V
ac
= 230 V; I
out
= 0.6 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Condition 10: V
ac
= 230 V; I
out
= 1.2 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Condition 11: V
ac
= 230 V; I
out
= 1.8 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Condition 12: V
ac
= 230 V; I
out
= 2.4 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Condition 13: V
ac
= 265 V; I
out
= 0.6 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Condition 14: V
ac
= 265 V; I
out
= 1.2 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Condition 15: V
ac
= 265 V; I
out
= 1.8 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Condition 16: V
ac
= 265 V; I
out
= 2.4 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Hold up test CH1: load current 2.4 A max; CH4: AC Input 230 V. . . . . . . . . . . . . . . . . . . . 37
Output soft start CH1: load current; CH2: output voltage, 24 Vmax . . . . . . . . . . . . . . . . . . 38
Output short circuit (hiccup mode) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Switching attempts during hiccup mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Single switching pulse (V
drain
and I
drain
) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Single switching pulse-no saturation of transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Burst mode operation at 100 V
ac
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Burst mode operation at 265 V
ac
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
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DocID024214 Rev 1
AN4248
Block diagram
1
Block diagram
Figure 1. L6566B block diagram
DocID024214 Rev 1
5/46
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