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DN91 PDF даташит

Спецификация DN91 изготовлена ​​​​«Diodes» и имеет функцию, называемую «Synchronous rectification improves set top box power supply efficiency».

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Номер произв DN91
Описание Synchronous rectification improves set top box power supply efficiency
Производители Diodes
логотип Diodes логотип 

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DN91 Даташит, Описание, Даташиты
A Product Line of
Diodes Incorporated
DN91
Synchronous rectification improves set top box
power supply efficiency
Yong Ang, Diodes Incorporated
Introduction
This design note describes the implementation of synchronous rectification on multiple-output
flyback converters. It is focused on the low side rectification approach as shown in Figure 1 to
facilitate a common drive for synchronous MOSFETs on low voltage 3.3 and 5V outputs. The note
concludes with practical measurement on a 27W four-output set top box power supply to
demonstrate that a 3% efficiency improvement over a Schottky solution can be achieved with the
aid of the ZXGD3101 synchronous MOSFET controller.
KBP 206 G
BR
R9
47 kΩ
C 14
10 nF
1 kV
T1
P 1 S 3 B1100
D2
C 11
150 μ F
C7
150 μ F
10Ω
NTC
2 × 5mH
L3
C 15
33 μF
400 V
R 11
47 Ω
GND
R10
68 Ω
PR 1507 G
D6
R 12
-20 V
R11Ω
D1
S2 S160
L4
3.3 μH
C2
330 μ F
C3
330 μF
+ 12 V
R tn
47Ω 10 n F
L1
+5 V
S1
R 2 C4
C5 C 6
1000 μ F
1000 μF
X2 -C
D5
C 13
22 μ F
Aux
Q2
R 5 L2
R tn
R13 GN D
L N 4 .7kΩ
U1
+ 3.3 V
R 3 10 n F
C9 C 10 1000 μF
S4 1000 μ F
47Ω C 8
HV Vcc
GND FB
D
U2
R 8 + 5V
200 Ω
R7
C 11
R5
20 kΩ
10 kΩ 0.1 μF
Q3
R6
Q1
F M M T491
U4
D R AIN Vcc
R EF
3 kΩ
RR EF
R4
10 kΩ
11 V Zener
D3
U3
Z T L431
R6
10 kΩ
C Y1
2.2 nF
GATEL
GATEH GND
ZXGD 3101
BIAS
R BIAS
1 .8kΩ
C 11
1μ F
Figure 1 - Flyback converter with synchronous rectification on 5V and 3.3V output
General description
Figure 2 shows a typical set top box power supply. It is a cost effective design with minimum
component count. The circuit is designed to operate in Discontinuous Conduction Mode (‘DCM’)
because there are lower switching losses. Furthermore, the flyback transformer size can be
reduced owing to the lower average energy storage whilst its smaller magnetizing inductance
also yields a better transient line/load response.
The primary stage is built around a monolithic high voltage current mode control IC, U1 running
at a fixed frequency of 50kHz. The secondaries of the transformer are rectified and filtered by D1,
C2, D2, C5, D3, C7, D4 and C9 to produce four outputs - +5V, +3.3V,+12V and -20V, where the +5V
Issue 1 - November 2008
© Diodes Incorporated 2008
1
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DN91 Даташит, Описание, Даташиты
DN91
main output channel is closed loop regulated through the optocoupler U2. LC post filters on each
output remove switching noise and minimize ripple level.
KBP 206 G
BR
NTC
10 Ω
R9
47 kΩ
C 15
33 μF
400 V
R11
47 Ω
C14
10 n F
1 kV
R 10
68 Ω
L3
2 × 5mH
D6
PR 1507 G
22 Ω
R 12
F1 X2-C
C 13 D 5
22 μF
R 13
L N 4. 7kΩ
U1
HV Vcc
D
GND FB
T1
P1 S 2
S1
S3
Aux
S4
S160
L5
D1
R2
47 Ω
C4
10 nF
D2
YG 811 R 04
L4
3 .3μ H
330 μ F
C2
L1
3. 3μ H
C5
1000 μ F
1000 μ F
C6
330 μ F
C3
+ 12 V
R tn
+ 5V
R tn
C 7 C8
D3
150 μ F
150 μ F
B1100
D4
YG811 R 04
R4
47 Ω
C Y1
2 .2nF
C1
10n F
200 Ω
R8
R3
L2
3. 3μ H
C 9 C 10
1000 μF 1000 μ F
C 11
R7
-20 V
+ 3. 3V
R5
20 kΩ
O pto
U2
10 kΩ
U3
Z T L431
0 .1 μF
R6
10 kΩ
Figure 2 - Typical set top box power supply
For set top box power supplies, it is common to stack secondary windings with the same polarity
and share a common source. This gives better voltage regulation between the outputs. In Figure
2, the winding for the 5V output will provide the return to ground whilst forming part of the
winding for the 12V output. This also helps to keep the total number of secondary winding turns
to a minimum. The wire size for the 5V winding needs to be chosen to accommodate its own
maximum load current plus the output current of the 12V output stacked on top of it.
The example set top box power supply was specified for a nominal input voltage of 230Vac and
load profile as shown in table 1.
Table 1 - Set top box power supply load profile
Nominal
voltage
5V
3.3V
12V
-20V
Power
Nominal load
current
2.5A
2A
500mA
60mA
26.3W
Maximum load
current
3.25A
2.5A
500mA
60mA
31.7W
Minimum load
current
1.25A
1A
125mA
20mA
11.05W
Function
Tuner/Demodulator
Audio & Video CODEC,
Video DAC, Memory, TVI
TDMS
Video and Audio Amp
Critically, much of the power supply heat generation and inefficiency is caused by Schottky
diodes, D2 and D4 on the 5V and 3.3V high current outputs. Replacing the diodes with more
efficient MOSFETs is recognized as a clear means of dramatically improving power supply
efficiency, thereby removing the need for bulky heat sinks and reducing the size and weight of the
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2 Issue 1 - November 2008
© Diodes Incorporated 2008









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DN91 Даташит, Описание, Даташиты
DN91
end product. This design note therefore examines the implementation of synchronous
rectification on these outputs.
More recently single output flyback has also been suggested for set top boxes. This approach
derives the low voltage 3.3V and 5V outputs from the main 12V flyback output through
synchronous or asynchronous buck post regulators. Although this simplifies the transformer
design and eliminates cross regulation issues, it incurs a higher BOM cost, component count and
possibly lower efficiency due to additional power loss in the post regulators. For information on
synchronous rectification on single output flyback refer to AN54.
Secondary side synchronous rectification approach
High-side synchronous rectification is not considered here as a common drive circuit controlling
a MOSFETs on each of the 5V and 3.3V outputs is not possible. For high side, a separate controller
on each output is required. The overall solution BOM component count and cost will be
prohibitive for a low power set top box.
Figure 1 shows a design where two MOSFETs are driven by a single ZXGD3101 Synchronous
MOSFET controller. Low side rectification positions Q2 and Q3 on the ground return of the 5V and
3.3V outputs respectively. This enables the MOSFETs to be driven by a common controller
because Q2 and Q3 share a common source point. The controller will derive its supply from the
12V output via an emitter-follower transistor Q1. RREF and RBIAS are chosen to be 3kand 1.8k
to set the controller turn-off threshold value to -20mV.
Either Q2 or Q3 can be the ‘Control’ MOSFET, whose Drain terminal signal needs be fed back to
the ‘Drain’ pin of the ZXGD3101. In the design shown in Figure 1, Q2 is selected to be the ‘Control’
FET because the 5V output has a higher load current; therefore the Q2’s peak current will be
higher than that of Q3. The controller monitors Q2’s Drain voltage for body diode conduction and
then turns on the MOSFET. The same gate voltage also drives Q3 on the 3.3V output, though the
gate voltage level is entirely independent of its current status.
The ‘Control’ MOSFET should ideally be the MOSFET that carries the highest current so that the
controller produces a sufficient gate voltage to minimize conduction loss. The gate voltage output
from the controller is proportional to Q2’s Drain voltage during MOSFET on time. Furthermore,
Q2 will also carry the current that flows through D1 on the 12V output as well as suppling the 5V
output due to the stacked winding design. This puts additional current stress on the MOSFET.
The sink and source current from the controller is used to drive both synchronous MOSFETs. The
switching time of Q2 and Q3 depends on their respective gate charge value. The controller turns
these MOSFETs off without reverse current flow in DCM. More care has to be taken using the
proposed configuration in a Continuous Conduction Mode (‘CCM’) converter to ensure that the
secondary side switches are turned off safely without imposing huge current stress on the
primary switch. This is because the increased likelihood of shoot through current occurring when
switching off the large current in a CCM converter as the available sink current is now shared by
Q2 and Q3.
Evaluation of results
Laboratory evaluation was conducted on a 27W set top box power supply to assess the
performance benefits of synchronous rectification. Set top box power supplies are typically
minimally designed and usually have efficiency around 75% with diode rectification which results
in hot internal component operation.
Although different types of MOSFETs could be selected for Q2 and Q3 based on their breakdown
voltage and Drain current requirement, a common part is used in this design because the 5V and
3.3V output have fairly similar load current. Based on the current level through Q2, a 16m, 79A,
Issue 1 - November 2008
© Diodes Incorporated 2008
3
www.zetex.com
www.diodes.com










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