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PDF DN91 Data sheet ( Hoja de datos )
| Número de pieza | DN91 | |
| Descripción | Synchronous rectification improves set top box power supply efficiency | |
| Fabricantes | Diodes | |
| Logotipo |  |
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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
0Ω
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
www.zetex.com
www.diodes.com
1 page  
DN91
Figure 5 shows the Gate voltage and Drain current waveforms of the ‘Control’ MOSFET at the
maximum and minimum loading profile. In Figure 5(a), the Gate voltage reaches 8.5V to obtain a
low MOSFET resistance at 100% loading. The Gate enhancement eases off gradually as the Drain
current through the MOSFET decreases due to the ZXGD3101’s proportional gate drive scheme.
The controller reduces the Gate voltage magnitude as the MOSFET current decreases. Although
this produces a larger on-state resistance, the subsequent increase in conduction voltage drop
ensures that the controller maintains the gate voltage above 2-3V as the current decreases
linearly down to zero. The MOSFET is driven off near the zero current point to ensure that there
is no reverse current flowing through Q2.
(a) Maximum output power 31.7W
(b) Minimum output power 11.05W
Figure 5 ‘Control’ MOSFET current and voltage waveforms (CH1: Primary Drain voltage; CHx:
Q2’s Gate voltage; CH4: Q2’s Drain current)
Set top box power supplies normally have tracking between critical loads where the minimum
and maximum loads on the +5V and +3.3V output occurs at the same time. In products where
there is no tracking between outputs, the 3.3V load current could be minimum loaded whilst the
5V output is fully loaded. In that instance, Q2 will conduct a current a few times higher than that
in Q3. As the Drain voltage level across the ‘Control’ MOSFET dictates that both Q2 and Q3 are
kept on when Q2 is still conducting substantial current, even though the current in Q3 has reached
zero as shown in Figure 6.
Figure 6 - Synchronous MOSFET current and voltage waveforms for uneven output loading (CH3:
Gate voltage; CH4: Drain current)
Issue 1 - November 2008
© Diodes Incorporated 2008
5
www.zetex.com
www.diodes.com
5 Page | ||
| Páginas | Total 8 Páginas | |
| PDF Descargar | [ Datasheet DN91.PDF ] | |
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