®
Design Idea DI-53
™
DPA-Switch
50 W DC-DC Dual Output Converter
Topology
Device
Power Output
Input Voltage
Output Voltage
Application
Forward Sync. Rect.
Telecom
50 W
36-75 VDC
DPA425R
5 V & 3.3 V
recirculate the transformer leakage energy during normal
operation, with Zener VR1 providing absolute clamping for
transient conditions.
Design Highlights
• High efficiency: 90% at 36 VDC using synchronous
rectification
CapacitorC21chargesthegateofQ2,theforwardsynchronous
rectifier MOSFET of the 5 V output. Resistor R21 limits gate
oscillation and R22 provides gate pull-down. Zener diode
VR20 limits the Q2 gate voltage during conduction and also
reverse charges C21 during the Q2 off time. The same drive
technique is used for the forward synchronous rectifier
MOSFET (Q4) of the 3.3 V output (with C22, R24, R25, and
VR21).
• Dual output with tight cross-regulation ±4% from zero to
full load on both outputs
• Output overload, open loop and thermal protection
• 300 kHz switching frequency to allow sufficient
transformer reset time with sync rectification
• 3.85 x 2.25 x 0.6 inch (~9.62 W/in3)
Operation
DPA-Switchgreatlysimplifiesthedesigncomparedtoadiscrete
implementation.Thisdesignusesacoupledoutputinductorand
synchronous rectification to achieve excellent cross-regulation
and high efficiency.
MOSFETs Q1 and Q3 are driven via resistors R23 and R26
from the transformer (T1) reset voltage and operate only
when Q2 and Q4 are off. Diodes D20 and D21 provide a
conductionpathfortheoutputinductor(L4)currentwhenthe
transformer reset is complete.
Resistor R1 programs the under/over voltages and linearly
reduces the maximum duty cycle with input voltage to prevent
core saturation during load transients. Components D1, D2, C9
and L2 implement a resonant clamp circuit to catch and
A winding on the coupled inductor L4, along with diode D4
and capacitor C9, provide the DPA-Switch bias voltage.
L4
5 V, 6 A
(8 A pk)
3.5 µH
9,10
7,8
3,4
T1
R23
10 Ω
8,9
D20
Q1
3.3 V, 6 A
(8 A pk)
R26
10 Ω
1,2
L1
4,5
+ V
36-75 VDC
1 µH
IN
Q3
2.5 A
R25
10 kΩ
1
C21-C22
4700 pF
R21
10 Ω
D21
R20
3.3 Ω
VR21
15 V
R1
619 kΩ
1%
D1
ES1D
2,3
Q4
R24
10 Ω
C23-C26
100 µF
10 V
C27
1 µF
C28
1 µF
RTN
10
C9
6,7
150 pF
200 V
5
6
Q2
10 kΩ
C20
3.3 nF
R22
VR20
15 V
D4
C7
1 nF
1.5 kV
R10
R12
15.9 kΩ
1%
L2
220 µH
8.66 kΩ
U2
R7
10 kΩ
1%
C1-C3
1 µF
100 V
U2
PC267
N1T
C9
4.7 µF
16 V
DPA-Switch
C16
100 nF
U1
D3
BAV19WS
DPA425R
R6
R12
D
S
L
150 Ω
5.1 Ω
CONTROL
C
R9
220 Ω
D2
ES1D
C15
10 µF
10 V
R4
X
F
C14
1 µF
1.0 Ω
VR1
C5
0.22 µF
SMBJ
150 A
R3
C6
68 µF
10 V
U3
LM431
AIM3X
R11
10.0 kΩ
1%
15 kΩ
-V
IN
1%
PI-3561-072203
Figure 1. DPA425R - 50 W, 5 V, 6 A and 3.3 V, 6 A DC-DC Converter.
.
July 2003
DI-53
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