®
Design Idea DI-52
DPA-Switch™
60 W DC-DC Converter
Topology
Forward Sync. Rect.
Device
Power Output
Input Voltage
Output Voltage
Application
Telecom
60 W
36-75 VDC
DPA426R
12 V
Design Highlights
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 re-circulate the transformer leakage energy during
normal operation, with Zener VR1 providing absolute
clamping for transient conditions.
• Low component count
• High efficiency: 91.5% at 36 VDC using synchronous
rectification
• Capacitor coupled synchronous rectification allows higher
output voltages without overstressing MOSFET gates
• No current sense resistor or current transformer required
• Output overload, open loop and thermal protection
• 300 kHz switching frequency to allow sufficient
transformer reset time
CapacitorC21chargesthegateofQ2,theforwardsynchronous
rectifier MOSFET. 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
(resets) C21 during the Q2 off time.
• 3.55 x 2.1 x 0.6 inch (approx. 13.4 W/cubic inch)
Operation
A similar drive technique is used for the catch synchronous
rectifier MOSFET Q1 (with C22, R23, R24, and VR21).
MOSFET Q1 is driven by the transformer (T1) reset voltage
and operates only when Q2 is off. Diode D20 provides a
DPA-Switchgreatlysimplifiesthedesigncomparedtoadiscrete
implementation. The capacitor coupled synchronous rectifier
driveusedinthisdesignisusefulforhighervoltageoutputs,still
allowing passive MOSFET drive without gate overvoltage,
which would result from direct resistor drive.
C7 1 nF
1.5 kV
R14
10 Ω
L4
40 µH
C25
1 µF
50 V
C23
C24
100 µF 100 µF
C20
C21
T1
12 V, 5 A
10,9
4,5
16 V 16 V
1 nF 2.2 nF
6,7
VR20
15 V
+ V
IN
36-75 VDC
VR21 R23
1
R21
15 V 10 Ω
D20
R22
10 kΩ
10 Ω
R20
0.5 Ω
1 W
9,8
L1
12 CWQ
10 FN
1 µH
2.5 A
R24
10 kΩ
D1
R1
619 kΩ
1%
RTN
ESD1
Q1
Si4486
Q2
Si4486
C22
1 nF
L3
2.2 mH
40 mA
D3
BAV19WS
4
C9
150 pF
200 V
5
2
R10
D4
BAV19WS
C1-C4
0.22 µF
U2
38 kΩ
R7
10 kΩ
1%
100 V
C8
1 µF
L2
220 µH
U1
U2
DPA426R
PC357
NT
C13
100 nF
D
S
L
DPA-Switch
D3
BAV19WS
CONTROL
C
D2
ESD1
R12
5.1 Ω
C17
R6
150 Ω
R4
R9
X
F
1 µF
1.0 Ω
220 Ω
VR1
C15
10 µF
10 V
C5
SMBJ
C6
R3
0.22 µF
150 A
68 µF
11 kΩ
10 V
1%
-V
U3
LMV431
AIM5X
IN
R11
10 kΩ
1%
PI-3550-062403
Figure 1. DPA426R - 60 W, 12 V, 5 A, DC-DC Converter.
.
July 2003
DI-52
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