Design Idea DI-52
™
DPA-Switch
60 W DC-DC Converter
Application
Telecom
Device
DPA426R
Power Output
60 W
Input Voltage
36-75 VDC
Output Voltage
12 V
Topology
®
Forward Sync. Rect.
Design Highlights
• 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
• 3.55 x 2.1 x 0.6 inch (approx. 13.4 W/cubic inch)
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.
Capacitor C21 charges the gate of Q2, the forward synchronous
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.
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
Operation
DPA-Switch
greatly simplifies the design compared to a discrete
implementation. The capacitor coupled synchronous rectifier
drive used in this design is useful for higher voltage outputs, still
allowing passive MOSFET drive without gate overvoltage,
which would result from direct resistor drive.
C7 1 nF
1.5 kV
T1
6,7
R14
10
Ω
C20
C21
1 nF 2.2 nF
VR20
15 V
L4
40
µH
10,9
4,5
C23
C24
100
µF
100
µF
16 V 16 V
C25
1
µF
50 V
12 V, 5 A
+
VIN
36-75 VDC
L1
1
µH
2.5 A
D1
ESD1
R1
619 kΩ
1%
1
R21
R22
R20 10
Ω
10 kΩ
0.5
Ω
1W
9,8
Q2
Si4486
VR21 R23
15 V 10
Ω
D20
12 CWQ
10 FN
RTN
Q1
Si4486
R24
10 kΩ
C1-C4
0.22
µF
100 V
C9
150 pF
200 V
4
5
2
C22
1 nF
D3
BAV19WS
L3
2.2 mH
40 mA
R7 U2
10 kΩ
R10
38 kΩ
1%
D4
BAV19WS
C8
1
µF
L2
220
µH
D
L
U1
DPA426R
DPA-Switch
C
U2
PC357
NT
D3
BAV19WS
R6
150
Ω
C15
10
µF
10 V
CONTROL
C13
100 nF
R12
5.1
Ω
C17
1
µF
D2
ESD1
S
X
F
VR1
SMBJ
150 A
-VIN
R4
1.0
Ω
C5
0.22
µF
C6
68
µF
10 V
R9
220
Ω
R3
11 kΩ
1%
U3
LMV431
AIM5X
R11
10 kΩ
1%
PI-3550-062403
Figure 1. DPA426R - 60 W, 12 V, 5 A, DC-DC Converter.
.
DI-52
www.powerint.com
July 2003
POWERINT [ POWER INTEGRATIONS, INC. ]
