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
VR1
SMBJ
150 A
X
F
R4
1.0
Ω
C5
0.22
µF
C6
68
µF
10 V
R9
220
Ω
-VIN
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
DI-52
conduction path for the output inductor (L4) current when the
transformer reset is complete.
TRANSFORMER PARAMETERS
Core Material
Bobbin
Ferroxcube P/N: EFD25, ungapped
10-pin EFD25 surface mount bobbin
Primary 5T + 5T, 4 x 26 AWG
Bias 5T, 1 x 30 AWG
12 V 6T, 4 x 26 AWG
Bias (2-5), Primary-1 (4-NC),
12 V (9,10-6,7), Primary-2 (NC-1)
Pin (1-4): 190
µH ±25%
@ 300 kHz
3.8 MHz (minimum)
1
µH
(maximum)
Key Design Points
• Transformer core reset is critical in this design. MOSFET
gate loading will affect the transformer-reset waveform.
Capacitors C20, C22 and C
Q1GS
will all load transformer
reset. Choose values to ensure sufficient reset at low line
and safe maximum drain voltage at high line. Also use
300 kHz operation for longest reset time.
• Capacitors C20 and C22 will capacitively drive MOSFET
gate capacitances C
Q2GS
and C
Q1GS
, respectively. C20 and
C22 should be chosen to ensure that gate drive voltage
attains turn-on threshold of MOSFET (Vg
TH
) at worst case
conditions (low line for forward MOSFET).
• Reduce transformer leakage inductance by filling each
winding layer across the entire width of the bobbin.
PI-3551-060503
Winding Details
Winding Order and
Pin Numbers
Primary Inductance
Primary Resonant
Frequency
Leakage Inductance
Table 1. Transformer Construction Information.
100
INDUCTOR PARAMETERS
Core Material
Ferroxcube P/N: EFD20-3F3
gap for inductance required
10-pin EFD20 surface mount bobbin
Main 18T, 3 x 24 AWG
Main (4,5-9,10)
Pin (4,5-9,10): 40
µH ±10%
@ 300 kHz
90
Efficiency (%)
80
Bobbin
Winding Details
70
60
36 VDC
48 VDC
72 VDC
Winding Order and
Pin Numbers
Inductance
50
0
1
2
3
4
5
Table 2. L4 Output Inductor Design Parameters.
Pout (W)
Figure 2. Efficiency vs. Output Power.
A
7/03
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