®
Design Idea DI-31
DPA-S w it c h™ 5 V, 70 W DC-DC
Converter with Synchronous Rectification
Topology
Forward
Device
Power Output
Input Voltage
Output Voltage
Application
DC-DC Converter
70 W
36-75 VDC
DPA426R
5 V
appliedtoMOSFETsQ1toQ6, eliminatingtheneedforgate
voltage clamp circuitry. The self-driven synchronous
rectification configuration is therefore very simple, with R4
to R6 filtering voltage spikes at the gates of Q1, Q2 and Q3
and D4 preventing the body diodes of Q4, Q5 and Q6 from
conducting.
Design Highlights
• Extremely low component count
• High efficiency, 90% using synchronous rectification
• No current sense resistor or current transformer required
• Output overload, open loop and thermal protection
• Accurate input under/overvoltage meets ETSI standards
• 300 kHz switching frequency – optimizes efficiency using
simple self-driven synchronous rectification
CapacitorC9,diodesD1-D2,andinductorL2formaresonant
snubber that recycles leakage and magnetizing energy stored
in T1, and also helps to reset T1. Zener diode VR1 provides
ahardvoltagelimitandonlyconductsduringoutputtransient
and overload conditions. Capacitor C12 and Resistor R7
damp secondary switching spikes and help to reset T1.
Operation
The 70 W converter shown in Figure 1 benefits from many of
the DPA-Switch integrated features. In particular, no external
current sense components are required. In a discrete
implementation,anexpensivecurrenttransformerandanumber
of additional components would increase the cost of this
converter significantly.
Key Design Points
• For nominal undervoltage set point VUV:
R1 = (VUV - 2.35 V)/50 µA. VOV = (R1 x 135 µA) + 2.5 V.
• Locate C10, C11, and R3 close to the U1 CONTROL pin,
with ground connections returned to SOURCE pin.
R1 programs the input UV/OV thresholds. The tight tolerance
of the UV/OV thresholds limits the range of gate drive voltages
C7 1 nF
C13, C14,
1.5 kV
C15, C16, C17
100 µF 10 V
(x5)
L3
3.8 µH
20 A
L1
C18, C19
100 µF
10 V
+ V
36-75 VDC
1 µH
IN
Q4-Q6
SI4842DY
R4-R6
10 Ω
5 V, 14 A
2.5 A
T1
1
9,10
6,7
L4
100 nH
20 A
C20
1 µF
10 V
D1
ESD1
R1
619 kΩ
2
D4
42CTQ30S
RTN
Q1-Q3
SI4842DY
C12 2.2 nF, 50 V
L5
U2
3
R7
1 Ω
C9
150 pF
200 V
5
4
R13
R15
10.0 kΩ
1%
D3
BAV19WS
2.2 mH
40 mA
D5
C3, C4,
C5, C6
1 µF 100 V
(x4)
C1, C2
1 µF
100 V
10 kΩ
D6
BAV19WS
BAV19WS
C22
100 nF
C8
1 µF
25 V
R11
R10
D
S
L
DP A-S w it c h
U1
150 Ω
5.1 Ω
L2
CCOONNTTRROOLL
DPA426R
R14
220 Ω
C
220 µH
U2
PC357N1T
C23
10 µF
C21 1 µF
X
F
10 V
R3
1.0 Ω
U3
VR1
SMBJ
150
C10
220 nF
LM431AIM3
R16
10.0 kΩ
1%
C11
68 µF
10 V
R2
D2
ESD1
6.8 kΩ
-V
IN
1 %
PI-3375-121602
Figure 1. DPA426 70 W, 5 V, 14 A DC-DC Converter.
December 2002
DI-31
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