Demonstration Note for CS5101

CS5101DEMO/D
Demonstration Note for
CS5101
Multiple Output, Telecommunications
Power Supply with Secondary Side
Control for Tight Output Regulation
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DEMONSTRATION NOTE
INTRODUCTION
The CS5101 demonstration board is a multiple–output
(5.0 V @ 7.0 A; 3.3 V @ 5.0 A), isolated, 50 W power supply
that accepts the standard telecommunications input voltage
range: 36 V to 72 V. The 3.3 V auxiliary output uses the
CS5101, a secondary side post regulator (SSPR) to provide
a tightly regulated output over full load to no load
conditions.
In this demonstration board, the main output is controlled
by the CS3843 current mode PWM controller. The CS5101
regulates the 3.3 V secondary output by adjusting the duty
cycle of the auxiliary switch using leading edge modulation.
This auxiliary output voltage is independently controlled by
a local feedback loop. This method of regulation is similar
to that of a magnetic amplifier, however, the CS5101 offers
several distinct advantages.
USING THE CS5101 DEMONSTRATION BOARD
The bench power supply is connected to the input voltage
terminals, shown as VIN and PGND in Figure 1. A load is
applied to the the output voltage terminals, shown as 5.0 V,
3.3 V and SGND in Figure 1. The loads should be set within
the parameters specified in Table 2.
Five test points TP1 through TP5 have been brought out
so that waveforms at critical nodes can be easily viewed (see
Figure 1). (Please note: when monitoring secondary side
waveforms, the ground lead on the oscilloscope AC plug
should not be connected to earth ground.)
TP1 is located at the drain of the primary–side FET. A
typical waveform at TP1 is shown in Figure 2a. When
measuring this point, reference the scope ground to the
primary side ground (PGND).
Table 2. Demonstration Board Specifications
Features
• High Efficiency
• Soft Switching of Auxiliary FET
• Programmable Overcurrent Protection
• Easy and Accurate Overcurrent Protection
• Tight Regulation Over Full Load/Line Range
• Lossless Shut–Down Feature
Parameter
Input voltage range.
36 V to 72 V
VOUT1
Main output voltage.
5.0 V ± 3.0%
VOUT2
Auxiliary output voltage.
3.3 V ± 2.0%
IOUT1
Main output current.
1.5 A–7.0 A
IOUT2
Auxiliary output current.
Isolation
Specifications
Specification
VIN
Table 1. Suggested Equipment for CS5101
Demonstration Board
Description
Description
POUT
0 A–5.0 A
Primary to secondary isolation
(min).
500 V
Total output power (max).
50 W
Output power without airflow
(max).
40 W
Power Supply
36 V–72 V, 2.0 A Output
Oscilloscope
100 MHz, 2–Channel
ICL
Current limit threshold (VOUT).
9.0 A
Multi Meters
At least two required
ISC
Short circuit current (VOUT2).
4.4 A
Load
Electronic load or power resistors
 Semiconductor Components Industries, LLC, 2001
May, 2001 – Rev. 0
1
Publication Order Number:
CS5101DEMO/D
CS5101DEMO/D
TP4
TP5
CS5101
U4
TP3
RAMP
L1
3.3 V
Q3
D5
TP2
T1
VIN
A
D6
D6
COUT2
SGND
5.0 V
L2
COUT1
TP1
SGND
L2
Q2
RSENSE (R7)
–
U3
PGND
+
CS3843
U1
VREF
U2
Figure 1. Simplified CS5101 Demonstration Board Schematic
TP2 is located at the top side of the secondary winding.
This voltage is similar to TP1, but is scaled down three to
one, based on the transformer turns ratio. This waveform is
illustrated in Figure 2b. The notch during the on–time
portion of the waveform is generated when the CS5101 turns
on Q3 and is caused by the transformer leakage inductance,
which appears in series with the secondary winding.
TP3 is located at the source of the FET (Q3). The on–time
of Q3 is reduced compared to TP2, as shown in Figure 2d.
The leading edge of this waveform is in sync with the notch
at TP2. The leading edge modulation scheme used in the
CS5101 causes a delay in Q3 turn–on to maintain regulation
on the 3.3 V output.
TP5 is located at the ramp node used to control the
CS5101, which is shown in Figure 2e. When measuring at
this point, connect the scope probe ground to TP4.
VIN
0V
a. Drain Voltage of the Primary Side Switch (TPI)
0V
b. Secondary Voltage Prior to Rectification and Filtering (TP2)
0V
c. Secondary Voltage After Rectification by D6
THEORY OF OPERATION
The main output of this 50 W power supply is 5.0 V at
7.0 A. This output is generated using a forward topology DC
to DC converter, which operates using peak current mode
control. Primary side regulation is accomplished with the
industry standard CS3843. The auxiliary output supplies
3.3 V at 5.0 A. It is regulated by the CS5101, which uses
leading edge modulation to control the on–time of Q3.
0V
d. Voltage Waveform at Q3’s Source (TP3)
e. Ramp Voltage Waveform (TP5)
Figure 2. Demonstration Board Test Point
Waveforms Under Full Load Conditions
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2
CS5101DEMO/D
The current limit threshold is set by the offsets of R17 and
R15. Under normal operating conditions, the current limit is
invoked when the DC voltage drop across the coil equals the
sum of these two offsets. Under a short circuit condition, the
output current will fold–back, since the offset associated
with R17 is negligible and the offset created by R15 remains.
The current limit set–point is determined by:
0V
a. Secondary Voltage Prior to Rectification and Filtering (TP2)
V
ILIM OFFSET
RL1
where VOFFSET is the DC offset voltage at the input of the
current sense amplifier and RL1 is the DC resistance of the
coil L1. On the demonstration board, the inductor DC
resistance is about 7.0 mΩ.
The fold–back value is determined by:
0V
b. Voltage Waveform at Q3’s Source (TP3)
0
c. Current Through L1
V
IFOLD OFFSET RL1
Figure 3. Demonstration Board Test Point Waveforms
Under No Load Condition on the 3.3 V Output
5.0 V6.2 k 1000
k6.2 k
7.0 m
REFR16
VR15R16
RL1
4.4 A
where VREF is the reference provided from the CS5101.
The output current limit set point is determined by:
Primary Side Control
The error amplifier (U3) feeds the output voltage back to
the CS3843 controller through the opto–isolator (U2). The
voltage on the secondary side of the transformer, at test point
TP2, is rectified by a Schottky pair (D6) and averaged by an
LC filter (L2 and COUTI), to create the main output (Figure
3).
V
ILIM OFFSET
RL1
V
R15
VOUT2R17
REFR16
VR15R16
OUT2
R17R18R19
R15R16
RL1
V6.2 k
3.3 V1.0 M
3.3 V390 k
1.05.0M6.2
1.0
390 k3.0
k
M6.2 k
k3.3 k
7.0 m
9.0 A
Secondary Side Control
The rectified voltage at the cathode of D6 (Figure 1) is
applied to the FET (Q3). Since the average of this rectified
waveform is 5.0 V, the CS5101 must reduce the duty cycle
of Q3, which is applied to the auxiliary output filter
comprised of L1 and COUT2. This is done by delaying the
turn–on of Q3 relative to the rising edge of the waveform
shown in Figure 2c. This waveform represents point A in
Figure 1.
During the off–time of Q3, its body diode prevents the
current through Ll from becoming discontinuous. As the
current through L1 decays through zero, the body diode of
Q3 conducts negative current. When the voltage at the drain
of Q3 rises, it immediately causes the source of Q3 to track.
The source will remain elevated until the current in Ll
reaches zero, as shown in Figure 3b. The CS5101 controls
the width of the second pulse, shown in Figure 3b, to
maintain regulation.
Output current limit for this circuit is set at 9.0 A to
guarantee specified maximum output current over the
variation of DC resistance in Ll.
9
R18
3.3 kΩ
R17
390 kΩ
8
CS5101
–
+
VREF = 5.0 V
R19
3.0 k
C12
R15
1.0 M
R16
6.2 kΩ
L1
Lossless Current Limit
Current limit is implemented without using a current
sense resistor. The inductor’s DC resistance is used instead
(Figure 4). The average voltage across the inductor is
determined by the product of its DC resistance and the DC
current through the coil. A low–pass filter averages the AC
voltage at the switching node. This voltage is compared to
the output voltage, which is fed to the CS5101 through R16.
Q3
D5
10 µH/0.007 Ω
3.3 V
+
COUT2
Figure 4. Lossless Current Limit Circuit
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3
CS5101DEMO/D
Efficiency Considerations
The conversion efficiency for the secondary output in this
design is calculated below The slight loss in efficiency is
primarily in the flyback diode (D5).
Power loss in the FET (Q3) occurs during conduction,
since the CS5101 ensures lossless turn–off and turn–on.
This FET loss is:
with a single pole. It has a lower crossover frequency than
the CS5101 voltage feedback loop.
Selection of the compensation components is described in
detail in the application note “Secondary Side Post
Regulator,” document number CS5101AN/D, available
through the Literature Distribution Center or via our website
at http://www.onsemi.com.
PQ3 RDS(on) I2 DC
0.04 25 A2 20% 0.2 W
CONSIDERATION FOR CRITICAL COMPONENTS
IN THIS DESIGN
where DC is duty cycle which is about 20% for a 3.3 V
output and 48 V input.
Power loss in the CS5101 depends on quiescent current,
gate drive current, and VCC.
Magnetics
An un–gapped transformer was chosen to provide
maximum primary inductance. The turns ratio was selected
so that the duty cycle at low line (36 V) does not exceed 50%.
The circuit uses a resonant reset technique, which eliminates
the need for an auxiliary catch winding. A single secondary
winding generates both the 5.0 V and 3.3 V outputs.
PQ4 (IQ IDR) VCC
(IQ QTOT fSW) VCC
(12 mA 30 nC 160 kHz) 18 V 0.302 W
Input Capacitors
Input capacitors are chosen to support maximum ripple
current. A 100 V rating for the capacitors was chosen to
ensure reliable operation at maximum input voltage (72 V).
Loss in the diode is:
PD5 VF IOUT (1 DC)
0.55 V 5.0 A (100 20)% 2.2 W
Loss in the inductor consists of conduction loss and core
loss. Inductor core loss is estimated from data supplied by
core vendor.
Output Capacitors
Output capacitors were selected based primarily on the
output ripple voltage specification. Output ripple is
determined by the effective series resistance (ESR) of the
output capacitors.
PL1 PCORE IOUT2 RLDC
0.125 W 25 A2 7.0 m 0.3 W
FETs
The MOSFET selection is based on a sufficient voltage
rating of 200V for Q2 and 55V for Q3. Because Q3 has a low
RDSON and switches with very low transient loss it does not
require a heat sink.
Total power loss and efficiency, for the 3.3V output, are
determined as follows:
PT PQ3 PU4 PD PL1
0.2 W 0.3 W 2.2 W 0.3 W 3.0 W
POUT
16.5 W
84.6%
3.0 W 16.5 W
PT POUT
Schottky Diodes
Schottky diodes D5 and D6 are selected for the maximum
current and voltage of the circuit. In this design they support
50 V (max) spikes. The snubber circuit formed by R29/C35
reduces these voltage spikes.
Compensating the CS5101
There are two control loops used in the demonstration
circuit. The 5.0 V output is regulated by the TL431 error
amplifier (U3) and its associated feedback components.
Compensation of this amplifier is handled using a simple RC
network (R23 and C18), which is common in current mode
control.
The 3.3 V output is controlled by the error amplifier in the
CS5101. This amplifier is compensated with a dual
pole–zero RC network (R13, C10, C11 and R22, C17),
which is common in voltage mode control. The crossover
frequency of the CS5101 controller is 5 to 10 times lower
than the main loop to ensure proper interaction between the
two control loops. The current limit circuit is compensated
Output Chokes
The output chokes were selected to support full output
load current with a 30°C temperature rise and to minimize
output voltage ripple.
CS5101 Voltage Limiting
The maximum voltage on the CS5101’s VCC input is
limited to 19 V by D4. This limits the maximum gate to
source voltage for Q3 to less than 20 V.
The complete demonstration board schematic is shown in
Figure 5, and the Bill of Materials, with part numbers,
vendors and contact numbers is contained in Table 3.
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CS5101DEMO/D
Table 3. Bill of Materials for the CS5101 Demonstration Board
Qty
Ref Des
Description
3
C1, C2, C3
47 µF, 100 V
6
C7, C12, C14,
C16, C18,
C31, C32
0.1 µF
1
C4
4700 pF
2
C11, C36
2
C5, C35
1
1
Manufacturer
Part Number
Phone
Fax
Nichicon
UPR2A470MPH
(708) 843–7500
(708) 843–2798
Panasonic
ECU–S1J104MEA
–
–
Panasonic
ECU–S1J472KBA
–
–
0.01 µF
Panasonic
ECU–S1J103KBA
–
–
150 pF
Panasonic
ECU–S2A151JCA
–
–
C8
68 F
Panasonic
ECU–S2A680JCA
–
–
C9
330 pF
Panasonic
ECU–S2A331JCB
–
–
2
C10, C15
0.47 µF
Panasonic
ECU–S1J474MEB
–
–
2
C13, C17
0.22 µF
Panasonic
ECU–S1J224MEA
–
–
6
C19–C24
680 µF/16 V
Nichicon
UPY1C681MPH
(708) 843–7500
(708) 843–2798
1
C33
47 µF/35 V
Nichicon
UPL1V470MEH
(708) 843–7500
(708) 843–2798
1
C34
1000 pF/200 V
Panasonic
ECQ82102JF
–
–
2
R1, R3
22 k
KOA Speer Electronics
CF–1/4–223–J
(814) 382–5538
(814) 382–8883
1
R2
2.0 k
KOA Speer Electronics
CF–1/4–202–J
(814) 382–5538
(814) 382–8883
1
R4
5.1 Ω
KOA Speer Electronics
CF–1/4–5R1–J
(814) 382–5538
(814) 382–8883
2
R5, R29
100 Ω
KOA Speer Electronics
CF–1/4–101–J
(814) 382–5538
(814) 382–8883
1
R10
33 Ω
KOA Speer Electronics
CF–1/4–330–J
(814) 382–5538
(814) 382–8883
2
R6, R7
0.2 Ω/1.0 W
KOA Speer Electronics
RSS–1–0R2–J
(814) 382–5538
(814) 382–8883
2
R8, R11
5.1 k
KOA Speer Electronics
CF–1/4–512–J
(814) 382–5538
(814) 382–8883
1
R9
510 Ω
KOA Speer Electronics
CF–1/4–511–J
(814) 382–5538
(814) 382–8883
3
R12, R16, R23
6.2 k
KOA Speer Electronics
CF–1/4–622–J
(814) 382–5538
(814) 382–8883
1
R13
300 Ω
KOA Speer Electronics
CF–1/4–301–J
(814) 382–5538
(814) 382–8883
2
R15, R28
1.0 M
KOA Speer Electronics
CS–1/4–105–J
(814) 382–5538
(814) 382–8883
1
R18
3.0 k
KOA Speer Electronics
CF–1/4–302–J
(814) 382–5538
(814) 382–8883
1
R19
3.3 k
KOA Speer Electronics
CF–1/4–332–J
(814) 382–5538
(814) 382–8883
1
R17
380 k
KOA Speer Electronics
CF–1/4–393–J
(814) 382–5538
(814) 382–8883
1
R20
1.3 k, 1.0%
KOA Speer Electronics
MF–55–0–1301–F
(814) 382–5538
(814) 382–8883
9
R12, R24, R25
2.0 k, 1.0%
KOA Speer Electronics
MF–55–0–2001–F
(814) 382–5538
(814) 382–8883
1
R22
200 Ω
KOA Speer Electronics
CF–1/4–201–J
(814) 382–5538
(814) 382–8883
1
R26
10 k
KOA Speer Electronics
CF–1/4–103–J
(814) 382–5538
(814) 382–8883
1
R27
18 Ω
KOA Speer Electronics
CF–1/4–180–J
(814) 382–5538
(814) 382–8883
1
U1
PWM
ON Semiconductor
CS3843AN8
(401) 885–3600
(401) 885–5786
1
U2
Opto–isolator
Motorola
MOC8102
–
–
1
U3
Reference
National Semiconductor
LM431ACZ
–
–
1
U4
SSPR
ON Semiconductor
CS5101N14
(401) 885–3600
(401) 885–5786
1
D1
15 Zener
Central Semiconductor
1N5245
(516) 435–1110
(516) 435–1824
2
D2, D3
Diode
Central Semiconductor
1N4148
(516) 435–1110
(516) 435–1824
1
D1
19 V Zener
Central Semiconductor
1N5249
(516) 435–1110
(516) 435–1824
1
D5
8.0 A/80 V Schottky
International Rectifier
8TQ080
(310) 322–2331
(310) 232–3332
1
D6
16 A/60 V Schottky
International Rectifier
30CTQ080
(310) 322–2331
(310) 232–3332
1
Q1
NPN, 100 V, 1.0 A
Central Semiconductor
TIP29C
(516) 435–1110
(516) 435–1824
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5
CS5101DEMO/D
Table 3. Bill of Materials for the CS5101 Demonstration Board
Qty
Ref Des
Description
Manufacturer
Part Number
Phone
Fax
1
Q2
NMOS, 200 V
1
Q3
NMOS, 55 V
International Rectifier
IRF640
(310) 322–2331
(310) 232–3332
International Rectifier
IRFZ34N
(310) 322–2331
(310) 232–3332
1
L1
10 µH/5.0 A
Allied Components Int.
CS226
(714) 630–3713
(714) 630–3562
1
L2
10 µH/ 7.0 A
Allied Components Int.
CS227
(714) 630–3713
(714) 630–3562
1
T1
Power Xformer
Gauss Transformer
GSPT–30EPC–H005
(714) 522–6889
(714) 522–7335
6
J1–J6
Turret Terminal
Millmax
2501–1–00–44–00–
00–07–0
–
–
2
H1
Clip–On Heat Sink
Aavid Engineering
576802804000
(603) 528–3400
(603) 528–1478
1
H2
1″ Heat Sink
Aavid Engineering
613002802500
(603) 528–3400
(603) 528–1478
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6
C31
0.1 µF
PGND
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7
C5
150
R3
22 k
C32
0.1
CS3843
U1
GND
R/C
COMP
VFB
IS
REF
C1
47 µF
OUT
R2
2.0 k
D1
15 V
R1
22 k
VCC
Q1
TIP29C
VIN (36 V to 72 V)
C6
1000
R6
0.2
+
R7
0.2
R26
10 k
Q2
IRF640
TP1
C3
47 µF
R5
100
+
R4
5.1
C2
47 µF
C4
4700
+
T1
C33
47 µF
R8
5.1 k
R27
22
C34
1000
C14
0.1
R29
100
Figure 5. Demonstration Board Schematic
C9
330
TP5
U3
TL431
Q3
IRFZ34N
C8
68
R36
1.0 k
D6
30CTQ060
R11
5.1 k
TP4
R12
6.2 k
R20
1.0 M
U2
MOC8102
R9
510
C35
150
TP2
D2
IN4148
R10
33
D4
19 V
+
C10
0.1
L2
C36
0.01
+
C23
680 µF
+
C19
680 µF
L1
R24
2.0 k, 1.0%
R25
2.0 k, 1.0%
R23
6.2 k
VREF
VFB
VCOMP
C11
0.01
C15
0.47
R13
300
R20
1.3 k,
1.0%
R21
2.0 k,
1.0%
C21
680 µF
+
R15
1.0 M
C16
0.1
C10
0.47
C20
680 µF
+
C24
680 µF
+
R16
6.2 k
IP
CS5101
U4
LGND PGND
IN
ICOMP
VG
RAMP
VSYNC
VO VCC VC
C22
680 µF
+
R19
3.3 k
C12
0.1
R18
3.0 k
D5
8TQ080
TP3
R17
390 k
C13
0.22
C7
0.1
D3 IN4148
SGND
5.0 V
SGND
3.3 V
R22
390
C17
0.1
CS5101DEMO/D
CS5101DEMO/D
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are trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes
without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular
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CS5101DEMO/D
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