Transcript
Application Report SLUA374C – January 2006 – Revised August 2006
-48-V Supply Selection Circuit for TPS2350, Simplifies Fault Detection in ATCA Systems Jim Bird ............................................................................................................... System Power MAN
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ABSTRACT This application note presents a method for controlling -48-V ORing circuitry to allow selection of the power source in dual feed -48-V telecom systems. Such control provides a powerful tool for diagnostic and load management functions. Hidden faults such as blown fuses and failed diodes can be found with this circuit. Fault detection in high availability and advanced telecommunications computing architecture (ATCA) systems is simpler and more effective. Either, neither or both of these supplies may be turned off with this circuit.
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Why Is This Circuit Needed? Present telecom power systems do not usually provide a method for selecting which -48-V feed provides power to the load. The ORing technique simply draws power from the supply with the greatest magnitude. Although ORing is simple, small, and inexpensive it tends to make certain faults quite difficult to detect. Figure 1 shows a typical diode ORing circuit with fuses on each power feed and each return, as required by the ATCA spec. This circuit is commonly found at the power inputs to telecom boards with power ratings from 20 W to 500 W. If a single fuse or diode fails the redundant design of Figure 1 prevents the load from detecting the failure. In fact, it would be possible for 2 diodes and 2 fuses to fail with no apparent impact to board performance, but with significant impact to the board availability/survivability. With no means to control which feed supplies the load, it becomes quite difficult to implement test routines capable of accurately exposing such failures. F1
D1
F2
D2
VRTN_B VRTN_A LOAD F3
D3
F4
D4
−48 V_A −48 V_B
Figure 1. Typical Diode ORing Circuit The ability to force power from one supply or the other enables diagnostic routines which can measure board performance, current, and voltage when a known supply is providing power. If supply A is selected and all status monitors report GOOD, and board performance is parametrically GOOD, it can be deduced that the A feed is good. The same test can be run for supply B by selecting supply B and rerunning the tests. When a feed is powered down for maintenance large transients can propagate through the system if the feed had been supplying a significant amount of power. With power feed select control, the loads can be gracefully shifted off the supply to be removed prior to feed shutdown. This reduces the probability of transient related problems during maintenance. If all power needs to be shut off then both feeds may be simultaneously disabled using this technique.
SLUA374C – January 2006 – Revised August -48-V 2006 Supply Selection Circuit for TPS2350, Simplifies Fault Detection in ATCA Systems Submit Documentation Feedback
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How Does It Work ?
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How Does It Work ?
Aoff_RTN 8
7
6
Boff_RTN
Aoff 5
4
3
9
11
12
R6 56 k
R5 56 k R4 56 k
−VINA
−VINB
C2 1000pF
FLTTIM RAMP −VINA −VINB 6 14 13 5
C1 .027 uF
R3 6.65k
4
R2 6.65k
OV
UV 3
R1 374k
GATB
8
9 GATA
SOURCE 7
R7 56 k
RSENSE 0.006 W SENSE 10
GAT 11
Fault FLT 2
RTN
U1 TPS2350
Power Good PG 12
Q1
Q5
Q3
Cbulk
D2 Rload −VINA_RTN
1
D1 −VINB_RTN
Q4
Q2
RT1 500
13
15
16
U2
14
10
1
2
Boff
RTN
At the heart of the power supply select circuitry is the TPS2350 hot swap FET ORing controller. The TPS2350 is a full featured hot swap controller for -48-V systems with sensing and control for low side ORing FETs. A detailed description of TPS2350 operation may be found in the datasheet, Texas Instruments Literature Number SLUS574A.
Figure 2. Power Feed Select Control Using the TPS2350
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-48-V Supply Selection Circuit for TPS2350, Simplifies Fault Detection in ATCA Systems SLUA374C – January 2006 – Revised August 2006 Submit Documentation Feedback
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Circuit Operation
To add power feed select circuitry to a standard TPS2350 application requires the addition of the following components as shown in Figure 2 and Table 1. Table 1. Additional Components Shown in Figure 2 COUNT
REF DESIGNATOR
DESCRIPTION
PART NUMBER
4
R4, R5, R6, R7
56 kΩ resistors
2
Q4, Q5
NFET
IRF530
1
U2
Quad Opto Isolator
TLP283-4-N
The full List of Materials for the circuit in Figure 1 is shown in Table 2.
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Circuit Operation The TPS2350 has 400 mV of supply select hysterisis and there is usually no need for back to back FETs in the select switches because a channel turns on before the FET body diode is forward biased. However, the power feed select circuitry in Figure 2 is able to override the internal supply select circuitry and force power to come from a supply of lower magnitude. For this reason Q5 is put in back to back with Q3, and Q4 is put in back to back with Q2. Q4 prevents body diode conduction in Q2 and Q5 prevent body diode conduction in Q3. Two opto isolators are used to turn off each channel. When Aoff is brought high relative to Aoff_RTN one opto pulls –VINA to RTN ( thus indicating to the TPS2350 supply select circuitry that there is no power at –VINA), and a second opto pulls Q4 gate to Q4 source. The TPS2350 pulls the off channel GATE to SOURCE which, under normal operation, is the lowest voltage available at the chip. But, since the channel select circuitry allows forcing a lower magnitude supply to provide power, SOURCE is not necessarily the lowest point around. For this reason an opto isolator is used to short the SOURCE and GATE of the off channel. Identical circuitry is used to turn off the –VIN_B when Boff is mode positive relative to Boff_RTN. If both Aoff and Boff are asserted both feeds are shut off.
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Design Concerns The ability to force power draw from a lower magnitude feed and then switch back to a higher magnitude feed allows operator induced overcurrents. In circuits not prepared for such events this can cause nuisance trips of overcurrent detectors or blown fuses. For Example; if a -42-V feed is forced to power the load there can be a large inrush current when the load switches from that -42-V feed back to a greater magnitude feed. If the load is switched to the maximum allowed -72-V feed there is 30 V between the power feed and the on board bulk capacitors. This can cause the overcurrent sensors to detect a fault and shut down. To prevent these nuisance trips it is recommended that large bulk capacitors be isolated by a resistor and diode, as shown in Figure 2. The bulk capacitor charging current is limited to a few tenths of an amp by RT1 which greatly reduces inrush stress on the FETs. During a power drop out the energy in the bulk caps can flow without limit through D2, keeping the board alive. Another technique for reducing inrush stress, bulk capacitor size, and cost is to store hold up energy at high voltage. An application note describing this technique can be found at the TI website, Texas Instruments Literature Number SLUA331.
SLUA374C – January 2006 – Revised August -48-V 2006 Supply Selection Circuit for TPS2350, Simplifies Fault Detection in ATCA Systems Submit Documentation Feedback
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Design Concerns
Table 2. List of Materials for -48-V Supply Select Circuit
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RefDes
COUNT
DESCRIPTION
MFR
Part Number
D1
1
Diode, Dual Schottky, 20 A, 100 V, D2PAK
Vishay
MBRB20100CT
D2
1
Diode, Schottky, 5.5 A, 100 V, DPAK
IR
50WQ10FN
C1
1
Capacitor, ceramic, 0.027 µF, 16 V, X7R, 10%, 805
Vishay
Std
C2
1
Capacitor, ceramic, 1000 pF, 16 V, X7R, 10%, 805
Vishay
Std
Q1, Q2, Q3, Q4, Q5
5
MOSFET, N-channel, 100 V, D2PAK
IR
IRF530S
R1
1
Resistor, chip, 374 kΩ, 1/10 W, 1%, 805
Std
Std
R2, R3
2
Resistor, chip, 6.65 kΩ, 1/10 W, 1%, 805
Std
Std
RT1
1
Thermistor, PTC, 500 Ω, 0.197 dia.
Vishay
2322 660 52893
RSENSE
1
Resistor, chip, 0.006 Ω, ½ W, 1%, 2010
Vishay
WSL2010.006<1%
R4, R5, R6, R7
4
Resistor, chip, 56 kΩ, 1/10 W, 1%, 805
Std
Std
U1
1
IC, redundant -48 V hot-swap controller, PW14
TI
TPS2350PW
U2
1
IC, quad, opto-coupler, MF4
Toshiba
TLP283-4
-48-V Supply Selection Circuit for TPS2350, Simplifies Fault Detection in ATCA Systems SLUA374C – January 2006 – Revised August 2006 Submit Documentation Feedback
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