External +-48 Volt to 5 Volt Power Supply ------------------------------------------- Initial Rev. 19-Jan-2026 Current Rev. 24-Jan-2026 The intent of this note is to sketch out the general design of an External +-48 Volt to 5 Volt Power Supply. The main reasons to use an External Supply are to: - Physically move the TRACO converter away from the low level wide bandwidth PMT signals and away from the PMT Analog to Digital Converter. - Provide more filtering on both the Input and Output of the TRACO converter. Both Common mode and Normal mode filtering are needed over an bandwidth from about 100 kHz up to 100 MHz. There is NOT enough space on the DK board itself to provide all of the TRACO Input and Output Filter that appears to be necessary. There is especially Not enough space to provide the required Output filter which uses physically large inductors because of the low voltage drop requirement with a 4 Amp current flow. Block Diagram of External Power Supply: --------------------------------------- Input Input Input Input Common Normal TRACO Power Power Mode Mode Power ---> Connector Protection Filters Filter Input Output Output >--- TRACO Normal Common Output Power Mode Mode Power Output Filter Filters Connector Stay with TRACO or Use a Different Converter: ----------------------------------------------- I've found at least 8 other companies that make similar converters. Some of these are quality companies like Vicor and Lambda and others are probably junk. Most or all of these converters seem to make the EN 55032 class B or A noise specification. That specification is no where near enough for the P-ONE application - that's why we need external Input and Output Filters. None of these commercial converters appear to have the circuit topology that is required to make them a seriously low noise converter - rather it looks like any of them will generate Common mode noise between their input and output. So the choices are: design and build a low noise converter ourselves or use one of the commercial converters with lots of external filtering. For now we will stay commercial and stay with TRACO just because we now know a lot about it and it appears to be a reasonable quality component. Filters on the Converter's Input and Output: -------------------------------------------- - Normal mode filters adjacent to both the TRACO's Input and Output are needed to remove any current spikes in these circuits that might otherwise saturate the Common mode filters and thus decrease the effectiveness of the Common mode filters. Recall that Common mode noise from the TRACO is the main cause of the noise problem. Recall that both the TRACO's Input and Output contain current spikes, e.g. about 30 mA spikes on the Input and 75 mV pp spikes on the Output. - Both the input and output filters need to be effective over a bandwidth from about 100 kHz up to about 100 MHz. The worst noise spike is expected to be at about 330 kHz the typical switching frequency (297 kHz min to 363 kHz max switching). - Relevant Characteristics of the TRACO Converter: Input to Output Isolation Test Voltage 2.25 kVolt DC Input to Output Isolation Resistance 1000 Meg Ohm min Input to Output Isolation Capacitance 3 nFd max Output Ripple and Noise 20 MHz Bandwidth 75 mV peak to peak typ Output Capacitive Load 5000 uFd max - TRACO Suggested Input Filter to meet EN 55032 Class B limits: 1x 39 uFd 250 V Aluminum Electrolytic 3x 470 nFd 250 V Ceramic 1x 470 uH Common Mode Choke 1x 1 nFd Ceramic between Vin - to Vout - - The input filters need to be able to handle about 265 mA of current and remain fully effective, i.e. no inductor saturation. Considerations: Main Cable is about 26 Ohms per km for each wire Assume 80% converter efficiency Assume the full 96 Volt power source in the Junction Box Assume the full 20 Watt 4 Amp load Want less than a 2 Volt drop in the Input Filters, i.e. Input filters are less than about 7.5 Ohms. So it is relatively easy to meet the current handling and Voltage drop (i.e. resistance) requirements of the Input filters while using components with reasonably small physical size. - The Output filters need to handle 4 Amps and have a low voltage drop, e.g. 50 mV maximum - so much harder requirements. 50 mV drop at 4 Amps is only 12.5 mOhms. - Wurth uses a number of materials in their Common Mode Chokes: WE-CMB Red Low frequency only WE-CMB Green NiZn Ferite High frequency only WE-ExB Yellow double hump boardband WE-CMBNC Black broadband WE-ExB appears slightly better than WE-CMBNC at high frequency WE-CMBNC appears slightly better then WE-ExB at low frequency WE-ExB parts seem to start 744844... WE-CMBNC parts seem to start 744801... through 744806... depending on the parts physical size All of the nice WE-CMBNC parts appear to be THD but that is OK in this power application that will have lots of THD components. I believe that the WE-CMBNC parts come in 6 different size families: XS, S, M, L, XL, XXL. I should try to select parts from size families that include other reasonable value parts just in case some specific part is not available or in case some other value makes a better filter. The nice Horizontal SMD parts seem to all start 744663 and I believe that they all use the WE-CMB Green NiZn Ferite material. I do not know how tall this External Supply can be. For now let's set the maximum component height at 22 mm i.e. the same as was used for the Rev A DK board. The Input Common mode filter will be in 3 stages. The issue is needing a high value of inductance to get enough reactance at the lowest frequencies but knowing that the high inductance chokes will have the highest parasitic capacitance which will kill their effectiveness at the high frequencies. Based on experience with putting choke cores on the power cable to the DK board set the smallest stage to about 50 uH and the largest to about 50 mH. The inductance will scale as turns squared and the parasitic capacitance will probably scale about with the number of turns. So set the middle choke to about the geometric mean, i.e. about 2 mH. Then see what is available in the WE-CMBNC series: 80 mH 7448020680 600 mA max 1 Ohm Peak Attenuation about 56 dB at 250 kHz has Attn of about >40 dB 10 kHz - 2 MHz 22 mm Tall, 14 mm x 18 mm pcb foot print 30 mH available is the same pcb foot print 3 mH 7448024503 4.5 A max 22 mOhm Peak Attenuation about 36 dB at 10 MHz has Attn of about >30 dB 800 kHz - 50 MHz has Attn of about >25 dB at 200 kHz 22 mm Tall, 14 mm x 18 mm pcb foot print 5, 2.5, 1.5 mH available is the same pcb foot print 0.4 mH 7448014501 4.5 A max 22 mOhm Peak Attenuation about 27 dB at 60 MHz has Attn of about >25 dB 10 MHz - 150 MHz 16 mm Tall, 7.5 mm x 15 mm pcb foot print 0.5, 1.0 mH available is the same pcb foot print Input Filter Normal mode choke - In the Rev A DK board I used 74477125 560 uH 650 mA 890 mOhms max that all sounds OK but its self resonant frequency is only 2.5 MHz which could be a problem but its XL is above 1000 Ohms from about 300 kHz up to about 20 Mhz which is probably OK. Is there anything better in a finite size ? Output Filter Common mode chokes one on the External Supply and one at the power input to the DK board 1 mH 7448027001 7 A max 6 mOhm Peak Attenuation about 28 dB at 30 MHz has Attn of about >25 dB 4 MHz - 80 MHz has Attn of about >20 dB 500 kHz - 160 MHz 22 mm Tall, 14 mm x 19 mm pcb foot print 5, 2.5, 1.5 mH available is the same pcb foot print 1.5 mH 7448026002 6 A max 8 mOhm Peak Attenuation about 30 dB at 20 MHz has Attn of about >25 dB 1.5 MHz - 90 MHz has Attn of about >20 dB 200 kHz - 170 MHz 22 mm Tall, 14 mm x 19 mm pcb foot print 5, 2.5, 1.5 mH available is the same pcb foot print Output Filter Normal node chokes: e.g. 7443321000 or 7443320470 Between all stages of the Input and Output filters we need shunt capacitors to "short out" any AC normal mode voltage so that the common mode filters can work effectively. Connectors and Their Pinouts: ----------------------------- - For now let's stay with the two row 25 mil sq pin 0.1" 0.1" shrouded headers with latches. This is what folks are used to, the cable mount contacts matches the Main Cable wires well, and the current handling capability matches the requirements. - On the Input side we need only two wires so we can use the minimum pin count in this family of connectors, i.e. 6 pins. 3M_3869-5302 Header 6 Pin 2x3 0.1" x 0.1" Through Hole Right-Angle 4 Wall 3M Part No. N3869-5302RB Cable Mount Housing 6 Pin AMP Part No. 2-87977-8 AMP Part No. 87631-1 - The pinout of the Input connector is: - On the Output side we can use 4 wires for the +5V bus and 4 wires for its Return. Using 8 pins keeps the connector small and removes the danger of swapping input and output connectors. Assuming that we use standard AWG 22 gauge wires then the voltage drop of this connection will be kept low. AWG 22 is about 16.5 Ohms per 1000 ft. Assuming that this is a 1 foot long connection with 4 wires in each direction then the overall resistance is about 8.25 mOhms or about 33 mV drop at 4 Amps. 3M_3889-5302 Header 8 Pin 2x4 0.1" x 0.1" Through Hole Right-Angle 4 Wall 3M Part No. N3889-5302RB Cable Mount Housing 8 Pin AMP Part No. 1-102387-4 - We may/will also want a "ground" connection to the metal housing of the module. The only use of this "ground" connection is to provide a connection point for the Varistor + Flash Tube over voltage protection on the Input power from the sJB. Note that no connection of either the Input or Output of this supply is made or allowed to this "ground" point. Specifically the Output Return current does not (and is not allowed to) flow through this "ground" connection. This "ground" connection is made via one or more of the M3 mounting screw holes in the pcb for this external supply. - The pinout of the Output connector is designed to allow low voltage drop distributed PCB trace routing to the pins of this dual row connector. The pinout of the Output connector is: Protection Devices: ------------------- - Need both reverse voltage and normal mode voltage spike protection. - Also need protection against over voltage across the TRACO converter itself, i.e. a high common mode voltage on the input power cable to the External PS board. This must be a very low leakage protection circuit, i.e. Varistor + Flash Tube. Move the Barnacle Connector to the External PS Brd: --------------------------------------------------- - It's rational to think about moving the Barnacle Connector from the DK to the External Power Supply. The point would be to have the Barnacle (which may pull as much power as the whole DK board) get its power directly from the TRACO output instead of getting its power from the Bulk 5 Volt bus on the DK Board. The intent would be to keep the Barnacle's turn On/Off spikes and other switching noise off of the DK board. The serial link between DK and the Barnacle would just passively loop through the External Power Supply board and go to a Barnacle connector on the External PS board. - But this is a bad idea for at least 2 reasons. It creates a path for TRACO noise to get onto the DK board and there is a difference in potential between the TRACO's negative output pin (the 5 Volt Return pin) and the Signal Ground on the DK board. This potential difference comes from the voltage drop in the chokes and wiring that connect the TRACO's negative output pin to the DK's Signal Ground. Output Voltage Trim: -------------------- The TRACO includes a pin to attach an output voltage trim resistor. As on DK Rev A include in this External PS pads to install an output voltage trim resistor and quite possibly install at build time a default value to compensate for the known voltage drop between Ext PS Output and the Bulk 5V bus on DK.