LV Power Supply Design¶
- Table of contents
- LV Power Supply Design
- Power requirement
- Current monitoring
- Control circuitry
- Full converter schematic
- Standalone box
- Output ripple
- Burning Bootloader
- Electrical Safety & Health
A low voltage powering system which provides isolated low voltage power supply to the readout electronics of the G-2 tracker. The power supply is capable of providing a maximum current of 3A per +5V and -5V output channels. In order to satisfy the isolation requirement, the power supply is split into an off-the-shelf AC-DC power supply and a bespoke DC-DC converter which provides the isolation. The flyback topology had been chosen as the basis of the DC-DC converter design. Flyback converters employ a transformer which provides galvanic isolation between the input and the output (i.e. ensuring that there is no DC path between them). The flyback converter uses a switch that is controlled by an external IC which transfers the energy from the primary winding of the transformer to the secondary winding. A simplified circuit of the flyback converter can be seen below.
Isolated Flyback Topology¶
When the switch is closed, the magnetic flux in the transformer increases, thus storing energy. No output voltage is generated at the load since the voltage induced at the secondary is negative and the diode is reverse-biased, hence blocking the current. When the switch opens, the magnetic field in the transformer collapses and a positive voltage is induced at the secondary winding. The diode becomes forward-biased and the current is allowed to charge the capacitor. The value of the capacitor depends on the switching frequency of the converter to provide a continuous current flow.
The major drawback of the flyback design is the current and voltage control. The isolation barrier makes it difficult to monitor the output and feed it back to the converter chip. This is commonly solved by using an opto-coupler or a third winding in the transformer to communicate the output over the isolation barrier in order to regulate it.
LT8302: The flyback converter chip¶
Instead of using the mentioned output sensing techniques (opto-coupler and third transformer winding) the LT8302 allows the sampling of the isolated output voltage directly from the waveform at the primary side, which simplifies the design of the isolated DC-DC converter. The LT8302 operates from an input range of 2.8V to 42V and can deliver up to 18 W of output power.
The converter has been designed to meet the greater power requirement of the positive supply. This allows the use of the same circuitry for both the positive and negative supplies. The maximum power output required of the positive power supply is 15 W (3 A @ 5 V). The output power of the LT8302 converter is dependent on the input voltage and the primary-to-secondary winding of the transformer. A transformer with a 4:1 turn ratio can comfortably provide the necessary power output at the selected input of 24 V. Traditionally, the turn ratio in transformers specifies the output voltage at a given input, however with the LT8302 this is not a concern as the output voltage is controlled by two external resistors.
Due to the difficulties of monitoring the output current over the isolation barrier, a decision has been made to monitor the input current to each of the converters (positive and negative channels individually). This is a compromise but it can give an idea on how well the converters are behaving and can detect problems. The current is measured using a current sense resistor, a current sense amplifier and an ADC.
The power supply board is controlled with an ATMEGA328P microprocessor. The microprocessor has 6 analogue inputs and 14 digital I/Os. Two of the analogue inputs are used to monitor the current outputs of the positive and negative converters and one digital I/O is used to enable/disable the converters. There is also an analogue input connected to an NTC, which is used to measure the temperature near the converter's IC (only available in Rev. 2).
The microprocessor is also connected to an I2C bus which is how the power supply board (microprocessor) receives its commands. This is particularly important when the power supply boards are in a rack, addressed by a standalone control board.
Full converter schematic¶
Low Voltage circuit schematic¶
The LV standalone box contains:
- Two LV boards
- An Arduino micro board to communicate with the LV boards via an I2C bus.
- Power supply, AC/DC, 75 W, 24 V, 3.2 A TXM 075-124.
- A 60 mm cooling fan.
The LV box operates with an AC input range of 100-240VAC / 50-60 Hz. 2.5 A max. A 2.0 A fuse is installed in the power entry module (under the ON/OFF switch).
On the front, the box has a USB-B connector which is connected directly to the Arduino board. Driver installations can be found here. The USB link emulates a serial comm and can be accessed via a serial terminal. The control commands are described in the next section.
The output of the LV boards is in the middle of the front panel. The LEDs on the side of the DB9 connectors indicate whether the board output channels are ON or OFF. (Right LED is for the +5V channel and the left LED is for the -5V channel). The pinout for the DB9 connector is shown below:
The bottom board (Board A) has an I2C address 0x1 and the top board (Board B) has an address 0x2.
The power supply boards can be controlled and monitored through the USB connection which emulates a serial port.
Commands are terminated with a carriage return and are followed by the address of the power supply board.
help - Arguments: None Return: List of commands on - Turns on an addressable power supply board. Arguments: Address (Int 1-254). Return: OK/Error. off - Turns off an addressable power supply board. Arguments: Address (Int 1-254). Return: OK/Error. status - Polls the status of an addressable power supply board. Arguments: Address (Int 1-254). Return: Input currents of the positive and negative channels, ON/OFF state and board's temperature. sc - Start a Slow Control console Arguments: Address (Int 1-254). Return: A Slow Control console is opened, if address is invalid Error would be returned. version - Returns the version of the LV firmware Arguments: None Return: Version number
+5V channel at 3A¶
-5V channel at 1.5A¶
The current LV boxes (as of March 2015) are shipped with the Rev. 1.1 PCBs. These PCBs have a few problems that have been manually fixed. All known problems have been corrected for Rev. 2.0.
A more compact PCB with added slow control.
|AC1, AC4, AC10, C7, C8, C35||0603||6||CC0603KRX7R9BB104||0.1uF||1362556|
|C21, C22, C23, C25, C26, C27||C1210||6||CL32A107MPVNNNE||100uF||766-1179|
|C3, C4, C9, C10, C20, C24||C1210||6||C3225X5R0J107M250AC||100uF||1907361|
|R_pu1, R_pu2, R_pu3||J1-0603||3||4.7K|
|R_pu4, Rref1, Rref2||J1-0603||3||10k|
|AR2, R1, R6||J1-0603||3||1M|
|DTVS1, DTVS2, DTVS3||DO-214AA||3||SMBJ24A-E3/52||9551026|
The ATMEGA2560 microchip used in the Rack version of the LV board has a large bootloader which the Arduino ISP programmer cannot handle. Therefore, a different programmer (more powerful) is needed. The Olimex AVR-ISP-MK2 (https://www.olimex.com/Products/AVR/Programmers/AVR-ISP-MK2/) is recommended. It is also very important to note that the new versions of the Arduino IDE do not work with this programmer. Version 1.0.2 has been proven to work. This is not due to the IDE itself but to the avr programmer software (avrdude) which comes with it. The avrdude version that comes with Arduino IDE 1.0.2 is 5.11.
Important: Bootloader burning is only required for brand new boards. Once the bootloader is burned the firmware can be flashed with any programmer.
Electrical Safety & Health¶
In accordance with Fermilab ES&H, the board design had followed the electrical design standards which puts measures to protect the circuit against
- Over current
- PTC resettable fuse are placed between the 24V plane and the converters. The fuses F1 and F2 have a tripping current of 1.5A and 1A, respectively. In Rev. 2.0, F1 and F2 are swapped and F3 was also added to protect the microprocessor regulator from over current. F3 has a tripping current of 1A.
- Over voltage
- A Transient Voltage Suppressor diode is placed in parallel with the load, between the power plane and ground plane. Voltage excursion above the diode's clamp voltage will activate the diode and the current will flow to ground. The short-circuit will instantly trip the fuse and the current will be interrupted. In Rev 1.0 there is only one TVS. However, more diodes were added in Rev 2.0 and placed after every fuse. The clamping voltage for the chosen DTVS is 38.9V.
- Over heating
- A fan was placed in the LV box to assist air flow.