Linear, Adjustable, Dual Power Supply
Overview
This linear bench power supply is designed to provide low noise regulated voltage for lab experiments. The device provides dual power supply: positive terminal and negative terminal, along with ground terminal. Each terminal is adjustable between 0-15 V. The device has multiple protection features such as adjustable over-current protection, over-heat protection, and limited negative current (current injection) protection.
User Panel
The voltage of each terminal is controlled independently using a panel-mounted analog rotary potentiometer which provides voltage to the error amplifier. Current-limit is adjustable between 0.1-1.5 (A) for each terminal independently through a panel-mounted rotary potentiometer. The panel displays three reading for each terminal (total of six readings): output voltage, output current, and current limit.
The user panel also has two LED indicators for each terminal: over-current indicator, and over-heat indicator for the corresponding terminal’s power transistor array heatsink. There are two spring-type terminal blocks for each terminal: positive, ground, and negative. Additionally, there are two female header-type connectors for each terminal. The output block can be turned on (i.e.: connected to the internal power circuit) using a single output switch. There is a discharge switch for each terminal that can be activated. Each discharge switch connects the corresponding terminal to ground terminal through 10 Ohm resistor when the output switch is turned off, hence safely discharging any capacitor (should one exist) in the application circuit.
Finally, the user panel has the main switch and a programming port for upgrading the display microcontroller(MC) software. There is brightness adjustment button, the button cycles between 8 degrees of brightness. If the brightness is not changed for 10 seconds, the brightness is stored in the display MC and restored next time the devices is turned on.
Technical Details and Specifications
There are four 60 (Hz) transformers: two power transformers for supplying current to output power transistors; one high voltage, low power dual transformer (24V X 2)for supplying the analog control circuitry which drives the power transistors; and a dedicated 10(V) transformer to supply the digital circuitry, display, fan, and relays.
Since this power supply is intended to be low-noise, no switching buck and boost regulators were used. Also, since the regulation is performed using a series element (three bjt power transistors for each terminal), the input power to these transistors was divided into two stages to avoid excessive heat dissipation at low output voltages. The first stage is composed of 12(V) transformer, it supplies the power transistors for output voltages between 0-8.3 (V). If the user dials the voltage above 8.3 (V), a 8(V) transformer is connected in series with the first one using a relay. There is a relay for each terminal to switch between 12(V) and 20(V) supply for the power transistors. The power transistor transformers, along with rectifiers and relays, are shown in the block “Transformer schematics ” in figure (4). The relays are represented using a high resistor for the disconnected terminal in LTSpice. A 12(V)X2 center-tap transformer supplies the 12 (V) stage for each terminal, and a dual core 8(V)X2 transformer supplies the additional 8(V) for each terminal. The main (MC) controls the stage relays. Switching of all relays is done using zero-cross detection to minimize EMP and arcing and to increase relay service life. Snubbers are also used in the AC side of the transformers to suppress EMP.
Three over-current protection mechanisms are implemented to ensure device safety at worst case short circuit. The 1st mechanism to take effect in event of a sever short circuit is a 22 uH inductor (rated for 6A) placed before the power transistor array. This inductor prevents the rapid discharge of the ripple capacitor (4700uF) through the power transistors during the first 3 microseconds and until the 2nd mechanism is activated. The 2nd mechanism is a transistor connected across the sense resistor of one of the power transistors. It is designed to keep the current below about (1.6-1.9 A) until the 3rd mechanism is activated. The 3rd mechanism is the current controlled feedback circuit using op-amp and it is the adjustable mechanism. It keeps the current at or below the current limit set by the user. Note that not all mechanisms are necessarily activated at every over-current incident, it depends on the severity of the short circuit. The current limit of the 3rd mechanism is adjusted by an adjustable current source driving small current from one power transistor and is connected before the current-sense resistor. this current creates a voltage drop below the emitter voltage of the power transistor. This voltage drop is compared with the voltage drop due to the current-sense resistor by an op-amp. This op-amp take control of the power transistors as long as the output current is at the current limit. The circuit does not implement short circuit foldback. The current limit value displayed is not very precise and the actual value can vary up to 10%, this is especially true at low values.
The main MC continuously monitors 4 values for each terminal: user input voltage to adjust the stage relays; output voltage and current to display for user; and heatsink temperature to control fan and shut down device in overheat event. The fan is turned on if heat sink’s temperature exceeds 50°, the corresponding heat sink overheat LED indicator flashes when fan is on. Shutdown temperature is 85° for single transformer (output below 8.3V) and is 65° for dual transformer (output above 8.3V). Shutdown of the power circuit in event of overheat is done using a relay that cuts power from the two power transformers (supplying the power transistors) and from the high-voltage low-power dual transformer that supplies the op-amps of the analog circuit control circuit. The digital circuit power supply transformer is connected directly to the AC to power the main MC and it can only be turned off from the main switch.
In the event of overcurrent, the corresponding LED indicator turns on along with a high-pitched sound. The main MC accumulates the time of overcurrent, if time reaches two seconds, the device is shutdown. If overcurrent is intermittent, then the main MC accumulates positive time for the periods of overcurrent and subtracts half the time when overcurrent is not detected. For example: if there is a periodic cycle of 300ms overcurrent followed by 300ms of normal operation, the device will eventually shutdown. If there is a periodic cycle of 300ms overcurrent followed by 600ms of normal operation, device will not shutdown.
At the event of shutdown, a message is displayed to the user: “ovr cur” in the corresponding terminal for overcurrent, and “ovr het” in the corresponding heatsink with temperature reading of both heatsinks displayed in the lower line of the display. The device is looked in the shutdown mode and can only be reset by turning the main switch off then on. The fan will keep functioning until both heatsinks are below 45°.
The device has a limited negative current capability. Each terminal can handle up to 60mA of negative current while keeping the output voltage regulated. In the event of negative current, the corresponding current display shows (-vE) reading (without numeric value of the current) along with a high pitch buzzer sound. The device does not shutdown on negative current as this measure has no protective benefit. Sustaining negative current for prolonged periods will eventually overheat a transistor in the circuit which will eventually fail.
Each terminal has 100uF stability capacitor with series resistor of 0.68(Ω) connected internally. This must be kept in mind for calculating transient short-circuit current.
Technical difficulties and lessons learned
The design and prototyping process went through many tweaks and adjustments. An example of the issues encountered was the EMP induced due to relay switching. This EMP reset the main MC. Solution was to connect parallel snubbers to reduce the voltage burst due to switching, fine-tune the cross zero detection to switch the relays in zero-cross points, and improve the pull-up circuit of the reset pin and SWCLK of the MC by using lower resistance and making it very close to the MC to avoid big loops that can catch the EMP. This application was the first time for me to use header files and multiple c files, this consolidated my understanding of the concept of encapsulation and the importance of it. This is also the second project where I used 32-bit MC (ATSAMC21E16A).
This linear bench power supply is designed to provide low noise regulated voltage for lab experiments. The device provides dual power supply: positive terminal and negative terminal, along with ground terminal. Each terminal is adjustable between 0-15 V. The device has multiple protection features such as adjustable over-current protection, over-heat protection, and limited negative current (current injection) protection.
User Panel
The voltage of each terminal is controlled independently using a panel-mounted analog rotary potentiometer which provides voltage to the error amplifier. Current-limit is adjustable between 0.1-1.5 (A) for each terminal independently through a panel-mounted rotary potentiometer. The panel displays three reading for each terminal (total of six readings): output voltage, output current, and current limit.
The user panel also has two LED indicators for each terminal: over-current indicator, and over-heat indicator for the corresponding terminal’s power transistor array heatsink. There are two spring-type terminal blocks for each terminal: positive, ground, and negative. Additionally, there are two female header-type connectors for each terminal. The output block can be turned on (i.e.: connected to the internal power circuit) using a single output switch. There is a discharge switch for each terminal that can be activated. Each discharge switch connects the corresponding terminal to ground terminal through 10 Ohm resistor when the output switch is turned off, hence safely discharging any capacitor (should one exist) in the application circuit.
Finally, the user panel has the main switch and a programming port for upgrading the display microcontroller(MC) software. There is brightness adjustment button, the button cycles between 8 degrees of brightness. If the brightness is not changed for 10 seconds, the brightness is stored in the display MC and restored next time the devices is turned on.
Technical Details and Specifications
There are four 60 (Hz) transformers: two power transformers for supplying current to output power transistors; one high voltage, low power dual transformer (24V X 2)for supplying the analog control circuitry which drives the power transistors; and a dedicated 10(V) transformer to supply the digital circuitry, display, fan, and relays.
Since this power supply is intended to be low-noise, no switching buck and boost regulators were used. Also, since the regulation is performed using a series element (three bjt power transistors for each terminal), the input power to these transistors was divided into two stages to avoid excessive heat dissipation at low output voltages. The first stage is composed of 12(V) transformer, it supplies the power transistors for output voltages between 0-8.3 (V). If the user dials the voltage above 8.3 (V), a 8(V) transformer is connected in series with the first one using a relay. There is a relay for each terminal to switch between 12(V) and 20(V) supply for the power transistors. The power transistor transformers, along with rectifiers and relays, are shown in the block “Transformer schematics ” in figure (4). The relays are represented using a high resistor for the disconnected terminal in LTSpice. A 12(V)X2 center-tap transformer supplies the 12 (V) stage for each terminal, and a dual core 8(V)X2 transformer supplies the additional 8(V) for each terminal. The main (MC) controls the stage relays. Switching of all relays is done using zero-cross detection to minimize EMP and arcing and to increase relay service life. Snubbers are also used in the AC side of the transformers to suppress EMP.
Three over-current protection mechanisms are implemented to ensure device safety at worst case short circuit. The 1st mechanism to take effect in event of a sever short circuit is a 22 uH inductor (rated for 6A) placed before the power transistor array. This inductor prevents the rapid discharge of the ripple capacitor (4700uF) through the power transistors during the first 3 microseconds and until the 2nd mechanism is activated. The 2nd mechanism is a transistor connected across the sense resistor of one of the power transistors. It is designed to keep the current below about (1.6-1.9 A) until the 3rd mechanism is activated. The 3rd mechanism is the current controlled feedback circuit using op-amp and it is the adjustable mechanism. It keeps the current at or below the current limit set by the user. Note that not all mechanisms are necessarily activated at every over-current incident, it depends on the severity of the short circuit. The current limit of the 3rd mechanism is adjusted by an adjustable current source driving small current from one power transistor and is connected before the current-sense resistor. this current creates a voltage drop below the emitter voltage of the power transistor. This voltage drop is compared with the voltage drop due to the current-sense resistor by an op-amp. This op-amp take control of the power transistors as long as the output current is at the current limit. The circuit does not implement short circuit foldback. The current limit value displayed is not very precise and the actual value can vary up to 10%, this is especially true at low values.
The main MC continuously monitors 4 values for each terminal: user input voltage to adjust the stage relays; output voltage and current to display for user; and heatsink temperature to control fan and shut down device in overheat event. The fan is turned on if heat sink’s temperature exceeds 50°, the corresponding heat sink overheat LED indicator flashes when fan is on. Shutdown temperature is 85° for single transformer (output below 8.3V) and is 65° for dual transformer (output above 8.3V). Shutdown of the power circuit in event of overheat is done using a relay that cuts power from the two power transformers (supplying the power transistors) and from the high-voltage low-power dual transformer that supplies the op-amps of the analog circuit control circuit. The digital circuit power supply transformer is connected directly to the AC to power the main MC and it can only be turned off from the main switch.
In the event of overcurrent, the corresponding LED indicator turns on along with a high-pitched sound. The main MC accumulates the time of overcurrent, if time reaches two seconds, the device is shutdown. If overcurrent is intermittent, then the main MC accumulates positive time for the periods of overcurrent and subtracts half the time when overcurrent is not detected. For example: if there is a periodic cycle of 300ms overcurrent followed by 300ms of normal operation, the device will eventually shutdown. If there is a periodic cycle of 300ms overcurrent followed by 600ms of normal operation, device will not shutdown.
At the event of shutdown, a message is displayed to the user: “ovr cur” in the corresponding terminal for overcurrent, and “ovr het” in the corresponding heatsink with temperature reading of both heatsinks displayed in the lower line of the display. The device is looked in the shutdown mode and can only be reset by turning the main switch off then on. The fan will keep functioning until both heatsinks are below 45°.
The device has a limited negative current capability. Each terminal can handle up to 60mA of negative current while keeping the output voltage regulated. In the event of negative current, the corresponding current display shows (-vE) reading (without numeric value of the current) along with a high pitch buzzer sound. The device does not shutdown on negative current as this measure has no protective benefit. Sustaining negative current for prolonged periods will eventually overheat a transistor in the circuit which will eventually fail.
Each terminal has 100uF stability capacitor with series resistor of 0.68(Ω) connected internally. This must be kept in mind for calculating transient short-circuit current.
Technical difficulties and lessons learned
The design and prototyping process went through many tweaks and adjustments. An example of the issues encountered was the EMP induced due to relay switching. This EMP reset the main MC. Solution was to connect parallel snubbers to reduce the voltage burst due to switching, fine-tune the cross zero detection to switch the relays in zero-cross points, and improve the pull-up circuit of the reset pin and SWCLK of the MC by using lower resistance and making it very close to the MC to avoid big loops that can catch the EMP. This application was the first time for me to use header files and multiple c files, this consolidated my understanding of the concept of encapsulation and the importance of it. This is also the second project where I used 32-bit MC (ATSAMC21E16A).
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