Two or more power supplies can be connected to supply higher voltages or current. The simplest method to create higher current is to connect the power supplies in parallel and leave only one supply in constant voltage mode. Some power supplies are equipped with analog control signals that allow auto-parallel or auto-tracking, a more elegant way to control multiple power supplies. Auto-parallel supplies can be controlled with a single master supply; a second advantage is that all of the master power supplies features can be used. e.g. remote sense, CV or CC mode, even analog programming. Auto-tracking allows multiple supplies to track the master, the slaves can have the same output characteristics or can be configured to be proportional to the master.
1) CONNECTING POWER SUPPLIES IN PARALLEL TO PRODUCE HIGHER CURRENT
Two or more power supplies being capable of CV/CC automatic cross over operation can be connected in parallel to obtain a total output current greater than that available from one power supply. The total output current is the sum of the output currents of the individual power supplies. The output of each power supply can be set separately. The output voltage controls of one power supply should be set to the desired output voltage (CV); the other power supplies should be set for a slightly higher output voltage. The supplies with the higher output voltage setting will provide constant current output, and will drop their output voltage until it equals the output of the CV supply.
Connecting two or more power supplies in parallel (Figure 1) provides higher currents. It is important to follow these precautions:
1. One unit must operate in constant voltage (CV) mode and the other(s) in constant current (CC) mode.
2. The output load must draw enough current to keep the CC unit(s) in CC mode.
Figure 1: Three power supplies connected in parallel to produce additional current.
Setting Voltage and Current. Program the current limit of each power supply to its maximum value. Next, set the output voltage of the CV supply to a value slightly lower than the CC unit(s). The CC units supply the maximum output current that they have been set to and drop their output voltage until it matches the voltage of the CV unit, which supplies only enough current to fulfill the total load demand. The current supplied to the load will be the sum of the current from each supply. To learn more about constant current and voltage modes see: How do I put the power supply in the constant current mode?
2) AUTO-PARALLEL OPERATION
Auto-parallel operation permits equal current sharing under all load conditions, and allows complete control of output current from one master supply. The control unit is called the master; the controlled units are called slaves. Normally, only supplies having the same model number should be connected for auto-parallel operation, since the supplies must have the same voltage drop across the current monitoring resistor at full current rating. The output current of each slave is approximately equal to the master's. Figure 2 shows the rear panel terminal connections for auto-parallel operation of three E36xx- series supplies.
Figure 2: Auto-parallel operation of three E36xx-series power supplies.
Setting Voltage and Current. Turn the slave unit's CURRENT control fully clockwise. Adjust the master unit's controls to set the desired output voltage and current. The master supply operates in a completely normal fashion and may be set up for either constant voltage or constant current operation as required. Verify that the slave(s) are in CV operation. For auto-parallel operation of three supplies, the combined output voltage is the same as the master unit's voltage setting, and the combined output current is three times the master unit's current. In general, the auto-parallel output current (Io) is Io = Im + Is1 + + Is2 + . . Isn = n (Im); where Im = master unit's output current and IS = slave unit's output current.
Proportional currents from auto-paralleled units require equal load plus lead voltage drops. Connect each supply to the load using separate pairs of wire with length chosen to provide equal voltage drops from pair to pair. If this is not feasible, connect each supply to a pair of distribution terminals using equal- voltage-drop wire pairs, and then connect the distribution terminals to the load with a single pair of leads.
Overvoltage Protection. Adjust the desired OVP shutdown limit using the master unit's OVP Adjust control. Set the slave units' OVP limits above the master's. When a master-unit shuts down, the master programs the slave units to zero voltage output. If a slave unit shuts down, it shuts only itself down. If the required current is great enough, the master will switch from CV to CC operation.
Remote Sensing. To remote sense with auto-parallel operation, connect remote-sense leads only to the master unit according to the remote-sensing instructions.
Remote Analog Voltage Programming. To remote program with auto-parallel operation, set up only the master unit for remote programming according to the remote-programming instructions.
3) AUTO-TRACKING OPERATON
Auto-tracking operation of power supplies is similar to auto-parallel operation except that the master and slave supplies have the same output polarity with respect to a common bus or ground. This operation is useful where simultaneous turn-up, turndown or proportional control of all power supplies is required. Figure 3 shows three supplies connected in auto-tracking with their negative output terminals connected together as a common or ground point. For two or more units in auto-tracking a fraction R2/(R1+R2) of the output of the master supply is provided as one of the inputs to the comparison amplifier of the slave supply, thus controlling the slave's output. The master supply in an auto-tracking operation must be the positive supply having the largest output voltage. Turn-up and turndown of the power supplies are controlled by the master supply. In order to maintain the temperature coefficient and stability specifications of the power supply, the external resistor should be stable, low noise, low temperature.
Figure 3: Auto-tracking operation of three E36xx-series supplies.
Determining Resistors. External resistors control the fraction of the master unit's voltage that is supplied from the slave unit. The output of slave one is determined by the ratio R2/(R1+R2) = (Vs1/Vm), and the output of slave two is determined by the ratio R4/(R3+R4) = (Vs2/Vs1).
Where Vm = master output voltage
Vs = slave output voltage
A 0.1 µF capacitor in parallel with R2 and a 0.1 µF capacitor in parallel with R4 can help ensure the stable operation.
Setting Voltage and Current. Use the master unit's VOLTAGE control to set the output voltage from both units. When the master is in CV operation, the master's output voltage (Vm) is the same as its voltage setting, and the slave's output voltage will be determined by the ratio of the resistors. The VOLTAGE control of the slave unit is disabled. Set the CURRENT controls of master and slave units above the required currents to assure CV operation of master and slave units.
Over voltage protection. Set the OVP shutdown voltage in each unit so that it shuts down at a voltage higher than its output voltage during auto-tracking operation. When a master unit shuts down, it programs any slave units to zero output. When a slave unit shuts down, it shuts down only itself.