For example, when a converter runs into a current limit, the output voltage will be adjusted to 0V. So the current signal of sharing bus will decrease, other power modules will continuously reduce their output current, causing the whole current sharing system to lose its function.
Specify a master power modules, and others are slave power modules. That is, the current signal of the master power module is used as a reference signal, and other power modules follow this signal to adjust their own voltage to achieve the effect of current balance. The disadvantage is that when the master power supply is damaged, the entire system loses the reference signal and becomes directly connected in parallel.
The system will lose the effect of active current sharing. This system can automatically select the master power module which has the maximum current by connecting a diode. The master module provides the signal to the current sharing bus, and other slave modules will get the signal as reference and adjust output voltage.
Because the master power module can be replaced. So if one of the power modules is damaged, the whole parallel system will not be affected. It would have the better reliability. There are many active current sharing ICs on the market that can be used as external controllers.
Generally, each power module needs to be connected with an external controller, and then each controller receives and processes the current signal from all power modules. The controller provides a signal to each power module and adjusts accordingly control signals to correct the imbalance of output current. Although this method can achieve higher current sharing accuracy, it usually has more complicated circuit design and relatively high cost.
When two power converters are connected in parallel, the output voltages of the two power converters will not be exactly the same due to component tolerance and trace, etc. Therefore, a power supply with the higher output voltage may cause the countercurrent to other power converters, and the damage might be occurred.
An ORing diode is placed in output path of converter, so that the current of each power module can only flow to the load end. When one of the power converters is failed, the diode can protect the other power modules from being affected and maintain the output function of the system. This article chooses to use an external controller to realize the parallel and current sharing of two independent power converters.
UCC is set to automatic master-slave method. The detection resistor and internal amplifier determine the master converter with the larger output current, and the others are the slave converters.
The current difference between the master and slave converter will generate an error signal. The sense pin and the external resistance promote the increase the output voltage, thus the output current of slave converter will increase to perform current sharing. Figure 5 shows the IC function block diagram.
Load share bus driver receives the signal from the current sense amplifier. When this signal is the highest among all power modules, it will generate a forward bias to the diode and become the master power supply. Load share bus receiver can monitor the load share bus voltage, and then send the signal of the main power supply to the error amplifier.
The two ends of the error amplifier come from the current signal of the master module and own current sense amplifier, and compares two signals. If the current signal of the master power is higher than the module's own output current, an error signal will be generated in the compensation component between EAO and GND.
The error signal is used by the adjust amplifier to make the necessary output voltage adjustments to ensure equal output currents among the parallel operated power supplies. The error signal of the adjustment amplifier is used to drive the transistor. The current flows through the adjust resistor RADJ and changes the output voltage of the module. This ensures that the output voltage of the master module has not changed.
Figure 6 shows the actual current sharing test board, which contains two sets of control circuits. In addition, LS terminals are reserved to meet the multiple parallel modules. The external components required by the IC should be determined according to the specifications of power module. The following is the design description of each parameter. Since the current detection resistor is connected in the current loop of the power converter.
It must be ensured that when the output current is maximum, and the voltage drop across the resistor is much less than the maximum output voltage adjustment range of the module.
For a start, bigger converters can be less efficient than their smaller counterparts. Bigger passives will be required, which take up space. Designing such power systems is not terribly straightforward, either — sometimes paralleling is the easier, quicker option. Also, using two smaller DC-DCs in place of one big one will spread the heat dissipated across a larger area on the PCB, helping reliability. Other reasons for using DC-DC converters in parallel include providing redundancy in high-reliability systems.
These setups use additional DC-DC converters; there are always more converters available than is required to provide the total load current — if one fails, the same amount of current can still be provided. Two DC-DC converters connected in parallel will not automatically share the load equally. Even if they are apparently identical, the output voltages will be slightly different due to component tolerances. The one with the higher output voltage will typically provide the entire load current, operating at its limit while its partner is doing relatively little work.
The converter doing the bigger share of the work will create a thermal hot spot, which could be problematic, and it will have a shorter expected lifetime under these conditions. Forcing DC-DC converters to share the load equally mitigates hot spots and optimises lifetime for all the modules. For parallel DC-DC converters operating in redundant configurations, if one module fails, the others must increase their output to compensate. Suppose the modules are loaded unequally and the one providing most of the current fails.
In that case, its partner must quickly increase its load from minimal to the maximum to compensate, which causes undesirable transients and a temporary drop in the output.
Sharing the load equally minimises the dynamic response required from each converter, resulting in less output disruption. There are several commonly used techniques for load sharing in parallel DC-DC systems. One of the most common techniques senses the output current from each converter and compares it to the average, using current sense resistors, sensing amplifiers and a summing amplifier.
Another popular method, the droop-share method, reduces the output voltage as a load current function if one converter produces more current than another. Other techniques, such as analogue current sharing, use only intelligent modules and actively adjust the output voltage of each supply.
There will always be discrepancies between switching frequencies in the converters used, even when operating identical parts from the same input bus. Learn more. Asked 8 years, 9 months ago. Active 5 months ago. Viewed 16k times. Tarang Shah Tarang Shah 1 1 gold badge 5 5 silver badges 12 12 bronze badges. Add a comment. Active Oldest Votes. Andy aka Andy aka k 21 21 gold badges silver badges bronze badges. The only possibility to balance out the indi- vidual currents is to use a special balance function like in R-5xxx or use converters with SENSE function and additional load- share-controllers as can be done for RP xxxxSG So like Andy aka said, it's not the best idea.
Johannes 10 10 bronze badges. Stephen Collings Stephen Collings Only the master closes the voltage loop; the slaves mirror the switching of the master in order to load share. Output diodes are required in this scenario, with separate voltage sense terminals connected on the load side of the diode. I want to do the same with a li-on battery-pack 3. Is this really a bad idea? Barleyman Barleyman 3, 8 8 silver badges 22 22 bronze badges. The motor controller is a FET and some logic for current measurement, and an inductor motor.
Simon Richter Simon Richter 9, 1 1 gold badge 16 16 silver badges 38 38 bronze badges. The problems you could encounter when doing it passively: Problems with power-up. What if supply 2 is slower? Oscillation of currents. If they respond to load changes poorly. Ecuashungo 15 4 4 bronze badges. Jeroen3 Jeroen3 20k 31 31 silver badges 67 67 bronze badges. Voltmonkey Voltmonkey 1. First, without knowing the design details of the converters being used, you cannot state that they will work in parallel without isolation diodes or ballast resistors, or controlled MOSFETs, etc.
Second, a zener diode will not give the protection you describe. Sign up or log in Sign up using Google.
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