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SQ60120QEw25xyz Datasheet(PDF) 11 Page - SynQor Worldwide Headquarters |
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SQ60120QEw25xyz Datasheet(HTML) 11 Page - SynQor Worldwide Headquarters |
11 / 14 page Product # SQ60120QEx25 Phone 1-888-567-9596 www.synqor.com Doc.# 005-0006429 Rev. F 10/16/2015 Page 11 Input:35-75V Output:12V Current:25A Part No.:SQ60120QEx25 Application Section APPLICATION CONSIDERATIONS Droop based current sharing is implemented by only regulating the output of first stage in the two-stage power conversion topology. The inherent impedance of the second stage balances current between multiple modules. This scheme ensures redundancy since there is no active current sharing circuit or common connection to fail. Graphs in this section show two units by way of example, but there is no fundamental limit to the number of units that can be placed in parallel. While the lack of output voltage regulation can seem to be a disadvantage, as we will discuss, it can actually reduce the overall voltage deviation when transient response is considered. Another hidden advantage of droop sharing is a dramatic stability improvement of any external post-regulators. Droop Damps Downstream Point-of-Loads: It is very common to have additional non-isolated point-of-load converters downstream of an isolated bus converter, called an Intermediate Bus Architecture (IBA). Each of these point-of-load converters requires damping to keep its input system stable. Since the point- of-load converter input current goes up when the bus voltage goes down, it presents an incremental negative resistance. This will be unstable when coupled with a low impedance source, parasitic or explicit inductance, high power, and low bus voltage. The usual solution is to add large amounts of bulk capacitance with inherent or explicit equivalent series resistance to provide damping (See Figure 4 in Input System Instability application note). The downside of this approach is that the capacitors are expensive and bulky. An alternate solution is to add an explicit series resistance, but this is undesirable because of the additional power loss (See Figure 3 in Input System Instability application note). A bus converter with a droop characteristic has an inherent series resistance, without the need for any additional components. Since this resistance comes from the transformer and output rectifiers of the bus converter, it does not represent any additional power loss. The value of this positive damping resistance can be derived directly from the slope of the bus converter output voltage droop characteristic vs. output current. Stability can be determined by evaluating equations 3-6 in the Input System Instability application note. Voltage Mismatch Impacts Share Accuracy: When multiple units having droop characteristics are placed in parallel, the current sharing accuracy is determined by the output voltage accuracy. A difference in voltage between two units will cause a differential current to flow out of one unit and into the other. Figure B shows an example with two units with output voltage mismatched by 0.5%. In this example, when Unit A is at 100% of its full rated load current, Unit B is only at 90%, effectively reducing the total available current by 5%. SynQor uses factory calibration of each unit to ensure that output voltage is well matched. Temperature Mismatch Self Balancing: The slope of the output voltage droop characteristic increases with increased temperature. So, if a paralleled unit were hotter than its neighbor, then it would take more of the load current. However, this situation is self correcting, because as a converter heats up, its droop increases due to an increase in output resistance. As shown in Figure C, this causes the hotter unit to share less current, which in turn cools down and restores equilibrium. Figure B: Droop Characteristics with Voltage Mismatch Figure C: Droop Characteristics with Temperature Mismatch (Self Balancing) Droop Characteristics with Voltage Mismatch -4.0% -3.5% -3.0% -2.5% -2.0% -1.5% -1.0% -0.5% 0.0% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Load Current (% of Rated Value) Unit A Unit B Droop Characteristics with Temperature Mismatch (Self Balancing) -4.00% -3.50% -3.00% -2.50% -2.00% -1.50% -1.00% -0.50% 0.00% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Load Current (% of Rated Value) Unit A (cooler) Unit B (hotter) |
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