Product # SQ60120QEx25

Phone 1-888-567-9596

www.synqor.com

Doc.# 005-0006429 Rev. F

10/16/2015

Page 11

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)