WHITE PAPER
Westcor MicroPAC Power-Shedding Technique
Increases LightLoad Efficiency
Written by: David A. Fletcher
Director of Engineering, Westcor
August 2012
Customers using the Westcor MicroPAC™ may take advantage of the power shedding
feature to increase light-load efficiency.
The MicroPAC has a factory selectable “power shedding” mode for managing parallel
arrays of power modules in an intermediate bus power-distribution system. This mode
optimizes efficiency across loads better than conventional approaches commonly
used to boost power supply efficiency, such as pulse skipping or low-power
“standby supplies.”
The power shed mode is similar to phase shedding often used to optimize light-load
performance for multi-phase buck regulators, typically used to provide low-voltage,
high-current power to microprocessors. In this case, however, the implementation is
quite different as the “phases” are actually independent power components and the
technique is employed at the intermediate bus stage rather than at the point of load.
The power shed mode makes it possible to eliminate not just the losses in the switching
FETs when “phases” are shed, but also the losses in entire modules when the individual
components are turned off.
Why Shed Power?
The original concept of improving light load efficiency for VI Chip® Bus Converter arrays
was developed by Mr. Ankur Patel (Vicor Product Line Engineer). Regardless of how
intermediate buses are implemented, approaches like this are necessary to optimize
power efficiency across a wide range of load current demands.
As previously mentioned, a phase-shedding technique is often employed at the point of
load in multi-phase buck regulators and improves the light-load efficiency by turning
phases off at light loads. The advantage of this approach is the power consumption of
switching losses is saved for every phase that is disabled.
The disadvantage of this approach is that while it works quite well for low-voltage loads,
it cannot be extended to high voltages or higher power levels, because the topologies
used do not contain parallel power trains that can be enabled and disabled.
VI Chip® BCM® Bus Converters resident within the MicroPAC are isolated fixed ratio
DC-DC modules producing standard intermediate bus voltages of 12, 14 and 48 Vdc.
The BCMs employ a sine amplitude converter topology with zero-voltage/zero current
switching which enables extremely high efficiency. These bus converters in parallel or
series can produce 12, 14, 24, 28, 36 and 48 Vdc outputs.
MicroPAC Solution
The principal of an intermediate bus architecture scheme based on power shedding is
best understood through a step process. Consider an example with multiple MicroPAC
converters. The worst case efficiency is one in which all four outputs of each MicroPAC
are connected in a parallel configuration for high power array applications.
vicorpower.com Applications Engineering: 800 927.9474
Page 1
In power shed mode the MicroPAC™ requires channel one to be active at all times
because this is the primary output and must be active to sense the initial current
demand from the load. The advantage to turning off un-used outputs at no load or
lighter load conditions is reduced power dissipation and an increase in the overall
efficiency. This occurs because only the outputs needed to support the load will be active.
MicroPAC offers the flexibility to enable this concept to be employed in much higher
power levels --up to many kilowatts.
Power Shed Mode Prerequisites
nSlots 1 to 4 must be populated
nAll outputs must be the same voltage
nAll slots must be configured in a parallel array
nCurrent rate slew rate not to exceed 20.8A/s
nThe PSM is not suitable for constant dynamic loads
Power Shed Bands
There are four operational modes for the power shed scheme.
Category
Customer Load
Output
1
0.0W – 250W
Output 1, active
2
250W – 500W
Output 1 and 2, active
3
500W – 750W
Output1, 2 and 3 active
4
750W – 1200W/1300W
Output 1,2,3 and 4 active
Power Shed Mode Functional Description
On power up with the power shed function enabled all four output channels are
initially enabled, channel one to four LEDs should be illuminated.
Circuitry internal to the MicroPAC monitors the amount of current drawn from the
MicroPAC and is proportional to the customer load.
If the load falls into category 1, the following will be observed. After 5 seconds output 4
will turn off, after 10 seconds output 3 will turn off, after 15 seconds output 2 will turn off.
If the customer load falls into category 2, the following will be observed. After 5 seconds
output 4 will turn off, after 10 seconds output 3 will turn off, output 1 and 2 will remain on.
If the customer load falls into category 3, the following will be observed. After 5 seconds
output 4 will turn off, output 1, 2 and 3 will remain on. If the customer load falls into
category 4, all output will remain on.
vicorpower.com Applications Engineering: 800 927.9474
Page 2
Category 1
Category 2
On
On
Output 1
Output 1
Off
Off
On
On
Output 2
Output 2
Off
On
Off
On
Off
Output 3
Output 3
Off
On
Off
On
Off
Off
Output 4
Output 4
Off
Off
Off
Off
5 sec
5 sec
5 sec
5 sec
5 sec
10 sec
10 sec
15 sec
Category 3
Category 4
On
Output 1
On
Output 1
Off
On
Output 2
Off
On
Output 2
Off
On
Output 3
Off
On
Output 3
Off
On
Off
On
Output 4
Output 4
Off
Off
Off
5 sec
When the MicroPAC™ is operating in categories 1 to 3 and detects an increase in load
current applied to the output which incurs into the next power band, the internal
microcontroller will turn on all outputs, regardless of the actual amount of load added.
With all of the outputs enabled, the microcontroller will turn off redundant outputs. In
the Power Shed Mode this is a constant cycle of detecting output load and continually
adjusting the outputs to satisfy that need.
vicorpower.com Applications Engineering: 800 927.9474
Page 3
No Load Power Dissipation with and without Power Shed Mode
Power (Watts)
Figure 1:
Plotted to the right is the
average no load power
dissipation when using four
12 V outputs. With the power
shed enabled the average power
dissipation is about 7.05 W;
With the power shed disabled
the power dissipation is around
28.20 W
30
27.5
25
22.5
20
17.5
15
12.5
10
7.5
5
7.06
7.055
7.05
7.045
1
2
3
4
7.04
Number of BCM’s
Power Shed Mode Disabled
Power Shed Mode Enabled
Plotted to the right is the
average no load power
dissipation when using four
48 V outputs. With the power
shed enabled the average power
dissipation is about 8.25 W;
With the power shed disabled
the power dissipation is around
33 W.
Power (Watts)
Figure 2:
35
32
29
26
23
20
17
14
11
8
5
8.26
8.255
8.25
8.245
1
2
3
4
8.24
Number of BCM’s
Power Shed Mode Disabled
Power Shed Mode Enabled
Further Considerations
Here are some considerations beyond the outlined steps to optimize the real-time
efficiency. there are inherent delays in power components and control circuits; so the
first BCM® Bus Converter in the system must maintain its output voltage before the
control circuitry turns on the un-used BCM during the fast load step transient. The
system should be capable of handling full power for a short time.
The power system designer should also consider the maximum slew rate of the load and
its repetition rate. These functions require smart management from control circuits. To
facilitate this, digital power-management control circuits can automatically detect the
load condition and smoothly switch to the appropriate converter.
The Power Behind Performance
Rev 1.1
4/2013
vicorpower.com Applications Engineering: 800 927.9474
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