Exclusive Technology Feature
ISSUE: May 2014
New Topology Eliminates Extra Stage Normally Required For PFC
by Tom Lawson, CogniPower, Malvern, Penn.
Power factor correction (PFC) is increasingly required for ac-dc power converters. For best power factor, a
power supply should always draw input current in proportion to input voltage. Conventional approaches to
improving the power factor add cost, size, and power losses, so PFC is generally applied only when the overall
efficiency of ac power distribution is critical, or in higher power supplies, where it is mandated. The usual way to
obtain a high power factor is to add a boost stage and a bulk storage capacitor between the power converter
and its ac input. All the power must then pass through that extra stage of power conversion.
But instead of adding on a separate PFC stage, it is possible to blend PFC as part of the regulation process.
CogniPower is patenting a topology for that purpose called the Compound Converter. In a Compound Converter,
the majority of power moves through only a single stage of power conversion. The topology provides an
immediate reduction in losses of 25% to 50%, depending on load. Efficiency improvements increase with
decreasing load, making it easier to hit the established targets for low-load efficiency.
This article offers an introduction to the Compound Converter, explaining its principles of operation, benefits,
and guidelines for designing converters using this new topology. Compound Converter designs can be
implemented using a discrete approach based on standard components. However, the details provided here
suggest the opportunities for integration of key Compound Converter functions in application-specific power ICs,
which would improve the performance, shrink the size and lower the cost of lower-power ac-dc supplies.
How It Works
For good output regulation, any power converter needs to respond promptly to changes in load. For good PFC, a
converter needs to respond promptly to changing input voltage. To meet both ends, an energy storage element
of some kind is essential.
In a Compound Converter, that storage element is an actively managed capacitor. Energy is stored during
periods of excess availability and energy is taken from storage during periods of insufficient availability. Less
than half of the total energy transferred passes into and out of storage, yielding improved efficiency. In
operation, the average amount of power transferred during any single complete ac cycle will be the right
amount to satisfy the load, the power factor will be near ideal, and the output regulation will be excellent. No
other power supply topology meets these goals near as efficiently without using much larger and more
expensive inductive elements.
You might think that complicated circuitry and controls would be needed to implement a Compound Converter.
In fact, there is only one element required that is not widely used in conventional power converters. That
element is a synchronous rectifier with an enable input (see Sync Rect in Fig. 1). Mastery of the art is not a
prerequisite for constructing such a circuit element.
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Fig. 1. Block diagram of a simple, isolated Compound Converter for an ac-dc converter with PFC.
One mode of control for the converter shown here would use constant on-time for operating the mains side
switch. At a peak of the ac input waveform, during one control cycle, energy moves through the transformer
and the synchronous rectifier until the regulation point is reached. Then, the control logic disables the
synchronous rectifier, causing the remaining inductive energy to pass through a diode and into the storage
capacitor.
Near a zero crossing of the ac input waveform, all the available inductive energy is transferred to the output
without reaching the regulation point. Then, the supplemental regulator acts to support the output using energy
from the storage capacitor. The result is that the storage voltage varies at twice the line frequency. If the
constant on-time is too long, the storage voltage will tend to rise over time.
If it is too short, the storage voltage will trend lower. An ordinary, heavily filtered control loop will serve to
adjust the on-time to keep the storage voltage within the desired range. With heavy loop filtering, the on-time
will not change enough during a single ac cycle to reduce the power factor. So, though the on-time is not
completely constant, it is effectively constant for PFC purposes.
Fig. 2 shows the input and output waveforms for a laptop power supply with a nominal 20-V output. The
storage voltage can be seen increasing during ac input peaks, and decreasing during zero crossings at the ac
input. The slow control loop causes the storage voltage to average about 40 V in this example. A more
capacitive storage element would show less variation in voltage, and would support the output on its own for
longer periods. That slightly larger storage capacity would serve to carry over missing ac cycles or for hot swap.
With a rechargeable battery for storage, UPS capabilities are obtained.
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Fig. 2. Representitive voltage waveforms for input and output and storage in a Compound
Converter.
Compound Converters have some other valuable properties. Among them are temporary 100% overcurrent
capacity, redundancy for fault tolerance, and excellent transient response. In addition, these converters can be
adaptively controlled to maximize efficiency, or to minimize output ripple. Regulation is improved by directly
comparing the output to the reference voltage, and by using a single set of operating rules that cover all
conditions, instead of changing modes for differing loads. Also, having the control intelligence residing on the
isolated, low-voltage side is best for digital supervision.
Circuit Design Details
The circuitry on the dc side can take many forms. Fig. 1 shows a linear supplemental regulator, which is all that
is required. For efficiency, the supplemental regulator would be set for a voltage slightly below that of the main
converter such that it would not operate until the main stage just began to fall out of regulation. A buck
converter is superior as the supplemental regulator for efficiency and flexibility. That supplemental buck
converter can be managed by the same controller, or it could be self-contained. The main stage power
movement can be flyback, forward, buck or SEPIC, among others. The transformer might not be needed for
some non-isolated applications, i.e., LED lighting.
The Compound Converter adapts well for zero voltage and/or zero current switching variations. There are dc-dc
applications for the Compound Converter where output voltage or current can be regulated independent of the
regulation of input current. Multiple output voltages can be supported. These applications and variations have
been demonstrated and are covered by pending patents. All 34 claims in the original patent were ruled
allowable by the European Patent Office. U.S. and international patents will issue in due course. Licensing
options are available.
The Compound Converter has been proven practical. Pre-production units have been developed and are being
qualified for demanding applications where smaller size, better PFC, higher efficiency, high reliability and low
cost are all required. You may still hesitate, since as we know, the devil is in the details. So, here is the next
level of detail for a practical, generalized control tailored for the mains side of a Compound Converter.
Behavior must be safe for startup and shutdown, overcurrent, short circuit, no load, high line, low line, and
extremes of ambient temperature, in all combinations and permutations. Plus, improved efficiency is great, as
long as there is no additional cost associated. Flexibility is nice, too, but only as long as it does not entail
significant additional complexity. Fig. 3 is the result of five generations of proof-of-principle demonstration
systems. The dotted border could outline an integrated circuit, or the controller could be built entirely from
discrete parts, as in the present reference designs.
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Fig. 3. Detail of a mains-side controller adapted for Compound Converter applications.
Because the control intelligence resides on the dc side of the power converter, a mechanism is necessary to get
the system started. Here, line frequency is used to pace start pulses that provide enough energy to power up
the controller on the dc side. A low-power, slow oscillator can perform the same function. Once powered, the
dc-side controller sends the state for the gate of the main FET back through a single-turn pulse transformer.
The appearance of those control signals overrides and disables the start-up oscillator. Once the controller is
running, power is saved because the start-up circuitry remains inhibited, and performance is improved because
start-up pulses are not mixed with the pulses needed for regulation. Should the dc-side controller cease
operation, the start-up process repeats, after a delay.
The bias supply to run the control and gate drive can be linear or switched mode. A switched-mode bias supply
entails an extra winding on the transformer, but will increase efficiency. For any specific application, it can be
determined whether the extra winding is justified by the power saved. In cases where extremely low standby
current or excellent low-load efficiency is required, the extra winding may be desired even if it saves only a few
milliwatts.
There is another option for cases where the power required to actually charge and discharge the gate
capacitance of the main FET is a significant factor. CogniPower has patented bidirectional power converters that
can more efficiently charge, and recover charge from, gate capacitance. That technology is built around
Predictive Energy Balancing, which is beyond the scope of this article. See the Resources tab at
www.cognipower.com for more information.
For quasi-resonant power converters, it is often desirable to have a completely fixed on-time. In that case, the
frequency of operation of the Compound Converter is modulated to regulate the storage voltage. The jumper for
constant-current mode shown in Fig. 3 would allow the same controller to be useful for ac-dc converters without
active PFC, where a set amount of current is loaded into the inductive winding during each control cycle. With
either jumper setting, and with an internal main FET, an integrated version of the Fig. 3 function could serve in
a standby supply or wall adapter producing a few watts. With an optional external FET, the same IC could serve
in power supplies that produce hundreds of watts.
The requirements are simple enough, and similar enough to standard functions for conventional power
converters, that an integration step is not necessary in order to build practical, cost-effective Compound
Converters. The dc-side control, too, can be implemented with generally available parts and building blocks.
That said, given ICs designed for the purpose, Compound Converters with excellent PFC should find their way
into lower-power converters, offering improved performance and efficiency, while shrinking physical size and
lowering the price.
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Further Reading
For more information on the Compound Converter, see “CogniPower Compound Converter for PFC AC/DC
More Efficient, Smaller, Less Expensive,” a presentation by Tom Lawson given at APEC 2014, available at
http://www.cognipower.com/pdf/ExhibitorSeminarAPEC2014.pdf.
About The Author
Tom Lawson has been involved with instrumentation since 1968. During the
1970s he worked in medical electronics with Bill Morong, the principal
inventor of Predictive Energy Balancing. During the 1980s and 90s he built his
own instrumentation company. Since rejoining Bill Morong, Lawson’s focus
has been on power conversion. Lawson started CogniPower in 2009 to begin
the commercialization process. Lawson is named on twelve issued patents
and four patents pending, spanning four decades.
For more on the design of power factor correction circuits, see the How2Power Design Guide, select the
Advanced Search option, go to Search by Design Guide Category, and select “Power Factor Correction” in the
Popular Topics category.
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