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Subject: The X converter
NNTP-Posting-Date: Sun, 24 Nov 2002 05:51:45 GMT
Organization: AT&T Broadband
Date: Sun, 24 Nov 2002 05:51:45 GMT
Here is a fun little circuit for your amusement. Shown below is a
non isolated switch mode power converter that I call the "X" converter
for obvious reasons. It has two capacitors and two inductors, each of
which are split into two equal windings. It is capable of producing
a non inverted output voltage from zero to many times the input
voltage (just the thing when your application needs a regulated 12
volt output from an unregulated 12 volt battery).
When you ponder the schematic below, keep the following in mind:
The output is non isolated and the same polarity as the input.
When the duty cycle for Q1 is zero the output is zero.
At 50 percent duty cycle the output is the same as the input.
L1A and L1B are two equal windings on the core of L1.
L2A and L2B are two equal windings on the core of L2.
The winding polarities for L1 and L2 are indicated by the
asterisks and are all positive when Q1 is on.
+Vi o-+ +-o +Vo
L1A @) L2A @)
| C1 || |
| \ / || |
|+-+ \ / _|_
o-|| Q1 X D1 /A\
|+-+ / \ |
| / \ || |
| C2 || |
L1B @) L2B @)
The "X" converter is an almost unknown power converter topology based
on split inductors. Its name comes from the x-shaped cross connection
of components in the schematic of the most basic form of the topology.
The split inductors add another degree of freedom that has kept the
"X" converter virtually undiscovered in spite of efforts to classify
all power converters with four or less energy storage elements.
There are several forms of this topology with positive or negative
output, with ground referenced switches, and with natural isolation.
This power converter has the same static conversion characteristics as
a buck-boost or a `Cuk converter, and it has reduced ripple current on
both input and output ports (sometimes referred to as "zero" ripple).
Although these ripple currents are not actually zero, it can be shown
that the "X" converter has the very interesting and useful behavior of
confining the bulk of its inductors' ramping magnetizing currents to
internal loops. Through this mechanism, sometimes called "current
steering", the "X" converter achieves ripple current characteristics
similar to an equivalent buck converter with an input filter.
Since the topology has four energy storage elements, its undamped dynamic
behavior is somewhat unwieldy. The control-to-output transfer function
has two left half s-plane complex pole pairs and a right half s-plane
complex zero pair due to the lattice topology and the switching action.
This combination yields 540 degrees of phase shift! With proper damping,
the zero pair can be pulled into the left half s-plane such that it
nearly cancels one of the pole pairs and the overall response appears
almost second order.
Getting ground referenced drive to Q1 may appear problematic due to the
flying nature of the switch, but this is easily done simply by adding a
small bifilar gate drive winding to L1B. Since first discovering it in
the mid 80's I have built several versions of the "X" converter circuit.
It is fun and easy to simulate in Pspice (both the frequency and time
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