Power converter with cross current sensing

Details Power converter with cross current sensing Power converter with cross current sensing

2002-12-17

2004-07-20

Riley, Shawn (Department: 2838)

Electric power conversion systems

Current conversion

Including d.c.-a.c.-d.c. converter

C363S131000

Reexamination Certificate

active

06765808

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to electrical power converters, and more particularly, to sensing and monitoring of electrical power in DC-to-DC switching-mode power converters.
2. Description of the Related Art
To convert one DC (Direct Current) level to another, a DC-to-DC switching-mode converter is commonly employed to perform the task.
FIG. 1
shows a conventional DC-to-DC switching-mode converter signified by the reference numeral
2
. The converter
2
has an input circuit
4
and an output circuit
6
separated by a transformer
8
. The input circuit
4
includes a switch
10
controlled by a control circuit
12
. One terminal of the switch
10
is tied to the primary winding
12
of the transformer
8
. The other terminal of the switch
10
is connected to a DC input V
IN
. The output circuit
6
includes an inductor
15
and a capacitor
16
connected in series. The common connection of the inductor
15
and the capacitor
16
drives a load
18
. The primary and secondary windings
12
and
20
of the transformer
8
have N
1
and N
2
winding turns, respectively. Disposed between the secondary winding
20
of the transformer
8
and the inductor
15
is a diode
14
. Further, connected across the inductor
15
and the capacitor
16
combination is another diode
17
.
During operation, an input DC voltage V
IN
is supplied to one terminal of the switch
10
. The control circuit
12
generates a periodic output which in essence periodically turns on and off the switch
10
. As a consequence, a time-varying current i
P
with periodic current pulses flows through the primary winding
12
of the transformer
8
. In this specification, the lowercase alphabets are used to denote parameters that vary with time. Because the primary and secondary windings
12
and
20
are inductively coupled together, a secondary current i
S
is thereby induced in the secondary winding
20
. The secondary current i
S
passes through the diode
14
which admits only positive current cycles but blocks away any negative counterparts. Since both the inductor
15
and the capacitor
16
respectively assume high inductive and capacitive values, they cooperatively contribute to a slow time-constant.
When the secondary current i
S
with a positive current cycle impinges upon the secondary circuit
6
, the diode
14
is forward biased. The secondary current i
S
, after passing through the forward biased diode
14
, charges sluggishly through the inductor
15
and capacitor
16
. At this juncture, the power converter
2
is said to be in the forward rectification mode.
When the diode
14
is not forward biased, to maintain continuous current flow, magnetic energy stored in the inductor
15
discharges into the capacitor
16
and flows through the diode
17
. The power converter
2
is then said to be in the freewheeling mode.
The alternating operating of the forward rectification mode and the freewheeling mode basically allows a DC voltage level to be maintained across the capacitor
16
. The DC voltage level is utilized as the DC output voltage V
OUT
driving the load
18
. Depending on the impedance of the load
18
, a DC current I
OUT
is established passing through the load
18
, in accordance with Ohm's law.
In practice, the load current I
OUT
needs to be monitored. Insufficient current flowing through the load
18
may render the load
18
inoperative or malfunctional. On the other hand, excessive current I
OUT
feeding the load
18
may damage the load
18
and also the power converter
2
. Different applications require different current monitoring schemes. For example, in some applications in which the load
18
may require over current protection and thus the upper limit of the output current I
OUT
must be detected and maintained. As another example, in a shared-load arrangement, the common current I
OUT
driving the shared load
18
needs also be ascertained for proper load current allocation. Furthermore, in usages where the instantaneous power needs to be known, the instantaneous value of the output current I
OUT
must also be instantaneously detected and reported.
Heretofore, monitoring of the output current I
OUT
has mostly been conducted on the secondary side of the transformer
8
by directly measuring the current path through the load
18
. A common approach is to place a shunt resistor in series with the load
18
. Another known approach is to couple a Hall effect device to the load
18
.
First, the use of a Hall effect device involves complicated circuit design and thus costly. In addition, a Hall effect device is spacious. The use of Hall effect devices in most instances is not practical.
The use of shunt resistors for current detection is a common practice but it involves considerable drawbacks. To understand the problems associated with using a shunt resistor, the basic principles of a DC-to-DC converter needs first be explained. Reference is now directed back to FIG.
1
. In the DC-to-DC converter
2
, if the transformer
8
is a step-down transformer, as is known in the art, the primary and secondary voltages v
P
and v
S
, across the primary and secondary windings
12
and
20
, respectively, assume a directly proportional relationship in accordance with the following algebraic expression:
v
P
v
S
=
N1
N2
(
1
)
However, the primary and secondary currents i
P
and i
S
relate to each other by an inversely proportional relationship as expressed by the following mathematical relationship:
i
P
i
S
=
N2
N1
(
2
)
In a step-down transformer, the secondary voltage v
S
is lower than the primary voltage v
P
. However, the secondary current i
S
is higher than the corresponding primary current i
P
. In most applications with a DC-to-DC converter, such as the converter
2
, the output voltage V
OUT
is much lower than the input voltage V
IN
, resulting in the output current I
OUT
much higher than the corresponding input current I
IN
. In practice, sensing a high current always posses technical complications and sometimes fraught with danger. Chief among all is the difficulty in the power management of the shunt resistor. Even though the shunt resistor is normally designed to have a small ohmic value, in terms of degree of difficulty in managing the power of the shunt resistor, the high output current I
OUT
passing through the shunt resistor more than compensates for the choice of a low resistive value for the shunt resistor in the first place. As is well known, power consumption of a resistor when current passes through the resistor has the following relationship:
 
P=I
OUT
2
R
  (3)
where P is the power consumed by the shunt resistor in Watts; R is the ohmic value of the shunt resistor; and I
OUT
is as defined above.
Very often, to meet the low resistive value R and high power dissipation requirements, the shunt resistor with a large physical size has to be selected. Modern day designs of power converters require compactness where the use of large components is not practical. Further, as shown in equation (3), the relationship between the power consumption P and the current I
OUT
is not linear. Rather, the power consumption P is proportional the square of the current I
OUT
passing through the resistor. A small increase in current always results in a significant increase in power dissipation.
Furthermore, as is also known in the art, heat also effects the resistive value of a resistor. Excessive self-generated heat from the shunt resistor may cause the shunt resistor drifting in resistive value and thus may yield inaccurate current reading of the output current I
OUT
. Sophisticated thermal management or temperature compensation circuitry may be implemented to rectify such shortfalls but it surely will result in high manufacturing cost and design complication.
U.S. Pat. No. 6,366,484, entitled “Cross-Current Sensing in Power Conversion,” issued to Jin on Apr. 2, 2002, addresses the aforementioned problem and discloses an arrangement which senses current in the primary circuit for the cu

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