Patent Issued for Primary-Side Based Control of Secondary-Side Current for a Transformer

By a News Reporter-Staff News Editor at Electronics Newsweekly -- According to news reporting originating from Alexandria, Virginia, by VerticalNews journalists, a patent by the inventor Melanson, John L. (Austin, TX), filed on March 31, 2009, was cleared and issued on October 16, 2012.
The assignee for this patent, patent number 8288954, is Cirrus Logic, Inc. (Austin, TX).

Reporters obtained the following quote from the background information supplied by the inventors: "The present invention relates in general to the field of signal processing, and more specifically to a system and method that includes primary-side based control of secondary-side current for a transformer.
"Power control systems often utilize a switching power converter to convert alternating current (AC) voltages to direct current (DC) voltages or DC-to-DC. Power control systems often provide power factor corrected and regulated output voltages to many devices that utilize a regulated output voltage. Switching power converters have been used as interfaces between triac-based dimmers and LOADs. The LOAD can be virtually any load that utilizes converted power, such as one or more light emitting diodes (LEDs).

"LEDs are becoming particularly attractive as main stream light sources in part because of energy savings through high efficiency light output and environmental incentives, such as the reduction of mercury. LEDs are semiconductor devices and are driven by direct current. The lumen output intensity (i.e. brightness) of the LED approximately varies in direct proportion to the current flowing through the LED. Thus, increasing current supplied to an LED increases the intensity of the LED and decreasing current supplied to the LED dims the LED. Current can be modified by either directly reducing the direct current level to the white LEDs or by reducing the average current through duty cycle modulation.

"FIG. 1 depicts a power control system 100, which includes a switching power converter 102. Voltage source 104 supplies an alternating current (AC) input voltage V.sub.in to a full bridge diode rectifier 106. Capacitor 107 provides high frequency filtering. The voltage source 104 is, for example, a public utility, and the AC voltage V.sub.in is, for example, a 60 Hz/110 V line voltage in the United States of America or a 50 Hz/220 V line voltage in Europe. The rectifier 106 rectifies the input voltage V.sub.in and supplies a rectified, time-varying, line input voltage V.sub.X to the switching power converter 102.
"The power control system 100 includes a controller 108 to regulate an output voltage V.sub.L of switching power converter 102 and control a primary-side transformer interface 116. Voltage V.sub.L is referred to as a 'link voltage'. Controller 108 generates a pulse-width modulated control signal CS.sub.0 to control conductivity of switch 110 and, thereby, control conversion of input voltage V.sub.X to link voltage V.sub.L. Switch 110 is a control switch. The controller 108 controls an ON (i.e. conductive) and OFF (i.e. nonconductive) state of switch 110 by varying a state of pulse width modulated control signal CS.sub.0. Switching power converter 102 can be any of a variety of types, such as a boost converter. Controller 108 utilizes feedback signals V.sub.X.sub.--.sub.FB and V.sub.L.sub.--.sub.FB to generate switch control signal CS.sub.0 to regulate the link voltage V.sub.L. Feedback signal V.sub.X.sub.--.sub.FB represents input voltage V.sub.X, feedback signal V.sub.L.sub.--.sub.FB represents link voltage V.sub.L.

"Power control system 100 includes an isolation transformer 112 to isolate the primary-side 122 and secondary-side 124 of power control system 100. Depending upon the type of switching power converter 102, the link voltage V.sub.L is either a multiple or a fraction of the input voltage V.sub.X. For a boost type switching power converter 102, the link voltage V.sub.L can be several hundred volts. Often the LOAD 114 does not require such high voltages. Transformer 112 steps down the link voltage V.sub.L to a lower secondary voltage V.sub.S. A lower secondary voltage V.sub.S can have several advantages. For example, lower voltages are generally safer. Additionally, LOAD 114 may have a metal heat sink, such as a heat sink to dissipate heat from one or more LEDs. The cost of insulation requirements implemented by regulatory associations for LOAD 114 generally decreases as the secondary voltage V.sub.S decreases below various voltage thresholds. Therefore, a lower voltage across LOAD 114 can lower manufacturing costs.

"The power control system 100 includes a primary-side transformer interface 116 between the switching power converter 102 and the primary-side of transformer 112. Since link voltage V.sub.L is a regulated, generally constant voltage over a period of time, the primary-side transformer interface 116 converts the link voltage V.sub.L into a time-varying voltage V.sub.P. The transformer 112 induces the secondary-side voltage V.sub.S from the primary voltage V.sub.P. A variety of topologies for interfaces 116 exist, such as half-bridge, full-bridge, and push-pull interfaces. The primary-side interface 116 includes one or more switches (not shown) arranged in accordance with the topology of interface 116. The controller 108 generates pulse width modulated switch control signals {CS.sub.1 . . . CS.sub.M} to control the respective conductivity of switches (not shown) in interface 116. The set of switch control signals {CS.sub.1 . . . CS.sub.M} allows the interface 116 to convert the link voltage V.sub.L into the primary-side voltage V.sub.P, which can be passed by transformer 112. Thus, the control signals {CS.sub.1 . . . CS.sub.M} control the coupling of the primary-side voltage V.sub.P to the secondary-side 124. Exemplary primary-side transformer interfaces 116 are discussed in chapter 6 of Fundamentals of Power Electronics--Second Edition by Erickson and Maksimovi , publisher Springer Science+Business Media, LLC, copyright 2001 ('Fundamentals of Power Electronics'). The power control system 100 also includes a secondary-side transformer interface 118 between the secondary-side of transformer 112 and LOAD 114 to convert the secondary voltage V.sub.S into an output voltage V.sub.OUT. A variety of interfaces 118 exist, such as half-bridge buck converter and full-bridge buck converters. Exemplary secondary-side transformer interfaces 118 are also discussed in chapter 6 of Fundamentals of Power Electronics.

"For a load 114 that utilizes a regulated secondary-side current i.sub.LOAD(t), controller 108 regulates the link voltage V.sub.L and the primary-side voltage V.sub.P to establish a particular value for secondary-side load current i.sub.LOAD(t). The secondary-side current i.sub.LOAD(t) is a function of an integral of the primary-side voltage V.sub.P over time. Controller 108 regulates the primary-side voltage V.sub.P by controlling the duty cycles of control signals {CS.sub.1 . . . CS.sub.M}. If the value of secondary side current i.sub.LOAD(t) is too large, controller 108 decreases the duty cycle of control signals {CS.sub.1 . . . CS.sub.M}, and, if the value of secondary side current i.sub.LOAD(t) is too small, controller 108 increases the duty cycle D of control signals {CS.sub.1 . . . CS.sub.M}. For a transformer with N.sub.P primary-side windings and N.sub.S secondary-side windings, the primary-side voltage V.sub.P and the secondary side voltage V.sub.S are related to each other in accordance with Equation [1], where N.sub.P and N.sub.S represent the respective number of primary-side and secondary-side windings: V.sub.PN.sub.S=V.sub.SN.sub.P [1].
"For an ideal transformer, the primary-side current i.sub.P(t) and the secondary-side windings current i.sub.S(t) are related in accordance with Equation [2]: i.sub.P(t)N.sub.P-i.sub.S(t)N.sub.S=0 [2].

"The secondary-side windings current i.sub.S(t) is one example of a secondary-side current. The secondary-side windings current i.sub.S(t) is the current in the secondary windings of transformer 112. The secondary-side load current i.sub.LOAD(t) also represents a secondary-side current. The secondary-side load current i.sub.LOAD(t) is a function of the secondary-side windings current i.sub.S(t) as modified by the secondary-side transformer interface 118.
"For a real transformer 112, the primary-side current i.sub.P(t) has a magnetizing current component, i.sub.M(t). Equation [3] depicts a relationship between the primary-side current i.sub.P(t) and the magnetizing current component i.sub.M(t).: i.sub.P(t)=i.sub.P'(t)+i.sub.M(t) [3].

"The current i.sub.P'(t) is related to the secondary-side windings current i.sub.S(t) in accordance with Equation [4]: i.sub.P'(t)=i.sub.S(t)N.sub.S/N.sub.P [4]. Thus, the primary-side current i.sub.P(t) is related to the secondary-side windings current i.sub.S(t) in accordance with Equation [5]: i.sub.P(t)=i.sub.S(t)N.sub.S/N.sub.P+i.sub.M(t) [5].

"In at least one embodiment, the magnetizing current on the primary-side of transformer 112 is not directly measurable. Accordingly, it is very difficult to monitor changes in the secondary-side load current i.sub.LOAD(t) without actually sampling the secondary-side current i.sub.LOAD(t).

"To regulate the secondary-side load current i.sub.LOAD(t), controller 108 utilizes feedback signal i.sub.LOAD(t).sub.--.sub.FB to generate switch control signals CS.sub.1 . . . CS.sub.M. Feedback signal i.sub.LOAD(t).sub.--.sub.FB represents the secondary-side load current i.sub.LOAD(t). Power control system 100 includes coupler 120 to receive feedback signal i.sub.LOAD(t).sub.FB. The feedback signal i.sub.LOAD(t).sub.--.sub.FB is, for example, a current or voltage that represents the value of secondary-side current i.sub.LOAD(t). Coupler 120 is, for example, an optical coupler that maintains isolation between the primary-side 122 and secondary-side 124 of power control system 100. In another embodiment, coupler 120 is a resistor. When using a resistor, insulation of LOAD 114 and other components can be used to address safety concerns. In any event, the coupler 120 and any auxiliary materials, such as insulation, add cost to power control system 100.

"Some conventional electronic systems, such as electronic system 100, limit the secondary-side load current i.sub.LOAD(t) to protect load 114. To simply limit secondary-side load current i.sub.LOAD(t), controller 108 can limit secondary-side load current i.sub.LOAD(t) by observing the primary-side current i.sub.P(t) without receiving feedback signal i.sub.LOAD(t).sub.--.sub.FB. The controller 108 can compare a peak target value of primary-side current i.sub.P(t) and the observed primary-side current i.sub.P(t), and limit the primary-side current i.sub.P(t) to the peak target value. Limiting the primary-side current i.sub.P(t) limits the secondary side load current i.sub.LOAD(t). However, because of many variables, such as the magnetizing current i.sub.M(t) and variations in the duty cycles of switch control signal CS.sub.1 . . . CS.sub.M due to ripple in the input voltage V.sub.X and link voltage V.sub.L, limiting the secondary-side load current i.sub.LOAD(t) based on a peak target value of primary-side current i.sub.P(t) results in controlling the secondary-side load current with, for example, a 15-50% margin of error. Thus, limiting the secondary-side current i.sub.LOAD(t) does not regulate the secondary-side current i.sub.LOAD(t)."
In addition to obtaining background information on this patent, VerticalNews editors also obtained the inventor's summary information for this patent: "In one embodiment of the present invention, an apparatus includes a controller to regulate a load current to a load coupled to a secondary-side of a transformer to an approximately average value based on an observed primary-side signal value. The controller is configured to generate one or more duty cycle modulated switch control signals to control a voltage on a primary-side of the transformer based on the primary-side signal value. The load current represents a current into the load and out of a filter. The filter is coupled to a rectifier, and the rectifier is coupled to the secondary-side of the transformer.

"In another embodiment of the present invention, a method includes regulating a load current to a load coupled to a secondary-side of a transformer to an approximately average value based on an observed primary-side signal value. Regulating the output current includes generating one or more duty cycle modulated switch control signals to control a voltage on a primary-side of the transformer based on the primary-side signal value. The load current represents a current into the load and out of a filter. The filter is coupled to a rectifier, and the rectifier is coupled to the secondary-side of the transformer.
"In a further embodiment of the present invention, an electronic system includes a controller. The controller is configured to receive a feedback signal from a primary-side of a transformer, wherein the feedback signal represents a current in the primary-side of the transformer. The controller is further configured to generate control signals for circuitry coupled to the primary-side of the transformer to regulate a load current on a secondary-side of the transformer to an approximately average value based on the feedback signal from the primary-side of the transformer without using a feedback signal from a secondary-side of the transformer."

This article is come from:
http://www.equities.com/news/headline-story?dt=2012-10-24&val=629995&cat=tech