1. Field of the Invention
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.
2. Description of the Related Art
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 Vin 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 Vin 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 Vin and supplies a rectified, time-varying, line input voltage VX to the switching power converter 102.
The power control system 100 includes a controller 108 to regulate an output voltage VL of switching power converter 102 and control a primary-side transformer interface 116. Voltage VL is referred to as a “link voltage”. Controller 108 generates a pulse-width modulated control signal CS0 to control conductivity of switch 110 and, thereby, control conversion of input voltage VX to link voltage VL. 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 CS0. Switching power converter 102 can be any of a variety of types, such as a boost converter. Controller 108 utilizes feedback signals VX—FB and VL—FB to generate switch control signal CS0 to regulate the link voltage VL. Feedback signal VX—FB represents input voltage VX, feedback signal VL—FB represents link voltage VL.
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 VL is either a multiple or a fraction of the input voltage VX. For a boost type switching power converter 102, the link voltage VL can be several hundred volts. Often the LOAD 114 does not require such high voltages. Transformer 112 steps down the link voltage VL to a lower secondary voltage VS. A lower secondary voltage VS 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 VS 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 VL is a regulated, generally constant voltage over a period of time, the primary-side transformer interface 116 converts the link voltage VL into a time-varying voltage VP. The transformer 112 induces the secondary-side voltage VS from the primary voltage VP. 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 {CS1 . . . CSM} to control the respective conductivity of switches (not shown) in interface 116. The set of switch control signals {CS1 . . . CSM} allows the interface 116 to convert the link voltage VL into the primary-side voltage VP, which can be passed by transformer 112. Thus, the control signals {CS1 . . . CSM} control the coupling of the primary-side voltage VP 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 Maksimović, 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 VS into an output voltage VOUT. 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 iLOAD(t), controller 108 regulates the link voltage VL and the primary-side voltage VP to establish a particular value for secondary-side load current iLOAD(t). The secondary-side current iLOAD(t) is a function of an integral of the primary-side voltage VP over time. Controller 108 regulates the primary-side voltage VP by controlling the duty cycles of control signals {CS1 . . . CSM}. If the value of secondary side current iLOAD(t) is too large, controller 108 decreases the duty cycle of control signals {CS1 . . . CSM}, and, if the value of secondary side current iLOAD(t) is too small, controller 108 increases the duty cycle D of control signals {CS1 . . . CSM}. For a transformer with NP primary-side windings and NS secondary-side windings, the primary-side voltage VP and the secondary side voltage VS are related to each other in accordance with Equation [1], where NP and NS represent the respective number of primary-side and secondary-side windings:VP·NS=VS·NP  [1].
For an ideal transformer, the primary-side current iP(t) and the secondary-side windings current iS(t) are related in accordance with Equation [2]:iP(t)·NP−iS(t)·NS=0  [2].
The secondary-side windings current iS(t) is one example of a secondary-side current. The secondary-side windings current iS(t) is the current in the secondary windings of transformer 112. The secondary-side load current iLOAD(t) also represents a secondary-side current. The secondary-side load current iLOAD(t) is a function of the secondary-side windings current iS(t) as modified by the secondary-side transformer interface 118.
For a real transformer 112, the primary-side current iP(t) has a magnetizing current component, iM(t). Equation [3] depicts a relationship between the primary-side current iP(t) and the magnetizing current component iM(t).:iP(t)=iP′(t)+iM(t)  [3].
The current iP′(t) is related to the secondary-side windings current iS(t) in accordance with Equation [4]:iP′(t)=iS(t)·NS/NP  [4].Thus, the primary-side current iP(t) is related to the secondary-side windings current iS(t) in accordance with Equation [5]:iP(t)=iS(t)·NS/NP+iM(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 iLOAD(t) without actually sampling the secondary-side current iLOAD(t).
To regulate the secondary-side load current iLOAD(t), controller 108 utilizes feedback signal iLOAD(t)—FB to generate switch control signals CS1 . . . CSM. Feedback signal iLOAD(t)—FB represents the secondary-side load current iLOAD(t). Power control system 100 includes coupler 120 to receive feedback signal iLOAD(t)FB. The feedback signal iLOAD(t)—FB is, for example, a current or voltage that represents the value of secondary-side current iLOAD(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 iLOAD(t) to protect load 114. To simply limit secondary-side load current iLOAD(t), controller 108 can limit secondary-side load current iLOAD(t) by observing the primary-side current iP(t) without receiving feedback signal iLOAD(t)—FB. The controller 108 can compare a peak target value of primary-side current iP(t) and the observed primary-side current iP(t), and limit the primary-side current iP(t) to the peak target value. Limiting the primary-side current iP(t) limits the secondary side load current iLOAD(t). However, because of many variables, such as the magnetizing current iM(t) and variations in the duty cycles of switch control signal CS1 . . . CSM due to ripple in the input voltage VX and link voltage VL, limiting the secondary-side load current iLOAD(t) based on a peak target value of primary-side current iP(t) results in controlling the secondary-side load current with, for example, a 15-50% margin of error. Thus, limiting the secondary-side current iLOAD(t) does not regulate the secondary-side current iLOAD(t).
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