chap12 - 12 Current Programmed Control So far we have...

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12 Current Programmed Control So far, we have discussed duty ratio control of PWM converters, in which the converter output is con- trolled by direct choice of the duty ratio d ( t ) . We have therefore developed expressions and small-signal transfer functions that relate the converter waveforms and output voltage to the duty ratio. Another control scheme, which finds wide application, is current programmed control [1–13], in which the converter output is controlled by choice of the peak transistor switch current The control input signal is a current and a simple control network switches the transistor on and off, such that the peak transistor current follows The transistor duty cycle d ( t ) is not directly controlled, but depends on as well as on the converter inductor currents, capacitor voltages, and power input volt- age. Converters controlled via current programming are said to operate in the current programmed mode (CPM). The block diagram of a simple current programmed controller is illustrated in Fig. 12.1. Control signal and switch current waveforms are given in Fig. 12.2. A clock pulse at the Set input of a latch initiates the switching period, causing the latch output Q to be high and turning on the transistor. While the transistor conducts, its current is equal to the inductor current this current increases with some positive slope that depends on the value of inductance and the converter voltages. In more complicated converters, may follow the sum of several inductor currents. Eventually, the switch cur- rent becomes equal to the control signal . At this point, the controller turns the transistor switch off, and the inductor current decreases for the remainder of the switching period. The controller must measure the switch current with some current sensor circuit, and compare to using an ana- log comparator. In practice, voltages proportional to and are compared, with constant of propor- tionality When the comparator resets the latch, turning the transistor off for the remainder of the switching period. As usual, a feedback loop can be constructed for regulation of the output voltage. The output voltage v ( t ) is compared to a reference voltage to generate an error signal. This error signal is applied
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440 Current Programmed Control
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12.1 Oscillation for D > 0.5 441 to the input of a compensation network, and the output of the compensator drives the control signal To design such a feedback system, we need to model how variations in the control signal and in the line input voltage affect the output voltage v ( t ) . The chief advantage of the current programmed mode is its simpler dynamics. To first order, the small-signal control-to-output transfer function contains one less pole than Actually, this pole is moved to a high frequency, near the converter switching frequency. Nonetheless, simple robust wide-bandwidth output voltage control can usually be obtained, without the use of compensator lead networks. It is true that the current programmed controller requires a circuit for measurement of the
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This note was uploaded on 01/17/2010 for the course EL 5673 taught by Professor Dariuszczarkowski during the Spring '09 term at NYU Poly.

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chap12 - 12 Current Programmed Control So far we have...

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