Compensation Resistor Stabilizes SMPS at High Duty Cycles

Posted on Feb 7, 2014

Current-control switching-mode power supplies (SMPS) are gaining in popularity because they allow pulse-by-pulse current control and monitoring, making them more reliable and robust than their voltage-controlled counterparts. Current control also eliminates a positive zero in some transfer functions, which makes the supplies more stable. However,

Compensation Resistor Stabilizes SMPS at High Duty Cycles
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at pulse duty cycles above 0. 5, current-control SMPS become unstable, oscillating at half of the switching frequency. 1 To stabilize the circuit, the designer must change the slope of the pulses going to the pulse-width modulation (PWM) comparator. 2 This can be done by adding a sawtooth voltage derived from the voltage across the timing capacitor to the voltage developed across the current-sensing resistor. Or, you can add a high-enough current slope to the slope-shaping resistor before summing the voltage across the slope-shaping resistor and the current-sensing resistor in the current-sensing transformer`s output circuit. This idea describes how to compute the value of the slope compensation resistor, Rsl, that will create the desired voltage pulse slope. The analysis uses Figure 1, which is part of a bridge converter that is a component of a current-control SMPS. Although the figure is a simplified schematic that denotes rectifiers as diodes and does not show the full bridge, it is suitable for this analysis. 1. This simplified schematic of part of a bridge converter that is a component of a current-control SMPS illustrates how designers can compute the value of a slope-compensation resistor to ensure stable operation. The voltage pulse slope is sent to the current-sensing input of U1, an LTC3722-1 synchronous dual-mode phase-modulated full-bridge controller with an internal current-ramping source whose peak current...

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