Bruce Carsten: Lifetime Achievement Award Winner

Sep 1, 2006 12:00 PM
By David Morrison, Editor, Power Electronics Technology

Developing Peak-Current-Mode Control

Working on high-power switching regulators in the early 1970s turned out to be a rather frustrating endeavor for Carsten. And after a time, Carsten opted to work on lower power dc-dc converter design. In making that change, he encountered a problem inherent in circuits using pulse-width modulation (PWM) control as then implemented using BJTs.

“Most of the requirements were for low-voltage inputs, and the variable BJT storage times led to transformer ‘flux walking’ problems in the push-pull converter topology used with PWM control. I can still remember one late night in 1974 disgustedly watching as the resulting current imbalance (from transformer flux walking) caused the slower transistor to heat up and get slower, increasing the current offset until the transformer saturated and blew the transistor. I thought, ‘Wouldn’t it be nice to sense the transistor currents and shut them both off at the same current? That would solve the current imbalance problem!’”

Carsten expected that, as he thought through this approach, he would discover a reason why it wouldn’t work. He did realize it would be necessary to adjust the transistor current at turn-off in response to load current changes. But that was feasible, and when he tested his idea with an actual circuit, he discovered the technique of constant-frequency peak-current-mode control “worked beautifully.” The only caveat was the need for “slope compensation” at conduction duty cycles greater than 50%.

Current-mode control provided several advantages over the conventional voltage-mode-controlled PWM. As Carsten explains, “It’s a faster control methodology (than PWM); you’re controlling the current into your output capacitor. It’s a first-order system rather than a second-order system, so you don’t need phase-lead compensation in the voltage-control loop. You have an automatic current limit built in just by letting the current reference get to a maximum level. Prior to that time, you had to sense current separately, and when it became excessive, ‘OR’ it in as a control function to limit the current—which didn’t always work fast enough.”

Current-mode control also would enable forced current sharing among paralleled converters. That capability would make multiphase or “polyphase” switching feasible—a technique Carsten had contemplated earlier, but which was impractical with PWM control. A few years later, Carsten would apply multiphase switching in his design of a telecom rectifier.

Carsten first applied peak-current-mode control at Glenayre in designing a 24-V to 12-V dc-dc converter, which delivered “a couple hundred watts” of output. But he found its benefits to be so compelling that he used current-mode control on almost all of his subsequent power-supply designs.

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