PWM: From a Single Chip To a Giant Industry

Oct 1, 2005 12:00 PM
By Gene Heftman, Contributing Editor

Knowing what you know today about the enormous penetration of power supplies and their component parts at every level of electronic products and systems, would you believe that this huge industry rode in on the back of a single integrated circuit some 30 years ago? The year was 1976 when the then Silicon General company introduced the SG1524 regulating pulse-width modulator (PWM) integrated circuit. Invented by Bob Mammano, now a staff technologist with Texas Instruments, it was the first device to incorporate all of the circuitry needed to generate the adjustable frequency, pulse-width-modulated, 180-degree-out-of-phase control pulses that drive the power transistors of switching regulator power supplies.

Prior to that time, switching power supplies were a curiosity except for their use in military systems. But the advantages of switchers were beginning to get a foothold. In a well-known textbook published in 1977, Switching and Linear Power Supply, Power Converter Design, the late Abraham Pressman, a highly regarded designer and consultant on power supply design, wrote, “Switching regulators are in the process of revolutionizing the power supply industry because of their low internal losses, small size and weight, and costs competitive with conventional series-pass or linear power supplies.”

Switching power supplies languished in commercial applications up until around 1976 because of their then complexity and high costs compared with linear supplies. Even Mammano, who had experience in the design of military switchers, says, “Switching power supplies looked threatening. There was a lot of technology that we were not familiar with, so we wanted to make a control chip that would bring a lot of that complexity onto a single chip.”

Len Sherman, senior scientist at Maxim Integrated Circuits, recalls, “When it was a discrete circuit, very few people could do a switching power supply. When the 1524 came out, it wasn't easy to design a switcher, but a lot more people could attempt to do one.”

The Pioneers

Looking back at the SG1524, it contained the basic functions for the control portion of a regulating power supply (Fig. 1). It had a 5-V regulator/reference, an error amplifier, an oscillator, a PWM comparator, a pulse-steering flip-flop, two uncommitted switching transistors, current limiting and shutdown circuitry. The uncommitted outputs were suitable for either single-ended or push-pull applications. It was rated for operation over the full military temperature range of -55°C to 125°C. Two other versions, the SG3524 and SG2524, had the same circuitry but operated over the 0°C to 70°C temperature range. All this circuitry was in a 16-pin dual in-line package (DIP). An interesting note is that a description of the SG1524 and its operation are the subject of the last three pages in Pressman's textbook.

Aside from being the first single-chip PWM IC introduced, what other significance can be attached to the device? According to Mammano, “Combining the analog functions, the references, error amplifier and op amps with digital circuits for doing PWM hadn't been done before on one chip. I always felt that was the most significant accomplishment.” And in today's technology, where mixed-mode (analog/digital) devices are commonplace, the SG1524 was one of the first mixed-mode ICs.

Other semiconductor companies were not standing still when the SG1524 made its debut. In short order, Motorola Semiconductor released its MC3420 based on much the same idea as the SG1524, and Texas Instruments came out with the TL494 (Fig. 2). The latter offered an improved oscillator design, which allowed easy synchronization of the control circuit to additional controllers or a system clock. In addition to these PWM ICs, manufacturers such as Signetics, with the NE5560, and Ferranti, with the ZN 1066, introduced their own versions on the market.

Once the door was opened to single-chip PWM controllers, new features and upgraded specifications flowed quickly as switching supplies made inroads on the power supply industry's mainstay linear regulators. For example, Silicon General brought out the SG1525, which increased the output drive capability from the original 100 mA to 200 mA, and provided a ±1% accuracy 5-V reference compared to the original's ±4%. Motorola beefed up its line with the MC3421, increasing the output drive capability to 250 mA compared to the original's 50 mA. And TI's TL497A was touted as being able to operate in three dc-dc voltage conversion modes — step up, step down and inverting — with up to 85% of the source power delivered to the load.

As PWM ICs added more functionality, a second class of power devices began to emerge in the form of support and supervisory chips. One of the most comprehensive was Silicon General's SG1543 output supervisory circuit, designed to replace several external circuits: overvoltage sensing with provisions to trigger an external crowbar shutdown, undervoltage sensing to either monitor the output or sample the input line voltage, and current sensing or current limiting.

A simpler IC, Motorola's MC3423 was essentially a crowbar-triggering device programmable through an external capacitor to provide delays that prevent the false triggering of an overvoltage protection circuit. Texas Instruments came out with the TL430 and TL431 adjustable shunt regulators designed to produce sharper turn-on characteristics than temperature-compensated zener diodes. These devices were analog ICs, and it should be noted that analog types were available years before the first PWM device arrived. Entire families of three-terminal series regulators, the µA7800 (for positive voltages) and the µA7900 (for negative voltages), were widely available from a number of semiconductor manufacturers.

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