The transistor was 18 years old in 1975, the year Solid-State Power Conversion (the forerunner of Power Electronics Technology) was founded; and by that time, the integrated circuit was some 17 years old, thanks to Jack Kirby, who invented it while at Texas Instruments in the summer of 1958. But in that period of less than two decades, transistors and ICs had made remarkable progress, and those components were setting the stage for the steady development of switching power supplies. A look at three products that appeared in the May/June 1975 issue of Solid-State Power Conversion provides a glimpse of the state of the art then, and perspective on the technology that exists now.
Discrete Power Devices
“Thirty years ago, discretes such as the devices described [in the ‘Power Transistors for Off-Line Converters’ box] were predominately bipolar. MOSFETs were just beginning to be introduced,” says Jack Wojslawowicz, now a senior principal engineer at Fairchild Semiconductor. He had started with RCA in the 1970s, and it was in the late 1970s and early 1980s when RCA began developing high-speed switching transistors for the switching power supply market that was coming to life.
Many of the devices were packaged in classical metal cans like the TO-3. Plastic packaging was around, but its reliability was suspect until a study by Boeing in the 1980s debunked the notion that hermetic packages were superior.
Today, low-voltage MOSFETs, such as Fairchild's Bottomless SO-8, n-channel MOSFETs, exhibit RDS(ON) values as low as 3.5 mΩ. Bottomless is Fairchild's term for its thermally enhanced packaging technology, which, like other proprietary variations of the 8-pin SOIC, allows greater power dissipation than a standard SO-8.
Ed Oxner of Siliconix (now a consultant at Siliconix) remembers the onslaught that occurred in October 1975 when Siliconix introduced VMOS, which everyone seems to agree was the first deliverable power FET in the marketplace. “We had an order from a major instrument company,” recalls Oxner. “We simply couldn't build them fast enough. I was in wafer-sort frantically trying to get good dice, but there were only one good die in perhaps many hundreds!”
The VMOS was so tantalizing because it delivered amperes, not milliamperes. The first VMOS, the Siliconix VMP1, was a 60-W device capable of handling 2 A. The great advances came about in VMOS due to the geometric “ditch” in the silicon, which made the far higher current possible — and with far less gate drive.
Oxner recalls that Hitachi invented what ultimately became the VMOS and International Rectifier's (IR's) HEXFET, and they were written up in Electronics Magazine about 1969. In fact, Hitachi had invented both the VGroove power FET, which was a precursor to what became Siliconix's VMOS, and the planar-power FET, which was the ancestor of IR's HEXFET. But, says Oxner, Hitachi never built them.
According to Carl Blake, IR's director of technical marketing for the computer and communications market, IR pioneered a cell structure and a manufacturing process that allowed a power MOSFET to be built reliably and with low on-resistance. When it was introduced in 1979, HEXFET had the lowest RDS(ON) available at the time — on the order of 0.5 Ω to 5 Ω. Ongoing improvements in process technology over the years have enabled the current generation of low-voltage power MOSFETs to achieve on-resistances of 0.5 mΩ to 5 mΩ, a three-order of magnitude improvement. And instead of 100 kHz, today's HEXFETs and MOSFETs can support switching regulators running at 2 MHz.
Since the LM125, LM126, LM127 were introduced in 1975 (see “Tracking Regulators”), there have been several improvements in linear regulator performance. As Alex Chin, technical marketing engineer at National Semiconductor, points out, output voltage accuracy back then was approximately 5%. Now we can expect 2.5% to 3% over input voltage, load current and temperature.
Today's state-of the-art linear regulators also exhibit extremely low dropout voltages. For example, National Semiconductor's LP33842 has a typical dropout of 115 mV at room temperature. What's more, this regulator can be stabilized with ceramic output capacitors. This is important because switching frequencies may range from 500 kHz to 1 MHz, and at these higher frequencies, effective filtering is essential to make sure the delivered output voltage is free of ripple components. The ceramic output capacitors exhibit lower ESR and therefore are effective filtering components.
To conserve power, modern regulators feature a shutdown mode in which their current consumption is reduced to very low levels. In the case of the LP33842, the regulator draws just 30 nA of shutdown current. In addition, the adjustable version of the LP33842 can go to 0.56 V. This reflects the current trend toward lower output voltages in linear regulators to accommodate the lower supply voltage requirements of logic chips.
While linear regulators have continued to develop in the past three decades, more efficient switching regulators have emerged and evolved during this time period. These ICs had their origins in the pulse-width modulation (PWM) controller ICs developed in the 70s. Bob Mammano, currently a staff technologist at Texas Instruments, designed the SG1524 PWM controller in 1975 while at Silicon General.
“My experience started with designing power supplies for the military, because in the early days of switching power supplies, it was they who had the dollars to pursue development,” says Mammano. “I left the aerospace industry to help found Silicon General, and in about 1975, we had the good fortune to come up with the SG1524.”
The SG1524 was the first commercially viable device to integrate all the functions for a PWM controller. Over the last 30 years, Mammano says regulator performance has improved dramatically; much greater accuracy, higher switching frequency capabilities and more protective functions are built in.
“What is really important is that along the way we added the brute force switching function itself — the high current part of the system,” Mammano says.
Today, switching regulators combine the PWM controller function with gate drivers, power MOSFETs, fault protection, and even tracking and sequencing functions. These highly integrated dc-dc converter ICs are capable of producing several amperes of output with high efficiency.
Switching Power Supplies
“Switching powers supplies — out of the question, they are simply too noisy.”
That's how many people reacted to the earliest switching power supplies. But, with respect to digital circuitry, they were wrong. Logic circuits could indeed tolerate that much noise, simply because having to be either in a zero or 5-V state meant they had an inherent immunity to noise.
“The power density of ac-dc converters took a dramatic turn in the 1970s as switching technology became the prevalent method of conversion. Power converter efficiencies in ac-dc converters are today well into the upper 90% efficiencies,” says Lou Pechi, director of market development for Power-One Inc.
The 500-W power supply in the pages of Solid-State Power Conversion (see “New 250-W and 500-W Switchers”) had a power density of 1.25 W/in.3, whereas comparable power supplies today such as Power-One's PFC500 Power Line achieve 4.44 W/in.3 and add features such as power factor correction, which would not have been available on most units in 1975.
“We had been in linears, but in 1975, we entered the switched-mode market launching. I recall our early 1000-W switcher, which weighed in at 75 lb. It was a two-person lift,” says David Martin, vice president of marketing at Lambda Americas. “Today, I carry a 2.5-kW switcher, such as our TL2500-48, weighing just 5.6 lb., in my laptop bag.”
Power Transistors for Off-Line Converters
These new 8-A, n-p-n power transistors have been especially designed for high-voltage inverters that operate directly off the rectified 120-V power line or bridge inverter configurations that operate from a rectified 240-V line.
The 2N6306, 2N6307 and 2N6308 have voltage [VCER(sus)] ratings of 350 V, 400 V and 450 V, a typical turn-on time of 500 ns, a fall time of 200 ns at 3 A, and a power capability of 125 W. (From $3.50/100 pieces; stock.)
Solid-State Power Conversion, May/June 1975, p. 50
The new LM125, LM126 and LM127 tracking regulators are designed to provide balanced positive and negative output voltages at currents up to 100 mA with inputs up to ±30 V. The LM125 provides tracking outputs of ±30 V and features output voltages balanced to within 1% and line and load regulation of 0.06%.
The LM126 provides ±12-V output balanced to within 1% and features line and load regulation of 0.08%, while the LM127 has 5 V and 12-V outputs. All three regulators are offered in a metal TO-5 can for operation over the military temperature range and industrial range. For the commercial 0°C to 70°C range, the regulators are available in the TO-5 can, a 14-pin Epoxy-B dual in-line package, and a 14-pin, ual in-line power package with an integral heat strap. (LM125 $2.60/100 pieces; stock.)
Solid-State Power Conversion, May/June 1975, p. 50
New 250-W and 500-W Switchers
Featuring a universal input, which permits operation from 115 to 230 Vac, 50 to 400 Hz, 230 to 260 Vdc or 115 to 180 Vdc, these new switchers supply 250 W or 500 W in two standard packages: 4.56 in. × 7 in. × 10.25 in. at 12 lb. for the 250-W units and 4.56 in. × 8.5 in. × 10.25 in. at 15 lb. for 500-W supplies. MS series (for “miniswitcher”) 250-W modules are offered in 5 Vdc at 50 A, 12 to 15 Vdc at 20 A and 22 to 28 Vdc at 10 A.
The 500-W family provides 5 Vdc at 100 A, 12 to 15 Vdc at 40 A and 22 to 28 Vdc at 20 A. The units are enclosed to shield against radiated EMI and an optional EMI input filter is available. MS series power supply efficiency exceeds 70%, and several supplies within one family may be paralleled for greater capacity or as a redundant source. (250 W: $490/1-50; 500 W: $605/1-50) Acme Electric Corp., Cuba, N.Y.
Solid-State Power Conversion, May/June 1975, p. 44