PWM Controllers Add EMI Reduction

July 1, 2005
Alliance Semiconductor has introduced a family of PWM controllers with integrated EMI reduction technology for low-noise switched-mode power supply (SMPS)

Alliance Semiconductor has introduced a family of PWM controllers with integrated EMI reduction technology for low-noise switched-mode power supply (SMPS) and inverter applications. The ASM8P18S4X family of PWM controllers, which marks the company's entry into the power semiconductor market, is designed to replace the popular series of “184X/384X” PWM controllers with versions that include a spread-spectrum based EMI reduction engine.

Although some PWM controllers currently on the market offer spread-spectrum modulation of the switching frequency for EMI reduction, the new devices from Alliance Semiconductor are said to target a wider range of SMPS applications. At the same time, these controllers employ the company's own spread-spectrum modulation technique, which differs from methods used in existing controllers.

The four ASM8P18S4X models now being introduced are designed to have the same “look and feel” as the popular “384X” controllers. The new chips feature the same pin configuration and packaging as the 384X devices. However, unlike the 384X devices, which employ current-mode control, the ASM8P18S4X uses voltage-mode control and integrates the EMI reduction engine on chip.

And unlike standard voltage-mode PWM controllers, the ASM8P18S4X features cycle-by-cycle current limiting. In addition, the controllers have a built-in undercurrent sense feature. The switching frequency ranges from 40 kHz to 400 kHz, and the controllers produce 1 A of output drive.

The ASM8P18S4X's spread-spectrum modulator has two features that help to suppress peak EMI noise. First, the modulation scheme and profile are deliberately and accurately controlled. In other words, the modulation profile is of triangular shape, with a fixed modulation rate that is internally set for optimized performance. Second, the modulation spread (spread range) is user-adjustable with a single external resistor. So, for the SMPS designer, the control of the spread-spectrum modulation characteristics (and EMI peak suppression) is reduced to simply adjusting one parameter (spread range).

The resulting EMI reduction can partially or completely eliminate the need for front-end chokes with the added benefit of improving system efficiency. For example, removing the choke of a 500-W supplycould lower I2R losses by 5%, saving 25 W from full-load power consumption.

Two of the four PWM controller models target off-line inverter and high-voltage applications: the ASM8P18S42 specifies a 15.4-V UVLO and a 95% maximum duty cycle; while the ASM8P18S44 specifies a 15.4-V UVLO and a 50% maximum duty cycle. The other two PWM models target dc-dc converter and low-voltage applications: the ASM8P18S43 specifies a 7.8-V UVLO and a 95% maximum duty cycle, while the ASM8P18S45 specifies a 7.8-V UVLO and a 50% maximum duty cycle.

The devices are available in 8-pin SOICs and DIPs with commercial and industrial temperature grades, and with standard tin-lead, RoHS and/or Green-compliant packaging. Alliance Semiconductor plans to expand this product family with higher performance product offerings that address new SMPS markets demanding effective, inherent EMI-reduction capability.

Samples are available today, with initial pricing set at $2 in quantities of 1000. Full data sheet/product specifications and a complete cross-reference guide are available at

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