Spread Spectrum ICs Cut EMI

Feb 1, 2003 12:00 PM
By Sam Davis, Editor, Power Electronics Technology

Conducted and radiated EMI emissions from switchmode power supplies can create havoc with nearby electronic systems. To prevent this, international EMI standards have prompted companies to employ costly suppression measures. Unfortunately, such an approach often leads to more expensive components that can add between $1 and $15 to total system costs. Instead, Alliance Semiconductor (Santa Clara, Calif.) has developed a lower cost all-electronic approach for EMI suppression: spread spectrum modulation of the power supply's PWM signal.

Fig. 1 shows how spread spectrum techniques suppress EMI by modulating the PWM fundamental frequency. As shown, the peak value of the first and second harmonics is reduced 6 dB to 10 dB. The new ICs using this technique frequency modulate the PWM train while maintaining the PWM signal's basic content.

Two spread-spectrum ICs for switchmode power supply applications, the AS80M1800 and AS80M1801, are now available from Alliance Semiconductor. The AS80M1800 is specified for PWM center frequencies between 50 kHz and 400 kHz. The AS80M1801 is for PWM center frequencies of 400 kHz to 1 MHz. These ICs frequency modulate the PWM frequency only about ±0.25% to ±2%, which is enough to provide the desired EMI suppression without affecting the performance of the supply's output filter.

Designers can program the percentage of modulation by controlling the IC's modulation rate and spread range (Fig. 2). A C-R-C network on the Loop Filter pin sets the frequency deviation (spread range), and the Modulation Rate pin sets that value. The spread range is the percentage deviation of the PWM center frequency. In operation, these ICs accept an input from the PWM controller whose duty cycle can vary from 10% to 90%. Then, the IC frequency modulates that signal according to the settings of the Mod. Rate and Loop Filter values, resulting in a spread-spectrum modulated drive for the associated MOSFET. The optimal combination of Mod. Rate and Loop Filter values provide the lowest EMI.

These devices reduce EMI by modulating the PWM gate drive signal to the power MOSFET (Fig. 1). This reduces the conducted and radiated EMI generated by the power MOSFET's high-frequency switching currents.

Features for both devices include a minimal gate drive delay, 7V to 20V input, ultralow power consumption, and 200mA MOSFET gate drive. They also have adjustable output short-circuit current limit with automatic fault recovery, undervoltage lockout (UVLO), no-load condition detection with frequency skip, and soft-start. The ICs are housed in an 8-pin SOIC.

They provide cost savings by minimizing the need for EMI shields and filters, and by simplifying the p.c. board layout. They use the most efficient modulation profile approved by the FCC. The modulation circuitry is implemented in a proprietary all-digital method.

You can use them in different power converter and inverter topologies, and their wide duty cycle range allows system design versatility. To ensure safe operation when the PWM drive is absent, they switch off the external power MOSFET by holding its gate voltage low.

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