Drivers for high-inductance, low-resistance coils
Article Abstract:
Pulse-width modulation (PWM) power amplifier technology is particularly superior to linear amplifier technology when the application involves driving magnetic coils with high inductance and low ohmic resistance. The amplifier's power dissipation depends solely on output current for a given dc supply voltage. For example, repetitive pulses that attain 500-A peak amplitude within 1 ms can excite the 0.5-mH, 0.05 gradient coils of a magnetic resonance imaging scanner. Whereas PWM amplifiers have no problem with this scenario, since feedback readjusts the output on/off ratio of the output bridge to produce the desired 25 volts, linear amplifiers encounter difficulties once a steady-state coil current has been reached. The linear amplifier's efficiency is only 8.4 percent; PWM power amplifiers can have about 12 kW junction and switching losses, yielding a 51 percent overall efficiency, or a six-fold advantage.
Publication Name: IEEE Spectrum
Subject: Engineering and manufacturing industries
ISSN: 0018-9235
Year: 1993
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PWM power amplifier fundamentals
Article Abstract:
Pulse-width modulated (PWM) power amplifiers differ from a switch-mode dc power supply in that the PWM amplifier tracks a variable reference voltage, while the power supply's output is anchored to a fixed reference. Fast-switching MOSFETs enable PWM power amplifiers to match the responsiveness of high-power linear amplifiers at much less the size, weight, cost and heat dissipation. The 80-MHz-and-above switching frequencies of MWM's enable dc-to-4KHz power bandwidth and a fraction of a millisecond rise time to full output. Less heat is dissipated in PWM amplifiers because they switch their power MOSFETs between fully on and fully off low-density modes. Other PWM power amplifier concepts are discussed.
Publication Name: IEEE Spectrum
Subject: Engineering and manufacturing industries
ISSN: 0018-9235
Year: 1993
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Kilowatts on order: wideband PWM amplifiers drive magnets efficiently, neutralize power-line harmonics, and control motion with precision
Article Abstract:
Pulse-width modulation (PWM) power amplifiers are relegating linear dc power amplifiers to obsolescence because of their ability to cut costs and create new applications for computer-controlled kilowatts. The key difference between the two types of amplifiers is that MWM amplifiers track a variable reference voltage; they also cost less, weigh less and suffer a fraction of the linear's internal heat dissipation. PWM applications cost less than $.32 per watt for continuous power, water cooling is not necessary and air-conditioning is no longer a significant additional cost. Active harmonic neutralization, active magnetic bearing and motion control all owe their existence to PWM amplifiers.
Publication Name: IEEE Spectrum
Subject: Engineering and manufacturing industries
ISSN: 0018-9235
Year: 1993
User Contributions:
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