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8200 Series - Next-Generation Switch-Mode Design for Greater Efficiency
Figure 1: 8200-series Switches: Signal at Zero Amplitude.
Figure 2: 8200-series Switches: Positive Signal.
Figure 3: 8200-series Switches: Negative Signal.
8200-series amplifiers feature an advanced switch-mode design that produces substantially less heat and offers lower distortion; higher ripple frequency and greater efficiency than earlier switch-mode amplifier designs. With these older designs, accurate timing of the switching output device is absolutely critical. If the switches have any dead time (with no switch on), large amounts of distortion will form. If they overlap, however, the circuitry will self-destruct with large amounts of shoot-through current.
8200-series amplifiers avoid these difficulties by using two sets of switching output devices instead of a single switching device. These two sets operate balanced in time and sample the same input waveform. One set is dedicated to the positive current portion of the waveform, and the other to the negative current portion. When there is no signal applied, or when a signal varying in amplitude reaches the “zero crossing” between positive and negative, the switching devices are being turned on and off simultaneously with a 50% duty cycle. The result is the formation of two balanced and cancelling high-frequency output currents with no net output at the no-signal condition. See Figure 1.
To produce a positive output signal, the output of the positive switching device is increased in duty while the negative switching device is decreased by the same amount. The 8200 series uses symmetrical interleaved pulse-width modulation (PWM), meaning both the leading edges and trailing edges of the pulse are varied according to the amplitude of the signal, and the spacing between pulse centers remains constant. Both the positive and negative switch pulses remain aligned on-center, and the net output current is positive. See Figure 2.
Likewise, to produce a negative output signal, the output of the negative switching device is increased in duty while the positive switching device is decreased by the same amount. Again, both switch pulses remain aligned on-center, and the net output current is negative. See Figure 3.
While older switching devices operate at 80-100 kHz, the Next Generation 8200 Series operates at a 500 kHz effective switching frequency, putting these noise artifacts well above typical MRI uses.
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