A low cost home-built gradient amplifier for matrix gradient coils
Huijun Yu1, Frank Huethe2, Sebastian Littin1, Kelvin J. Layton1, Feng Jia1, Stefan Kroboth1, and Maxim Zaitsev1

1Dept. of Radiology, Medical Physics, University Medical Center Freiburg, Freiburg, Germany, 2Dept. of Clinical Neurology and Neurophysiology, University of Freiburg, Freiburg, Germany


To address the increased cost of high numbers of amplifiers for matrix gradient coils, a low cost home-built 100A/100V gradient amplifier is proposed. The water cooling amplifier consists of several parts: a controller that interfaces to a waveform generator and provides the current control signal; an H-bridge circuit that generates constant current to the load; and a ripple cancellation filter that reduces the inevitable ripple current. The switching frequency and peak ripple current of amplifier is 100 kHz and 300 mA, respectively. The amplifier can be operated with maximum continuous current (100A) for at least 1 min.


Matrix gradient coils were recently proposed for encoding showing their potential to offer high flexibility in generating dynamic customized spatial encoding magnetic fields1-3. The generation of specific spatial encoding magnetic fields may be very flexible if each coil element is driven by its own amplifier. However, to drive a high number of coil elements, such a system might not be feasible due to the high costs of an increased number of commercial gradient amplifiers. Moreover, typical commercial gradient amplifiers are designed for high inductive loads. A single coil element of a matrix coil cannot be driven directly because of the very low inductance; an additional inductive load is often needed in series with the coil element to match the output requirements of a commercial gradient amplifier. Additionally, if the number of coil elements is increased, the current requirement for each gradient power amplifier would be reduced, while maintaining the same current density inside a matrix coil. Here we present the design and implementation of a prototype amplifier, capable of 100A/100V with the requirement of low cost.


Figure 1 shows the block diagram of the gradient amplifier. The amplifier consists of a controller, an H-bridge circuit and a ripple cancellation filter. The controller contains a fiber-optic receiver, a field-programmable gate array (FPGA), a digital to analog converter (DAC), a voltage-controlled pulse width modulator (PWM), an analog Proportional-Integral (PI) controller and an error amplifier. The amplifier was designed using a standard constant current source topology, where the current demanded by the DAC is compared with the sampled current through the coil via a current sensor (LA 100-P, LEM, Swiss). Current waveform signals are sent in real-time via a commercial high speed digital IO board (PCIe-6536, National Instruments) to the amplifier. The optical fiber is used to achieve galvanic isolation between the amplifier and the PCIe-6536 board. The frequency of the PWM (LTC6992-2, Linear Technology) was set to 100 kHz to achieve a high bandwidth of the amplifier. A phase lock loop (PLL) including a phase detector and a PI controller was developed to synchronize multiple amplifiers with an external 100 kHz reference clock and compensate the frequency error of the analog PWM chip. To drive different loads, a digital potentiometer and octal single pole, single throw (SPST) switches are connected to different capacitors to tune the proportional and integral parameters. The H-Bridge circuit contains two transformer-isolated half-bridge drivers with a 4 A output capability (ADuM4223, Analog Devices) and four 150A/150V MOSFETs (IXFN180N15P, IXYS). The ripple cancellation filter was implemented based on the design of Sabate et al.4.

Results & Discussion:

A prototype gradient power amplifier was constructed with water cooling, shown in Figure 2. Different loads (422uH, 123uH and 12uH) were successfully tested with continuous current up to 100 A for up to 1 min. The bandwidth and peak ripple current of the amplifier is approximately 10 kHz and 300 mA, respectively. The total cost is less than 1 000 €. Figures 3 and 4 show the currents and voltages of a trapezoid and triangle waveform, respectively, for the 123uH load. Due to the lack of a feedforward module, the controller can’t adjust the current very fast during the transition period as shown in Figures 3 and 4; this might degrade the image quality. The stable time of the PLL module is in the range of a few tens of microseconds, this might limit use of the amplifier for some sequences. To improve the performance of the amplifier, a full digital controller including a feedforward module, a feedback module and an interleaved PWM module will be implemented.


This work is supported by the European Research Council Starting Grant ‘RANGEmri’ grant agreement 282345.


1. Juchem C, et al., NMR IN BIOMEDICINE (2015); 2. Jia F, et al., Proc. ISMRM21 (2015), 3091; 3. Littin S, et al., Proc.ISMRM22 (2015), 1022; 4. Sabate J, et al., APEC (2004), 2: 792-796.


Fig.1 Block diagram of gradient amplifier. The amplifier consists of three major components. Firstly, a controller that interfaces to a waveform generator and provide current control signal. Secondly, H-Bridge circuit generates constant current to the load. Thirdly, a ripple cancellation filter reduces the inevitable ripple current.

Fig. 2 The home-built amplifier prototype includes controller, H-bridge circuit with water cooling and ripple cancellation filter.

Fig. 3 Test result of amplifier prototype measured with oscilloscope (HRO 64Zi, Teledyne Lecroy). Green line-Ch4: output current -trapezoid waveform, 20 A/V, 5 V/div. Yellow line-F1: differential output voltage, 50 V/div.

Fig. 4 Test result of amplifier prototype measured with oscilloscope (HRO 64Zi, Teledyne Lecroy). Green line-Ch4: output current -triangle waveform, 20 A/V, 5 V/div. Yellow line-F1: differential output voltage, 50 V/div.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)