Medical ultrasound imaging can be used to visualize internal organs without the need to penetrate the skin or bring radiation to the human body like some other imaging systems such as endoscopy, X-ray computed tomography (CT) or X-ray imaging. In addition, ultrasound imaging is less expensive and easier to move than non-invasive magnetic resonance imaging (MRI). The growth in the use of ultrasound technology will be an important trend in the coming years.
Ultrasonic systems (see picture) usually contain several key basic modules, which will increase the demand for power supply. The ultrasound probe contains up to hundreds of piezoelectric transducers. These piezoelectric transducers can convert the output electrical signal into sound waves or the input acoustic waves into electrical signals.
In order to generate the electrical signal to be input to the transducer, a transmit beamforming device is needed. The transmit beamformer is usually implemented by FPGA or DSP. The transmit beamformer generates electrical pulses, which are timed and adjusted to illuminate specific parts of the human body.
Figure important components of an ultrasound system
These electrical signals from the transmit beamformer pass through a high-voltage pulse generator or a high-voltage D / A converter (DAC), and the energy generated is used to excite the transducer element. There is a transmit / receive switch between the pulse generator or DAC. During transmission, it ensures that the low-voltage receiving circuit is protected from high-voltage damage, because the receiving loop also needs to be connected to the transducer.
Piezoelectric transducer elements convert these electrical signals into acoustic wave signals that can penetrate the human body. These acoustic wave signals are reflected at the edges of various internal organs and returned to the transducer.
As soon as these returned sound waves pass through the transducer, they are converted into electrical signals. These lower voltage received signals need to be amplified, filtered and converted to digital format to generate images. Thanks to the development of the semiconductor industry, all these tasks can be completed in one device: the analog front end (AFE).
The digital output will be loaded into the receive beamformer, which will reconstruct the data. After the image has been reconstructed, you can perform some video post-processing on it and form a format that can be output to the display so that the technician can view the results.
Point-of-load power supply
Ultrasound imaging systems need to consider providing multiple power supply voltages. Understanding the power supply requirements early in the design stage can save time in building a power supply architecture. When determining the system bus voltage, it is best to know the highest voltage required. The ultrasonic transmission pulse generator may require the highest voltage in the system, which requires several amperes of current to drive the transducer.
12V is the most popular mid-level bus voltage option. It may be necessary to implement another power supply to generate higher positive and negative voltages to power the pulse generator. If ultrasound has battery-backup characteristics, a higher voltage may be required to charge lead-acid or series / parallel Li-ion (Li-ion) batteries, then an intermediate bus voltage of 28-30V is a good choice.
There are many easy-to-use DC / DC switching regulators on the market that can accept this voltage as input and provide point-of-load (POL) power for displays, digital processors, and analog circuits. In addition, the ultrasound imaging system may be very susceptible to noise interference because it has sensitive high-precision analog circuits and requires clear display images. Dc / dc switching regulators with frequency synchronization can be driven by the main clock frequency to eliminate beat frequencies that may interfere with analog and video signals.
Processor power
Ultrasound imaging system uses FPGA, DSP and microcontroller to realize beamforming transmit and receive functions, Doppler processing and graphics processing. Powering the processor is simple.
However, the power supply of high-performance processors must consider some important factors. Processor manufacturers may specify minimum and maximum voltage ramp-up times, and many DC / DC regulators provide adjustable soft-start pins to program voltage ramp-up times. If no soft-start pin is available, please select a DC / DC regulator with a suitable preset soft-start time.
The processor data sheets may also provide recommended time specifications for the power-up timing of the core, I / O, phase-locked loop (PLL), and peripherals. Although there are several different ways to power the upper and lower voltage rails, sequential timing is the most commonly used. This method is easy to implement, just connect the POWERGOOD pin of the regulator to the enable pin of the next regulator.
Sequential timing also supports interleaving multiple voltage rails during power-up to minimize inrush current instead of turning on all voltage rails at the same time. Some low-dropout regulators (LDOs) and switching regulators provide a special timing or tracking pin that can be adapted to almost any timing mode.
The lower processor core voltage drives the demand for higher precision DC / DC converters. The new regulator claims to achieve a reference accuracy of ± 1% or higher over the full temperature range. Low-cost regulators may specify ± 2% or ± 3% reference voltage accuracy. Please check the manufacturer's data sheet to ensure that the voltage regulation accuracy meets the processor's requirements. Early dc / dc converters that have been reused for many years may not meet the voltage accuracy requirements of the latest processors.
Analog circuit power supply
The main analog components of the ultrasound system are AFE, ultrasound transmit pulse generator and ultrasound transmit / receive switch. These analog circuits, such as DACs, analog-to-digital converters (ADCs), and operational amplifiers (op amps) in high-precision instrument applications are very susceptible to noise and power supply ripple.
Noise is classified according to spectral noise density and output noise voltage (μVRMS). Power supply ripple suppression (PSRR) refers to the amount of ripple from the output ripple at the output. Most designers choose linear regulators to provide clean voltage rails to maximize system performance. The linear regulator that promotes its PSRR and noise performance specifications on the front page of the data sheet is optimized for powering noise-sensitive analog integrated circuits.
AFE, such as AFE5808A, contains 8-channel voltage control amplifiers, with low noise amplifiers, gain programmable amplifiers, CW mixers, ADCs and other analog circuits. The device requires several voltages to power various circuit modules, such as 1.8, 3.3, and 5V.
LDO regulators are suitable for powering these AFE voltage rails because the current required is small. In order to further improve efficiency and save board space, a highly sensitive analog circuit can be powered by a switching regulator and LDO packaged together. For example, the TPS54120 is a low-noise power supply that integrates a dc / dc converter and LDO. It supports a maximum current of 1A. It can provide an efficient switching regulator and high PSRR LDO for 5 or 12V buses. The output noise is as low as 17μVrms .
Ultrasonic transmit pulse generators, such as the LM96550, contain 8 high-voltage pulse generators with integrated diodes that can generate bipolar pulses up to ± 50V, peak currents up to 2A, and pulse rates up to 15MHz. Because the transmission pulse generator also implements digital-to-analog conversion, another set of voltage rails is needed to power the level shifter and low-voltage logic.
A split-rail power supply is needed, and a buck converter (such as TPS54060) in an inverter buck / boost structure can easily obtain positive and negative voltage rails from the mid-level bus voltage. It is a good design to use a low-current LDO on each voltage rail to help eliminate any switching noise from the inverter buck / boost converter.
The ultrasonic transmit / receive switch protects the AFE input of the low-noise amplifier (LNA) from the high-voltage pulses of the transmit channel. The input voltage is transmitted by the output of the sending pulse generator, and the output voltage of the LNA is ± 0.7V, while the current is clamped at 1mA. Similar to the transmission pulse generator, this switch requires a separate rail power supply for analog power supply and negative high-voltage bias power supply. The bias supply must be the maximum negative supply voltage connected to the switch.
Ultrasound imaging systems include analog and digital integrated circuits, which have different power supply considerations. Linear regulators can provide low noise performance for analog circuits. Switching regulators provide higher efficiency and are generally more suitable for powering high-current, high-performance DSPs.
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