Originally Published MPMN January 2006

PROFILE

Synchronized Frequency Power Supply Solves Design Challenge in Ultrasound Equipment

Marshall Wright, Ryan Benhard, and Art Alva

Ultrasound machines are widely used as diagnostic tools in the field of medicine. They work by scanning the body with a probe that emits high-frequency sound wave pulses. When these sound waves contact areas of concern in the body they are reflected back to the probe. The probe transmits these reflected sound waves to the central processing unit (cpu) of the ultrasound machine, where an image is generated based on the distance and intensities of the reflected sound waves. A two-dimensional image is then displayed on a monitor screen. This allows the physician to view an accurate depiction of the area of concern. The condition can then be properly evaluated and diagnosed.

However, as one manufacturer of this equipment discovered, ultrasound is particularly sensitive to interference. This is because the devices use analog waveforms to see inside the body. Analog waveforms can be subject to distortion stemming from different types of electrical interference such as EMI and RFI. This interference can cause variations that could be misinterpreted as data by the ultrasound’s cpu and cause degraded or false images to be generated and displayed. Inaccurate images may lead to an inappropriate diagnosis or treatment.

The ultrasound manufacturer’s engineering team had designed filters into the system to protect the cpu from any internally produced interference generated at the operating frequency of the equipment. However, the engineers also realized that the power source, a switching power supply connected to power their equipment, could also be a major source of interference. Since the power supply and the ultrasound system can operate on different frequencies, a complex array of harmonic noise could result. Designing filters in the cpu to eliminate the resulting harmonic noise would be very difficult. The solution was to develop a power supply that operated at a switching frequency matching the frequency of the ultrasound machine.

Design Challenge

Designing a power supply with a switching frequency that matches the operating frequency of the ultrasound equipment is not hard. But, because the operating frequency of the ultrasound equipment could vary during normal use, the power supply also had to be able to sense the ultrasound’s operating frequency and adjust its own switching frequency to match. This frequency matching or synchronization would have to be done dynamically, in a real-world environment.

Based on these requirements, the ultrasound manufacturer developed a specification for the “synchronized frequency” power supply that they envisioned. Among the requirements were the following:

• Medical-grade combination power supply and battery charger

• Universal input

• Output 1: +15 V dc, 2.0–3.4 A

• Output 2: +12.6 V dc, current source for battery charging

• Synchronized switching frequencies at either 62.5 KHz or 83.3 KHz

The ultrasound manufacturer invited seven power supply makers to submit a bid. Of the companies, six said it could not be done. Elpac (Irvine, CA; www.elpac.com) was confident it could meet the challenge.

The firm’s technical approach included adding a dedicated timing circuit to the switching power supply’s normal circuitry. This timing circuit would receive a signal from the ultrasound system via a RATE/SYNC line that would indicate the operating frequency of the ultrasound in real time. Based on this signal, the timing circuit would set the power supply’s system clock to operate at the matching frequency. When the frequency of the ultrasound system changed, the timing circuit in the power supply would receive a new RATE/SYNC signal and adjust the power supply. The ultrasound system’s operating frequencies would always be synchronized and the production of harmonics that could adversely affect the ultrasound image would be eliminated.

Based on their approach, Elpac’s engineering team was selected for the project. Prototypes were built and delivered in less than eight weeks. After customer approval, regulatory agency approval was obtained and volume production was started.

However, the project was not yet finished. One week before the product launch, the manufacturer realized that a design change was needed in its equipment, and that this change would require a change in the power supply. The product launch included an international sales meeting bringing in people from all over the world. The plan was to have them leave with a working piece of the ultrasound equipment to begin selling worldwide. Canceling this meeting was not an option. In order to successfully launch the product and provide the salespeople with equipment working to the new specification, two hundred power supplies modified to the new requirements were needed.

Elpac was able to make the necessary changes in less than four days and deliver the modified supplies to the client in time for the crucial meeting.

Copyright ©2006 Medical Product Manufacturing News