The battle against heat
As previously mentioned, high operating temperatures degrade the SNR performance of the CMOS sensor and reduce its lifetime. Moreover, high operating temperatures pose a risk to patient safety. To maintain superior image resolution, the X-ray flat panel detector is placed in direct contact with the patient’s body. Human skin starts to suffer burns at temperatures as low as 40°C (100°F). Therefore, the exterior of any medical device that could potentially come in contact with skin must stay below this limit. Thus, high operating efficiency and the ability to spread the heat that is generated over a wide area is critical in multiple areas: sensor lifetime, image clarity and patient safety.
Keeping that slim figure
From surgical system attachments to handheld examination tools, the increasing complexity of next- generation medical devices is at odds with the space available to fit the very components, supporting these additional capabilities. In the case of a flat panel X-ray detector, existing hospital infrastructure has already allocated a fixed-sized slot known as a bucky slot where the analog X-ray film cassette was formerly located. These film cassettes generally follow ISO4090 guidelines, permitting external dimensions of 46cm x 38.6cm x 1.5cm for an X-ray image size of 43cm x 35cm (14 x 17 inches). A suitable power management solution needs to be compact and efficient to comply with such restrictive size limitations and minimize operating temperature rise.
As part of the regulatory requirements in the US and Europe, medical devices must be proven to be compliant with CISPR11, also known as EN55011. Since switching regulators radiate electromagnetic fields, the designer must gain a full understanding of the switching regulator’s impact on EMI compliance or select a power solution that is tested to meet radiated EMI limits by the manufacturer. Otherwise, expensive and time consuming product iterations could result in order to achieve compliance. The most stringent radiated EMI limits are assigned to medical equipment intended for use in office buildings, Group1 – Class B devices whose radiated limit is identical to EN55022 Class B (CISPR22 Class B) limits assigned to information technology equipment intended for use in office buildings and homes.
A long product lifetime
A power solution with proven reliability is a necessity for medical devices. In the case of an X-ray panel sensor, the panel must capture the image correctly the first time otherwise the patient and health care professional face a lamentable repeated radiation exposure. At minimum, a delayed diagnosis causes a delay in the start of treatment, an unacceptable situation in modern medical standards.
Another factor to consider is how long will the electronic components selected be available? After a lengthy regulatory approval process enduring CE, UL, IEC and FDA hurdles, each electronic medical device should be manufacturable for a long period of time - upwards of 15 years. This length of time is much longer than the consumer product cycles many power management semiconductor vendors service as their primary market. Product requalification solely due to obsolete components is a burden to engineering resources and the bottom line.