Medical Device Architecture: Controlling Electrical Isolation and Risk in Very High Voltage Applications

June 19, 2012

By Paul Aceto, Principal Electrical Engineer

Electronic medical devices must provide protection against patient and operator electrical hazards per IEC 60601-1. Electrical isolation for typical voltages (under 1 KV) is easily accomplished using single component optical or transformer based digital isolators. However when a device spec requires a voltage in the order of 10KV isolation becomes an electrical engineering challenge of its own. Recently Ximedica faced this exact challenge and found a solution that surpasses IEC 60601-1 requirements which require protection against a Single Fault Condition and in fact went well beyond the requirements.

About Electrical Isolation:

Electrical isolation falls into two categories: Dielectric Withstand and Creepage/Clearance. Dielectric Withstand refers to the actual voltage that a component can tolerate before breaking down. Creepage and Clearance refer to the distances between conductors either along a surface or through air, respectively. Typical isolation components are designed to withstand voltages up to 5 KV. Creepage and Clearance distances are determined by the component package and typical components provide less than 15 mm separation between high and low voltage.

Mini Case Study:

On a recent development program Ximedica faced a challenge. The design specs for the device required isolation components that could withstand voltages greater than 10KV, double the isolation that components generally handle.

Therefore the team put their heads together and came up with the following solution-

The transmit and receive sides of the isolation section were separated into two optical components (Avago HFBR series) connected by fiber optic light pipes. The Dielectric Withstand voltage of the fiber is virtually unlimited and the Creepage/Clearance distances are limited only by the length of the fiber. The physical separation of the two halves of the design greatly reduces the risk of a single fault condition causing harm to the patient.

Additionally, the transmit and receive components were mounted on different circuit boards, located in separate halves of a compartmentalized metal chassis. This not only provides the required high voltage isolation, it also shields the low voltage side from high voltage switching transients, eliminating conducted interference.

While this solution may be ideal for digital signals, optical analog signal isolation can be of a further challenge. Analog opto-isolators are notoriously non-linear and are avoided in all but the least demanding roles. The technique described above may be employed in either of two ways for optical analog signal isolation:

  1. For high precision at a higher cost, convert the analog signal to serial digital data and reconstruct on the other side of the barrier using a D/A (digital to analog) converter.
  2. If high precision and high data rates are not required, a cheaper solution is to transmit a PWM (pulse width modulated) signal whose duty cycle is proportional to the signal voltage. On the isolated side, the signal can be reconstructed using a simple low-pass filter.

Learn more about Ximedica’s engineering team and other challenges solved.