Power Isolation Design Challenges: Meeting Medical Safety Standards

When designing medical products, an important consideration is meeting the IEC 60601-1 safety standard and isolation ratings for products that come into contact with the patient. This standard specifies many aspects of medical electrical (ME) equipment to protect patients and operators from dangerous high voltages and currents. One of the most critical aspects of personnel safety is minimizing leakage currents through the patient , including AC currents, which is also the most challenging part of product design. Since the isolation barrier always takes the form of capacitance, it is often necessary to minimize the isolation barrier to limit AC leakage currents caused by signal voltages and switching power supply voltages, which cause capacitive currents. This capacitive or AC current exists regardless of the electrical isolation mechanism used, such as inductive isolation or optical isolation, because there is always a barrier capacitance. The most stringent leakage current requirement is the leakage current through the patient caused by the applied part, such as the applied part of a CF floating heart device, the probe pads of an electrocardiograph (ECG/EKG), etc. The combined AC/DC leakage current must be less than 10μA in normal operation.

Patients and operators must also be protected from leakage paths through the enclosure or accessible parts of the device. These currents to which patients or operators may be exposed are called “touch currents.” Under normal circumstances, the touch current from medical system components or between components in the patient’s environment should not exceed 100μA. This touch current is limited to 500μA in a single fault condition (SFC), which refers to the failure of a single protection method or the presence of some abnormal condition.

Medical electrical equipment must have a double means of protection (MOP) to prevent applied parts and other accessible parts from exceeding leakage and touch current limits. Protection methods include insulation, air gaps, creepage distances, impedance and protective grounding. There are two basic categories of medical electrical equipment. Class I medical electrical equipment refers to equipment that adopts basic insulation measures and the accessible parts are protectively grounded to provide additional safety protection. Class II medical electrical equipment refers to equipment that not only prevents electrical shock through basic insulation, but also provides additional safety protection through double insulation or reinforced insulation. There is no limitation on whether Class II safety requirements are met through protective grounding or relying on installation conditions.

Double insulation consists of basic insulation and supplementary insulation. Double insulation provides a dual method of protection, while reinforced insulation is a single insulation system that also provides a dual method of protection. To meet the requirements of the dual method of protection of the patient (MOPP), the component must be able to withstand the AC test voltage. For a component with a 5kVRMS rating using solid insulation material, the AC test voltage means a working voltage of 707VPK or 500VRMS. It is generally accepted that medical isolation measures must use insulation material with a thickness of 0.4mm to meet the minimum distance requirements. This is one criterion that solid insulation materials must meet to meet double insulation or reinforced insulation requirements. Another applicable criterion is that the insulation must consist of at least two layers of insulation material, each of which must pass the appropriate dielectric strength test. In the case of reinforced insulation, the dielectric strength test must be sufficient to ensure that the dual method of protection works.

IEC 60601 specifies that when the device is connected to the patient, the patient leakage current limit is 10μA DC in normal operation and 50μA DC in single fault conditions. The acceptable range of AC leakage current to the patient is 10μA to 1mA, depending on the type of equipment, normal operation or single fault conditions, and whether there are single or multiple applied parts. The safest applied parts for the patient are F-type isolated (floating) applied parts, in which the patient connection point is isolated from the rest of the medical electrical equipment. The isolation must prevent any current higher than the allowable patient leakage current from flowing, even if an unexpected voltage generated by an external source is connected to the patient and thus applied between the patient connection point and ground. F-type applied parts are further classified as BF type (floating parts for human use), or CF type (floating parts for cardiac use). See Table 1, which summarizes the allowable current to the patient. The table includes the current of the individual applied parts as well as the total leakage current to the patient. The total leakage current to the patient is the leakage current when all applied parts required for the operation of the medical device are in contact with the patient.

Table 1: Permissible leakage current and touch current flowing through the patient

Touch current is also listed in Table 1. Touch current is leakage current from the enclosure or parts of the equipment (excluding the patient connection point) to which any operator or patient may come into contact during normal use. Touch current flows to earth or another part of the enclosure through an external path other than the protective earth conductor. This term has the same meaning as "enclosure leakage current", which is now consistent between IEC 60601 and IEC 60950 and correctly reflects the fact that leakage current also applies to parts that are normally protectively earthed.

According to the IEC 60601-1 standard, components on the isolation barrier that meet the requirements of the dual patient protection method must maintain 4kVRMS isolation for a duration of 1 minute. The standard also defines the patient protection method (MOPP), which describes the isolation protection required to reduce the risk of electrical shock to the patient. There are also some requirements for the operator protection method (MOOP). Medical electrical equipment requires a dual protection method to reduce the electrical risk to the patient and operator when a fault occurs that renders one protection method ineffective. The isolation protection requirements include provisions for creepage/clearance distances, insulation and protective earth connections, pollution degree and total leakage current. The dual patient protection method requires double the creepage distance and air clearance. Many medical electrical products are powered by standard 120VAC and 240VAC power supplies, and the standard working voltage is often increased to 250VAC. The creepage distance required for the single patient protection method is 4mm, and 8mm for the dual patient protection method. This 250VRMS is equivalent to 354VDC (or peak), and for the dual patient protection method, a test voltage equal to 4kVRMS is required.

As shown, 4kVRMS is a common isolation requirement for two MOPPs directly applied to a part of the patient. There is an additional optional requirement that can be applied to many medical instruments to isolate voltages up to 5kVPK, which reiterates the need for 4kVRMS isolation voltage. This refers to the situation when the medical equipment and applied parts must be defibrillation-proof. A defibrillation-proof applied part is one on which protection is provided to prevent discharge from a cardiac defibrillator from causing adverse effects to the patient. Essentially, a defibrillator is a charged capacitor in series with an inductor that acts as a current limiter. When started, this will produce a gradually decaying sine wave, and the peak voltage of the first ring may be significantly higher than the charge voltage on the capacitor itself. The IEC 60601 standard uniformly stipulates that 5kVPK is the maximum value of this voltage overshoot, so it is necessary to protect the patient from electric shock if the isolation barrier is broken during defibrillation of the patient.

Figure 1: Complete isolated RS485/RS422 µModule transceiver + 1W power supply

This type of design environment is extremely challenging. When developing products for the medical market, it is required to meet the requirements of IEC 60601. Linear Technology offers a growing family of isolation devices to help meet medical isolation requirements.

Linear Technology will launch a 5kVRMS isolation device series that provides an integrated power supply to provide up to 1W of power and an isolated data interface, eliminating the need for any external components. The series is based on the 2.5kVRMS isolated RS485 micromodule (μModule) transceiver LTM2881, which provides uninterrupted communication at rates up to 20Mbps and can withstand high-voltage transient events. The product includes an isolated 1W DC/DC converter with an efficiency of up to 62%, providing ample power with a 5V regulated output. Everything from decoupling capacitors, diodes to switch termination resistors is integrated into the module. The isolation barrier is composed of two layers of dielectric material and can withstand voltages up to 5kVRMS, which will meet the requirements for creepage distance and clearance. The LTM2881 provides ±15kV ESD protection on the transceiver pins and across the isolation barrier.

An excellent fail-safe receiver ensures that the receiver output is in a logic high state when the input is shorted, left open, or terminated (but not driven). The receiver thresholds are balanced to maintain the data duty cycle when long network wiring is used. In addition, the LTM2881 protects itself by disabling the driver and receiver in the event of excessive power dissipation.

At the same time, the 1.62V to 5.5V logic power supply pin makes it easy to connect to digital components, and when using a 3.3V or 5V power supply, it can still maintain TIA/EIA compatible RS485 signals. The LTM2881 also provides a low current shutdown mode, so that when communication is not required, it only draws less than 5μA of current.

These new devices form a rugged solution that provides continuous communication, even withstanding transient events greater than 30kV/μs. The LTM2881 provides a low EMI solution with a nominal barrier capacitance of 6pF and meets EN 55022/CISPR 22 Class B emissions requirements if good layout practices are followed. These new devices are also available in RS232 and digital logic isolator versions.