Medical-grade isolation in DC/DC converters using ‘3D power packaging’

The importance of medical equipment has never been so evident, as hospitals around the world struggle with waves of patients affected by virus infections. Manufacturers of ventilators, analysers and a host of other therapeutic and diagnostic equipment have stepped up production in response and in turn, the need for power supply products has increased. Despite the urgent need, power supplies must still be safe and the international standard IEC 60601-1 set out the requirements.

There are many aspects to consider in power supply designs meeting IEC 60601-1. The latest version of the standard is intended to be more pragmatic than previously, allowing relaxed power supply ratings depending on the intended end-use. For example, ‘patient’ or ‘operator’ environments with different ‘measures of protection’ (MOPs) and three levels of patient connection, allowing different maximum mains leakage current: type B (Body, earthed), type BF (Body, Floating) and type CF (Cardiac, Floating).

With battery-powered equipment, it could be thought that no dangerous voltage or current leakage is present but if the product charger could be connected while in use, this might not be so. Also, other connections such as communications interfaces to portable equipment provide a connection to a potentially unknown source which could be mains-powered, so the highest level of isolation specifications from IEC 60601-1 for patient connections might still apply. Conversely, an AC/DC might only need to meet the headline specifications of a commercial product in medical environments such as an analysis laboratory, staffed with trained operators and with no patients ever-present.

Some isolation scenarios

Mains-powered equipment with a direct patient connection is the scenario with the most risk and a power supply with two means of patient protection, 2 x MOPPs, is necessary. The patient connection must also be isolated from ground by 1 x MOPP in the form of specified solid insulation or creepage/clearance distance. This is to allow for the patient becoming electrically ‘live’ from other faulty equipment - the 1 x MOPP isolation then prevents lethal current flowing through the patient to the ground of the product considered. Although a fully-specified medical-grade AC/DC will have this isolation, another possibility is to power just the circuitry connected to the patient from a separate medically-isolated DC/DC converter with a minimum of 1 x MOPP, fed from the main AC/DC. This provides the required isolation to ground and also allows the AC/DC to have a lesser grade of overall isolation - minimum 1 x MOPP, which could be achieved with an IT-grade power supply with reinforced isolation. The DC/DC can ensure that leakage current limits are also met for the most stringent CF applications because its coupling capacitance is typically very low, normally a few pF. Note that the IT-grade power supply, typically certified to IEC 62368-1, should also meet other requirements such as EMI levels to IEC 60601-1-2 and have double AC line fusing unless in a fixed, hardwired installation.

As mentioned, AC operated or portable equipment will often have a communications interface, perhaps an ethernet connection or USB to a laptop for data logging. These connections are termed ‘signal input/output’ (SIP/SOP) and must be assumed to have dangerous voltage applied under external fault conditions and no guaranteed isolation to ground. There must therefore be medical-grade isolation between these lines and the product patient-connect circuitry, even if the main AC/DC is 2 x MOPP rated. Again, this can be achieved using a separate 2 x MOPP DC/DC converter which guarantees safety and low leakage for the patient, also potentially allowing an ‘operator-rated’ or IT-grade AC/DC. This scenario is shown in Figure 1. The DC/DC may be relatively small and low-cost, as the power requirement for just the patient-connected circuitry is likely to be low.

Figure 1:  Medical-grade DC/DC converter isolates signal inputs and outputs from patient connections

Selection criteria for medical-grade DC/DC converters

Unlike AC/DC converters, DC/DCs are available with a wide variety of isolation specifications with and without agency certifications. A high isolation voltage rating may just be for transient immunity and form no allowable safety barrier. For medical isolation, safety certification to the relevant national version of IEC 60601-1 is mandatory, and one or two MOOPs or MOPPs should be stated. Note that system voltage should also be specified – it is possible to obtain 2 x MOPP certification to 30VAC system voltage, which might have some applications, but would not be sufficient for even 1 x MOOP at the ‘normal’ 250VAC rating.

The selection of low-power 2 x MOPP/250VAC DC/DCs available is limited, not least because it is difficult to achieve the required internal solid insulation and 8mm creepage necessary, in a small package.

One example of a medical-grade surface-mount DC/DC converter that achieves certified 2 x MOPP/250VAC isolation is the R05CT05S  (211-7166) from RECOM (Figure 2). The part is rated at 0.5W, adequate for many patient-applied circuits and has a selectable 3.3V or 5V output with fixed 5V input. Outputs can also be set to the typical head-voltages for low-drop linear regulators, 3.7V or 5.4V. The DC/DC is extremely compact at just 10.3mm x 7.7mm footprint, 2.65mm high, making it suitable for space-constrained applications. It is rated for 5kVAC test voltage in medical applications and could also be used for 800VAC working voltage with reinforced isolation according to IEC 62368-1. Coupling capacitance is just 3.5pF typical, so leakage current at 250VAC is negligible. The R05CT05S operates to 140°C with derating and has the useful features of enabling, trim, synchronisation and under-voltage lockout.

Figure 2: DC/DC converter from RECOM with 2 x MOPP/250VAC medical rating

Advanced packaging has enabled medical isolation

The challenge to achieve the insulation, creepage and clearance to 2 x MOPP/250VAC ratings in the R05CT05S while maintaining electrical and efficiency performance has been met by the use of advanced ‘3D’ fabrication and packaging techniques. Switching at 8MHz allows the use of a miniature planar transformer incorporating the necessary solid interwinding insulation, while advanced circuit design yields good efficiency, despite the high switching frequency. Control is implemented in a custom die, wire-bonded to the DVE SOIC-16 Leadframe and over-moulding completes the package.

Conclusion

Medical-grade DC/DC converters are valuable tools as enablers for system safety compliance and flexibility in the choice of other isolation components, such as a main AC/DC converter. Products such as the RECOM R05CT05S are an example, with the highest 2 x MOPP isolation rating for 250VAC medical systems and with leakage current to meet the most critical CF patient connection requirements. This is all achieved in a miniature size using RECOM’s signature ‘3D’ power packing technology.