By: Lorenzo Cividino, Director of Field Technical Support, SL Power Electronics
READ THIS ARTICLE TO FIND OUT ABOUT
• The changes in the IEC 60601-1-2 standard’s 4th edition that affect medical power supplies
• how the MB65 power supply helps designers to quickly achieve compliance with the new standard
• The effect of operating temperature on the lifetime of medical power supplies
Regulatory changes in the medical industry take place at a slow but steady pace. In the past few years, however, several updates to the medical safety regulatory standard based on IEC 60601-1 3rd edition have been introduced. This primary standard also has collateral standards, one of which defines the requirements for electromagnetic disturbances and compliance requirements (EMC).
Then in February 2014, the IEC issued the 4th edition of IEC 60601-1-2, ‘General requirements for basic safety and essential performance – Collateral Standard: Electromagnetic disturbances – Requirements and tests’. Under review by the nations of the European Union, it is expected to be adopted in or around 2016 as a European Norm (EN standard).
This marks an important change: designers of medical equipment should certainly plan ahead and prepare for its introduction, as it will have a noticeable impact on the professional and home healthcare markets.
The force of the changes will be felt particularly strongly in the areas of:
• AC input-voltage power drop-outs
• ESD immunity
• Susceptibility to electric and magnetic fields
For power supplies used in medical devices, the main changes in the IEC 60601-1-2 4th edition are shown in Tables 1 and 2.
Table 1: comparison of new 4th edition of IEC 60601-1-2 with 3rd edition (for enclosure port)
IEC 60601-1-2 4th Edition – Enclosure Port Comparison to 3rd Edition
|Phenomenon||Basic EMC standard or test method||Immunity Test Levels: Professional healthcare facility||Immunity Test Levels: Home category||Immunity Test Levels: IEC 60601-1-2 3rd Edition|
|Electrostatic Discharge||IEC 61000-4-2||±8kV contact, ±2kV, ±4kV, ±8kV, ± 5kV air||±8kV contact, ±2kV, ±4kV, ±8kV, ±15kV air||±2kV, ±4kV, ±6kV contact, ±2kV, ±4kV, ±8kV air|
|Radiated RF Electromagnetic Fields||IEC 61000-4-3||3V/m 80MHz - 2,7GHz 80% AM at 1kHz||10V/m, 80MHz - 2,7GHz, 80% AM at 1kHz||3V/m for non-life supporting ME equipment 10V/m for life-supporting ME equipment 80MHz - 2,5GHz 80% AM at 1kHz|
|Rated Power Frequency Magnetic Fields||IEC 61000-4-8||30A/m, 50/60Hz||30A/m, 50/60Hz||30A/m, 50/60Hz|
Table 2: comparison of new 4th edition of IEC 60601-1-2 with 3rd edition (for AC input power port)
IEC 60601-1-2 4th Edition – Input AC Power Port Comparison to 3rd Edition
|Immunity Test Levels:||Immunity Test Levels:||Immunity Test Levels:|
|Phenomenon||Basic EMC standard or test method||Professional healthcare facility||Home category||IEC 60601-1-2 3rd Edition|
|Voltage Dips||IEC 61000-4-11||0% UT; 0,5 cycle At 0, 45, 90, 135, 180, 225, 270 and 315 degrees||0% UT; 0,5 cycle At 0, 45, 90, 135, 180, 225, 270 and 315 degrees||<5% UT; 0,5 cycle|
|Voltage Dips||IEC 61000-4-11||0% UT; 1 cycle and 70% UT; 25/30 cycles Single phase: at 0 degrees||0% UT; 1 cycle and 70% UT; 25/30 cycles. Single phase: at 0 degrees||70% UT; 25/30 cycles null|
The MB65 AC-DC power supply from SL Power Electronics was developed to provide system designers with an easy means to achieve compliance with the new 4th edition standard. It is especially well suited to home-healthcare equipment.
In particular, the continuous AC input range now goes to 85V AC, down from 90V AC. This offers extra margin for home-healthcare environments, in which power quality is variable, and less well controlled than in hospitals and clinics.
Another area seeing a noteworthy change is in the level of ESD protection required. This has increased from 6kV to 8kV (contact discharge), and from 8kV to 15kV (air discharge). Clearly, the regulatory bodies have recognised that new, high levels of ESD may be generated by the synthetic and natural materials used in the home and in professional institutions.
The MB65 also meets the emissions requirements for home-health environments; in fact, it stays inside the threshold for conducted and radiated emissions for Class B compliance with margin – no small accomplishment, as shown in Figure 1. This simplifies the design of medical equipment at the system level, since it eliminates the need for external components or circuitry dedicated to filtering or shielding Class B emissions. This can provide worthwhile cost savings in the end equipment.
Designed for long life, the 65 watt MB65 has a compact 2” x 3.5” footprint. It provides a high output power with convection cooling, and offers excellent safety isolation, including two means of patient protection (type BF).
This model family is also designed to operate at higher temperatures up to 70°C while still providing 40W of continuous output power. While supporting this high output, careful attention was paid to power losses, since these serve to limit product life and maximum operating temperature. In order to achieve power density of 7W/in3 and still allow for up to 65W of output power with convection cooling in such a small unit, power conversion efficiency needed to be maximized.
The MB65’s high efficiency of 88-90% lowers power losses. This in turn markedly lowers the scale of the internal temperature rise in normal operation, and also reduces the requirement for cooling.
In fact, power losses from the power supply are the dominant source of heat that raises internal temperature. Figure 2 shows
the power losses for different power supplies: one with 85% efficiency, a second with 88% and a third with 90%. While the 3-5 basis point difference in efficiency may not seem important, the better comparison is that in power losses. The more efficient product has 30-37% lower losses. The less efficient products have a higher internal temperature because of their higher losses, and the reliability and expected life of the more efficient product will be much higher.
Temperature and failure rate
In general, devices and components fail faster at higher temperatures. The expression below shows the temperature factor as a function of temperature with respect to 30°C. It is a non-linear function, and clearly shows the dramatic impact temperature has on component failure rates. For example, the failure rate is 1.6 times higher when operating at 40°C than at 30°C.
Fig. 1: MB65S24K, typical conducted emissions at full load, 230V AC
Fig. 2: comparison of power efficiency and power losses
Temperature factors, πT are derived by the following expression:
Where: T0 = Reference temperature in °k
T1 = Operating temperature in °k (°C + 273)
Ea = activation energy, varies from 0.05 ~ 0.70, use 0.40 for
electrolytic caps and discrete semiconductors
K = Boltzman constant = 8.62 x 10-5
Source: Telcordia Technologies, Reliability Prediction Procedure for Electronic Equipment, SR-332 May 2001.
The curve in Figure 3 plots the temperature factor for failure rate. It can be used to determine the failure rate difference when comparing operating temperature conditions. For example, at 70°C, πT = 6.0. If you were able to reduce the operating temperature to 60°C, πT = 4.0, so the failure rate would drop to 66.7% of that at 70°C. While the product failure rate is a function of many variables, there is a marked impact from reducing the temperature of components.
Fig. 3: plot of failure rate vs temperature, typical for electrolytic capacitors and power semiconductors
Product operating life is often determined by its life-limiting components such as electrolytic capacitors. The failure mechanism of electrolytic capacitors is the loss
of capacitance and increased ESR as the electrolyte seeps out through the end seal of the capacitor. high temperature accelerates this process. The following expression has been experimentally determined to express the relationship between temperature and deterioration of the capacitor.
Lx= Lo • Ktemp = Lo • B(To –Tx)/10
B = Temperature acceleration factor ≈ 2
To = Manufacture’s maximum rated temperature for the
selected capacitor (°C)
Tx = Actual ambient temperature of the capacitor (°C)
Source: Nippon Chemi-Con, Aluminium Electrolytic Capacitors 2008.
Based on a 105°C-rated capacitor, at its 105°C rated operating temperature and base life, KT Factor = 1. At 70°C, KT Factor is 11.3, so for a 5,000 hour rated capacitor, operating at 70°C, its rated life is 65,500 hours, or approximately 6.4 years. The life of the capacitor is strongly affected by its operating temperature. For products
requiring long operating life, long life (5,000-10,000 hours) rated capacitors should be selected, and their operating temperature kept to a minimum.
In the MB65, high-quality components are used at relatively low operating temperature, since the high conversion efficiency of the power supply keeps power losses to a minimum. This model family is also designed to operate when necessary at higher temperatures up to 70°C while still providing 40W of continuous output power. In fact, the MB65 leads the way for reliable, compact power at a competitive cost while compliant with the EMC requirements of the 4th edition of the IEC 60601-1-2 standard.
Fig. 4: electrolytic capacitor life estimate, life factor as a function of capacitor temperature