Power supplies in medical applications face requirements and standards above and beyond what is specified for more pedestrian applications. These include greater isolation between the output voltage and both the ground and the power main as well as less EMI/RFI emissions. It is expected that these devices will consume a growing share in the overall market for power supplies in the coming years because of the increased production of smaller, portable medical devices in the West, as well as the greater use of medical equipment of all types in Asia.
The governing regulation for medical power supplies worldwide is IEC (International Electrotechnical Commission) 60601-1. A major subdivision of the regulation is based on where the equipment comes in contact with the patient. Type B (for body) is for equipment that merely operates in the vicinity of the patient; Type BF (body floating) is for equipment that touches a patient; and type CF (cardio floating) is for power supplies intended for equipment that may come in contact with the heart.
Notice the word “floating” in types BF (body floating) and CF (cardio floating). What the word connotes here is that the medical power supplies that meet these standards, unlike most electronic equipment, are not grounded. Rather, they “float” with respect to the ground. Medical power supplies are held to very high standards by IEC 60601-1 as to their maximum allowed leakage currents, representing the maximum allowed current, measured in microamps, which could inadvertently flow through the patient to ground.
The most difficult trade-off in medical power supplies is low-leakage current and good filtering of EMI/RFI, and that’s because the capacitors needed for an effective filter also tend to allow current leakage. Designers have found much success in getting around this conundrum by using zero-voltage switching (ZVS). This solution involves slowing the rise and fall times of the high frequency pulses inherent in switchers, and has the effect of lowering EMI. Thus, the power supply requires less filtering, thereby lowering its leakage current. This technique would be impossible to achieve using the traditional analog feedback loops once so common in electronic equipment. Modern medical power supplies employ onboard digital controllers, which can be programmed to implement ZVS. These controllers are also employed to change the pulsing in response to changes in the current that the medical device requires, and to fluctuations in the AC mains.
Even within the medical field, some types of situations require greater protection from EMI than others. For example, in diagnostic situations, medical equipment is often required to record the subtlest of changes in signal created or reflected from the patient’s body. In these instances, even a miniscule amount of EMI may overwhelm the equipment’s sensors, necessitating that the best filtering possible is required.
Medical power supplies can be inside of the units they power, mounted on a circuit board or to the chassis. They can also be external, and some of these are of the Power over Ethernet (POE) variety, which can supply power over Ethernet cables, obviating the need for a separate power cable. Units can be specified to offer one, two, or more different voltage outputs, depending on the needs of the designer.
SL Power Electronics’ MINT1045A2475K01 is an example of an open-frame AC-to-DC power supply that provides 24 volts DC at 1.9 amps, yet takes up only 2 inches by 3 inches and is only 1.0 inches high. And perhaps most importantly for critical medical applications, it provides two levels of MOPP protection. The datasheet for the MINT1045A2475K01 reveals that this unit is a member of a family of similar products that offer 12, 15, 28, or 48 VDC. All members of this family provide input/output isolation of 4500 VAC, and both input/ground and output/ground isolation is rated at 1900 VAC. What these latter values convey is the amount of external, unexpected voltage that the units can endure without failing. Both the MINT1045A2475K01 and the 12- and 15-volt models are rated for leakage currents of 30 microamps.
Because the design and construction of medical power supplies impose many specialized and easily overlooked obstacles not ordinarily encountered even by most power supply designers, their fabrication is best left to specialists. Engineers not well-versed in the highly esoteric issues involved should always avoid the temptation of trying to save costs by designing power sources themselves. The consequences could be a medical product that underperforms due to EMI issues, and the very real possibility of exposing patients to dangerous and perhaps fatal shock hazards.
Notice the word “floating” in types BF (body floating) and CF (cardio floating). What the word connotes here is that the medical power supplies that meet these standards, unlike most electronic equipment, are not grounded. Rather, they “float” with respect to the ground. Medical power supplies are held to very high standards by IEC 60601-1 as to their maximum allowed leakage currents, representing the maximum allowed current, measured in microamps, which could inadvertently flow through the patient to ground.
Multiple Levels of Protection are Mandatory
The IEC 60601-1 standard is also concerned with enforcing multiple levels of safety, or means of protection (MOP), so that it takes more than one failure in the power supply to cause it to potentially do harm. These safety enforcement standards are divided into means of operator protection (MOOP) and means of patient protection (MOPP). Because the patient may be less healthy than the operator, and because the equipment will have a tighter, more intimate contact with the patient than it will with the operator, MOPP requirements are stricter than MOOP standards. These protections include robust insulation, large physical spacing requirements (with creepage defined as the space required between two points for it to be considered a MOP), as well as electrical isolation between elements of the mains voltage, the high-voltage section (which are all switchers), and the unit’s DC output.The most difficult trade-off in medical power supplies is low-leakage current and good filtering of EMI/RFI, and that’s because the capacitors needed for an effective filter also tend to allow current leakage. Designers have found much success in getting around this conundrum by using zero-voltage switching (ZVS). This solution involves slowing the rise and fall times of the high frequency pulses inherent in switchers, and has the effect of lowering EMI. Thus, the power supply requires less filtering, thereby lowering its leakage current. This technique would be impossible to achieve using the traditional analog feedback loops once so common in electronic equipment. Modern medical power supplies employ onboard digital controllers, which can be programmed to implement ZVS. These controllers are also employed to change the pulsing in response to changes in the current that the medical device requires, and to fluctuations in the AC mains.
Even within the medical field, some types of situations require greater protection from EMI than others. For example, in diagnostic situations, medical equipment is often required to record the subtlest of changes in signal created or reflected from the patient’s body. In these instances, even a miniscule amount of EMI may overwhelm the equipment’s sensors, necessitating that the best filtering possible is required.
Leakage Current is a Critical Parameter
As in all switching power supplies, a central component of a medical power supply is the transformer that conveys the power of the mains to rest of the unit. No matter how well constructed a transformer is, there is some stray capacitance between its two coils, and this is a prime source of leakage current, which must be reduced at all costs because of the electrocution risk it engenders. A common, but expensive way of reducing that leakage is to impose an extra DC-DC converter in series with the first in the power chain. Then, the effective capacitance through which the leakage current originates is comparable to the value of two capacitors in series, which is always less than either capacitor by itself. The inclusion of the DC-DC converter has the additional benefit of adding an extra MOP, or means of protection. As described earlier, MOP’s are potential lifesavers, and are required for certification under 60601-1.Medical power supplies can be inside of the units they power, mounted on a circuit board or to the chassis. They can also be external, and some of these are of the Power over Ethernet (POE) variety, which can supply power over Ethernet cables, obviating the need for a separate power cable. Units can be specified to offer one, two, or more different voltage outputs, depending on the needs of the designer.
SL Power Electronics’ MINT1045A2475K01 is an example of an open-frame AC-to-DC power supply that provides 24 volts DC at 1.9 amps, yet takes up only 2 inches by 3 inches and is only 1.0 inches high. And perhaps most importantly for critical medical applications, it provides two levels of MOPP protection. The datasheet for the MINT1045A2475K01 reveals that this unit is a member of a family of similar products that offer 12, 15, 28, or 48 VDC. All members of this family provide input/output isolation of 4500 VAC, and both input/ground and output/ground isolation is rated at 1900 VAC. What these latter values convey is the amount of external, unexpected voltage that the units can endure without failing. Both the MINT1045A2475K01 and the 12- and 15-volt models are rated for leakage currents of 30 microamps.
Because the design and construction of medical power supplies impose many specialized and easily overlooked obstacles not ordinarily encountered even by most power supply designers, their fabrication is best left to specialists. Engineers not well-versed in the highly esoteric issues involved should always avoid the temptation of trying to save costs by designing power sources themselves. The consequences could be a medical product that underperforms due to EMI issues, and the very real possibility of exposing patients to dangerous and perhaps fatal shock hazards.