Using GaN devices can reduce the size of external medical AC/DC power supplies
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Using GaN devices can reduce the size of external medical AC/DC power supplies

Posted Date: 2024-01-22

Author: Bill Schweber

Despite continued advances in battery technology and low-power circuits, completely independent battery-only designs may not be feasible, feasible, or acceptable for many applications. Medical systems fall into this category of applications. Instead, the device usually must run directly from the AC line, or at least be connected to an AC outlet when the battery is low.

In addition to meeting basic AC/DC power supply performance specifications, medical power products must also meet regulatory requirements that meet less obvious performance requirements such as electrical isolation, rated voltage, leakage current and measures of protection (MOP). These standards were developed to ensure that electrical equipment does not pose a risk to the operator or patient even if the power source or load fails. At the same time, designers of medical power supplies must continue to improve efficiency and reduce size and weight.

This article will discuss the use of external AC/DC power supplies for medical instruments and review relevant regulatory standards. Then introduce XP Power's products. Designers can use these products to meet various regulatory standards while also using gallium nitride (GaN) power devices to reduce the physical size of power supplies by nearly half.

Basic requirements for power supply design

When selecting an AC/DC power supply, first consider standard power supply performance specifications. The power supply must provide a nominal DC voltage and the rated current required to support the load at that voltage. The universal power supply must accommodate a wide range of AC input voltages from 47 Hz to 63 Hz (typically 85 VAC to 264 VAC).

While input and output voltages and current ratings are critical, they are not sufficient to fully define a power supply. Other considerations include:

・Dynamic performance attributes such as startup delay, startup rise time, hold time, line and load regulation, transient response, ripple and noise, and overshoot
・ Overload, short circuit and over-temperature protection
・Efficiency requirements are a function of the power supply's maximum power rating and must have specific values ​​on the load curve, including the full load point, low load point, and no-load point
・Power factor (PF) is close to 1, the specific PF value depends on the power level and control regulatory standards
・Electromagnetic compatibility (EMC), which describes a power supply's maximum electromagnetic interference (EMI) and radio frequency interference (RFI) and susceptibility to electrostatic discharge (ESD), radiated energy, sudden energy events, line surges, and magnetic fields
・Safety, which defines the basic protection requirements for users and equipment, including isolation voltage between input and output, input and ground, and output and ground

Requirements for medical power supplies

When evaluating power supplies for medical applications, additional standards and regulations further complicate the matter. This primarily involves patient and operator safety, ensuring that in the event of a single or even double failure, the power supply does not endanger either.

Most of the concerns are related to stray or leakage current. Line standard voltage (110/230 V; 50 Hz or 60 Hz) may induce ventricular fibrillation at currents as low as 30 mA, even for a fraction of a second. Currents less than 1 mA (AC or DC) can cause cardiac fibrillation if the current flows directly through the heart, such as through a cardiac catheter or other electrodes.

These are some of the standard thresholds often cited when electrical current flows through the human body through contact with the skin surface. In contrast, the dangerous currents in the event of internal contact are much lower:

・1 mA: Barely noticeable
・16 mA: The maximum current that an average-sized person can grasp and "let go"
・20 mA: Respiratory muscle paralysis
・100 mA: Ventricular fibrillation threshold
・2 A: Cardiac arrest and internal organ damage

The level of risk is also a function of the path that the current takes between two points of contact on the body, such as across or through the chest or from the arm to the foot. Therefore, it is critical to minimize the leakage current through the insulation medium of the AC isolation transformer.

If the insulation quality is good enough, leakage current appears to be negligible. However, while this leakage may be a physical "leakage" current due to imperfect properties of the insulating material, it may also be due to capacitively coupled currents that can pass even through special insulating materials.

A simplified model of an ideal transformer shows perfect electrical isolation (resistive) between its primary and secondary sides (Figure 1).

Figure 1: A basic model of a transformer shows that there is no current path between the primary and secondary sides. (Image source: Power Sources Manufacturers Association)

In an ideal transformer, no current flows directly from the AC source to the powered product, creating a complete current loop back to the AC grid, even if a component or wiring failure provides a new current path on the secondary side. However, no transformer is perfect, and there is interwinding capacitance from primary to secondary (Figure 2).


Figure 2: A more realistic model showing the basic interwinding capacitance (Cps1) between the primary and secondary sides. (Image source: Power Sources Manufacturers Association)

More complex models add additional sources of interwinding capacitance (Figure 3).


Figure 3: In addition to the first interwinding capacitor (Cps1), there are other transformer capacitors. (Image source: Power Sources Manufacturers Association)

This unintended capacitance can cause leakage current to flow, and this capacitance value is related to many variables such as wire diameter, winding pattern, and transformer geometry. The capacitance values ​​range from as low as one picofarad (pF) to several microfarads (µF). In addition to transformer capacitive leakage, other sources of unintended capacitance include spacing on the printed circuit board, insulation between semiconductors and grounded heat sinks, and even parasitics between other components.

Transformer leakage current due to capacitance is not the only issue that medical power supply specifications address. Basic AC safety and insulation are top priorities. Depending on the voltage and power level, the power supply may require a second, independent isolation layer in addition to the main isolation layer (or physically hardened isolation layer). Insulation performance also degrades over time due to extreme temperatures, high voltage stress, and voltage surges, but it still maintains its rated value.

The first isolation layer is often referred to as the "base isolation layer." Such as wire insulation. The second isolation layer is usually an insulating enclosure, like many wall-mounted and desktop power supplies.

Standards and Measures of Protection (MoP)

The main standard governing medical electronic equipment and safety is the IEC 60601-1 standard. The latest edition (4th edition) expands the focus on patients and requires the combination of one or more Measures for Operator Protection (MOOP) and "Movement for Patient Protection" (MOPP) to form an overall Measure of Protection (MOP) ).

Regulatory standards also establish levels of protection around how MOOPs are delivered. These standards are divided into Class I and Class II and specify the structure and insulation of power supplies in detail. Class I products have a conductive chassis connected to a safety ground. To simplify local wall plug compatibility, the power supply is equipped with an IEC320-C14 receptacle for the convenience of a user-supplied power cord with a safety ground conductor (Figure 4, left).

In contrast, a Class II power supply has a two-wire power cord with a safety ground connection (Figure 4, right). Since there is no grounded chassis, there are two layers of insulation (or a single layer of reinforced insulation) between the user and the internal current-carrying conductors.


Figure 4: Class I (left) and Class II (right) equipment are connected to a grounded three-wire or ungrounded two-wire AC line, typically for use with a standard IEC receptacle and a user-supplied power cord. (Image source: XP Power)

Therefore, any AC/DC power supply designated for medical applications and certified as Class I or II must be specifically designed and tested to relevant standards. Fortunately, power suppliers like XP Power understand the technical, manufacturing and certification issues required to provide power supplies that meet these standards.

Size matters too

Technical requirements and regulatory requirements for medical AC/DC power supplies have nothing to do with physical size, but size does matter. In situations where space is limited, such as in ambulances or clinical settings where space for mobile carts and desks is limited, large power supplies can complicate on-site operational arrangements.

In this case, reducing the size of the AC/DC power supply can be beneficial, but it can also be a challenge. Minimum power supply size is limited due to compliance with regulatory standards regarding insulation, creepage, clearance, etc.

Another issue with shrinking the power supply is heat dissipation. If a power supply has insufficient volume and package surface area, its internal temperature will be higher than that of a larger power supply, degrading the performance of the active, passive, and insulating components inside. Forced air cooling is not recommended due to possible airflow obstruction, long-term reliability issues, and increased ambient noise.

In addition, the heat inside the power supply may cause the temperature of the power supply housing to rise beyond the allowable range, posing danger to patients and operators. The key to shrinking the power supply is to use appropriate circuit switching components to minimize heat generation.

Compared with silicon (Si) devices, GaN-based switching devices have obvious advantages in this regard. Such devices have smaller series resistance, faster switching times, and lower reverse recovery charges, thereby reducing losses and making switching power supplies more efficient, cooler, and more compact.

For example, XP-Power's AQM200PS19 power supply. This power supply belongs to the AQM series. This power supply is rated 19 V/10.6 A and is classified for Class I operation. The power supply measures approximately 167 × 54 × 33 mm, half the volume of a conventional power supply with the same rating, and weighs only 600 g (Figure 5).


Figure 5: The AQM200PS19 is a 200 W, Class I power supply capable of delivering 19 V at up to 10.6 A with an efficiency of up to 92%. (Image source: XP Power)

This external power supply fully complies with international medical standards. Its electrical parameters include patient leakage current of less than 100 µA, typical efficiency of 92%, standby power consumption of less than 0.15 W and PF > 0.9.

The power supply is available in Class I and Class II models and is rated for operating temperatures from 0°C to 60°C. This power supply features a fully sealed enclosure with an IP22 protection rating and a smooth surface for easy cleaning in medical environments.

For more powerful systems, XP Power offers the AQM300PS48-C2 300 W, Class II power supply. This power supply is rated at 48 V/6.25 A and consumes less than 0.5 W in standby power. Although slightly larger, this power supply is still compact, measuring only 183 × 85 × 35 mm and weighing 1050 grams.

Rated at 250 W, XP Power's AQM250PS24 is a 24 V/10.4 A, Class I power supply with standby power consumption of less than 0.15 W. The size of this power supply is 172 × 67.1 × 32 mm.

Conclusion

External stand-alone AC/DC power supplies for medical devices must meet strict regulatory, operational, performance, safety and efficiency requirements. XP Power's AQM series of medical-grade external power supplies use GaN devices, and their overall package size is only half the size of traditional silicon device power supplies, exceeding the above standards.


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