• Component Selection
• Electromagnetic Interference (EMI – FCC Part 15)
• High Voltage Transients (IEC 61000-4-2)
• Battery Protection (IEC 62133)

COMPONENT SELECTION

The component selection process is often overlooked but critically important. Even two passive components like capacitors with the same capacitance values can perform significantly differently in various conditions. Besides the component value, other important device parameters include:

• Device Parasitics (inductance, capacitance, resistance)
• Operating Temperature (typical and absolute maximum)
• Operating Voltage (typical and absolute maximum)
• Operating Currents (typical and absolute maximum)
• Thermal Performance (heat dissipation – typical and absolute maximum)

Choosing elements such that they operate in environments below their absolute maximum (or minimum) ratings with a safety margin is called derating. For example, choosing a capacitor with a maximum voltage rating of 7.5 V (or higher) when the capacitor would only see at most 5 V, results in a 50% margin, which increases the reliability of the component. A 50% margin can increase Mean time between failures (MTBF) by about 30% [1]. Note that MTBF is the predicted time between inherent failures of a system during normal operation.

Furthermore, the manufacturing process needs to enforce the reliability requirements. If due to parts obsolescence or sub-standard supplier work, lower quality components with lower margin may be substituted, thereby degrading the reliability of the device (despite the upfront scrupulous design work). Therefore, requirements on the component tolerances must be communicated from the design engineering to the manufacturing engineering departments in a documented and traceable fashion for quality assurance purposes.

EMI

SOLID GROUND PLANE

An insufficient ground plane is the single largest factor for sub-par EMI performance. Using a 4-layer PCB allows for a dedicated ground layer. However, during the Printed Circuit Board (PCB) routing phase, much of the internal layer can have traces, which decreases the effectiveness of the ground plane. This is especially problematic for high frequency signals, since their electromagnetic fields will have to find another return path, thereby causing coupling issues. Since some traces may be required in the ground layer, traces in that internal layer should be minimized. A general rule of thumb is to ensure the ground plane has gaps smaller than 10 mm [2]. The ground plane should be solid across high frequency traces (Fig. 1). A ground plane area at least three times the high frequency signal trace width should be enforced on each side of the trace [3].

SHIELD/GUARD VIAS

To ensure high frequency signals are kept within their traces, placing vias around the critical traces is recommended (Fig. 2). This is technique is extremely effective for frequencies up to at least 5 GHz. A general guideline is to ensure that the distance between the vias is at most a quarter of the system’s resonant wavelength. However, it is often more effective to locate the shield vias at most a 1/10^th (or preferable a 1/20^th) apart.

In the example above, a high frequency trace originating from a wireless Microcontroller Unit (MCU) and terminating at an onboard antenna is protected.

Another important characteristic of the trace shown in Fig. 2 is its trace width, which dictates its inherent characteristic impedance. To maximize power transfer from the MCU to the antenna, the characteristic impedance needs to match the impedance of the microcontroller and antenna, which typically is 50 Ω. Determining the optimal trace width is based on the PCB composition and out of scope for this paper, but a simple calculator can be found on our website [4].

STITCH VIAS

Stitching vias are like guard vias. Stitching vias allow for greater EMI performance because various ground layers can be connected, yielding a lower parasitic series resistance to ground for all signals on the PCB (Fig. 3). The general rule for spacing should be followed as specified in the Shield/Guard Vias section. In a standard PCB design software, the size between vias (grid) as well as the via size can be selected. Below we show the control window in Altium (Fig. 4).

HIGH VOLTAGE TRANSIENTS

High voltage transients are defined as voltages several orders of magnitude larger than expected voltages (i.e., 10 kV) for a short period of time (a fraction of a second). Despite the short duration of transients, they can destroy a circuit. Static electricity from a human hand as well as USB cable removal/insertion can cause the transients. An effective safeguard is electrostatic discharge (ESD) protection, implemented as Transient Voltage Suppressors (TVS), which are special diodes. Note that using a conventional diode does not successfully protect the circuit because of their high parasitic capacitance that prevent the diode from reacting quickly enough to curb the transient; TVS have a capacitance on the order of picofarad. Applying such protections is recommended whenever a transient is expected, such as USB connectors or user buttons. There are four considerations when choosing a TV:

1. Standoff Voltage higher than normal operating voltage
2. Clamp voltage (for given peak current) is below the protected IC’s pin’s max voltage rating
3. Specified peak current exceeds expected peak current
4. Bidirectional protection is chosen (if required)

Be wary that an excellent PCB schematic alone (Fig. 5) is insufficient to protect against the transients: an effective PCB layout is an equally important step. For example, a +/- 15 kV IEC-61000-4-2 Air-Gap Discharge ESD event with a nanosecond pulse results in a pulse current of 15 A. A ½ inch PCB trace represents L = 10 nH of parasitic inductance, which translates to a clamp voltage that is 450 V¹ in additional to the diode’s clamp voltage [3]. This ineffective PCB layout would, therefore, render the ESD protection useless since most components are not rated for handling 450 V. Even if the components do not fail immediately, the product would have a lower MTBF reliability rating.

¹450V = (L*dI/dT = 10 nH * 45A/10⁹s)

BATTERY PROTECTION

Modern medical equipment commonly employs secondary rechargeable Lithium-based batteries for various reasons. Ventilators, for example, commonly employ backup power sources to mitigate power supply failure, as mandated by IEC Standard 80601-2 [5]. Other devices are fully wireless-based and, therefore, depend on a single primary rechargeable battery for connectivity applications. Even in non-safety-critical applications, medical devices are developed with adequate safety standards to minimize the risk of failures, such as fire or explosion that would endanger the user. IEC 62133 is a cross-industry standard for exporting devices with lithium batteries in accordance with international compliance. IEC 62133-2 specifies requirements and tests for the safe operation of lithium-based batteries [6]. IEC 62133-1 is applicable to nickel-based batteries.

Some of the battery tests include:

• Free fall
• Crush
• Over charging
• External short circuit

The batteries must be able to tolerate these tests with no fire or explosion results. Each battery will have a different safe voltage current, operating/charging temperature, and number of cells. Clearly, these safety standards levy requirements not only the battery manufacturer, but also the device manufacturer (battery integrator).

The Lithium-based battery should either have the protection circuit built-in or integrated onto the PCB. The choice will depend on the product’s mechanical and cost requirements. A PCB-based solution can be slightly more cost-effective in some cases but would increase the verification burden onto the application integrator. Specifically, the protection circuit must protect against:

• Overcharge
  • A 3.7 V Li-Ion battery, for example, can typically be safely charged only to a certain level, such as 4.2 V.
• Over-discharge
  • A 3.7 V Li-Ion battery must not be discharged below a certain voltage. 3 V is a common cutoff voltage for this battery class.
• Charging too quickly
  • A charge rate is typically recommended that should not be exceeded. 0.5 C or 1 C are common charge rates.
• Discharging too quickly
  • A maximum discharge current rate as a function of the battery capacity is typically specified. 2 C is a common, but not a universal, parameter value.

Exceeding any of these limits can increase the probability of critical device failure and, thereby, endanger the user. Fig. 6 shows a simplified protection circuit that could be integrated into the application PCB.

SUMMARY

Ubiquitous pitfalls exist in designing modern medical devices that are highly reliable, safe, cost-effective, and functionally competitive. As a result, a myriad of tradeoffs must be balanced to deliver a competitive product that can secure market share. We discussed tangible steps to help accomplish that vision. Any questions on this article (or any related topic) may be directed to the author.

REFERENCES

[1] Reliable Design of Medical Devices by Richard C. Fries.

[2] MAX 13202E Datasheet. https://datasheets.maximintegrated.com/en/ds/MAX13202E-MAX13208E.pdf.

[3] PCB Design and Layout Fundaments for EMC by Roger Hu.

[4] PCB Characteristic Impedance Calculator. https://simplonics.com/simulations/.

[5] IEC 80601-2 Standard.

[6] IEC 62133-2 Standard.

[7] Lithium-Ion Cell Protection Examples. https://www.digikey.com/en/maker/blogs/lithium-ion-cell-protection

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