Capacitors in medical devices are quite different than those in consumer products. Medical versions are established proven designs that may be used in high reliability implanted devices. They are compact and sized to fit on high-density circuit boards often with restricted headroom. To better select capacitors for electrical tasks, let's look at a few circuits and problems common to medical devices and make recommendations for particular capacitors.

Step-down switcher

Simple step-down switchers are typically used along with N-channel Mosfets to regulate voltage. The switching regulator's operating frequency is usually set by adding an external oscillator capacitor, C_osc in the post regulator switch circuit. The best choice is a stable NP0 MLCC (multilayered ceramic capacitor) in the range of 50 to 300 pF, selected based upon the operating frequency. NP0 (negative positive zero) is one way to say the capacitance value of the device remains stable over the operating temperature range. Use NP0 capacitors where operating frequency stability is important.

The main input supply to the regulator is filtered with a combination of MLCC and tantalum capacitors, C₁. Typical values for the tantalum capacitors are 330 µF or 470 µF. In addition, the devices may require voltage derating based on guidelines supplied by the manufacturer. The tantalum capacitor provides peak current to the drain of the topside Mosfet, so polarity must be observed. An MLCC capacitor is placed in parallel with the tantalum capacitors. A good choice is an X7R MLCC with a value of 100 nF at rated voltage. The output lines require a bank of low ESR (equivalent series resistance) tantalum capacitors, (C₄), typically with a value of 470 µF that are capable of handling the output ripple. ESR can be viewed as a lumped AC resistance with a magnitude effected by frequency. The lowest ESR occurs at the self resonant frequency of the capacitor. Generally, the lower the ESR, the better the capacitor's ability to handle any ripple noise. These applications require reduced power losses and high converter efficiencies, made possible by the low ESR values of tantalum chip capacitors. They can also handle high currents in bulk energy storage.

Tantalum capacitors tested by the manufacturer for their response to surge currents are particularly useful in switching regulators. Surge testing subjects each capacitor to a rapid turn-on from a low-impedance power bus to assure reliability for high in-rush-current applications.

Some regulator ICs have built-in features that provide voltage to the output driver and control circuits. They typically use tantalum capacitors to decouple the power to ground lines. A tantalum capacitor (C₈) with a range from 2.2 µF to 4.7 µF is a good choice. In this case, a soft-start circuit on the regulator requires additional external decoupling capacitors (C₃). The best choices for these are MLCC X7R 100-nF types.

Holding dc power steady

Many circuit boards in medical instruments are a mix of digital and analog components that operate at different voltage levels. Dc-to-dc converters turn battery power into several different stable-voltage levels.

Proper operation in handheld blood analyzers, glucose monitors, and blood-pressure monitors require switching regulators to operate at high efficiency with low-load currents. Battery-operated applications need low loss electronic components to extend device operating time. For capacitors, this means checking the dc leakage (DCL) rating or the insulation resistance (IR). A low DCL improves battery runtime. It's calculated by dividing operating voltage by insulating resistance. In general, MLCC capacitors have low DCLs. Tantalum capacitors screened for lower-than-rated DCL levels can also be used to match specific application needs and can replace high-valued MLCC's.

Input capacitors use a parallel combination of bulk tantalum capacitors and MLCC decoupling capacitors for filtering high-frequency noise. Low-ESR capacitors are the better choice for reducing sensitivity to voltage transients.

Select output capacitors on the basis of the inductor value for maximum output current. Low-ESR ceramic capacitors are needed on outputs to keep output-voltage ripple small and ensure stable regulation in the control loop.

Buck-boost charge pump regulators

Portable medical instruments are typically powered by a lithium ion or two to three-cell alkaline batteries. A charge-pump-regulator circuit maintains stable dc-output voltages during battery charge and discharge cycles which may span voltage levels above and below the 3.3V often needed for LEDs. Charge pumps boost voltage to required levels and are commonly used to drive white LEDs that provide backlighting for displays.

The recent SiP1759 buck-boost charge pump is an example of an IC that regulates output from 1-cell Li Ion or two to three-cell NiMH voltages. The IC has fixed 3.3-V and adjustable output voltage options. Of course, several capacitors are part of the overall circuit. The output capacitor (C_out) is selected based upon the resonant frequency and the inductor value. The accompanying table is typical for a resonant frequency of 50 kHz. A low-ESR MLCC capacitor is a good choice for the application. The input capacitor (C_in) should handle the maximum rms input current. Low-ESR tantalum capacitors can also be used in some applications for the input capacitor.

When selecting tantalum bulk-input capacitors, derate the voltage and test them to ensure no circuit damage occurs during voltage surges. Surge voltage for a tantalum capacitor is the peak or maximum voltage that can be applied to the capacitor for a short period. The recommended surge rating typically is 1.3 times the rated voltage.

MLCCs are used on the input to decouple any noise on the voltage rail to ground. Common MLCC types include Class II, X5R, and X7R with a value of 100 nF at rated voltage.

High-voltage medical applications

Galvanic isolation between input and output is an important requirement in high-voltage medical applications. Isolation allows grounding outputs to meet safety standards yet assures reliable operation. An isolation transformer combined with capacitors can be used when input and output voltages differ. For example, the output on flyback converters is isolated from the input by combining a power transformer with high-voltage capacitors across the primary to secondary coil.

Ripple noise or residual pulsation is unwanted in output power. High-voltage capacitors used in these circuits to reduce ripple may generate potentially unsafe transients due to noise generated by the switching regulator.

Up to now, traditional leaded through — hole capacitors were a preferred choice for high-voltage applications, such as portable defibrillators and medical power supplies. These require high-breakdown voltage and protection from high-voltage arc-over, which can damage capacitors when applying potentials greater than 1,000 V. To eliminate arc-over, designers typically coat circuit boards, or arrange components in such a manner as to isolate the high-voltage section of the board. This approach, however, tends to require larger boards, as do leaded through-hole capacitors commonly used in high-voltage sections.

To address the problem, a recently developed a series of high-voltage HV Arc Guard MLCCs prevents arc-over while conserving space and eliminating the need for leaded capacitors connected in series. The new capacitors also provide double the voltage breakdown capability as standard or through-hole capacitors while requiring much less space thanks to their surface-mount 0805 to 1812 case sizes. They are available with initial voltage ratings up to 1.5 kV. The HV Arc Guard devices are also useful in multiplier circuits.

When boards flex

Flexing or bending of printed circuit boards by as little as a few millimeters is often enough to damage MLCCs. The rigid devices usually develop a crack near their lower mounted corners. It can happen even in production and be so small the board still passes inspection. The initial effect, capacitance loss, can progress to a lowered insulation resistance and possibly a catastrophic short circuit.

A new and more flexible Open Mode Design (OMD) series of capacitors better tolerate board flexure and at the same time eliminate catastrophic short circuits caused by excessive flexing. Extensive board-flexure testing shows that OMDs are more reliable than standard construction MLCCs.

Board-flexure testing calls for solder mounting parts on a test coupon and maintaining a solder-fillet height of 25 to 50% of the capacitor's total thickness. Test coupons are subjected to a flexure bending of 1±0.5 mm until reaching bending depths of 2, 5, and 8 mm. Capacitance is monitored and recorded for each 0.5-mm increment. Test results helped develop new MLCC designs with sufficient margin to protect the part from shorting across two opposing electrodes.

CAPACITOR VALUE	TYPE	PART SERIES	ESR
4.7 to 10 µF	16V MLCC or 35V Ta bulk capacitor	VJ series MLCC 594D series TA	< 150 m-ohm
0.01 to 0.1 µF	16V MLCC decoupling capacitor	VJ series	< 150 m-ohm