ESD Testing Method, for Class II Input Medical Devices

ESD Testing Method, for Class II Input Medical Devices

GlobTek, May 6, 2019, Dave Love


Abstract:

With the latest 4th edition of the medical EMC standard 60601-1-2, 4th Ed, the ESD test levels were set to 15KV Air Discharge for all categories of medical equipment. Previously, the 3rd edition only required the more stringent 15KV air discharge test level, for life support and critical medical devices. Use of correct ESD test protocol, is imperative to obtain accurate test results.

+++++++++++++++++++++ ++++++++++++++++++++++++++++++++++

At the higher test level, any mistakes in the testing method become more pronounced, and may cause damage to the DUT (device under test). The more common error may made during testing at the high 15KV level, is failure to discharge accumulated charge voltage when testing Class II input medical devices, between subsequent ESD test events.

When a Class I input device is tested, the output is always attached to earth, which is the return path for ESD, therefore no high voltage can accumulate across the xfmr bridging Y1 series capacitor network. However, due to the floating nature, of the output when the input classification is Class II (or Class II with functional earth), the voltage across the xfmr bridging Y1 series capacitor network (XBYSCN), may take on a large DC value following a 15KV air discharge ESD event.

In this scenario, the power supply under test, will ring at thousands of voltages amplitude between typically 4MHz and 10MHz precisely after the high ESD energy dump into the DC output of the power supply. The fundamental impedances during the ringing event are as follows:

  1. XBYSCN

  2. Input Common mode choke inductance

  3. AC line common mode impedance, which may be either capacitive or inductive at the ring frequencies, but is typically measured to be inductive, when using a network analyzer between 4MHz and 10MHz.

  4. ESD gun series resistance, and series capacitance

When the ringing stops the polarity of the DC across the XBYSCN, will typically be the same polarity of the ESD gun. If the CMC has a spark gap in parallel with it (or 2), the spark gap will help limit the ringing energy, by dissipating said energy in the break-over spark gap. Should the spark gap be wide (over 1mm), or missing altogether, the residual DC voltage after the ring-down event may be in the 5000V area.

Applying a second ESD pulse of the same magnitude immediately after the first, may result in the residual DC voltage after the ring-down event increasing to perhaps 7000V. Eventually the voltage will build up after 3rd or 4th ESD pulse test event, so the first ring ½ cycle peak voltage will exceed 12KV. With 12KV or greater directly across the XBYSCN, virtually all power supplies will experience failure of (and/or logic) optocoupler, xfmr, output voltage error-amp, input SMPS controller.

However, if the residual DC voltage after the ring-down, is discharged according to the requirement in the ESD standard (standard referenced by 60601-1-2, 4th), each subsequent ESD pulse test event, will start from zero volts across the XBYSCN, thus preventing ratcheting up of the voltage stress across the xfmr isolation barrier.

This discharge requirement is very well explained in the ESD standard, in the 2008 version of the standard, the clause number is 7.2.4.1, titled “Ungrounded Equipment”. Although not using the lexicon Class II to describe the input character of the system, certainly the Class II medical device system employing an external power supply fits the category of ungrounded equipment.

In the Rationale, of paragraph 2 of this section in the standard:

The standard further instructs how

to prevent the negative outcome of EUT failure, by discharging the

residual charge in further paragraphs within the section.

And


And finally,

At GlobTek, in our EMC lab, we typically use a discharging brush with 2x 470K-ohm resistors, but several EMC labs we work with have elected to use the long time interval between successive discharges method. A long time, would be greater than 3 minutes dwell time between discharges.

It should also be noted, (perhaps obviously), that if a ESD discharge event is near a metal part which can not be reached with the discharging brush, since the metal part has a narrow creepage or clearance path to the metal discharge between 2 insulating plastic pieces, attempting to discharge accumulated ESD charge from the plastic housing is a totally useless method. It is essential, if the ESD jumps to the local metal component, that ESD is discharged from that component in some fashion.

Since the failed component when the discharge procedure is not done properly (or typically, was not done), is in the power adapter, a correct discharge point will be on the output connector of the power adapter.

This may require removing the DC power plug, and discharging this point, and then reinserting the connector, before the next ESD test event. This method is typically faster than waiting 3 minutes between subsequent ESD test events.



Summary:

When 15KV ESD testing levels are applied, to medical equipment systems using a Class II input configuration, it is necessary to remove accumulated ESD charge between successive ESD pulses. This is generally not well known information by many EMC test engineers, since most EMC immunity tested devices, are tested to a 8KV ESD test level, or if they are medical devices are Class I input.

Therefore if a Class II input medical system is reported as failing the ESD test regime, it is typically necessary to first determine if the EMC test lab performed the required residual ESD discharge process between subsequent ESD test events, as detailed in 61000-4-2.



REF:
GlobTek’s latest line of power supplies offer elevated ESD immunity withstand with specified air discharge levels of 18KV for Class II input, and 20KV for Class I input.





Immunity Test

3th ed criteria

4th ed criteria

Enhanced GT EL6 GTM96XXX
Family Product Capability

 




ESD
61000-4-2

6KV Contact
8KV Air

8KV Contact
15KV Air

10KV Contact
18KV Air

Radiated RF
61000-4-3

3V/m, 80-2500MHz, General Medical Equip
10V/m, 80-2500MHz, Life Support Equip

3V/m, 80-2500MHz, Professional Healthcare Facility
10V/m, 80-2500MHz, Home Healthcare Environment

Same as 4th ed Criteria

EFT
61000-4-4

2kV on AC input, 5KHz Repetition
3x -- to Line, Neutral and Earth

2kV on AC input, 100KHz Repetition
3x -- to Line, Neutral and Earth

4kV on AC input, 100KHz Repetition
3x -- to Line, Neutral and Earth

Surge (Lightning)
61000-4-5

2kV

common mode, 1kV differential mode on AC input lines (minimum 5

surges at each phase angle)

500V,1kV,2kV common mode, 500V,1kV differential mode, on AC input lines

500V,1kV,2kV,4kV common mode, 500V,1kV,2KV differential mode on AC input lines #

Conducted RF
61000-4-6

3Vrms, 0.15-80MHz on AC input

3Vrms, 0.15-80MHz on AC input

Same as 4th ed Criteria

Dips and Dropouts
61000-4-11

Dip to 40% for 5 cycles (100 ms)
Dip to 70% for 25 cycles (500ms)
Dropout to 5% for 10 ms
Interrupt >95% for 5 sec

Dip to 0% for 1 cycles @ 0°phase angle
Dip to 70% for 25/30 cycles @ 0° phase angle
Dropout to 0% for 0.5 cycles @ 0°,

45°, 90°, 135° , 180°, 225°, 270° & 315° phase angles


Interrupt 100% for 250/300 cycles

Same as 4th ed Criteria



# GTM96060 Enhanced Surge capability limited to 1.5kV for differential mode

Request Info

World map 中文 (Chinese)
Call
Email
💬
Text
💁
Chat
?
询价
我们可以帮助您?
我们可以帮助您?