Medical products need to comply with relevant parts of EN 60601-1 (e.g. EN 60601-1-2)

Good resource sites

www.601help.com

Indicators And Buttons

Red or yellow indicator lights, buttons or switches must not be used for general items as these have special meaning.

Connectors

Connectors on patient leads must not be of a type in common use, e.g. 9-pin D-type, keyboard connector, etc., to prevent the possibly of the patient being inadvertently connected to an unsafe device or port.

IP Rating

Is there is a requirement for the product to prevent the ingress of liquids which the design therefore needs to ensure?

Component Choices

As the developer of an electrical medical device, you will probably use components manufactured by other companies. Those components, which may influence the safety of your product, are known as safety critical.  When you submit your product to a laboratory for testing to IEC 60601-1, the test engineer will want to have proof of the compliance of these components. All components carrying mains voltage are considered safety critical. Such components include: mains transformers, mains switches, fuses, fuseholders, appliance inlets, EMI filters. Other components having a specific safety function are also considered safety critical, e.g. thermoswitches, optocouplers, interlock switches, etc. Battery charging circuits are also considered safety critical.

It is generally in your interest to use components which already have approvals to North American (e.g. UL, CSA) and European (e.g. TUV, VDE, Demko, etc.) test houses. In most cases you should ask the component manufacturer for a copy of the component approval certificate. The engineer who tests your product will ask to see this. Be wary of component manufacturers’ use of phrases such as ‘conforms to’, ‘compliant with’ or ‘designed to as this often means that there is no third party approval of the component.

Electrical Design Requirements

IEC 60601-1 requires that two levels of protection be employed in various areas of the product to meet the requirements of the standard. If one level of protection fails, the product would then still have another level of protection to contain any electrical shock hazards and shield patients and operators from harm.

IEC 60601-1 permits three building blocks to be used in various combinations to meet the “two levels of protection” requirement. These building blocks are insulation, protective earthing, and protective impedance. For example, a protective earth (one level of protection) used in combination with basic insulation (one level of protection) provides the two levels of protection that are required. Alternatively, a product’s plastic enclosure that has reinforced insulation (considered two levels of protection) between the outside of the enclosure and its circuits again achieves two levels of protection.

Protection against Electrical Shock

For devices powered by an external source, the product may be classified as Class I or II. Class I is a product that is provided with a reliable protective earth (PE), such as a complete metal enclosure, that is protectively tied to the ground pin of the three-pronged power plug. Construction is such that accessible metal parts cannot become live in the event of a single fault. Class II is a product without a PE and where double or reinforced insulation is relied upon to provide protection against electric shock. For example, a product has an external brick power supply that provides double insulation. The Class II symbol is a double-walled square, indicating the product’s double insulation.

Degree of Protection (Applied Part) against Electric Shock

This product classification deals with the definition of applied parts—those parts or circuits that deliberately come in physical contact with the patient. The classification applies to each applied part. They are classified either as type B, BF or CF, depending on the degree of protection they offer against electric shock

Type BF and CF are ‘floating’ and must be separated from earth.

Patient connections, particularly Type BF and Type CF, must be isolated from other secondary circuitry used for signal input/output ports. This protects the patient if, as a result of faults on external equipment, dangerous voltages appear on communication lines.

Connections:

Ground (0V)

+5 V

+12 V (typically for disk drives and cooling fans)

-5 V (typically for peripherals on the ISA bus)

-12 V (typically for RS-232 serial port negative rail)

Power Good – pin used to prevent digital circuitry operating while the power supply turns on and output is not yet properly stable.

Typical power rating 63.5 watts (most of it on the +5 V rail)

Power is turned on and off at the mains (no energy saving low-power mode).

Wikipedia

ATX Power Supply

Connections:

Ground (0V)

+3.3 V

+3.3 V sense (should be connected to the +3.3 V at the motherboard to allow for remote sensing of the voltage drop in the power supply wiring)

+5 V

+12 V

-5 V – absent in current power supplies (optional in ATX and ATX12V ver. 1.2, and deleted as of ver. 1.3)

-12 V

Power good (PWR_OK) – low when other outputs have not yet reached, or are about to leave, correct voltages (typically remains low for 100–500 ms after the PS_ON# signal is pulled low)

Power on (PS_ON#)- pulled up to +5 V by the PSU, motherboard drives low to turn on the PSU.

+5 V standby (+5VSB) – provides a small amount of standby power when the computer is “off”

Positive supply voltages must be within ±5% of their nominal values at all times. Negative supply voltages must be within ±10%.

Wikipedia

Good Design Resources

Solar Power Answers

Pink bags and bubble wrap and anti static tubes

These products are anti-static, in that they are specially coated with a material which prevents static charge build up.  It works great at what is does,  BUT it does not protect fully against static discharge damage.  If you are carrying a high voltage static charge and pick up a bag containing components, one of these products will not provide better protection than a non treated plastic bag against that charge damaging the components.  Many people assume the pink bags protect components from this type of static damage – they help because they are a plastic bag providing some level of insulation, but their coating doesn’t shield the components inside from a static discharge.

Silver and black static shielding bags

These are conductive bags (at voltages level which can cause static damage).  They provide full static protection for components.

Using A Bluetooth Module vs A Chip Based Design

Bluetooth OEM modules have usually gone through all the testing so you don’t have to.  You can slap one on your PCB and your ready to go, often with a faster development route too.  The catch is that it costs more in production than using the discrete components instead.  It can be even more frustrating that the Bluetooth IC the module uses is capable of running the embedded software your application needs, but you have to use a separate microcontroller because either the Bluetooth module implementation doesn’t allow it or if you do you would break the Bluetooth stack certification, requiring re-testing.

As a general rule of thumb, if your production volume is likely to be say 10k or 20k then going with a module is often an overall cheaper choice once your factor in the testing costs.  However if your production volume is likely to be say 100k then the testing costs will typically be overcome by the savings in production cost.

The Cause

Until relatively recent times digital PCB design (and especially when prototyping) could be viewed as simply a means to electrically interconnect components and unless you designed RF circuits there was little else to worry about. However the PCB itself, or the means of connecting the components used (i.e. prototyping), is now is a very common cause of a loss of signal integrity. The reason is mainly due to the rise and fall times of output signals having decreased as devices are designed to operate faster and faster and to use smaller and smaller silicon manufacturing processes. This problem is not actually due to the operating frequency of a device or the frequency at which a signal is changing, it is due to the speed at which a signal output changes state from high to low and low to high. A signal doesn’t instantaneously change from high to low or low to high, it takes a certain amount of time which will be specified as the rise and fall time in a devices data sheet. Previous signal rise and fall times of many 10’s of nano seconds have now become times measured in just a few nano seconds or for many devices they are measured in pico seconds.

So you may be thinking, this can’t possibly be an issue for me, my board is only operating at a few MHz and I’ve even slowed my data bus down to a few KHz. Well unfortunately that doesn’t matter. If you work with a DC signal the only thing you really care about in a wire or PCB track is its resistance, which for short lengths will be close to zero. However, when using that wire or PCB track with a fast AC signal it starts to behave like a capacitor and inductor. Capacitors and inductors exhibit resistance to alternating current called reactance. The impedance of the wire or track is the vector sum of resistance and reactance, essentially the total resistance seen at a particular frequency. What happens when you send a signal with a fast rising and falling edge down a wire or PCB track, if the impedance of the gate driving the wire or track isn’t exactly the same as the one receiving the it, is it that some of the pulse bounces (literally) back to the driving gate. As there is still an impedance mismatch, the signal continues to bounce between the two until it finally dampens out. This bouncing becomes worse as the speed of signal rise and fall times increases. Basically, the faster rise and fall times of signals from modern semiconductors combined with wire or PCB trace inductance and capacitance causes noise signals of a greater magnitude than before. Greater magnitude means the bouncing signals can reach the threshold voltage required for the receiving device to ‘see’ another clock pulse, or an incorrect data level at the moment it is sampling the data line.  The solution is to design your PCB to use impedance controlled tracks on these clocked connections.

Single Connections

Download the free Saturn PCB Design Toolkit – its a great tool for this.  Use it as follows:

Select Conductor Impedance

Select Imperial or Metric (you can select later and it will auto convert all values)
Set copper weights and plating thickness (e.g. 18um copper + 18um plating = 35micros (1oz))
Select internal or external layers (typically microstrip or embedded microstrip)
Enter substrate options (prepreg dielectric constant – typical PCB’s are FR4)
Enter track width in conductor width
Enter prepreg height in conductor height (the distance between the copper layers excluding their thickness)
Click ‘solve’
Now adjust the track width until you get the impedance (Zo) you need.  This is the thickness to make your tracks

100ohm Single Track Impedance (General signals, SPI bus etc)

General signals, SPI bus, etc will generally perform well with a 100ohm track Impedance.  Some examples track widths with a GND plane under the tracks:

2 layer 1.6mm PCB (1.48mm FR4) = 0.61mm (100.3150ohms)

4 layer PCB Pool – Internal to Internal (0.71mm FR4) = 0.27mm (100.2273ohms)

4 layer PCB Pool – Internal to External (0.38mm FR4) = 0.12mm (100.9915ohms), or if too small then 0.15mm = (95.0631ohms)

4 layer PCB Train – Internal to Internal (0.99mm FR4) = 0.4mm (99.8636ohms)

4 layer PCB Train – Internal to External (0.22mm FR4, layer 2-3) = 0.05mm – not possible

4 layer PCB Train – Internal to External (1.245mm FR4, layer 1-3) = 0.49mm – not possible

75ohm Single Track Impedance

Also relevant to 75ohm radio antenna connections.Example track widths with GND plane under track

2 layer 1.6mm PCB (1.48mm FR4) = 1.29mm (75.0457ohms)

4 layer PCB Pool – Internal to Internal (0.71mm FR4) = 0.6mm (74.7938ohms)

4 layer PCB Pool – Internal to External (0.38mm FR4) = 0.3mm (74.8156ohms)

50ohm Single Track Impedance

Also relevant to 50ohm radio antenna connections.  Example track widths with GND plane under track

2 layer 1.6mm PCB (1.48mm FR4) = 2.65mm (50.1165ohms)

4 layer PCB Pool – Internal to Internal (0.71mm FR4) = 1.25mm (50.0581ohms)

4 layer PCB Pool – Internal to External (0.38mm FR4) = 0.65mm (49.9609ohms)

Differential Pairs

Download the free Saturn PCB Design Toolkit – its a great tool for this.  Use it as follows:

Select Differential Pairs

Select Imperial or Metric (you can select later and it will auto convert all values)
Set copper weights and plating thickness (e.g. 18um copper + 18um plating = 35micros (1oz))
Select internal or external layers (typically microstrip or embedded microstrip)
Enter substrate options (prepreg dielectric constant – typical PCB’s are FR4)
Enter track width in conductor width
Enter clearance between tracks in conductor distance
Enter prepreg height in conductor height (the distance between the copper layers excluding their thickness)
Click ‘solve’
Now adjust the track width and spacing until you get the impedance (Zdifferentail) you need.  This is the thickness to make your tracks

USB 90ohm Differential Pair Track Impedance

USB 2.0 requires 90ohms differential impedance (max 45ohms per track)

Max trace-length mismatch between High-speed USB signal pairs should be no greater than 3.81mm.

Example track widths with GND plane under track

4 layer PCB Pool – Internal to External (0.38mm height – FR4 thickness to GND plane)

35um copper, track spacing 0.15mm and 0.38mm track width = 90.525ohms Zdifferential
35um copper, track spacing 0.2mm and 0.43mm track width = 90.174ohms Zdifferential

To stick with the max 45ohms per track (not practical for many designs):
35um copper, track spacing 1.4mm and 0.75mm track width = 89.118ohms Zdifferential

4 layer PCB Pool – Internal to Internal (0.71mm height – FR4 thickness to GND plane)

35um copper, track spacing 0.15mm and 0.61mm track width = 90.385ohms Zdifferential

4 layer PCB Train – Internal to Internal (0.99mm height – FR4 thickness to GND plane)

35um copper, track spacing 0.15mm and 0.8mm track width = 90.156ohms Zdifferential

2 layer 1.6mm PCB (1.48mm FR4 thickness to GND plane)

35um copper, track spacing 0.15mm and 1.12mm track width = 90.184ohms Zdifferential

10/100Mbps Ethernet 100ohm Differential Pair Track Impedance

Ethernet requires 100ohms differential impedance (max 50ohms per track)

Example track widths with GND plane under track

4 layer PCB Pool – Internal to External (0.38mm height – FR4 thickness to GND plane)

35um copper, track spacing 0.15mm and 0.3mm track width = 100.462ohms Zdifferential
35um copper, track spacing 0.2mm and 0.34mm track width = 100.923ohms Zdifferential

To stick with the max 50ohms per track (not practical for many designs):
35um copper, track spacing 2.5mm and 0.65mm track width = 99.835ohms Zdifferential
If you use 1 of the plane layers with GND above and below (0.71mm & 0.38mm from track to GND planes) you get:
35um copper, track spacing 1.5mm and 0.42mm track width = 99.0.16ohms Zdifferential

Ensure Your PCB IS Made With The Right Stackup

Its a good idea to include the required stackup on one of your copper layers to ensure it is used.  Something like this:

http://www.eecs.umich.edu/~prabal/projects/hijack/