Fundamentals Part 2 › emcfastpass › IS_hardware › ... · Assignment Solution IS Parameters: •Voc = 4.2V (battery) + 1.34V (COSH sensor assuming Alphasense) = 5.6V •Capacitance

Intrinsically Safe Design

Module 6

Fundamentals Part 2

To do…

• Assignment solution

• Parts that improve IS:

– Infallible resistors

– Fuses

– Blocking diodes

– Voltage Clamping with Zener diodes

• Summary and example use cases

Assignment Solution

Class I Zone 0 IIC T6 rating

• T6 rating is aggressive and not common for lithium ion products due to high short circuit currents and resulting temperatures; may need extensive potting to achieve which will have implications for the size of the product and may also complicate the battery assembly. Since industry standard for gas detectors is T4, is there a specific reason why T6 is targeted, or is this just for marketing?

Assignment Solution

H2S and CO sensing using single COSH sensor

• Will need vendor declaration of maximum voltage that will be contributed to overall product design; if the sensor elements do not share an electrode, could potentially add significant voltage making other parts of the design difficult due to limited capacitance.

Assignment Solution

Minimum 1000mAh lithium polymer battery no larger than 50mm x 30mm x 6mm

• Specifying a battery capacity and size at this point is not good. Need to create power budget and look at available cell configurations. Candidate cells must first be short-circuit tested to determine viability. 6mm might be a limiting factor even without considering IS.

Assignment Solution

Must fit in 80mm x 50mm x 13mm carbon fibre enclosure that has just completed tooling and ready for prototype testing• Designing the case before major components have

been verified is extremely dangerous. The final battery selected may not meet the dimensions required, and we do not know any details about the rest of the circuit. LCD may not be possible in which case a different model would be needed. Material selection may not meet 1 GOhm minimum surface resistivity so product will require ESD warning. Not having freedom in the form factor makes IS design extremely difficult.

Assignment Solution

2” TFT color display

• This could be a show-stopper due to high voltage requirements, power consumption, and processor/memory requirements.

Assignment Solution

100dB piezoelectric buzzer (use model XYZ with +/-5V drive)

• A doubler / inverter circuit will be required and so the buzzer circuit will require physical and electrical isolation from the rest of the circuit; 10V potential will have low capacitance budget, so must ensure driver circuit can operate. Buzzer itself will need testing to ensure it’s safe. Buzzer port must be sealed for IP54 to inner circuit.

Assignment Solution

Silent mode with vibrator alert

• Vibrator motor inductance required and current limiting resistor likely needed.

Assignment Solution

USB chargeable from any USB device

• If this is an external connection then required blocking diodes would cause too much voltage drop. Would have to have boost circuit that would lead to a very complicated design; using standard USB would also require in-product mains protection. Fitting components would be challenging.

Assignment Solution

Charger input at bottom of device between beeper and vibrator

• In addition to issues with USB voltage, it will be virtually impossible to put the charger between the two components due to spacing requirements and protection components for mains.

Assignment Solution

1 week operating life between charges

• Need to assess power budget first and determine actual battery.

Assignment Solution

User-replaceable main fuse

• Since fuse must be potted, would require custom, separate PCB that would plug in infallibly so main battery power could not bypass. This would add size and cost for very little gain since a blown fuse would likely mean that something has catastrophically failed in the device anyway.

Assignment Solution

Release date: 6 months from today

• Given initial IS component evaluation, electrical design, and the certification process, this is extremely aggressive and does not allow for any findings, failures, or unexpected results.

Assignment Solution

IS Parameters:• Voc = 4.2V (battery) + 1.34V (COSH sensor assuming

Alphasense) = 5.6V• Capacitance limit = 54uF from table• Short circuit current will be well above 3.3A, so CLR of 5.6V

/ 3.3A = 1.72R (1% tolerance)• Peak current draw can likely be under 200mA, so use

200mA 1206 fuse• CLR power = (0.2A x 1.7)² x (1.8 x 1.01) x 1.5 = 316mW so

choose 1206 0.5W • Buzzer requires energy test: design 0R 2010 placeholder

series resistor for now

Assignment Solution

IS Parameters:

• Buzzer circuit assuming 10V max will have capacitance max of 3uF; 3uF visible to main circuit through some reduction factor.

• Vibrator motor requires L and DCR characteristics; design 0R 2010 placeholder series resistor for now

• Mains protection required: given small product size, suggest doing this externally. if USB charger is essential, a dongle with protection and voltage boost could be designed so this would not be in the product.

Assignment Solution

• Let’s walk through creation of the block diagram

Infallible Resistors

• Only way to reduce transient currents

• Assessed at rated value +/- tolerance

• For infallibility: operate at 2/3 power, voltage and current; physically sized and spaced so cannot be bypassed

• Infallible resistors can only be opened with a countable fault


• Must not be bypassed by other components or traces in normal operation or under fault

• Power reduction: dissipate (share) some of the power delivered to circuits

– In series: with two or more

– In parallel: with four or more

• Low resistance almost always protected by fuse


• 2010 (1W) and 2512 (2W) SMT packages are available; 1206 (0.5W) option as well

• Note that we “defy Ohm’s law” in a way if a fuse is involved:• Small values of resistance: use P=I²R where I = fuse rating x

1.7

• Larger R: use P = V²/R

– Ballpark 0 to 2 Ohms and 15+ Ohms with an unusable amount in the middle


• Fuse and resistor order does not really matter

• Always assume the resistor can short to ground somewhere unless something specifically done to prevent that


• Series combinations split power almost in half

• Need infallible spacing between the two


• Parallel combinations take up more space since four are required


• A great trick: use a resistor or a few resistors to “add a layer” to your circuit board (or avoid blind/buried vias)

• Particularly useful when one or two traces would otherwise require a via but via is not allowed due to exposure to atmosphere.

Fuses

• Assessed at rated value x 1.7

• Must be rated for peak product voltage and breaking capacity

• UL or IEC certified

• Cannot be bypassed in normal operating or fault conditions

• Cold resistance is considered infallible but most likely must be measured

Fuses

• Cannot be faulted (except opened)

• Must be potted if energized in hazloc

• Fuse value has no bearing on transient currents due to fusing time which is orders of magnitude longer than spark time (even very-fast acting fuses)

Fuses

• Designs may have multiple fused paths to different (isolated) parts of the circuit

Fuses

• Lots are available!

• Since they must be potted, 0603 size is acceptable; 1206 may have more options

• Littelfuse, Vishay and Bourns have good ones

• Check to see that datasheet lists all of the requirements.

• Make sure specs meet electrical requirements

• Let’s look at 466 series from Littelfuse

Fuses

• Fuse value has no bearing on transient currents due to fusing time which is orders of magnitude longer than spark time (even very-fast acting fuses)

Fuses

• “Cold resistance” of the fuse is accepted infallible resistance for transients.

• Datasheet value likely not accepted so 10 samples must be tested over temperature.

Fuses

• Watch temperature derating curves: cold fuses pass more current and some agencies will perform other calculations based on the worst case

Fuses

• Watch out for additional derating factors from manufacturer.

• So how do we estimate this few resistance?

– 1.1 Ohms x 0.75 / 1.12 = 0.74 Ohms

• A good estimate, but we have tested this fuse and the lowest value was 0.64 Ohms.

Fuses

• Cold resistance may only be a few tenths of Ohms, but that is significant both for transient reduction and voltage drop.

• Example: a product with a lithium ion rechargeable battery will be assessed at 4.2V for spark which means 3.3A of short circuit current is allowed.

• 4.2V / 1.1 Ohms = 3.8A (pretty close)

• But 4.2V / 0.64 Ohms = 6.6A

Fuses

• Recommend always designing in additional current-limiting resistor (at least 0R).

• Remember 1.3W at 40°C is a magical number (and scales for higher ambient temperatures)

Fuses

• Don’t forget the potting! Minimum 1mm thick in all directions.

• Must stick to part

• PCB or enclosure counts if potting adheres

Fuses

• A potential alternative

Fuses

• But...

– Not SMT

– Huge!! (13mm x 8mm diameter?!?)

– No 200mA, though nice 125mA

– Cold resistance minimum specified at both -20°C and -40°C, so no testing required

Blocking Diodes

• Excellent for completely blocking / hiding current (including energy)

• Not infallible (can be opened or shorted) but can be used as a component on which IS depends and thus only countable faults can be used (max 2)

• Must be rated for current that is available with 1.5x safety factor

Blocking Diodes

• Do not typically have power spec

• Series diode barriers always require three since two can be shorted by countable faults

• Any diode can be used, but non-Schottkys typically have too much forward drop

• Often protected by fuse

Blocking Diodes

• Typical application 1: power / charging lines

• Typical application 2: “hiding” capacitance or otherwise blocking energy within circuit

• Big challenge is always dealing with Vf

• Always unidirectional

Blocking Diodes

• On power lines, 1A fuse is about maximum

• 1A x 1.7 x 1.5 = 2.55A so use 3A parts

• Total voltage drop could be almost 1.5V

Blocking Diodes

• One of our favourites: B340A (3A rated)

• We must still consider temperature.

• Introducing: R-theta-junction-ambient, RθJA

Blocking Diodes

• So we end up calculating power dissipated with P = IV at the forward voltage

• Use 0.5V @ 25°C with 2.55A fault current

Blocking Diodes

• So P = 2.55A x 0.5V = 1.275W

• Temperature rise is P x RθJA

• T = 1.275W x 50°C/W = 63.75°C

• So room temperature rating is 25°C + 64°C = 89°C

• Maximum ambient rating (T4): 135°C – 89°C + 25°C = 71°C

• Lots of margin for linear assessment (but)

Blocking Diodes

• What about isolating different circuits?

• SOD-123 parts with 1A rating are good

• Check out LSM115J for ultra-low Vf

• Still might have problems, so try this:

Blocking Diodes

• This is all generic but likely specific applications won’t be too far off

• Follow the same process of assessment.

• Often delicate balance between IS and electrical (surprise!)

• Critical introduction of RθJA temperature rise calculation

Voltage Clamping Zeners

• Most common voltage shunt (clamp) device is Zeners which clamp voltage to Vz + tolerance

• Must be rated for max current that would flow if Zener were shorted

• Must also be rated for power = current x (Vz + tolerance) x 1.5 (can get very big!)

• Not infallible (can be opened or shorted); in shunt assembly, only one countable fault


• Often fuse protected due to power

• Big disadvantage: leakage current

• Watch out for required bias current

• Ultimately, this allows more capacitance and/or special higher-voltage zones for special circuits


• Example: clamping within a design

• 4.2V Li Ion, 200mA, 3.3V LDO, LCD 5.5V DC-DC


• Infallible connection requirements



• Calculations:

– Spark voltage 4.2V

– Choose 3.9V + 5% Zeners = 4.1V (< 4.2V)

– Note that 4.1V Zeners x 1.05% = 4.31V, so overall product voltage would be assessed higher than battery (sometimes ok, but often need as low voltage as possible)

– P = [3.9V x 1.05] x [0.2A x 1.7] x 1.5 = 2.1W


• 2.1W is high, though some 3W and maybe 5W SMA/SMB parts are available – just watch derating curves

• SOD-123 parts are nice, but typically 0.5W

• Work backwards:

– 0.5W = (Vz + 5%) x I x 1.5

– I = 82mA

• Fuseable with 62mA, but expensive + potting


• So why not current limiting resistor? No problem on high impedance lines!

• R = 3.8V / 0.082mA = 47R (with 1% tolerance)

• P = 82mA² x 47R x 1.5 = 0.474W OK for 1206


• But what if we have more than one line between the two domains?


• Any one part can receive power from multiple paths

• Need to have n x R resistance for n paths

• Not required to be the same, but Req must remain greater or equal to original calculation


• Current leakage: Real Zeners leak under bias voltages much less than Vz (as much as 2V!)

• Some are worse than others

• Datasheet spec can help, but exact conditions may not be listed

• Good idea to test under exact conditions (input resistance, voltage, temperature)


• Bias current: all Zener require minimum current to operate and “be Zeners”

• Input resistance impacts this significantly based on voltage divider with forward-active Zener resistance

• Agencies don’t recognize this (yet) but we must show due diligence and properly design for it.


• First: consider what the Zeners are doing

• Clamp LCD voltage back to VCC

• Handle power from VCC through all available paths


• So what if we need to do this to supply more power to LCD? Back to unprotected path!


• So now... Ry is single path entry from the other side, so can be minimum value first designed if isolated.

• BUT we must ensure D1 and D2 have sufficient bias current to be on.


• Power sharing is possible and could start with three parts

• But diodes won’t share equally

• Requires agency approval and test

• Did this once and got about 80:20 split


• Rule of thumb: if you can fit 3, put the pads in but design for / populate two

• Don’t forget they work in the reverse direction, too, so you get free negative voltage clamping to the regular diode forward voltage

End of part 2

Documents

Fundamentals Part 2 › emcfastpass › IS_hardware › ... · Assignment Solution IS Parameters: •Voc = 4.2V (battery) + 1.34V (COSH sensor assuming Alphasense) = 5.6V •Capacitance