Hint for @Ash

You can use CMOS Schmitt triggers when you don't need precision thresholds. That saves on component count/complexity significantly.

Typical chips: 74HC14 (in 5V logic), 4093, 40106, 4584

CD4093 is 2-input NAND, so it can perform some useful logic functions at the same time.

Medved :

The PNP transistor is pretty much what is done in my 2nd circuit (P-MOSFET here, same idea)



Driving everything with the comparator outputs directly would mean connecting everything to the same point. With open collector comparators this is definitely a problem, as the intermediate voltage on the dividers etc will appear on the gates of the MOSFETs that shut down signals. To make this work at all, voltages of the dividers would have to be carefully considered vs. the MOSFETs Vgs th.... More design constraints and worse reliability

With push/pull comparators this could work, but i dont see a justification

Small signal MOSFETs are cheap, use them to break up everything to separate lines and invert as necessary, each with its own simple design requirement, resulting in fewer constraints and compromises in the design



The protection logic paths have to remain independent, because their outputs control different things in the circuit :

 - Undervoltage and overvoltage shut down all operation modes. They short the gate drive to the MOSFETs (or if a gate driver IC is used, its inputs or enable input)

 - EOL lamp only prevent the 50% opreation - Full power with the remaining lamp is still allowed. So it shut down the switching generator IC only



RRK :
Here i do want some moderate precision for the UV and OV protections. Since i already have Vref and comparators for that, i can as well use the additional comparators in the chip for the other functions



You all may be interested to see the following part of some other project i am currently working on

@Ash

I see. As a fresh idea for your design you may consider using modern capacitive isolators instead of optocouplers. Very convenient, good isolation ratings, logic I/O on both sides, saves a lot of components/board space. Original TI's ISO series used to be expensive, now every major Chinese manufacturer like Chipanalog or 2Pai seems to have an inexpensive clone With some versions, there is even a luxury of having a tiny isolated DCDC converter right on a chip, be careful at layout though, as they work at really high frequency, some 20-50MHz!

I dont use those in my designs. Not after i seen one of them in some other commercial product short between the 2 isolated sides after one side got burned out (from an external applied vdd reverse polarity)

It can be expected that the chip will burn out, but an optoisolator would never short through the isolation

Are you sure you are talking about capacitive insulators, and not some related technologies like Analog Device's magnetic couplers? Not sure about Chinese clones of course, but at least TI's parts consist of two chip dies, connected with some tiny wires inside the case. If you burn one die, the second will stay intact, and so will the second pair of insulating capacitors.

More, many times the insulation is not employed as a safety-related feature, but mere as a means to break ground loops or to protect serial ports like RS-485 or just to shift ground voltage levels. In that case, even if you burn the insulator there is no significant safety risks.

And you can of course employ a sacrificial 5V TVS on the power bus nearby to reduce the chance of this happening.

If an optocoupler is abused to the point that the case is charred, one can expect the insulation is compromised too. Though, TBH I have not seen this happned before in practice, even if semiconductors nearby naturally exploded ) Probably can happen with some intelligent high-speed parts having extra +3.3/+5V power to the receiver side like 6N137-139.

Back to topic, browsing through the parts for work I found out that thermal fuses for some high temperatures (say 150+ degrees C) and high current of 20+ Amps are easily available. So making a ventilated metal box with a couple of 100W resistors (some power margin) and strategically placed thermal fuses will solve the problem cheaply/easily/safely and with zero worries about the interference to car electronics. Added benefit is that you are not limited to 100/50% power ratios.

https://www.setfuse.com/Products/Over-Temperature-Protection/Thermal-Link-OTCO-Organic-Type/RP-series.html



 

That isolator was Si8641, rated as basic insulation (it was in a SSOP package. The same component in a SOIC-W would be rated as reinforced insulation). It was burned internally without visible external damage. Showed some medium resistance on a megger between the sides

(And dead short between VDD and GND on the side that was burned out, but this can be expected)



For your resistor suggestion :

Connect the resistor from the minus side, and route the lamp wires such that they cannot short to each other (so the wires come right up to the lamp holder from 2 sides, and not in one cable). This will reduce significantly the chance of full 12V getting to the resistor

Protect the circuit by a motor overload breaker set to the precise current - like PKZM0-25 or similar

For the resistor construction :

 - As the resistor element use a packaging steel strip. Use the thinnest possible so it have high resistance/unit length. The element will be of large length (about 4m if folded by two in a 2m long duct)

 - The resistor is installed on busbar insulators.

 - Cable duct made of metal with ventilation slots as the outer housing. Install a unit of ~2m length (as much as can fit) under the car

It may be possible to fit 2x or 4x the resistor element running in parallel along the duct. The 4x configuration is the best here, as it allows to fit 2x 4m long resistors (so the resistors for both lamps) in a single 2m long duct

The heat dissipation <100W in a 2m long metal duct is not much, it is not going to get to any high temperatures

Okay, the devil is in the details. Skyworks does not appear to to be overly honest on which parts are certified for which purpose in their datasheet. Well, even Chinese friends put it more clearly! For QSOP part the isolation rating is very minuscule (down to 1kV!) so it will not qualify and will be plain dangerous even for basic insulation, which requires a mandatory ground connection anyway.

We of course are using wide-SOIC reinforced 5kV class chips for anything between live and low-voltage circuits, SO8  for functional insulation only.

Still, you are right that digital insulators are at somewhat elevated risk of getting shot-through on a several abuse. TI even has a special application note on this.