Use IGBT or power MOSFET to replace 30A automotive relay?

[vl78]

Hi everyone,

I'm a student in college and I'm working on the electronics for my project team. I would like to know if anyone has any knowledge regarding the replacement of relays with either IGBTs or power MOSFETs?

I would like to replace 30A automotive relays for 12-14 volts DC.

I've attached a hasty DXP screenshot of the schematic for one of the relays. This is the main relay and goes out to the injectors. I know they draw a lot of current instantaneously when they are open, but I'm not sure how to measure how much that is - if someone could help me out with that too, I would be so greatful!

Thanks!
Vivian

 

[ClydeCrashKop]

Take a look at this topic.

https://www.electro-tech-online.com/threads/question-about-headlights-relay-in-car.131751/

 

[3v0]

You might do some looking and find the same parts now being used for this purpose on modern cars. They will standup to the automotive electrical system and should not be too expensive because they are produced in great quantitative.

 

[shortbus=]

When you say "injectors" do you mean fuel injectors? If so, they are normally driven with a low side switch. The plus voltage is constant on them and switched to ground. Relays are not even close enough to being able to switch fast enough. There are a lot of "injector driver" ICs available, just Google.

 

[vl78]

Hi,

I'm really sorry! I think I must have been unclear - I meant that this relay simply gives power to the entire car. The injectors are driven with a low signal from the ECM and are powered off of a fuse from the main relay. I wanted to replace the relays because I think they are unnecessarily bulky and I had read that IGBTs are good for high voltage, high current, low switching frequency applications, which is what I want. I found this:

https://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00182201.pdf

Does this look like it would suit my needs? And if so, how would I wire it? It has a gate, an emitter, and a collector, but I am not sure I know what those are.

Thanks!
Vivian

 

[alec_t]

Although relays are bulky they are rugged. You may not save much space by using IGBTs or MOSFETs because those would probably need heatsinks plus ancillary components to protect them from voltage spikes.

 

[shortbus=]

Plus an IGBT or mosfet in a high side circuit will need a boot strap circuit and a charge pump circuit. A continuous high side switch has a many pronged need. There are many sizes of relays available, maybe look into the different types of relays. Just be sure to choose one that is automotive rated.

 

[ClydeCrashKop]

Plus an IGBT or mosfet in a high side circuit will need a boot strap circuit and a charge pump circuit.

I found this SUP90P06-09L-E3-ND MOSFET P-CH 60V 90A TO220AB with Logic level input. If using P channel MOSFETs are as easy and obvious as N channel, as suggested in this thread https://www.electro-tech-online.com/threads/question-about-headlights-relay-in-car.131751/ why are we using N channel with boot strap circuits and a charge pump circuits?

 

[vl78]

Although relays are bulky they are rugged. You may not save much space by using IGBTs or MOSFETs because those would probably need heatsinks plus ancillary components to protect them from voltage spikes.

alec_t, how can I figure out whether I need heatsinks?

shortbus=, I do not think I am trying to make a high side circuit. I think I mentioned that the injectors are given a ground signal from the ECM, so I think ClydeCrashKop's question is my question - why would this project require an nMOSFET with bootstrap circuits and charge pumps if the thread suggests that using a pMOSFET would be as simple as using an nMOSFET?

Thank you for your responses so far everyone! I am learning a lot!

-Vivian

 

[alec_t]

how can I figure out whether I need heatsinks?

Calculate the peak load current and multiply by the voltage drop across your switching device (BJT/FET) at that current to get the peak power dissipated by the device. If it's more than a fraction (1/2?) of 1W you'll probably need a heatsink (depending on how brief the peak is). The datasheet for the device will tell you the maximum allowable device temperature and you can use that figure, plus heatsink specifications, to decide what heat-sinking is needed.

 

[shortbus=]

Hi,

I'm really sorry! I think I must have been unclear - I meant that this relay simply gives power to the entire car. The injectors are driven with a low signal from the ECM and are powered off of a fuse from the main relay. I wanted to replace the relays because I think they are unnecessarily bulky and I had read that IGBTs are good for high voltage, high current, low switching frequency applications, which is what I want.
Vivian

Vl78 and Clyde, why I said a high side switch is how I interpreted the above post. I was under the impression that you were supplying the high side rail of the injectors. Replacing a relay with a mosfet or IGBT. All fuel injectors that I'm aware of use low side switches (from the ECM) to switch the injectors, with a high side rail that is always on with the ignition switch.

Guess I should have read Clyde's link. As a side note though, there must be a reason that the OEM still uses relays for this application.

 

[tcmtech]

Typically in modern vehicles the key switch itself is not a high current carrying device any more. It instead turns on a number of high current relays mounted in other locations throughout the dash and in the engine compartment.

By doing it this way less high current wire is need throughout the vehicle as was needed in the older vehicles that switched most of everything directly through the key switch itself.

That and today's vehicles have far higher peak current demands as did older vehicles which makes it necessary to divide the loads up using multiple relays instead of one very heavy main switch.

 

[vl78]

Typically in modern vehicles the key switch itself is not a high current carrying device any more. It instead turns on a number of high current relays mounted in other locations throughout the dash and in the engine compartment.

By doing it this way less high current wire is need throughout the vehicle as was needed in the older vehicles that switched most of everything directly through the key switch itself.

That and today's vehicles have far higher peak current demands as did older vehicles which makes it necessary to divide the loads up using multiple relays instead of one very heavy main switch.

tcmtech, yes, we are powering several other relays off of the main relay. I am not sure how this changes whether I would like to replace relays with IGBTs/MOSFETs?
alec_t, thank you for the explanation!
shortbus=, that's okay. I am sure there is a reason, but I also don't see why I shouldn't try this out.

Okay everyone, here is a question: there are two combinations that I think would work, but I am not sure if I completely understood the role of each of these devices...

1. IGBT

low side driver

2. power MOSFET

low side driver

Merry Christmas everyone
Vivian

 

[tcmtech]

So what exactly are you trying to gain by doing this anyway?

You wont gain anything on the weight saved or the power saved and not likely on the life expectancy of the devices either.
Plus if given a sufficiently high enough voltage spike IGBT's and Mosfet tend to go short circuit full on which could present some serious problems if critical devices can not be shut off where as relays ten to die in a open circuit mode and will not turn on what they are supposed to.

 

[vl78]

tcmtech, why won't I save weight? I don't see how using a combination of those two small removable components won't save weight over a removable automotive relay.

Vivian

 

[tcmtech]

And what is the 2 -4 ounces saved from a dozen or so relays in your car going to improve?

Vacuum your floor. Don't put that last cup of gas in the tank when you fill up, run the engine a half quart low on oil, don't fill the window wiper fluid tank all the way, Take a laxative an hour before you drive, tell your fat friends and family to walk!

I can name more things that will cut far more weight off your car over what a few relays will ever add up too so the rationality of the weight savings argument seems pretty lame to me!

So once again what are you really after here?

 

[vl78]

tcmtech, I'm not sure that you read my first post at all... I am on a project team. This is not anyone's personal car. This is a racecar. We are cutting weight from every subsystem of the car, including the electrical system. The only portion of the system we could possibly save weight from is the fusebox. If you don't want to help, it's okay, but please don't start ridiculing me without reading what I had previously posted. It is very counterproductive.

Vivian

 

[tcmtech]

Fair enough what other sub systems are you planning to use solid state switches on and what are their average and peak current loaded they can draw? ?

BTW a typical IGBT has around 1 volt or slightly higher voltage drop across it at full conduction.So from there can your sub systems work effectively as needed with slightly less voltage opposed to full system voltage?

Second on the weight aspect at that 1 volt drop and at a rather low 5 amp load that IGBT will still be needing to dissipate 5 watts which will require a heat sink of some fashion which means that either the IGBT has to be solidly mounted to a part of the vehicle that can dissipate that heat or it needs a heat sink which even if made of aluminum will make its total weight and physical size greater than that of a basic cube type power relay.

I did a bit of online searching and using a standard 12 volt 30 amp cube type relay as the base line I found they weigh in at around 25 - 40 grams (~.85 - 1.6 Oz) and use between .25 and 1.5 watts peak power to operate. The real kicker after that is that with a solid state based device only dissipating a mere 5 watts a correctly sized heat sink even if being ran at 60+C over ambient temp will weigh more than 45 grams alone!

Upon further research into power Mosfets and any variation of them capable of working at 60 volts or higher and a honest 30 amp continuous duty current carrying capacity I can not find any combination of devices that would make a full electrical equivalent to a SPST 30 amp rated relay that would be equal or less than the 25 gram units I found at Digikey and match or exceed it in performance, function, or realistic durability in a automotive service type application.

I could be wrong though.

 

[vl78]

tcmtech and everyone else who replied, thanks a lot for all of your help! I learned a lot of things in my little journey of trying to force something to work... but in the end, you were right tcmtech! I decided it would be best for us to use small 30A automotive relays with PCB mount sockets. They have been around for a long time, and reliability and safety need to come first! I hope everyone has a great New Year's Eve tonight!

Vivian