High Current DC Switching

[abuhafss]

Hi

I am looking to build a high current SSR to power on a 12V Starter Motor which could extract more than 50A.

Here is my idea:


Is it okay? Of course, a suitable mosfet to be selected.
Or do I have to consider some other facts?

 

[(*steve*)]

No, that will never turn off.



This will work better. R1 1k, R2 1k D1 1N4001, Q1 & Q2 2N3904

 

[Bluejets]

Use the solenoid designed for the job. Starter motors if the direct drive can draw up to 400 amps at start depending on the load.

 

[abuhafss]

Use the solenoid designed for the job. Starter motors if the direct drive can draw up to 400 amps at start depending on the load.

The actual problem is that the solenoid's winding become hot and after sometime would get burned.

 

[abuhafss]

No, that will never turn off.

This will work better. R1 1k, R2 1k D1 1N4001, Q1 & Q2 2N3904

Would you please be kind enough to explain why it won't turn off, though the simulation is doing it perfectly.

Your suggested schematic would switch on the mosfet when there is no control voltage on the base of Q2.

 

[Bluejets]

The actual problem is that the solenoid's winding become hot and after sometime would get burned.

So what do you think will happen to a 50A mosfet then?

 

[(*steve*)]

Would you please be kind enough to explain why it won't turn off, though the simulation is doing it perfectly.

The simulation shows it turning on, not off. There is no path to ground to discharge the gate.

Also, you're using the transistor as an emitter follower, which is not ideal.

Your suggested schematic would switch on the mosfet when there is no control voltage on the base of Q2.

You can use another transistor to invert the input. It may be easier to use a proper gate driver.

 

[(*steve*)]

So what do you think will happen to a 50A mosfet then?

*Poof*

Large amounts of smoke followed by the MOSFET failing short circuit. Possibly a subsequent failure involving melting the leads allowing the circuit to open.

 

[abuhafss]

The simulation shows it turning on, not off. There is no path to ground to discharge the gate.

Also, you're using the transistor as an emitter follower, which is not ideal.



You can use another transistor to invert the input. It may be easier to use a proper gate driver.

Got it, thanks.

 

[abuhafss]

So what do you think will happen to a 50A mosfet then?

The solenoid's winding doesn't get burned because of the 50A+ current! That current is extracted by the starter thru the solenoid's contact terminals.
The solenoid's winding current is only 3+ amperes.
In my circuit (after improving as per Steve's suggestion) if I use a 100A mosfet, wouldn't it do the job?

 

[abuhafss]

By the way, I remember, I read somewhere using a few Highside switches in parallel to switch high current load. But I could not find any schematic. Any tips or starting point?

 

[Bluejets]

In my circuit (after improving as per Steve's suggestion) if I use a 100A mosfet, wouldn't it do the job?

That depends on your application.
As we cannot see what your application is, then it is difficult or rather, almost impossible to comment.
What has been commented on until now is what you have told us, i.e. 12v starter motor and any advise given has been in relation to the general use of the same.

 

[abuhafss]

I have already mentioned it. It's just to switch on the starter motor briefly.

 

[(*steve*)]

In my circuit (after improving as per Steve's suggestion) if I use a 100A mosfet, wouldn't it do the job?

The problem is that the peak current may be well in excess of what you're expecting. Measure the DC resistance of the starter to determine the peak current.

Then, if it will exceed the maximum current for a single mosfet, you will need to connect multiple in parallel.

A problem you then have is switching the MOSFETs both fast enough to avoid them dissipating so much power while they're switching that they die, but also switching them at exactly the same time so that the load isn't carried by just one of the MOSFETs.

 

[davenn]

In my circuit (after improving as per Steve's suggestion) if I use a 100A mosfet, wouldn't it do the job?

until you get the 200+ amps draw by the starter

 

[davenn]

here is a true SSR
150A continuous; max continuous with forced air cooling 225 A, 275 A with a 5% duty cycle

http://www.hellroaring.com/SSR-15275-30V-12C.PDF

 

[AnalogKid]

Referring to the schematic in post #1, you can solve the turn-off problem with a 1K resistor from the emitter of Q1 to GND. This preserves the logic polarity of the original design. The current through Q1 is only 12 mA, so the fact that the voltage drop across an emitter follower is slightly higher than that of a saturated switch should not be a problem. Vgs will be above 10V, a typical value for full enhancement of a power MOSFET.

ak

 

[Bluejets]

So to get things clear, you are switching a solenoid, not the starter motor. And it's now just a short time, not like previously where you said it overheats after sometime?

 

[(*steve*)]

This preserves the logic polarity of the original design

But for high current switching the MOSFETs will probably linger in the saturation region long enough to cause some serious problems.

Given that it's only turned on and (presumably) off once, you can get away with much slower switching, but you wouldn't want to have any contact bounce!

Of course all this depends on the actual load being switched. I think he wants to switch the starter, but it sounds like this was prompted by some failure of the starter solenoid. As much as I like a solid state solution, starter solenoids are a known quantity.

 

[duke37]

I would think that a FET solution would be cheaper than a solenoid but the car makers use a solenoid. There must be a reason for this.

A dismantled solenoid will show how severe the duty is.