No current sense resistor required for MOSFET switch protection

I've got a lot of mileage out of sleazier tricks than that one!

Cheers

Phil Hobbs

One-offs can be tweaked to work. My stuff has to work 100's of thousands of times out of the box.

I do prefer current-limited games, as opposed to loops which can be real squirrelly to get stable. But these can be done quite precisely if one plans ahead. ...Jim Thompson

Is designing to the min/max specs on a datasheet a "sleazy trick"?

Is adhering to a rule somebody taught you 50 years ago a virtue?

John

So this is your crusade to keep a tight lid on this one?

Maybe I'm a life jacket salesman.

More likely an amateur with a silly circuit.

John

That's fine. In my non-pick-and-select world, beta generally can range over 8:1 when you account for process corners and temperature (and sometimes Early effect). ...Jim Thompson

Silly and doesn't work. Pray for flame. It can be quite educational :-) ...Jim Thompson

I don't think it's particularly sleazy, no--since it's guaranteed by the data sheet, it ought even to be OK for Joerg's crowd. I build all kinds of things that won't work if the beta is out of spec, though usually it's noise and offset currents that I have to worry about rather than big bias shifts.

My only teensy worry about yours is what its behaviour looks like at the corner cases--it has only 300 mV V_CE nominal, which is pretty near saturation. Saturation makes the beta go through the floor, so the collector current could drop pretty badly if the LM1117 were near its lower spec limit for output voltage. (I haven't read the datasheet, but I sort of doubt that the beta is guaranteed at 300 mV V_CE.)

The point I was making in my first post wasn't that it was a bad plan for biasing a voltage reference as in your circuit, just that nobody in his right mind would do it in the source of a power MOSFET in a SMPS.

It does meet oparr's description, though, and I wouldn't be surprised if his circuit were something like that--the TL431 would be to keep a constant voltage across the base bias resistor. He's cagey enough about it that either (a) he thinks it's a genuine crown jewel or (b) he doesn't want to get his tailfeathers singed by bragging about a kluge.

Cheers

Phil Hobbs

I haven't the faintest idea as to what my "circuit" is being likened to. This thread is a followup (sort of) to an earlier thread "High side current monitor with latch" in which Joerg gives me some advice (albeit old school) regarding MOSFET protection.

After throughly investigating the characteristics of MOSFETs, it dawned on me that they are probably the easiest of all transistors to protect. It also dawned on me that others, including Joerg, were probably aware ot this already but have decided to keep this to themselves. Hence this thread.

It is my opinion, based on both characteristics and test results, that a MOSFET can be operated very close to its absolute maximum thresholds, without damage, when used in conjunction with a single small signal BJT. Not being one to ever give anything on a silver platter (too expensive), that's about all you need to know from me.

Furthermore, I'm c>

I think you grossly overestimate your own competence. And I will maintain your competence rating at "Dubious" until you prove otherwise.

It was probably a severe goof-up on my part to allow you thru my hotmail rejection filter, I perceived you initially as asking for help on a real technical question.

Further observation proves you to be a blow-hard. ...Jim Thompson

Put your sour grapes to good use....A new exotic w>

How many "red herr>

You've presented nothing of substance to "keep a lid on".

You nothing more than a severely egotistical j*rk-off.

I'll remove you from my hotmail whitelist, so don't bother to continue to j*rk-off, I won't see your response. ...Jim Thompson

What! Me worry? We're just playing mind games with each other. I have something some want and another set hoping and praying that I keep my mouth shut.

All out of silver platters.....Sorry!

Well it a>

Old car-battery lead platters, more like.