Hi. I undertstand the 2 zeners at 12 volts are because the voltage on the gates would be higher and you need them to keep it at 12 volts. But lets say the transformer gives less than 300V. Wich would be the minimum voltage the transformer should provide in order to have at least these 12 volts at the gates of the MOSFETS?

EDITED: I know the zeners are to guarantee there is no more than 12 volts at the gates, but also I think there is a minimum voltage requiered at the gates for appropriate operation. So the question is, how low could be the voltage on the transformer to let the mosfets operate properly? I mean, if I go below 300 volts there will be a point that the voltage on the gates won´t be enought. How low can I go?

Thank you

best regards

 

Wich would be the minimun voltage the transformer should provide in order to have at least these 12 volts at the gates of the MOSFETS?

You're thinking about this wrong. The zener diodes are to prevent the maximum Vgs from being more than 12V, thereby (conservatively) protecting the gate oxide from breaking down. If you happened to use a transformer that produced voltages less than 12V, the zener diodes would have nothing to protect, but wouldn't affect operation negatively.

BTW, that schematic is crap. No component designators, too many humps on the resistors (and not drawn symetrically), unnecessary wire jogs, and the MOSFET symbols aren't drawn correctly. There's no polarity arrow and the gate is drawn incorrectly. It should be dashed to indicate an enhancement mode device. When hand drawn, it's typical for the gate to be drawn as a solid line.

 

word is minimum, not minimun

zeners are there to progrect gate so that Vgs does not exceed 12V. in normal operations those voltages are lower, perhaps 3-4V.

 

Yes, I undertand it is to protect from higher voltages. I´m going to edit the post to clarify that.

But I understand there is a minimun value to drive the gates of the mosfets. So, how low could the transformer go to provide the miniun requiered voltage in the gates?

 

I´m going to edit the post to clarify that.

You shouldn't edit it in a way that makes the responses to the original text seem clueless. That would be rude.

But I understand there is a minimun value to drive the gates of the mosfets. So, how low could the transformer go to provide the miniun requiered voltage in the gates?

It depends. Vgs(th) is up to 4V.

BTW, your misspelling of minimum was already pointed out, but you still managed to misspell it 2 different ways; and you misspelled required. Also, MOSFET is an acronym and should be all caps.

 

again... it is spelled minimum, (with "m" at the end...).

i do not understand what you are trying to get. you can simulate circuit. and you still did not even mark the components.

 

here you go:

 

suppose Vgs is 10V (easy math), then setting RV1 to 1.8k would mean output is 15V (Voltage across R7 is same as Vgs and voltage across RV1 is half of that since value of RV1 is half of R7).

with no significant load current, voltage drop across R2 is small. btw this is always small ( <=3.2V even at max load) because Vbe of Q3 is about 0.6 - 0.7V and Vd6 is 2.5V. lets call that 1V for now. in this case voltage at gate of Q1 is 15V + 1V + 10V = 26V.

 

So the question is, how low could be the voltage on the transformer to let the mosfets operate properly? I mean, if I go below 300 volts there will be a point that the voltage on the gates won´t be enought. How low can I go?

It depends on the output voltage range and the current required.

You misspelled MOSFETs and enough.

 

You are try to use a circuit that was designed for 300V with less than 12V?

 

300VAC rectified and filtered is going to be 422.5VDC
(300V - 0.6Vf x 2) x 1.414 = 422.5032VDC when filtered.

Edited to correct the math.

 

values show that this is for low current. judging by current limiter, this is never supposed to go over 100mA so max Q1 dissipation is some 40W which is a lot but ok for linear regulator.

btw. not sure why people insert current limiting circuits into voltage feedback loop. this creates output that changes with load an 0-3.2V is rather large swing.
something like this uses (almost) exact same parts but does not have that problem. the difference is that Q3 need to have high Vceo voltage, spomething like PBHV8560Z. and one could add a red LED into Q3 collector circuit to show that current limit is tripped.

 

300VAC rectified and filtered is going to be 420.8VDC
(300V-0.6Vf x 4) x 1.414 = 420.8064VDC when filtered.

The diode drop is only times 2.

 

300VAC rectified and filtered is going to be 420.8VDC
(300V-0.6Vf x 4) x 1.414 = 420.8064VDC when filtered.

Nope. The diode forward voltage drop (Vf) is not multiplied by root2, it is subtracted.

Assuming the 1N4007 Vf is 0.9 V at 1 A (Fairchild datasheet, 2003), then:

(300 x sqrt2) - (2 x 0.9) = 422.464 V

ak

 

not sure why people insert current limiting circuits into voltage feedback loop.

Why do you say that?
TS's circuit does not have the current limiter circuit in the feedback loop.
The sensed voltage is by RV1 and R7 (post #12) across the output to the load, which is not affected by the voltage drop across the current sense resistor R2.

 

Thank you for your answers and help.

Sorry, the circuit is not mine. Also english is not my language, excuse me for the typos, I´ll be more carefull.

The question is, if the voltage of the transformer is less than 300V, on the gates Vgs it could be too low to operate the circuit. It just for try the circuit below it´s designated voltage

 

Aside from all of the critical remarks, the way to determine the minimum gate voltage required is to use the data sheet curves. Certainly reading the data sheet is indeed tedious, BUT the actual product data sheets have a whole lot of useful information, that includes very useful graphs that show (usually), resistance, or conductance, versus the gate voltage, at different temperatures.
So my very serious answer is to download the manufacturer's data sheet pages.

As for that actual circuit, my question is what is the intended application?? And why that particular circuit??
It is a power supply of some kind, and my guess is that the goal is to provide some amount of current at some voltage.
It will help some of us to provide better answers if we know what the intention actually is.

 

Why do you say that?

you are correct of course... my attention is divided and was not paying attention. then post was done on "cruise control"

 

if the voltage of the transformer is less than 300V, on the gates Vgs it could be too low to operate the circuit.

Off hand, it would appear that circuit will work down to near the maximum Vgs(th) of the MOSFETs (4V for the IRF840).

How low do you want to go?

 

The diode drop is only times 2.

OOPS. Yes. Correcting that now.

300VAC rectified and filtered is going to be 422.5VDC
(300V - 0.6Vf x 2) x 1.414 = 422.5032VDC when filtered.

Edited to correct the math.