mosfet load switch

[MikeMl]

The arrow as shown is for a "P" channel. The arrow would point towards the gate if were a "N" channel.

**broken link removed**

No. For a PFET, the diode cathode is always closest to the source lead! The diode symbol should always be reversed biased. For the correct placement of a PFET in the OP's circuit, the diode cathode should be in the lead closest to the + supply terminal AND the diode should point toward the + supply terminal!

 

[indulis]

PFET (P channel JFET) and P channel MOSFET symbols are completely different.

The original post has, without question, a P channel MOSFET symbol, in fact it’s shown as an enchantment mode P channel MOSFET.

A "P" channel FET will have the arrow on the gate pointing away from (out of) the gate, whereas a N channel's gate arrow will point towards the gate.

In a P channel MOSFET, the body diode anode is the drain and the cathode is the source, so the source should have been connected toVin.


JFET symbols:

 

[MikeMl]

ALL ABOUT CIRCUITS has no credibility with me; THEY CANT even figure out which direction CURRENT FLOWS! Look at this data sheet from International Rectifier. This is the way industry does it.

 

[andy257]

Mike i found your cuircuit did the job but as Hero pointed out depending on the amount of current drawn alters the switch on slightly. It actually improves it in my case because to get a 9V from standard resistor values is impossible.

Crutschow - I still cant visualise how the zener is acting like a 9v voltage source. I understand the gate must be -ve in relation to the source pin plus the threshold. In my case around 10 - 11V. I cant see how the zener pulls the gate low when conducting which is why i thought it suddenly went low originally.

 

[indulis]

That IR data sheet agrees exactly with what I said...

The arrow as shown is for a "P" channel. The arrow would point towards the gate if were a "N" channel

In a P channel MOSFET, the body diode anode is the drain and the cathode is the source, so the source should have been connected to Vin.

As shown in the original post, the P MOSFET is in fact backwards (drain and source need to be swapped) and, as shown, the body diode is conducting...

 

[MikeMl]

Mike i found your cuircuit did the job ...

Which one, the one with the LM431?

 

[andy257]

Yeah, using the LM431. It behaves differently depending on the load i use. With a simple led on the output i was getting pretty close to 9V switch on. when i used a larger load in the order of amps it alters the switch on.

 

[indulis]

The zener is nothing more than a regulator... as Vin increases the gate source voltage will be about the same (MOSFET off). As Vin becomes greater than the zener voltage (9V) the gate will be held there and source voltage will continue to increase until the gate is sufficiently below the source and the MOSFET will start to turn on. When Vgs is lage enough the MOSFET will be hard on. How hard it is on will depend on Vin.

 

[Hero999]

The arrow would be pointing the other way if it was a PFet symbol.

Both of the transistors in that schematic are P channel.

You are wrong, indulis is correct, the original poster use the right type of transistor with the correct symbol, the drain and source connections were wrong.

The transistor which you added to the schematic has the correct drain and source conections.

 

[MikeMl]

Yeah, using the LM431. It behaves differently depending on the load i use. With a simple led on the output i was getting pretty close to 9V switch on. when i used a larger load in the order of amps it alters the switch on.

I re-simed the circuit with different load resistances, ranging from 4Ω to 40KΩ, and I see no sensitivity to load resistance. Are you sure you have it hooked up as shown here?

 

[andy257]

Hi Mike,

i did not use the IRF7204, and i changed the 2.2M feedback resistor to a much lower one because i wasnt getting the snap i required. I think i changed it to something like 50k. The switch off voltage is very snappy, but the switch on voltage still seems a little slow.

 

[indulis]

Most TL431's like to have at least 1mA of cathode current...

 

[MikeMl]

What is the threshold voltage (Vt) of the PFet you substituted>

 

[andy257]

Hi Mike,

Can you post the model for the tl431 used in LTspice.

thanks

 

[MikeMl]

Hi Mike,

Can you post the model for the tl431 used in LTspice.

thanks

Here it is. Unzip it into a new subdirectory, and have fun with it.

 

[andy257]

must be a problem with the file becuase it says its invalid? Downloads fine but wont unzip.

can you repost

thanks

 

[MikeMl]

must be a problem with the file becuase it says its invalid? Downloads fine but wont unzip.

can you repost

thanks

I downloaded it, unzipped it and ran the simulation. I'm using an older version of WinZip

 

[andy257]

Ive tried both my home pc and work pc and both say the file is invalid/corrupt.

 

[mneary]

I'm using WinRAR 3.50 which has no problem with it.

Try deleting it and downloading again.

 

[andy257]

Hi All,

Is there a formula to calculate the hysterisis values?

What i find is that when i have a load connected to my circuit as soon as the threshold value is reached the mosfet turns off but not fully. When the load is disconnected the voltage at the gate actually increases thereby turning the mosfet back on. I have not put a scope on this yet but i would guess that is probably oscillating.

The circuit is that which mike has posted in previous threads (TL431) driving a P mosfet as a high side load switch. The circuit is being powered from a cap (acting as a backup battery). Once the voltage on the cap drops to a determined level the load should be disconnected.

What i would like to do is be able to define a turn on voltage and a turn off voltage. However i have to give the off voltage sufficient hysterisis to prevent it turning back on which is why i would like to be able to calculate it. Due to the amount of current the load pulls there are losses to take into account which alterts the threshold.