Voltage rating of p-channel MOSFET

Tahmid · 2nd July 2008, 07:03 AM

Hi,
I had a confusion on the voltage rating of p-channel enhancement MOSFETs. I have collected p-channel MOSFET :IXTH11P50 with -500v Vds from IXYS semiconductors. But in the datasheet it is mentioned that it has a Vgs of +/- 20v.
Since it is p-channel, current will go in through the source and out through the drain once the gate is pulled low. Here, both drain and gate are at ground level, so doesn't the MOSFET have to have a Vgs of -500v to be used at 500v?
I tested with 12v and 20v and the p-channels worked fine, but with 200v/300v they were not working properly, rather, shorted, with no gate control.
Why is this? If so, why is it mentioned -500v Vds in datasheet?

All help would be appreciated in this regard.
Thanks.

Eva · 2nd July 2008, 07:21 AM

Everything is right except your assumptions about how P-channel works. Think further

Tip: It's always Vgs what drives a MOSFET.

Tahmid · 2nd July 2008, 07:23 AM

Hi,
I did not quite understand what you said. Could you please elaborate on what is wrong?
Thanks.

areza · 2nd July 2008, 11:26 AM

working principal is same for both p and n channel fet, its the supply polarity changes to, +500v become -500v and +20v become -20v, so GND become +500v, +20v/0v will turn off the p channel , - 20v will turn it on, only n channel is superior then p channel,

Tip 2 , think again

Tahmid · 2nd July 2008, 11:30 AM

Hi,
Is the -20v in reference to source or drain? (I think its to source as it is Vgs). But, if in reference to source, which in this case in positive, wouldnt -20v simply mean 20v in reference to drain?
Thanks.

Ouroboros · 2nd July 2008, 01:42 PM

No!
A P-channel MOSFET is connected just the same as an N-channel one, expect that all the voltages are -ve with respect to the 0V line.

So you have a 0V (local ground) that is connected (say) to the source. The drain is connected through the load to the -ve power rail (perhaps at -50V), and the important thing is that the gate is still driven with respect to the source, but is also at a negative potential. That is, you might have 0V on the gate (w.r.t source) to turn the FET off, and -10V (w.r.t. source) to turn it on.

Do a search for 'MOSFET complimentary source follower' or something like that on Google. I'll sure you'll find the relevant simple schematics showing you how things are connected.

S.G. Mahbub · 2nd July 2008, 01:47 PM

Tahmid,
You are learning / trying to learn very fast. I appreciate your inquisitiveness.

Thanks again for your million dollar confusion. The answer is very useful for all mosfet users and I am sure, most of them will be benefited from the solution.

Data sheet is alright. vgs and vds ratings are also correct. Thing is that, you have to know the working mechanism of P- Channel Mosfet.

The rating for Vgs is correct and means that with the source at 500V, you cannot pull the gate to ground. You can use the P-channel at up to its rating (500V in this case; although, I tend not to go the maximum ratings). The point is that the gate cannot be pulled more than 20 V below the source. Thus, you need some way to level shift its drive.
Eva is correct- it is always vgs which drives a Mosfet. So, think about Level shifted PMOS Gate Drive. That will give you the solution. If you fail, solution will be provided to you.
Thanks

Tahmid · 3rd July 2008, 08:10 AM

Thank you,
I got the point and am trying with level shifting method. I hope to make it soon and hope that my remaining p-channel MOSFETs will not blow up.

2nd July 2008, 07:03 AM	#1
Tahmid diyAudio Member Join Date: Jun 2008 Location: Dhaka	Voltage rating of p-channel MOSFET Hi, I had a confusion on the voltage rating of p-channel enhancement MOSFETs. I have collected p-channel MOSFET :IXTH11P50 with -500v Vds from IXYS semiconductors. But in the datasheet it is mentioned that it has a Vgs of +/- 20v. Since it is p-channel, current will go in through the source and out through the drain once the gate is pulled low. Here, both drain and gate are at ground level, so doesn't the MOSFET have to have a Vgs of -500v to be used at 500v? I tested with 12v and 20v and the p-channels worked fine, but with 200v/300v they were not working properly, rather, shorted, with no gate control. Why is this? If so, why is it mentioned -500v Vds in datasheet? All help would be appreciated in this regard. Thanks.

2nd July 2008, 07:21 AM	#2
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	Everything is right except your assumptions about how P-channel works. Think further Tip: It's always Vgs what drives a MOSFET. __________________ I use to feel like the small child in The Emperor's New Clothes tale

3rd July 2008, 08:10 AM	#8
Tahmid diyAudio Member Join Date: Jun 2008 Location: Dhaka	Thank you, I got the point and am trying with level shifting method. I hope to make it soon and hope that my remaining p-channel MOSFETs will not blow up. __________________ The tallest building in the world starts from the first brick. পৃথিবীর সর্বোচ্চ দালানও প্রথম ইট বসানো থেকেই তৈরি হয়েছে।

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
voltage rating for Ci?	rfenergy	Chip Amps	10	21st June 2008 09:24 AM
Using a cap above its voltage rating?	h_a	Parts	10	6th October 2007 03:46 PM
5 channel transformer rating for LM4780/LM4781	wjp007	Chip Amps	5	10th June 2007 12:12 PM
Transformer rating and using the ESP Mosfet Kit...	PaulO-PEBoard	Solid State	12	29th March 2007 08:45 PM
Where to buy a low gate voltage p channel MOSFET or make an alternative?	Corn-Picker	Everything Else	6	16th December 2003 10:15 PM


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

2nd July 2008, 07:23 AM	#3
Tahmid diyAudio Member Join Date: Jun 2008 Location: Dhaka	Hi, I did not quite understand what you said. Could you please elaborate on what is wrong? Thanks.

2nd July 2008, 11:26 AM	#4
areza diyAudio Member Join Date: Jun 2007	working principal is same for both p and n channel fet, its the supply polarity changes to, +500v become -500v and +20v become -20v, so GND become +500v, +20v/0v will turn off the p channel , - 20v will turn it on, only n channel is superior then p channel, Tip 2 , think again

2nd July 2008, 11:30 AM	#5
Tahmid diyAudio Member Join Date: Jun 2008 Location: Dhaka	Hi, Is the -20v in reference to source or drain? (I think its to source as it is Vgs). But, if in reference to source, which in this case in positive, wouldnt -20v simply mean 20v in reference to drain? Thanks.

2nd July 2008, 01:42 PM	#6
Ouroboros diyAudio Member Join Date: Dec 2003 Location: Nottingham UK	No! A P-channel MOSFET is connected just the same as an N-channel one, expect that all the voltages are -ve with respect to the 0V line. So you have a 0V (local ground) that is connected (say) to the source. The drain is connected through the load to the -ve power rail (perhaps at -50V), and the important thing is that the gate is still driven with respect to the source, but is also at a negative potential. That is, you might have 0V on the gate (w.r.t source) to turn the FET off, and -10V (w.r.t. source) to turn it on. Do a search for 'MOSFET complimentary source follower' or something like that on Google. I'll sure you'll find the relevant simple schematics showing you how things are connected.

2nd July 2008, 01:47 PM	#7
S.G. Mahbub diyAudio Member Join Date: May 2008	Tahmid, You are learning / trying to learn very fast. I appreciate your inquisitiveness. Thanks again for your million dollar confusion. The answer is very useful for all mosfet users and I am sure, most of them will be benefited from the solution. Data sheet is alright. vgs and vds ratings are also correct. Thing is that, you have to know the working mechanism of P- Channel Mosfet. The rating for Vgs is correct and means that with the source at 500V, you cannot pull the gate to ground. You can use the P-channel at up to its rating (500V in this case; although, I tend not to go the maximum ratings). The point is that the gate cannot be pulled more than 20 V below the source. Thus, you need some way to level shift its drive. Eva is correct- it is always vgs which drives a Mosfet. So, think about Level shifted PMOS Gate Drive. That will give you the solution. If you fail, solution will be provided to you. Thanks

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read