Why is my MOSFET dropping so much voltage? - Page 2

AndrewT · 19th April 2006, 08:49 AM

Hi,
leakage on both electros are a problem.
You only have 1.1mA flowing through your current source.
This gives about 56mW on each Zener. It's better to bias them with 10% (or more) of rated capacity (about 100mW).
Then allow for leakage in the caps and add these together to find the Current Source requirement.

You've got a little capacitance multiplier in there.

While the input voltage varies from your set point the FET is dissipating all the spare energy. Particularly at start up.
As Eva & Xpro said:- lower your time constant.

arnoldc · 20th April 2006, 02:28 AM

Hi Alex and Andrew, thanks for the replies.

Unfortunately, you're talking to a newbie here who only uses Duncan PSUD to make PSU

The protection diode's direction is positive to the gate?

I will change to 100K and 10uF to change the time constant.

AndrewT · 20th April 2006, 07:25 AM

Hi,
once you have the modification done, go and measure the volts drop across the 100k.
It will give you the leakage.

I then suggest you consider reducing the 560r slightly to increase the Zener current, about 270r to 330r should be about right.

This will raise your output voltage very slightly, but more importantly will allow small variations in leakage current to have less effect on the Zener voltage due to it's being biased more heavily.

x-pro · 20th April 2006, 10:19 AM

Quote:

Originally posted by arnoldc
Hi Alex and Andrew, thanks for the replies.

Unfortunately, you're talking to a newbie here who only uses Duncan PSUD to make PSU

The protection diode's direction is positive to the gate?

I will change to 100K and 10uF to change the time constant.

Here is the updated schematics for you:

Cheers

Alex

arnoldc · 20th April 2006, 11:47 PM

Hi Andrew,

changed to 100K and 10uF, I get 2.4V across 100K.

changed 560R to 280R (560R ||) and got 2.2V across 100K, with a slightly higher voltage- 218.3V vs 212.4V

does that look right?

Hi Alex,

thank you for the updated schematic. I'll give it a try. I've seen a schematic with the zener on the mosfet the way you described it, but for a power amp (I think Rod Elliot's site). Is the protection scheme the same?

Most schematic (series pass) do not have that, but rather have an inverted 1N4007 between Drain and Source, but I suppose the IRF840 already have that internally.

ifrythings · 21st April 2006, 05:02 AM

The zener between the gate and source is to protect the gate from being zap by a high voltage that will break down the insulation.

P.S the IRF840's maximum gate voltage is +/-20V so you need to put a lower voltage zener in to protect it.

arnoldc · 21st April 2006, 05:10 AM

hi ifrythings, geez i saw that in the spec sheet

now that make me wonder, why couple of high voltage designs i saw on the net does not actually include that protection

ifrythings · 21st April 2006, 05:57 AM

They leave it out to save a few pennies

AndrewT · 21st April 2006, 07:43 AM

Hi,
these numbers look OK.

Quote:

changed 560R to 280R (560R ||) and got 2.2V across 100K, with a slightly higher voltage- 218.3V vs 212.4V

I am surprised that each Zener gave an extra 1.2V by increasing the bias, but it confirms that they were underbiased. As a short term test:- try paralleling a third 560r to see what effect it has on Zener voltage. I would expect the increase to be less than 0.5V per Zener.

I am slightly perplexed by the lower leakage through the 100k

Quote:

got 2.2V across 100K cf. 2.4V across 100K

.

arnoldc · 21st April 2006, 07:52 AM

ifrythings, geez at the expense of the MOSFET

Thanks Andrew, will make change and report back.

19th April 2006, 08:49 AM	#11
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, leakage on both electros are a problem. You only have 1.1mA flowing through your current source. This gives about 56mW on each Zener. It's better to bias them with 10% (or more) of rated capacity (about 100mW). Then allow for leakage in the caps and add these together to find the Current Source requirement. You've got a little capacitance multiplier in there. While the input voltage varies from your set point the FET is dissipating all the spare energy. Particularly at start up. As Eva & Xpro said:- lower your time constant. __________________ regards Andrew T.

20th April 2006, 07:25 AM	#13
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, once you have the modification done, go and measure the volts drop across the 100k. It will give you the leakage. I then suggest you consider reducing the 560r slightly to increase the Zener current, about 270r to 330r should be about right. This will raise your output voltage very slightly, but more importantly will allow small variations in leakage current to have less effect on the Zener voltage due to it's being biased more heavily. __________________ regards Andrew T.

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20th April 2006, 02:28 AM	#12
arnoldc diyAudio Member Join Date: Apr 2003 Location: Makati	Hi Alex and Andrew, thanks for the replies. Unfortunately, you're talking to a newbie here who only uses Duncan PSUD to make PSU The protection diode's direction is positive to the gate? I will change to 100K and 10uF to change the time constant.

20th April 2006, 11:47 PM	#15
arnoldc diyAudio Member Join Date: Apr 2003 Location: Makati	Hi Andrew, changed to 100K and 10uF, I get 2.4V across 100K. changed 560R to 280R (560R \|\|) and got 2.2V across 100K, with a slightly higher voltage- 218.3V vs 212.4V does that look right? Hi Alex, thank you for the updated schematic. I'll give it a try. I've seen a schematic with the zener on the mosfet the way you described it, but for a power amp (I think Rod Elliot's site). Is the protection scheme the same? Most schematic (series pass) do not have that, but rather have an inverted 1N4007 between Drain and Source, but I suppose the IRF840 already have that internally.

21st April 2006, 05:02 AM	#16
ifrythings diyAudio Member Join Date: Oct 2005	The zener between the gate and source is to protect the gate from being zap by a high voltage that will break down the insulation. P.S the IRF840's maximum gate voltage is +/-20V so you need to put a lower voltage zener in to protect it.

21st April 2006, 05:10 AM	#17
arnoldc diyAudio Member Join Date: Apr 2003 Location: Makati	hi ifrythings, geez i saw that in the spec sheet now that make me wonder, why couple of high voltage designs i saw on the net does not actually include that protection

21st April 2006, 05:57 AM	#18
ifrythings diyAudio Member Join Date: Oct 2005	They leave it out to save a few pennies

21st April 2006, 07:52 AM	#20
arnoldc diyAudio Member Join Date: Apr 2003 Location: Makati	ifrythings, geez at the expense of the MOSFET Thanks Andrew, will make change and report back.

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