Winfield Hill <hill@rowland.harvard.edu> wrote:
> Ralph Barone wrote...
>> 
>> Chris Jones <lugnut808@spam.yahoo.com> wrote:
>>> On 26/02/2015 16:55, Ralph Barone wrote:
>>>> Can anybody suggest a cheap, easily obtainable >= 400 V, < 2A thru hole
>>>> MOSFET, preferably one that's designed more for analog use than for sub-ns
>>>> switching. Thanks.  I'm looking to squeeze a cathode follower into an
>>>> existing vacuum tube amp and adding one MOSFET is easier than adding
>>>> half a 12AT7.
>>>> 
>>> 
>>> Be a little careful if you are running a MOSFET with high VDS and
>>> significant power dissipation (some drain current) at the same time.
>>> 
>>> Modern MOSFETs often have a positive coefficient of drain current with
>>> temperature, for a given gate-source voltage. Even different parts of the
>>> same die have that property, so that if one side of the chip gets hotter,
>>> it will dissipate a greater fraction of the power than the cooler side,
>>> and so it will get hotter still. If things work out badly, a hot spot can
>>> form leading to failure. Some MOSFETs are specifically characterised for
>>> linear operation at high voltage, and have DC safe operating area graphs.
>>> For example I am using one of these:
>>> http://ixdev.ixys.com/DataSheet/DS100086%28IXTN60N50L2%29.pdf
>>> Perhaps they also make something smaller that would suit your
>>> application. Perhaps you are even running the device at low enough power
>>> that an ordinary switching MOSFET would be fine.
>>> 
>>> Chris
>> 
>> I haven't done all the calcs (hell, I'm not even sure WHAT I'm going
>> to do yet), but I suspect drain current will be under 2mA with a supply
>> voltage of 250 V, so a device rating of a couple Watts should be
>> ballpark OK.
> 
>  Your IRFD310 is rated at 1W, but that's for a junction-temp
>  rise of 125 degrees, and they assume you manage to keep the
>  case at 25 degrees, which you can't do if the junction is
>  at 150 degrees.  Bottom line your 2mA and 250V will probably
>  make the junction VERY hot.  A larger part with higher Pd
>  rating would be better for your power dissipation issue.
>  The two suggested TO-220 parts would have no trouble.
> 
>  Like vacuum tubes, MOSFETs have high Zin, but unlike tubes
>  they have very high input and output capacitances.  IRFD310
>  has Ciss = 170pF, which is about 100x higher than a 12AT7.
>  The IRS820 is only 2x higher, 360pF, but it's rated at 50W.
> 


Point taken. The 250 V I had quoted was total supply voltage, of which the
device should only see around half, but I suppose a TO-220 isn't any harder
to use that a TO-92 or DIP. 

PS: Thank you for AoE. In a previous lifetime I designed a firing angle
meter for an HVDC converter using many hints from your book.

"Winfield Hill" <hill@rowland.harvard.edu> wrote in message 
news:mcpu4601og@drn.newsguy.com...
> Like vacuum tubes, MOSFETs have high Zin, but unlike tubes
> they have very high input and output capacitances.  IRFD310
> has Ciss = 170pF, which is about 100x higher than a 12AT7.
> The IRS820 is only 2x higher, 360pF, but it's rated at 50W.

It's funny that Cin and Gm are substantially higher, so that the 
coefficient of performance in a circuit of limited Zo can end up much 
worse.  Well, that's just to say the impedances aren't matched, so not 
really saying much.

If they made RF transistors with the ratings of a 12AT7, it would be much 
better.  Sadly there's no market for a 10mA, 400V transistor that's 
capable of 500MHz, so the only thing you'll see is couple-ampere switching 
types.

It irritates me more that, although they could make switching transistors 
with specs similar to LDMOS RF parts, they simply *don't*.  The R_g and 
Ciss are always so bad.  So if you ever wanted a 1-10MHz switching part 
for a couple bucks... nevermind.  I suppose GaN is moving in these days, 
but it's still a tiny market segment.

Tim

-- 
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com 

Ralph Barone wrote...
>
>Chris Jones <lugnut808@spam.yahoo.com> wrote:
>> On 26/02/2015 16:55, Ralph Barone wrote:
>>> Can anybody suggest a cheap, easily obtainable >= 400 V, < 2A thru hole
>>> MOSFET, preferably one that's designed more for analog use than for sub-ns
>>> switching. Thanks.  I'm looking to squeeze a cathode follower into an
>>> existing vacuum tube amp and adding one MOSFET is easier than adding
>>> half a 12AT7.
>>> 
>> 
>> Be a little careful if you are running a MOSFET with high VDS and
>> significant power dissipation (some drain current) at the same time.
>> 
>> Modern MOSFETs often have a positive coefficient of drain current with
>> temperature, for a given gate-source voltage. Even different parts of the
>> same die have that property, so that if one side of the chip gets hotter,
>> it will dissipate a greater fraction of the power than the cooler side,
>> and so it will get hotter still. If things work out badly, a hot spot can
>> form leading to failure. Some MOSFETs are specifically characterised for
>> linear operation at high voltage, and have DC safe operating area graphs.
>> For example I am using one of these:
>> http://ixdev.ixys.com/DataSheet/DS100086%28IXTN60N50L2%29.pdf
>> Perhaps they also make something smaller that would suit your
>> application. Perhaps you are even running the device at low enough power
>> that an ordinary switching MOSFET would be fine.
>> 
>> Chris
>
> I haven't done all the calcs (hell, I'm not even sure WHAT I'm going
> to do yet), but I suspect drain current will be under 2mA with a supply
> voltage of 250 V, so a device rating of a couple Watts should be
> ballpark OK.

 Your IRFD310 is rated at 1W, but that's for a junction-temp
 rise of 125 degrees, and they assume you manage to keep the
 case at 25 degrees, which you can't do if the junction is
 at 150 degrees.  Bottom line your 2mA and 250V will probably
 make the junction VERY hot.  A larger part with higher Pd
 rating would be better for your power dissipation issue.
 The two suggested TO-220 parts would have no trouble.

 Like vacuum tubes, MOSFETs have high Zin, but unlike tubes
 they have very high input and output capacitances.  IRFD310
 has Ciss = 170pF, which is about 100x higher than a 12AT7.
 The IRS820 is only 2x higher, 360pF, but it's rated at 50W.


-- 
 Thanks,
    - Win

Chris Jones <lugnut808@spam.yahoo.com> wrote:
> On 26/02/2015 16:55, Ralph Barone wrote:
>> Can anybody suggest a cheap, easily obtainable >= 400 V, < 2A thru hole
>> MOSFET, preferably one that's designed more for analog use than for sub-ns
>> switching. Thanks.  I'm looking to squeeze a cathode follower into an
>> existing vacuum tube amp and adding one MOSFET is easier than adding half a
>> 12AT7.
>> 
> 
> Be a little careful if you are running a MOSFET with high VDS and
> significant power dissipation (some drain current) at the same time.
> 
> Modern MOSFETs often have a positive coefficient of drain current with
> temperature, for a given gate-source voltage. Even different parts of the
> same die have that property, so that if one side of the chip gets hotter,
> it will dissipate a greater fraction of the power than the cooler side,
> and so it will get hotter still. If things work out badly, a hot spot can
> form leading to failure. Some MOSFETs are specifically characterised for
> linear operation at high voltage, and have DC safe operating area graphs.
> For example I am using one of these:
> http://ixdev.ixys.com/DataSheet/DS100086%28IXTN60N50L2%29.pdf
> Perhaps they also make something smaller that would suit your
> application. Perhaps you are even running the device at low enough power
> that an ordinary switching MOSFET would be fine.
> 
> Chris

I haven't done all the calcs (hell, I'm not even sure WHAT I'm going to do
yet), but I suspect drain current will be under 2mA with a supply voltage
of 250 V, so a device rating of a couple Watts should be ballpark OK.

George Herold <gherold@teachspin.com> wrote:
> On Thursday, February 26, 2015 at 12:55:36 AM UTC-5, Ralph Barone wrote:
>> Can anybody suggest a cheap, easily obtainable >= 400 V, < 2A thru hole
>> MOSFET, preferably one that's designed more for analog use than for sub-ns
>> switching. Thanks.  I'm looking to squeeze a cathode follower into an
>> existing vacuum tube amp and adding one MOSFET is easier than adding half a
>> 12AT7.
> 
> IRF820? (in my parts box)  Digikey has a nice search engine.
> 
> George H.

Thanks (and thanks to bitrex for the BUZ74 recommendation).  Your mention
of parts box reminded me to look in mine where I found an IRFD110, which
didn't fit the bill, but helped me find IRFD310, which also does. Now back
to design before I check price and availability.

On 26/02/2015 16:55, Ralph Barone wrote:
> Can anybody suggest a cheap, easily obtainable >= 400 V, < 2A thru hole
> MOSFET, preferably one that's designed more for analog use than for sub-ns
> switching. Thanks.  I'm looking to squeeze a cathode follower into an
> existing vacuum tube amp and adding one MOSFET is easier than adding half a
> 12AT7.
>

Be a little careful if you are running a MOSFET with high VDS and 
significant power dissipation (some drain current) at the same time.

Modern MOSFETs often have a positive coefficient of drain current with 
temperature, for a given gate-source voltage. Even different parts of 
the same die have that property, so that if one side of the chip gets 
hotter, it will dissipate a greater fraction of the power than the 
cooler side, and so it will get hotter still. If things work out badly, 
a hot spot can form leading to failure. Some MOSFETs are specifically 
characterised for linear operation at high voltage, and have DC safe 
operating area graphs. For example I am using one of these: 
http://ixdev.ixys.com/DataSheet/DS100086%28IXTN60N50L2%29.pdf
Perhaps they also make something smaller that would suit your 
application. Perhaps you are even running the device at low enough power 
that an ordinary switching MOSFET would be fine.

Chris

On Thursday, February 26, 2015 at 12:55:36 AM UTC-5, Ralph Barone wrote:
> Can anybody suggest a cheap, easily obtainable >= 400 V, < 2A thru hole
> MOSFET, preferably one that's designed more for analog use than for sub-ns
> switching. Thanks.  I'm looking to squeeze a cathode follower into an
> existing vacuum tube amp and adding one MOSFET is easier than adding half a
> 12AT7.

IRF820? (in my parts box)  Digikey has a nice search engine.

George H.

Ralph Barone <address_is@invalid.invalid> Wrote in message:
> Can anybody suggest a cheap, easily obtainable >= 400 V, < 2A thru hole
> MOSFET, preferably one that's designed more for analog use than for sub-ns
> switching. Thanks.  I'm looking to squeeze a cathode follower into an
> existing vacuum tube amp and adding one MOSFET is easier than adding half a
> 12AT7.
> 

BUZ74

-- 

Can anybody suggest a cheap, easily obtainable >= 400 V, < 2A thru hole
MOSFET, preferably one that's designed more for analog use than for sub-ns
switching. Thanks.  I'm looking to squeeze a cathode follower into an
existing vacuum tube amp and adding one MOSFET is easier than adding half a
12AT7.