Simplistic Mosfet HV Shunt Regs - Page 220

king4joy · 23rd December 2011, 08:15 AM

I will try and see how big heat sinks I need first...if heat sinks not too big, I can fit it in the amp box....but then I need to put the last cap in the amp box as well.....

vgeorge · 23rd December 2011, 08:25 AM

If you do not drop too much voltage a small sink for Q3 will do.
I think that a 0.1uf cap at the input will do also. Try and see.

king4joy · 23rd December 2011, 08:35 AM

I think I just need to frop the minium for the board...say 20V...a 0.1uF 400V is not big....thankyou....you think it is not good to put the board in the PSU box??

king4joy · 23rd December 2011, 08:36 AM

frop should read drop.....sorry

vgeorge · 23rd December 2011, 11:00 AM

It' s always better to have the regulator close to the load.

king4joy · 24th December 2011, 04:42 AM

What are the chances that Q4 and Q5 will need heat sink?? if they will not need heat sinks at all...I can put them closer...and make the board smaller...I plan to have Vout = 270-290V max.....

Salas · 24th December 2011, 09:02 AM

There are good chances they will not need mini sinks for under 300V if the box ain't much hot inside.

AndrewT · 24th December 2011, 10:33 AM

What are the dissipations of Q4 & Q5?
Use that information to estimate the Tj from the datasheet temperature de-rating.

Salas · 24th December 2011, 10:48 AM

2mA @ ~150V each for 300V out.

AndrewT · 24th December 2011, 10:55 AM

King,
look at Salas posted pic.
2mA & 150Vce (or Vds) gives a dissipation of 300mW.
Find 300mW on the pic.
Draw a horizontal line at the 300mW height.
Where does the 300mW line cross the lower of the two plots? I'd guess about 115degC to 120degC.
The plot is telling you that at Ta ~115degC, the Tj = 150degC.
Now reduce Ta by 20C degrees to 95degC.
What is the new Tj? (answer, Tj=130degC)
Now reduce the Ta to what you estimate is the internal ambient temperature of the air around the regulator.
What is the estimated Tj?
If Tj < 100degC the device should survive long term.
If Tj < 70degC the device should never deteriorate in your lifetime.

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23rd December 2011, 08:15 AM	#2191
king4joy diyAudio Member Join Date: Apr 2011	I will try and see how big heat sinks I need first...if heat sinks not too big, I can fit it in the amp box....but then I need to put the last cap in the amp box as well.....

23rd December 2011, 08:25 AM	#2192
vgeorge diyAudio Member Join Date: Mar 2004 Location: poros island	If you do not drop too much voltage a small sink for Q3 will do. I think that a 0.1uf cap at the input will do also. Try and see.

23rd December 2011, 08:35 AM	#2193
king4joy diyAudio Member Join Date: Apr 2011	I think I just need to frop the minium for the board...say 20V...a 0.1uF 400V is not big....thankyou....you think it is not good to put the board in the PSU box??

23rd December 2011, 08:36 AM	#2194
king4joy diyAudio Member Join Date: Apr 2011	frop should read drop.....sorry

23rd December 2011, 11:00 AM	#2195
vgeorge diyAudio Member Join Date: Mar 2004 Location: poros island	It' s always better to have the regulator close to the load.

24th December 2011, 04:42 AM	#2196
king4joy diyAudio Member Join Date: Apr 2011	What are the chances that Q4 and Q5 will need heat sink?? if they will not need heat sinks at all...I can put them closer...and make the board smaller...I plan to have Vout = 270-290V max.....

24th December 2011, 09:02 AM	#2197
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	There are good chances they will not need mini sinks for under 300V if the box ain't much hot inside.

24th December 2011, 10:33 AM	#2198
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	What are the dissipations of Q4 & Q5? Use that information to estimate the Tj from the datasheet temperature de-rating.

24th December 2011, 10:55 AM	#2200
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	King, look at Salas posted pic. 2mA & 150Vce (or Vds) gives a dissipation of 300mW. Find 300mW on the pic. Draw a horizontal line at the 300mW height. Where does the 300mW line cross the lower of the two plots? I'd guess about 115degC to 120degC. The plot is telling you that at Ta ~115degC, the Tj = 150degC. Now reduce Ta by 20C degrees to 95degC. What is the new Tj? (answer, Tj=130degC) Now reduce the Ta to what you estimate is the internal ambient temperature of the air around the regulator. What is the estimated Tj? If Tj < 100degC the device should survive long term. If Tj < 70degC the device should never deteriorate in your lifetime.