Fail Safe Power Supply - Page 3 - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs

Pass Labs This forum is dedicated to Pass Labs discussion.

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
Reply
 
Thread Tools Search this Thread
Old 23rd February 2007, 01:10 PM   #21
diyAudio Member
 
Join Date: Feb 2006
Location: peak district, england
Thanks to everyone for their contributions on this post.

I have corrected the fault with the placement of the output mosfet protection diode and the schematic shown at the start of the thread is now OK. I have also ammended the links:
psu.html
psu.html
  Reply With Quote
Old 23rd February 2007, 05:55 PM   #22
diyAudio Member
 
Join Date: Feb 2007
Location: Lake Tahoe, NV
I have been looking at your revised circuit and started thinking about the time needed to shutdown T2 after an overcurrent trip. It appears that both the original and modified circuit would exhibit turn-off times in the 1.0 millisecond range (this is a back-off-the-envelope estimate). This is controlled by the time it takes to pulldown C6 via R6 in order to reduce the Vgs of T2 to the threshold voltage (or thereabouts).

I would like to suggest the following modification for your consideration:

Make R6 and R5 into a serial connection between C6 and T2's gate.
Move the Triac connection to the common node between R6 and R5.
Connect D1's cathode to T2's gate.

This would give a much faster shutdown for T2. R5 and R6 would provide a known bleed path for the output and input C's, respectively, during powerdown, after an overcurrent trip.

If you like the Triac's original load when R6=68 Ohms; then something like R6=R5=100 Ohms might be suitable. This preserves the original 200 Ohm resistance between C6 and T2's gate under normal Cap-Mult operation. I don't believe that the values for R5 or R6 are very critical and can be changed to suit a wide range of circuit prejudices.

Thanks again for sharing your circuit idea.
VSR
  Reply With Quote
Old 24th February 2007, 08:51 AM   #23
EUVL is offline EUVL  Europe
diyAudio Member
 
Join Date: Oct 2003
I would have thought 1ms is quick enough for protection ?
It probably will take more than 1ms to drain the cap downstream of the mosfet even after the latter is turned off.

Perhaps you could post a figure to illustrate your proposal better ?


Thanks,
Patrick
  Reply With Quote
Old 24th February 2007, 09:30 AM   #24
EUVL is offline EUVL  Europe
diyAudio Member
 
Join Date: Oct 2003
I have a bit more thought about your proposed changes.

Let's assume a rail voltage of 16V (low by any standard) and a trip current of 4A. Further assume C7 to be 10,000uF, It will take something like 35ms to drain C7 by the load current (4A), if my quick-and-dirty calculation is not wrong. So much longer than 1ms. Hence the 1ms reaction time at the MOSFET gate is IMHO not critical.

I am not sure whether the proposal to drain C7 via D1 is really necessary or effective, as I would assume that the amp is still connected and drawing current. (I do not recall a discussion in this thread to incorporate a relay to separate the PSU from the amp.) The amp at trip current of 4A would probably be much faster at draining C7 than through R6, and putting D1 between Vout and gate of T1 would put unnecessarily strain on R6 and the triac ??


Regards,
Patrick
  Reply With Quote
Old 24th February 2007, 02:38 PM   #25
diyAudio Member
 
Join Date: Feb 2006
Location: peak district, england
I have just checked the Vgs trace on tripping and the 'output protection diode' doesn't do anything! I have played around with the value of the capacitors and their discharge loads, and Vgs never exceeds the expected 4V or so. Still it isn't doing much harm either. I am not sure about the normal power down situation though.

Click the image to open in full size.

The image above shows the power-up and trip characteristic. There are two significant time constants involved in the trip characteristic. These are the discharge of the output reservoir C7 and the discharging of C6 via R6 when the triac fires. With the circuit shown, the output reservoir C7 has an RC time constant of 11mS with a load current of 4A, whilst C6 and R6 have an RC time constant of 32mS. So the R6,C6 time-constant is the dominant one. Please note that on triggering, it is only C6 and it's discharge path R6 which count. The main reservoir capacitors remain fully charged and node R1, R2,C5 is discharged to ~ 50%

You can reduce the switch-off time by reducing the value of R6. The only caution is to make sure that the peak current through though triac does not exceed its rating. For a 20V supply, the gate voltage ~ 24V and the peak triac current will be 24/68 ~ 350mA which is fine for the device shown ( 4A rating). Reducing R 6 to 22 Ohms will give the same response as the output reservoir under a 4A load. Speeding things up further will require a reduction in the value of the output reservoir, C7.

I am not sure how fast it needs to be. Unlike a fuse, the circuit will not permit excessive currents. A quick-blow fuse will only blow quickly (mS) if the current is several time greater than it should be:

Click the image to open in full size.
  Reply With Quote
Old 24th February 2007, 05:23 PM   #26
diyAudio Member
 
Join Date: Feb 2007
Location: Lake Tahoe, NV
I thought the axiom that a faster response to an overcurrent condition is preferred to a slower response would be logical and noncontroversial in the context of this thread.

In particular the suggestion I proposed in my previous post had no cost impact and required a minor rearrangement of previously included parts in Allan Newby's revised circuit as presently posted at the beginning of this thread. The proposal did not seem to change the dynamics of the circuit under normal operating conditions. It made the speed of response to an overcurrent trip dependent on the triac's Ton rather than the time to remove energy from a filter cap. Thus, instead of responding in a 1.0 millisecond time frame it would respond in a 10 microsecond time frame.

If the point is the current response time is adequate. Then this presupposes that an exhaustive failure analysis for the current load (amplifier) has been accomplished. It further presupposes that the analysis is some how definitive for the different loads (amplifiers) that other potential user's of Allan's idea might choose to implement. Even with such an analysis; I still believe that faster is preferable, especially if it costs the same as slower.

Thank you for your indulgence,
VSR
  Reply With Quote
Old 24th February 2007, 06:38 PM   #27
diyAudio Member
 
Join Date: Feb 2006
Location: peak district, england
Is this shrerelec's variation?

Click the image to open in full size.

If so, the triac does indeed pull the gate down in microseconds. The 2200F output reservoir with will still take 11mS or thereabouts to discharge, but nevertheless the mod suggested by sherelec is a welcome improvement. Faster has got to be better.
  Reply With Quote
Old 24th February 2007, 09:50 PM   #28
EUVL is offline EUVL  Europe
diyAudio Member
 
Join Date: Oct 2003
> Is this shrerelec's variation?

I don't think so.

As it is shown, when the triac is on, it will also drain C7 through D1 without any current limiting resistor, and is likely to damage the triac.

I think you want to put R5 where you now have R6 (i.e. between C6+ and T2 gate) and R6 between T2 gate and Triac. Cathode of D1 also goes to T2 gate. That way, C7 is discharged both through the amp and D1-R6-Triac-Ground.

But I personally would remove D1 altogether.

I agree that it is an improvement, and I would also implement it as such. Especially when the time constant of C6-R6 is till rather long.


Patrick
  Reply With Quote
Old 24th February 2007, 11:05 PM   #29
diyAudio Member
 
Join Date: Feb 2007
Location: Lake Tahoe, NV
To Allen Newby,

Sort of, the schematic you have posted is a good representation of my thinking, but D1 needs to be connected to T2's gate.

There are two additional tweaks that should help this circuit weather electronic mishap:

Make D1 a 10-15V 1W Zener (leave the anode/cathode orientation the same as the current 1N4004). This will further protect T2 against a hard short across C7 by insuring that the Vgs does not exceed +20V.

Add a resistor in series with T1's base so that a hard short across the input C's will not allow the base current to exceed 0.1 times the max collector current rating for T1, under reverse breakdown of T1's B/E junction. In this scenario the body diode will force the current sense resistor to be the main ballast impedance between the output C's and the input C's. I do not believe the triac has a way to respond to this condition, nor does it matter; as the body diode will be forced into conduction independent of T2's gate drive.

Finally I have misspelled your name (twice!) in my previous post. I beg your pardon.
VSR
  Reply With Quote
Old 28th February 2007, 11:57 AM   #30
diyAudio Member
 
Join Date: Feb 2006
Location: peak district, england
Thanks to the various contributions, I think we now have a final version which although aimed at A class amps, could be used in almost any application?

I have updated the write ups:

psu.pdf
PSU.html

The image shown at the start of this thread has also been updated.

If I get time, I will alter and post the pcb artwork.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
in a CLC input supply: is it safe to use... jarthel Power Supplies 2 8th July 2007 11:39 PM
low cost, small, safe bias supply I_Forgot Planars & Exotics 36 1st June 2006 02:57 PM
What makes a power amp fail... pflodin Solid State 38 1st August 2004 10:50 AM
Safe Working voltage of output Transistors in class AB amps with +/- supply rajeev luthra Solid State 3 31st July 2004 06:58 AM


New To Site? Need Help?

All times are GMT. The time now is 09:42 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2