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Old 22nd August 2012, 07:42 AM   #41
Mooly is offline Mooly  United Kingdom
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I did some quick but real tests last year on the switching times over in 'tother thread on all this

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Old 22nd August 2012, 08:11 AM   #42
dai is offline dai  Wales
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I worked for siliconix in swansea over 20 years ago and was involved in the constuction
of a switching speed tester The largest devices available then were capable of
rise times of sub nanoseconds when swiching tens of amps as long as the gate was driven hard enough !
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Old 22nd August 2012, 09:20 AM   #43
AndrewT is offline AndrewT  Scotland
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Originally Posted by ChocoHolic View Post
Type: Schrack, RP11006, 250VAC/10A,....
What is the DC breaking capacity?
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Old 22nd August 2012, 09:54 AM   #44
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Originally Posted by ChocoHolic View Post
These photovoltaic MosFet drivers are pretty nice.
The only downside is that it cannot drive large MosFets fast, this will lead to limitations for high power applications.
Turn OFF can be speed up with a simple PNP on the floating side. Speeding up the turn ON is also possible, but boosting complexity in a way, which would kill the charming simplicity of the photovoltaic MosFet drivers...
The Avago ASSR-V621 PV coupler has a switch off circuit already built in.
Off in 30 microseconds with a 1 nF load. What more do you want?

Best wishes
David

Last edited by Dave Zan; 22nd August 2012 at 09:57 AM.
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Old 22nd August 2012, 10:42 PM   #45
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@ Cowboy_film_fan:
Figure 14 of the IPB025N10N3 data sheet tells us that it needs approx 30nC to overcome the plateau which turns the switch from conductive to isolating or vice versa.
For two gates this makes 60nC. With a 14uA charging current the switch will operate more than 4ms in linear mode.
Now have a look to figure 3 of the data sheet. Safe Operating Area.
Assuming an inductive behavior of the load, then the SOA for 4ms is just sufficient to protect an 8 Ohms speaker driven from an amp with +/-40V rails.
Already with a 4 Ohms speaker load the design is not bullet proof anymore...
Real life might be slightly more forgiving, because the speaker load will not act fully inductive.
Ignoring SOA typically leads to the situation that one guy says 'it can work' while another guy has to struggle with customer rejections and 'unexplainable defects'.
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Old 22nd August 2012, 11:41 PM   #46
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Quote:
Originally Posted by Dave Zan View Post
The Avago ASSR-V621 PV coupler has a switch off circuit already built in.
Off in 30 microseconds with a 1 nF load. What more do you want?

Best wishes
David
I would like to turn ON reasonably fast as well.

I agree that turning OFF is more critical, because you will have the most unfortunate load to the MosFet, when it has to interrupt a burned amp from the speaker.
But also the situation that the MosFet has to switch ON at high music levels should not be ignored. Fortunately here the inductive component of the load is calming the situation and with identical gate drive speed I would guess that turning ON is causing approximately a factor four less stress to the MosFet compared to turning OFF.

These photovoltaic drivers can be a great simplistic solution, just be aware of the resulting limitations.
'It can work', but does not automatically work for all applications.
Simply follow the reasoning of my previous posting about the SOA and check the situation in your application.

Usually you will not need a relay than can easily handle 85Vrms/50Arms or more as shown in posting #5.
Of course the shown Rolls Royce contradicts with commercial aspects.
But here I am for DIY hobby - not at all for publishing cost optimized solutions.
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Old 23rd August 2012, 02:35 AM   #47
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Originally Posted by ChocoHolic View Post
I would like to turn ON reasonably fast as well.
My question was mainly rhetorical because you discussed a switch off circuit when the ASSR already had one - but your answer is very educational!
The FET capacitance is more than I expected. The first idea that occurs is to drive the PV couplers with a simple capacitor + resisitor to increase the current pulse at turn on and reduce the switch time.
This probably needs more care if the FETs are used as rail protection. The on board capacitors could be a difficult load at turn on.

Best wishes
David
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Old 23rd August 2012, 10:39 AM   #48
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Originally Posted by Mooly View Post
I did some quick but real tests last year on the switching times over in 'tother thread on all this

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The measurements are great value for the discussion here as well, because they are showing one key point.
If you use small MosFets like inside the ASSR you can reach reasonable switching times already with small photovoltaic driving currents and consequently reasonable breaking capabilities even with small MosFets.
If you use the same gate drive but just add giant MosFets, your Rds will drop as desired, but the breaking capability will increase by far underproportional due to slower switching.
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Old 23rd August 2012, 10:57 AM   #49
Bonsai is offline Bonsai  Taiwan
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I used 2 photo voltaic drivers in parallel per channel on my e-Amp protection board to speed things up. If you are looking for straight muting function + speaker short or output device fail protection, switch the return line of the speaker, then you are down to a few 10's of microseconds. However, as pointed out by Dave, this scheme will not protect against a speaker hot lead short to chassis. I have a solution for that, but it adds a bit of complexity so, I take my chances with the scheme I wrote about on my website. With the MOSFETs, you are still well ahead of any EMR solution
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Old 23rd August 2012, 06:00 PM   #50
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I am particularly not interested in the switch-on time, since the amp will be muted anyway.Who switches on his amp at full power?
Switching off has to be fast,that's true but these PV couplers have a circuit built in that discharges much faster than it charges.
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