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Old 19th June 2015, 04:08 AM   #1
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Default question on speaker protection circuit

Attached is the speaker protection part of my design. My question is whether I should put a 10M resistor across D91 to breed off the voltage when the input LED is off. I just afraid if there is no resistor, the capacitance of the gate of the transistors will hold the voltage and keep the FET on for a long time. And it will defeat the speaker protection.

I also attached the datasheet of Avago FET driver.

Thanks
Attached Images
File Type: jpg Parallel FET protection.jpg (54.8 KB, 121 views)
Attached Files
File Type: pdf AVAGO Photovoltaic MOSFET Driver ASSR-V621 V622.pdf (224.1 KB, 13 views)
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Old 19th June 2015, 08:51 AM   #2
AndrewT is offline AndrewT  Scotland
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I think you are missing the current drain that allows the FETs to switch to OFF/OPEN

The current from the LEDs drives the FETs to ON/CLOSED
This is fast part of the switching.

The slower part is the going to OFF when there is a need to isolate the speakers.
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Last edited by AndrewT; 19th June 2015 at 08:53 AM.
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Old 19th June 2015, 08:57 AM   #3
sajti is online now sajti  Hungary
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Quote:
Originally Posted by AndrewT View Post
I think you are missing the current drain that allows the FETs to switch to OFF/OPEN

The current from the LEDs drives the FETs to ON/CLOSED
This is fast part of the switching.

The slower part is the going to OFF when there is a need to isolate the speakers.
I see "turn off circuit" integrated in the FET driver. This is responsible for the fast turn off, so I would not use any other parts for it.

Sajti
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Old 19th June 2015, 09:09 AM   #4
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I saw the "turn off circuit". But what does that mean? It specified turn off time, BUT it specified at 10mA and CL=1nF. Is CL the gate source capacitance of the MOSFET it is driving? It really does not explain anything. that's why I post this.

I hope someone actually used this part can join in.
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Old 19th June 2015, 09:12 AM   #5
sajti is online now sajti  Hungary
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Quote:
Originally Posted by Alan0354 View Post
I saw the "turn off circuit". But what does that mean? It specified turn off time, BUT it specified at 10mA and CL=1nF. Is CL the gate source capacitance of the MOSFET it is driving? It really does not explain anything. that's why I post this.

I hope someone actually used this part can join in.
Hi,

the specified turn off time is turn OFF with 1nF load (MOSFET input capacitance), and turn OFF from 10mA led current.

Sajti
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Old 19th June 2015, 09:18 AM   #6
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Originally Posted by sajti View Post
Hi,

the specified turn off time is turn OFF with 1nF load (MOSFET input capacitance), and turn OFF from 10mA led current.

Sajti
Thanks

Then I don't have to put the resistor across the D91 to help collapsing the voltage then.

Good.

Thanks
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Old 19th June 2015, 02:14 PM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Alan0354 View Post
I saw the "turn off circuit". But what does that mean? It specified turn off time, BUT it specified at 10mA and CL=1nF. Is CL the gate source capacitance of the MOSFET it is driving? It really does not explain anything. that's why I post this.

I hope someone actually used this part can join in.
Have you read any of the Threads discussing this?
Have you read Bonzai's PDF on this topic?
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Old 19th June 2015, 02:18 PM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Alan0354 View Post
Thanks

Then I don't have to put the resistor across the D91 to help collapsing the voltage then.

Good.

Thanks
If you leave the gate charged up, then the mosFET will never turn off. Your SS relay will turn ON and once on will never turn off.
You NEED to turn it OFF to make the SS relay effective.

The Turn OFF should be quick, nut it usually ends up being a bit slower than the Turn ON.

The time it is turning ON, or turning OFF, times the current that is flowing is the effective energy building heat inside the device.
Slow turn ON, or OFF, leads to excessive heating.
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Old 20th June 2015, 07:21 AM   #9
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I read 30 pages of this thread:Output Relays

The V621 has a turn off circuit and Mooly in post #277 shows the turn off time of 400uS without additional resistor. I doubt adding a 4.7M will speed up any at all. Looks like the V621 take care of the turn off time.
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Old 20th June 2015, 09:09 AM   #10
AndrewT is offline AndrewT  Scotland
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post1 show a LED activated voltage generator.
Are you still referring to that schematic?

post9 link took me to post521
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