Thank you for providing this information! Because I neglected to read the data sheet for the optocoupler I was unaware that it provides isolated gate voltage supplies to the MOSFETs.
I think I will try this speaker protection circuit.
I have never been totally satisfied with the electromechanical relays commonly used for speaker protection for several reasons:
- Relay contacts tend to get oxidized and this frequently causes a channel to drop out entirely or have excessive distortion from the non-linear properties of the oxidized relay contacts (they behave like diodes or zener diodes in series with the audio signal).
- Very few mechanical relays are rated to interrupt DC voltages >48V. It is difficult to avoid arc-over across the relay contacts when both the DC voltage and current are large. Therefore a fault which applies the entire amplifier power supply rail voltage and current may cause a mechanical relay to fail in a spectacular and catastrophic manner.
- Mechanical relays may open too slowly to prevent speaker damage.
-EB
Thank you for providing this information! Because I neglected to read the data sheet for the optocoupler I was unaware that it provides isolated gate voltage supplies to the MOSFETs.
I think I will try this speaker protection circuit.
I have never been totally satisfied with the electromechanical relays commonly used for speaker protection for several reasons:
- Relay contacts tend to get oxidized and this frequently causes a channel to drop out entirely or have excessive distortion from the non-linear properties of the oxidized relay contacts (they behave like diodes or zener diodes in series with the audio signal).
- Very few mechanical relays are rated to interrupt DC voltages >48V. It is difficult to avoid arc-over across the relay contacts when both the DC voltage and current are large. Therefore a fault which applies the entire amplifier power supply rail voltage and current may cause a mechanical relay to fail in a spectacular and catastrophic manner.
- Mechanical relays may open too slowly to prevent speaker damage.
-EB
Thanks bonsai,yes exactly.
Although this is my friend's question who have 2 pieces upc1237.
how do i use the SOA data to determine how much audio current a particular mosfet can pass?
https://www.infineon.com/dgdl/Infin...N.pdf?fileId=5546d462533600a4015364c38ddf29b1
https://www.infineon.com/dgdl/Infin...N.pdf?fileId=5546d462533600a4015364c38ddf29b1
Just look at the absolute maximum rating table, for this mosfet it says that at the transistor temperature of 25 C it can pass a maximum of 129 Amps, and at 100 C degrees it can still pass 91 amps maximum.
This mosfet is more than capable of being in the output of the protection circuit for power levels well beyond 1KW
Audio current is AC so the RMS value will be lower than for DC.
You will just have to leave one half of the speaker mute circuit off. I don't think having separate speaker mute boards is a good idea personally
Yes, but you will need to feed each opto LED with a separate resistor - ie don't just stack the optos on top of each other as one of the LED's may hog all the current. The other method is to put them in series - this is what I've done in Model 1721 and it switches c. 60A in <100us.
the SOA indicates with a VDS of 0.1v it can pass 10A.
then there is the question of those thin legs.
If you look at the graphs in the data sheet, you will see they are using a short pulse - some of the tests are 60us. The mosfets can only conduct high current when they are fully ON and there is just a few hundred mV across the drain<>source legs.
The legs are thin, but they are very short! The package in many cases is the limiting factor in total Rds(on). Many of these high power TO220 mosfets use multiple bonding wires to the source connection, or they use special clip bonding to be able to handle these high currents.
If you short the output of a big amp to 0V, you can easily get many 10's of amps flowing in the output devices.
Protecting an amplifier is a complicated business. You should never push the output transistors anywhere their limits. Ideally you would decide what resistance constitutes a short and set the current limit for the current the power supply could deliver into such a (partial) short. But it is unlikely that the transistor dissipation could handle that current for more than a fraction of a second. This particular FET has an amazing power rating but that is not realistic on a passive heat sink. Most silicon transistors in a TO-220 package are rated about 90W, and even that would be unrealistic. A single pair of transistors in a TO-220 package in an AB amplifier are limited to rail voltages of about 50V rail-rail, ie 25V peak into 4 Ohms is just over 6 Amps peak, so a 7Amp limit is reasonable. If you use a higher limit, you are just protecting your speakers in the event of an amp failure, and/or you enjoy rebuilding your amp repeatedly.
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In amplifiers, the mosfets are are normally turned on hard in normal operation, so there is only a few mV across the drain<> source. The problem comes with switching under gross fault conditions (eg output of amp shorted) where you have high currents and high voltages = high power disspation. If you switch the mosfet quickly, you can limit the dissipation.
I'm using 100A TO220 mosfets and they conduct >60A 20mS every second pulses without any sweat and will switch 60A when a fault is sensed in under 100us.
I'm using 100A TO220 mosfets and they conduct >60A 20mS every second pulses without any sweat and will switch 60A when a fault is sensed in under 100us.
Class D amps do not have the SOA problems of AB amplifiers, so yes, you can get a lot more from TO-220 transistors in a class-D amp. But regardless, the protection should be designed to detect a fault condition and not by the limit of the amplifier, assuming the limits of the amplifier are beyond any normal load conditions.
I didn’t write in ‘Class D amplifiers’. All my comments are related to class AB amps.
In class D amplifiers the OP devices switch on and off at high frequency so the switching losses are significant. In a class AB protection circuit the misfits go hard on and the losses are straight conduction losses. The switching events are very few and far between.
In class D amplifiers the OP devices switch on and off at high frequency so the switching losses are significant. In a class AB protection circuit the misfits go hard on and the losses are straight conduction losses. The switching events are very few and far between.
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