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ChocoHolic 30th November 2009 06:17 PM

MosFet Relays
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Also tired about struggling with old or low quality speaker relais?
May be this solution is more adequate for the 3rd Millenium.
It offers fully isolated speaker switching within few microseconds and distortion well below my measurement accuracy.
Don't bother about the huge PCB. The shown PCB is a trial PCB for multiple functional blocks which I intend to use in my next class D amp and also for getting familiar with the layout software...

ChocoHolic 30th November 2009 06:22 PM

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The schematic is rather simple.
Classic anti series connection of two MosFets allows to block or pass through AC. Gate drive is derived by a transformer, ensuring nice isolation.

ChocoHolic 30th November 2009 06:28 PM

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The distortions are below my measurement possibilities.
Especially I am lacking a power amplifier, which would have a harmonic spectrum below -120db... :p
I adjusted one of my amps for class A operation, and
even with this the distortion spectrum remains unchanged when adding the switch. Here the spectrum of the class A amp with this switch delivering 10W into 5 Ohms.

ChocoHolic 30th November 2009 06:41 PM

How can I tell -120db from the above? From the above I cannot.
But I analysed the voltage drop accross the switch itself, which is approx. 1/1000 of the amplifier output voltage.
Analysing this drop showed nothing but the fundamental and the noise of
the mic amp, which is 60db below the drop of the switch.
So the distortions of the switch are below -60db of 1/1000 of the amp output.
Means referencing to the amp output the switch introduces distortions below -120 db.

ChocoHolic 4th December 2009 10:42 AM

Speaker Relay
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...just for completing this monologue about a Solid State Speaker Relay...
Different from what some people might guess - such a MosFet solution can be very rugged. You have to settle a proper gate drive, which ensures a fast transition of the MosFets from isolating to conductive and vice versa.
Please note that the schematic shows the possibilty of multiple winding connections, furtheron the gate drive transformer has a myor influence. The types on the picture are not the best choice for this application.
If you ensure reasonable fast switching, i.e. voltage-current-overlap of 1us or less, then you can benefit of a huge MosFet SOA.
The shown circuit employs IRFB4321 types and I am not able to test it at its limits. My max. load that it can supply in a controlled way is around 75 ampere. In the attached picture you can see the switch operating to short circuit a power transformer, which delivers 125Vp unloaded and delivers about 75Ap during short circuit condition.
Please note that 125Vp is not recommended, because at this voltage the protection varistors do already start to clamp and generate continuous heat.
The shown circuit is fine for amps delivering in unbridged configuration approx. 85Vp, means OK for amps with rails up to +/-85V.
Generally this switch is suited not only for solid state amps but also for class D, in the range of 1kW...2kW and up to 3kW in bridged configurations with rails up to +/-85V. For bridged configurations you have to ensure that the switches for both Amp outputs are controlled with the same ON/OFF-control signal, otherwise you might overload the protection varistors.
And it is just a question of the MosFets+varistor to go higher.

White trace, measured with 10:1 probe: Transformer output voltage, 50V/div
Red trace, measured across 1 mOhm shunt: Current load, 50A/div

ChocoHolic 6th December 2009 09:01 AM

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The first schematic is the Rolls Royce, which might look complicated.
But we can simplify things to
2 MosFets
1k Resistor
9V Battery

For first real life trials you can even skip the protection varistor (BTW: Overvoltage protection could also be ensured by simple diodes from the MosFet drain to the amplifier rails).

The attached schematic shows all you need to start playing around.
Battery not connected ==> Speaker off
Battery connected ==> Speaker on
Feel free to start playing around and listen.

The MosFets should have low Rdson, the voltage rating should be as high as the rails of your amp.

How does the circuit work? If turned ON the N-chanel of the MosFet is able to conduct in both current directions! The only unpleasant thing is the body diode during the turned OFF situation. If you use only one MosFet, then you could block only on current direction. So I simply use two MosFet in series, connected in opposite direction. This is done since decades in bilateral CMOS-Switches. Modern MosFets allow such low Rdson that you can neglect the influence on distortion and damping factor.

tiefbassuebertr 6th December 2009 07:26 PM

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I like to know, whether one can use MOSFETs relais (bi-directional) for inrush current limiting by use of very large main transformers?
I want to replace the relais in the attached circuit through a wear free device
Here are some weblinks:
MOS-FET Rectifier

ChocoHolic 7th December 2009 05:22 PM

Basically yes, but do NOT start playing like proposed in posting #6 in a high voltage circuit.

If you want use the MosFet Relay for this application you will have to use 600V MosFets with low Rds_on and use a protection varistor which is intended to operate at 250V or 275V mains.
Furtheron your driving transformer must ensure the isolation for such voltages.

The nice thing is that you can test the relay and ensure by measurements also for proper gate signals under low voltage conditions and then if everything is Ok, you may step to higher voltages.
In any case: Take care! Mains voltage applicatons can easily kill a homo sapiens and even easier a homo incautus.

Elvee 8th December 2009 07:19 AM

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Hi Chocoholic

You seem to have done a good job ensuring artifacts in the ON conditions are minimal.
But you might have overlooked the OFF state. Large MOSfets have pretty large and non-linear parasitic capacitances, and they could cause unwanted leakage into the output when OFF.
Choco1 shows the isolation vs. frequency of a circuit similar to yours. At 3KHz, in the region of maximum audibility, the isolation is ~60dB, which is clearly insufficient.
This applies to a single throw switch. The situation would much improve in multiple-throw configurations.
Choco2 shows the situation in the time domain, with a slew-limited square wave.

Nrik 8th December 2009 08:06 AM

Interesting stuff.

Elvee: Could you simulate with an output-coil and zobel filter before the switch?

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