georghifi,
You appear to be misinterpreting what EPDR means. An EPDR of 0.9 ohms does not mean the amplifier sees an 0.9ohm load, it means that the power dissipation in the amplifier is equivalent to what it would see with a 0.9 ohm load. It sounds the same, but it is much different. When the load and current are in phase, the output transistor has the lowest drop across it (rail to load) when the current it at a maximum, hence power in the device at this voltage is not high. When the current is out of sync, you can have a maximum current when the voltage drop across the transistor is higher, hence the peak power across the transistor is much worse. Again, this is not "load" impedance, this is a mathematical formula to generate a convenient number to represent dissipation in a linear amplifier.
Here is the kicker. This problem is much worse for BJT transistors because BJT transistors experience secondary breakdown at high voltage and current, especially at high temperature. So in addition to taking into account the additional heat dissipation, you need to take into account secondary breakdown mechanism, which may mean doubling up devices, where one would normally be okay. MOSFETs don't experience secondary breakdown and don't mind that peak power, as long as average power is taken into account.
Here is the 2nd kicker, this problem does not exist at all for Class-D amplifiers.
You appear to be misinterpreting what EPDR means. An EPDR of 0.9 ohms does not mean the amplifier sees an 0.9ohm load, it means that the power dissipation in the amplifier is equivalent to what it would see with a 0.9 ohm load. It sounds the same, but it is much different. When the load and current are in phase, the output transistor has the lowest drop across it (rail to load) when the current it at a maximum, hence power in the device at this voltage is not high. When the current is out of sync, you can have a maximum current when the voltage drop across the transistor is higher, hence the peak power across the transistor is much worse. Again, this is not "load" impedance, this is a mathematical formula to generate a convenient number to represent dissipation in a linear amplifier.
Here is the kicker. This problem is much worse for BJT transistors because BJT transistors experience secondary breakdown at high voltage and current, especially at high temperature. So in addition to taking into account the additional heat dissipation, you need to take into account secondary breakdown mechanism, which may mean doubling up devices, where one would normally be okay. MOSFETs don't experience secondary breakdown and don't mind that peak power, as long as average power is taken into account.
Here is the 2nd kicker, this problem does not exist at all for Class-D amplifiers.