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their much smaller size, no filaments and greater reliability with reduced
vulnerability to physical damage.
Audio
Applications
When used in an audio power amplifier, the advantages of the power MOSFET
over the power transistor are much more difficult to describe and would require
greater complexity than can be gone into here. However, they can be
summarised as follows – the most important point is that the power MOSFET
has a negative temperature coefficient whereas the power transistor has a
positive temperature coefficient. This means that when a power transistor is
handling power it heats up further and consumes more power. This
characteristic, called thermal runaway, will result in the destruction of the power
transistor if some means is not provided to control it. The power MOSFET on
the other hand, although heating up due to the power flow through the device,
does not continue to draw more and more power just because its temperature has
risen. But in fact has a tendency to stabilize itself – provided adequate head
sinking is available to remove the heat generated during normal operation.
Incidentally this is less heat sinking than is required for a similarly power rated
standard transistor.
Secondary
Breakdown
Then there is the appearance of secondary breakdown and ‘hot spots’ in a power
transistor. This is related to thermal runaway. In order to understand this, one
must imagine that the chip silicon inside the power transistor is in fact many
smaller transistors connected in parallel. Now, if one of these smaller transistors
or a spot on the chip has a greater gain (or amplification factor) than the rest,
then that spot will heat up faster and to a greater temperature than the remainder
of the transistor chip. This means that whole power dissipation capability of the
transistor has been severely reduced and is a major cause of these unexplained
output stage failures in large power amplifiers, i.e. over 80W
rms
.
The power MOSFET is largely immune to this problem because if a small part
of this chip has a higher gain than the rest then its temperature will rise slightly
causing that spot to reduce gain and hence stabilization occurs. The power is
more evenly distributed throughout the chip and therefore reliability is
maintained.
It can be seen from the above that the transistor power amplifier has to have a
much larger margin of power dissipation capability and heat sinking in its output
stage than the power MOSFET amplifier.
The transistor power amplifier of 100W
rms
output into 8
Ω
can require a driver
stage capable of delivering 10W at 1kHz and up to 20W at 20kHz into the input
of the output device. The power MOSFET only requires a maximum of 0.01W
so a major saving in driver stage componentry and associated noise and
distortion can be eliminated.
