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er a speaker is attached. The heatsink
temperature climbs steadily to 70°C
at around 3°C per minute. The thermal protection then activates and the
amplifier cycles on and off about once
per minute as the heatsink cools and
heats again.
This occurs with the lid off the enclosure and even when the amplifier
circuit board is totally removed from
the enclosure. My basic understanding is that Class-D amps are supposed
to run cool, so this behaviour seems
odd to me.
siliconchip.com.au
I have checked the component
placement and orientation, and
checked all the PCB voltages against
the instructions. They are within the
expected ranges. The supply rails are
±47V DC. I measured the amplifier gain
and it is around 15 times, as expected.
Q1 and Q2 appear to be the source of
the heat as they are hotter than the
heatsink and Q3.
Do you know why it is doing this
and how I can solve it? (J. C., via email)
• The most likely scenario is that the
amplifier switching frequency is too
Australia’s electronics magazine
high and/or the dead time is incorrect,
resulting in much greater switching
losses than normal.
Check the switching frequency and
the components at pins 1, 2 and 4
(which set the frequency) and the dead
time setting resistors at pin 9.
Presumably there is no significant
DC offset at the output, since you have
the speaker protector connected and
it would switch the output off if that
were the case. So the frequency and
dead time are the leading suspects for
this overheating problem.
SC
November 2018 103
