UM10155_2 © Koninklijke Philips Electronics N.V. 2006. All rights reserved.
User manual Rev. 02 — 5 September 2006 10 of 12
Philips Semiconductors UM10155
Discrete Class D High Power Audio Amplifier
6.3 Sonic performance versus Efficiency trade-off
The dead-time that is allowed between Q13 and Q14 conduction determines the
efficiency of the Demonstrator Board for a great deal; a long dead-time leads to better
efficiency. On the other side, long dead-time also leads to worse sonic performance; it has
important adverse consequences for e.g. THD.
Short dead-time causes quiescent current to go up, thus reducing efficiency and
improving sonic performance. The higher quiescent current is the result of more
cross-conduction of Q13 and Q14. So, most of the power that is lost in this way is
dissipated in Q13 and Q14, which may heat-up excessively. This may cause thermally
instable behavior of the MOSFETs. For that reason, the Demonstrator Board is shipped in
low quiescent current mode – during initial evaluation the risk of damaging the MOSFETs
through thermal runaway is thus reduced.
Nevertheless, by modifying one resistor value, the dead-time can be shortened and
evaluation in enhanced sonic performance mode can take place. Changing R9 to a lower
value decreases dead-time. It is left the responsibility of the experimenter to prevent
thermal runaway, but assistance from Philips will be available through the local sales
representatives.
To reduce the risk of thermal runaway in the enhanced sonic performance mode, a
thermal feedback loop can be implemented: the operating temperature of the MOSFETs
can be fed-back to the dead-time control by changing the fixed R9 value, mounting a PTC
resistor on the heatsink of the MOSFETs and connecting the PTC in parallel with R9.
Again, any changes are left the responsibility of the experimenter, but assistance from
Philips will be available through the local sales representatives.
