Here below is described a circuit proposal for a high efficiency amplifier which can be adopted for both HI-FI and CAR-RADIO applications.
Constraints of implementing high power solutions are the power dissipation and the size of the power supply. These are both due to the low efficiency of conventional AB class amplifier approaches.
The TDA7294S is a monolithic MOS power amplifier which can be operated at 90V supply voltage (100V with no signal applied) while delivering output currents up to ±6.5 A.
This allows the use of this device as a very high power amplifier (up to 100W as peak power with T.H.D.=10 % and RL = 4 Ohm); the only drawback is the power dissipation, hardly manageable in the above power range.
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The typical junction-to-case thermal resistance of the TDA7294 is 1 °C/W (max= 1.5 °C/W). To avoid that, in worst case conditions, the chip temperature exceeds 150°C, the thermal resistance of the heatsink must be 0.038 °C/W (@ max ambient temperature of 50 °C).
As the above value is practically unreachable; a high efficiency system is needed in those cases where the continuous RMS output power is higher than 50-60 W. The TDA7294 was designed to work also in higher efficiency way.
For this reason there are four power supply pins: two intended for the signal part and two for the power part.
T1 and T2 are two power transistors that only operate when the output power reaches a certain threshold (e.g. 20 W). If the output power increases, these transistors are switched on during the portion of the signal where more output voltage swing is needed, thus “bootstrapping” the power supply pins (#13 and #15).
The current generators formed by T4, T7, zener diodes Z1, Z2 and resistors R7,R8 define the minimum drop across the power MOS transistors of the TDA7294S. L1, L2, L3 and the snubbers C9, R1 and C10, R2 stabilize the loops formed by the “bootstrap” circuits and the output stage of the TDA7294S.
By considering again a maximum average output power (music signal) of 20W, in case of the high efficiency application, the thermal resistance value needed from the heatsink is 2.2°C/W (Vs = ±45V and RL=
All components (TDA7294S and power transistors T1 and T2) can be placed on a 1.5°C/W heatsink, with the power Darlington’s electrically insulated from the heatsink.
Since the total power dissipation is less than that of a usual class AB amplifier, additional cost savings can be obtained while optimizing the power supply, even with a high heatsink .
PCB suggestion for the assembly fo circuit potency audio amplifier
PCB components side for the montage of parts