10

Application information

High-efficiency

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.

Here below (Figure 18) is described a circuit proposal for a high efficiency amplifier which can be adopted for both

HI-FI and CAR-RADIO applications.

The TDA7294 is a monolithic MOS power amplifier which can be operated at 80 V supply voltage (100 V with no

signal applied) while delivering output currents up to ± 10 A.

This allows the use of this device as a very high power amplifier (up to 180 W 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.

Figure 22 shows the power dissipation versus output power curve for a class AB amplifier, compared with a high

efficiency one.

In order to dimension the heatsink (and the power supply), a generally used average output power value is one

tenth of the maximum output power at T.H.D. = 10 %.

From Figure 22, where the maximum power is around 200 W, we get an average of 20 W, in this condition, for a

class AB amplifier the average power dissipation is equal to 65 W.

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 exceedes 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 pratically 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: 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 TDA7294. 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 TDA7294.

TDA7294

Application information

DS0013 - Rev 8

page 17/31