Radio Transmitter

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Saturday, April 18, 2009


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 above 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 80V supply voltage (100V 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 180W 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 20 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 below, 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 oC/W (max= 1.5 oC/W). To avoid that, in worst case conditions, the chip temperature exceedes 150 oC, the thermal resistance of the heatsink must be 0.038 oC/W (@ max ambient temperature of 50 oC). 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: 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 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.

In figures above, the performances of the system in terms of distortion and output power at various frequencies (measured on PCB shown in fig. 19) are displayed. The output power that the TDA7294 in high efficiency application is able to supply at
Vs = +40V/+20V/-20V/ -40V; f =1 KHz is:
- Pout = 150 W@ T.H.D.=10 % with Rl= 4 Ohm
- Pout = 120 W@ ” = 1% ” ” ”
- Pout = 100 W@ ” =10% with Rl= 8 Ohm
- Pout = 80 W @ ” = 1% ” ” ”

Results from efficiency measurements (4 and 8 Ohm loads, Vs = ±40V) are shown by figures 23
and 24. We have 3 curves: total power dissipation, power dissipation of the TDA7294 and power dissipation of the darlingtons. 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 2.2oC/W (Vs =±40 V and Rl= 4 Ohm).
All components (TDA7294 and power transistors T1 and T2) can be placed on a 1.5oC/W heatsink, with the power darlingtons 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 headroom.

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