Clarity 2x300W: Class-T amplifier （4）

Published:2011/8/3 2:32:00 Author:Li xiao na From:SeekIC

By Ton Giesberts

Protection

To protect the amplifier, the driver IC monitors the supply voltages and the currents through the transistors. The VPPSENSE input is used to monitor the main positive supply voltage for overvoltage and undervoltage; the VNNSENSE input is used in the same way for the main negative voltage. If the supply voltage is outside the allowable limits, the output stage is disabled (mute mode). If the supply voltage returns to within the allowable limits, the output is again enabled. For the calculation of the associated component values, please refer to the data sheet. Theoretically, the amplifier could become ’stuck’ in situation in which it constantly detects an overvoltage. However, that is very unlikely, since both detection inputs need more than roughly 68 V before they respond. This thus primarily amounts to protection for the IC itself, since several of the power supply capacitors are only rated for 63 V.

The calculations for overcurrent protection are certainly more interesting that those for voltage protection, since they determine the minimum load impedance the amplifier can handle at maximum output power. Since the output stage operates in switch mode, the MOSFETs used in the circuit determine the maximum load capacity of the amplifier. Here we have selected a relatively heavy-duty ST Microelectronics type, the STW38NB20. This transistor, which is housed in a TO-247 package, can handle up to 38 A and has a maximum drain-source voltage of 200 V. The maximum channel resistance with a gate-source voltage (U_GS) of 10 V is 0.065 £2 (ID = 19 A). A disadvantage of MOSFETs with this sort of specifications is that their input capacitance (C_iss) is rather large, in this case as much as 3800 pF. That explains why the drivers in the IC must be able to deliver rather substantial currents in order to switch the MOSFETs sufficiently quickly. We primarily chose these transistors in order to reduce the risk of unpleasant surprises when using speaker systems with unknown impedances. Naturally, the break-before-make time could be made shorter if transistors with significantly smaller gate capacitance are used, which would reduce the distortion level. However, our choice was in favor of a design that can tolerate low impedances.

Overcurrent detection is provided by the two low-inductance resistors R6 and R11 (R27 & R32), which are connected in series with the transistors as sense resistors. R6 is used for positive half-cycles in series with the drain of Tl, while Rll is used for negative half-cycles in series with the source of T2. The response threshold of the protection circuit is set in combination with R21. The IC directly measures the voltages across the sense resistors and uses these voltages to generate a current through R21. The maximum output is determined by comparing the voltage across R21 with the overcurrent threshold voltage VJQQ. C13 (C36) filters the voltage from the rectifier. The relationships between these components are given by the following two equations:

Imax = 3580 x (VTOC - Ibias X R21)) ÷(R21 X R6)

R21 = (3580 X VTOC) ÷ (Imax X R6 + 3580 x Ibias)

Here VTOC is the threshold voltage for overcunent detection (typically 0.97 V) andIbjasis20μA.

The first equation can easily be rearranged to allow the component values to be calculated. The second equation can be used to determine the value of R21 (R42). We have chosen a maximum output current of nearly 20 A, so that a load of somewhat less than 3Ω just avoids triggering the mute mode.

The mute mode can only be reset by briefly switching the level at the Mute input or briefly switching off the amplifier. When the mute mode is active, the HMUTE output is High, and this signal drives a LED that can be fitted to the front panel if desired. A red high-efficiency LED should be used for this purpose, since reducing the value of R43 would overload the output.

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