Clarity 2x300W: Class-T amplifier （2）

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

By Ton Giesberts

Modulator

The modulator amplifies the signal from the input stage to the output level. It is the second part of the overall amplification, or better said, the actual gain stage. The processor provides a switching
waveform that depends on the level and frequency of the signal. With no input signal, the average value of the switching frequency is approximately 700 kHz. It can vary over a maximum range of 200 kHz to 1.5 MHz. Two complementary MOSFET drivers with level shifters convert this signal to the proper level for driving the gates of the MOSFETs. The supply voltage for the drivers (10 V) is provided via pin VN10. It is first decoupled as much as possible by C38 and C39. These capacitors must therefore be placed as close as possible to the associated pin of the IC. On the negative side, LOICOM (connected to the source of T2) and VN10 provide the supply connections for the driver. On the positive side, bootstrap capacitor C7 (C20) is charged via D5 (D12) to nearly 10 V when the output is negative, and it ’rides up’ on top of the output voltage when Tl starts to conduct. This voltage is fed to VBOOT. which together with HOICOM (the source of Tl is also the bridge output) forms the other supply connection for this driver. At the clipping level, C8 (C21) provides an extra buffer, since the switching frequency is lower at the clipping level. R13 (R14) limits the charging current of C8 (C21) when the amplifier is switched on.

MOSFETs

Two n-channel MOSFETs (Tl & T2 or T3 & T4) form a half-bridge circuit for each channel. The level-shifter outputs alternately drive each of the MOSFETs into conduction. A ’dead time’ (break-before-make) is maintained to ensure that the two MOSFETs can never both be conducting at the same time (no ’shoot-through’ current). This time can be set using two jumpers (JP1 and JP2). We strongly recommend against experimenting with the selected setting. It might be possible to reduce the dead time if you use MOSFETs with considerably smaller gate capacitance (smaller amplifier power), but certainly not here! Gate resistors R78 and R9 (R28 & R30) limit the slew rate, and thus limit the amount of overshoot due to switching. They also somewhat reduce the amount of power that would otherwise be dissipated in the drivers (these are 1-W resistors) for charging and discharging the gate capacitances of the MOSFETs. Dl and D2 (D8 and D9) decrease the gate discharge time. This reduces the fall time of the pulses, and thus reduces the chance that both Tl and T2 may both be conducting at the same time. R8 and RIO (R29 & R31) are added for safety reasons. If no IC is fitted, they insure that the gates of the MOSFETs remain discharged. Without these resistors, leakage currents and noise voltages could occasionally have disastrous consequences. R6 and Rll (R27 and R32) are low-inductance resistors that are necessary for current limiting, which is described later on. R12 and C4 (R33 & C7) form a snubber network that eliminates high-frequency spikes in the output signal. This network is thus placed as close as possible to T2 (T4). Diodes D6 and D7 (D13 & D14) are connected between the source and drain of each of the transistors to suppress overshoots. Such overshoots are primarily caused by the coil in the output filter when large currents flow. These diodes (in SMD packages) are also placed as close as possible to the associated leads, primarily to protect the IC. D3 and D4 (D10 & Dll) are additional diodes connected between the sources and drains (respectively) of the MOS¬FETS to suppress overshoots. All of these diodes (D1-D14) must be ultra-fast-recovery types. C5 and C6 (C18 & C19) decouple the half-bridge circuit and are especially included to suppress spikes on the supply voltage lines. This also has a beneficial effect on the operation of the MOSFETs. These capacitors must also be placed as close as possible to the leads of the MOSFETs. C6 (C19) must be an electrolytic capacitor with extremely low ESR and very good HF characteristics. Here you should not use a substitute for the prescribed type unless its specifications are just as good or better. A normal electrolytic capacitor would probably explode or have a very short life. The pulse-width modulated signal at the output of the half bridge is fed to the output terminals via the LC filter L1/C9 (L2/C22).

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