6 Pairs of VMOS output stage for the modular high end audio amplifier

Project presentation

Abstract

The amplifier output stage presented here is part of my modular audio amplifier system. It is designed to be equipped with six pairs of vertical MOSFETs in TO-247 package, for example IRFP240 and IRFP9240. Of course, different pin compatible transistors can be used as well.

This output stage is basically like the four pairs vertical MOSFET output stage, just with two extra pairs of transistors for higher safe operating area. The 4x output stage will likely never be realized, but of course the larger 6x output stage can be equipped with four pairs of transistors only.

Schematic

Prevention of cross-conduction

The schematic is mostly the same like the BJT output stage, except added series resistors prior to the protection network and quick discharge diodes in parallel to the gate resistors for each MOSFET. The quick discharge diodes seem very uncommon in audio circuits. I haven't seen them so far in audio amplifiers output stages, although some designers may have added some before. The MOSFETs gate to drain capacitance is a function of the gate to drain voltage. The lower the voltage, the higher the capacitance. This means, the closer the output swings to the supply rails, the more charge is stored in this capacitance. In extreme cases, like when the output stage is driven into clipping, this charge can be substantial. Discharging CGD is hindered by the gate series resistors and therefore pretty slow and this can lead to cross-conduction of the output stage transistors, i.e. simultaneous conduction of both P and N device, possibly resulting in destruction of the output stage. Since the gate resistor hinders quick discharge, it needs to be bypassed by the quick discharge diodes. During normal operation, those diodes do not conduct any current. Only in extreme cases, they can save the output stage from destruction.

Ferrites

I plan to use low value gate series resistors in order to optimize performance. This is risky because lower gate resistors increase the risk of oscillation. In order to maintain HF stability, I plan to put ferrite beads around the gate resistors. The ferrites have low impedance at low frequency and increasing impedance with higher frequency. The perfect ferrites for this application seem to be Würth 74270021. The Würth ferrites perfectly fit over my 0.6W resistors and their impedance characteristic matches the application well.

I have seen ferrites being put on the gate terminal of the transistors in other designs. This is basically a good countermeasure against oscillation as well, but since I'm also using gate to drain snubber networks, I will put mine in series with the gate resistor in order not to defeat the purpose of the snubber network.

DIY toroid air core inductor

This output stage features my DIY toroid air core inductor. The air core inductor has lowest stray field and unlike any inductor with a ferrous core, perfect linearity, lowest distortion and cannot saturate.

Assembly

Conclusion

I have no conclusions yet since I did not build this output stage. The PCBs are waiting for assembly and once I finished other modules, I will focus on this one.