Re: Some technical clipping questions about Class D amplifiers

Originally Posted by
thch
not usually true. power class D amps usually are NMOS only, though CMOS and PMOS versions exist. (FYI, for NMOS amps, a portion of the load energy gets pumped into a capacitor to allow the upper FET to turn on.) current demand is based upon the load, and the amount of time the devices stay on vs off. practical designs require both FETs be off for a brief time, in this case, any current flowing in the inductive speaker will flow through the FET's "body diode".
thus the output, before filtering, is either +Vdd, 0V, or -Vss. after the output filter, it can be any level in between.**
it is not noramlly desired to operate with a duty cycle of greater then 80%. this is due to the need to prevent both devices from conducting at the same time. as such, many class D amps have "overmodulation limiting", which limits the duty ratio. failing that, you would just have a FET which is on for a prolonged period of time, and the amp would have clipping in a similar manner as a class B amplifier. if the output current is allowed to rise, the power dissipated by the device may grow higher then intended. this is especially true if the applied Vgs cannot keep the device from saturating*. when this happens, the voltage across the device increases, and you get the FET to burn up (since it was handeling a high current as well)
class D is actually a wide range of implementations. in the case of overmodulation protected amplifiers, the maximum error signal is limited, either with a hard limit or a soft one. this prevents the amp from wanting to put out even its maximum rail voltage.
*FETs "saturate" when the current through them cannot increase without changing Vgs. this is backwards from a BJT which "saturates" when the voltage across the device cannot become lower.
** and technically, the voltage before the filter will be clamped a little bit above the supply voltage when both FETs are off and current is flowing in the flyback path made by one of the FET's "body diodes"
awsome info,you ouight to be teaching electronics on the side to us .
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