EMI processing of Class D power amplifier IC
Time:2022-05-18
Views:2158
The high frequency component of class D amplifier output is worthy of serious consideration. If not correctly understood and handled, these components will produce a lot of EMI and interfere with the work of other equipment.
Two types of EMI need to be considered: signals radiated into space and signals conducted through speakers and power lines. The modulation scheme of class D amplifier determines the baseline spectrum of conducted EMI and radiated EMI components. However,
some board level design methods can be used to reduce the EMI emitted by class D amplifier, regardless of its base line spectrum.
A useful principle is to minimize the loop area carrying high-frequency current, because the strength related to EMI is related to the loop area and the proximity of the loop to other circuits. For example, the layout of the entire LC filter (including speaker wiring) should be as close as possible and kept close to the amplifier. The current drive and return circuit printed wiring should be concentrated to minimize the loop area (twisted pair wiring for speakers is helpful). Another thing to note is that large transient charges will be generated when the gate capacitor of the output stage transistor is switched. Usually, this charge comes from the energy storage capacitor, forming a current loop containing two capacitors. The EMI effect of transients in the loop can be reduced by minimizing the loop area, which means that the energy storage capacitor should be charged as close to the transistor as possible.
Sometimes it is helpful to insert an RF choke coil in series with the amplifier power supply. Properly arranged, they can limit the high-frequency transient current to the local loop close to the amplifier without conducting long distances along the power line.
If the gate drive non overlapping time is very long, the induced current of the speaker or LC filter will forward bias the parasitic diode at the transistor end of the output stage. When the non overlapping time ends, the diode bias changes from forward to reverse. Before the diode is completely disconnected, there will be a large reverse recovery current spike, resulting in a troublesome EMI source. Minimize EMI by keeping the non overlapping time very short (it is also recommended to minimize audio distortion). If the reverse recovery scheme is still unacceptable, a Schottky diode can be used in parallel with the parasitic diode of the transistor to transfer the current and prevent the parasitic diode from conducting all the time. This is helpful because the metal semiconductor junction of Schottky diode is essentially not affected by the reverse recovery effect.
An LC filter with a ring inductor core minimizes stray field transmission line effects caused by amplifier current. A good compromise between cost and EMI performance is to reduce the radiation from the low-cost drum core through shielding, which can acceptably reduce the linearity of the inductor and the sound quality of the loudspeaker if care can be taken to ensure this shielding.
Two types of EMI need to be considered: signals radiated into space and signals conducted through speakers and power lines. The modulation scheme of class D amplifier determines the baseline spectrum of conducted EMI and radiated EMI components. However,
some board level design methods can be used to reduce the EMI emitted by class D amplifier, regardless of its base line spectrum.
A useful principle is to minimize the loop area carrying high-frequency current, because the strength related to EMI is related to the loop area and the proximity of the loop to other circuits. For example, the layout of the entire LC filter (including speaker wiring) should be as close as possible and kept close to the amplifier. The current drive and return circuit printed wiring should be concentrated to minimize the loop area (twisted pair wiring for speakers is helpful). Another thing to note is that large transient charges will be generated when the gate capacitor of the output stage transistor is switched. Usually, this charge comes from the energy storage capacitor, forming a current loop containing two capacitors. The EMI effect of transients in the loop can be reduced by minimizing the loop area, which means that the energy storage capacitor should be charged as close to the transistor as possible.
Sometimes it is helpful to insert an RF choke coil in series with the amplifier power supply. Properly arranged, they can limit the high-frequency transient current to the local loop close to the amplifier without conducting long distances along the power line.
If the gate drive non overlapping time is very long, the induced current of the speaker or LC filter will forward bias the parasitic diode at the transistor end of the output stage. When the non overlapping time ends, the diode bias changes from forward to reverse. Before the diode is completely disconnected, there will be a large reverse recovery current spike, resulting in a troublesome EMI source. Minimize EMI by keeping the non overlapping time very short (it is also recommended to minimize audio distortion). If the reverse recovery scheme is still unacceptable, a Schottky diode can be used in parallel with the parasitic diode of the transistor to transfer the current and prevent the parasitic diode from conducting all the time. This is helpful because the metal semiconductor junction of Schottky diode is essentially not affected by the reverse recovery effect.
An LC filter with a ring inductor core minimizes stray field transmission line effects caused by amplifier current. A good compromise between cost and EMI performance is to reduce the radiation from the low-cost drum core through shielding, which can acceptably reduce the linearity of the inductor and the sound quality of the loudspeaker if care can be taken to ensure this shielding.
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