滑动摩擦力的公式Understanding Output Filters
for Class-D Amplifiers
By John Widder and Yun Tao Zhao, ST Microelectronics
This article - the first in a three-part ries on Class-D amplifiers - explains the purpo of the low-pass filter components in Class-D amplifiers and how to calculate their values. Class D amplifiers generally u a low-pass filter to attenuate the switching noi in the output waveform while passing the audio signal to the loudspeaker, but many engineers are not familiar with the functions performed by the various components in a Class-D amplifier filter or how to calculate the proper values. This article explains the purpo of the filter components and how to calculate their values.
The heart of a Class-D amplifier filter is a L-C low-pass filter. The corner frequency of the filter is chon so that the filter will have minimal effect on the desired output frequency range while attenuatin
g the switching noi as much as possible.
热伤风症状Low-Pass Filter
The optimum value for the filter inductor is L = R L/2πf C where f C is the desired corner frequency of the filter and R L is the load (speaker) resistance. Note that the inductor value is dependent on both the desired corner frequency and the speaker impedance so the inductor value will changes if the speaker impedance changes.
Practical designs require the u of standard component values so small adjustments usually have to be made to the ideal inductor and capacitor values. Rather than calculating the inductor and capacitor values independently and then adjusting their values, it's better to calculate the inductor value, lect the clost standard inductance value, and then calculate the required capacitance using the lected inductor.C = 1/((2πf C)2 • L) The quality factor (Q) of a filter is the ratio of the center frequency to the filter bandwidth.
Q = RL√(C/2L) A high Q produces an underdamped curve and a low Q produces an overdamped curve. The Q of the filter should be in the range 0.6 > Q > 0.8 to avoid underdamped or overdamped behavior. If you u the equations above the filter should have a Q of about 0.7, which provides goo
今年是第几个教师节d performance and allows for impedance variation in the speakers. Note that the Q of the filter will change if the speaker impedance is changed without adjusting the filter component values, which can result in an underdamped or overdamped respon.
Figure 1: A low pass filter for a Class-D amplifier
Component Selection
Not only is it important to choo the correct
L-C filter values, it is also important to choo the correct types of components for the class-D amplifier in order to avoid loss and minimize harmonic distortion.
The DC current rating of filter inductors must be greater than or equal to the maximum current that it will e. The change in inductance versus load current should not be more than 10%. The core material can affect the amplifier's harmonic distortion and should have very low hysteresis loss.
The capacitor should be a multilayer polyester, polypropylene or polycarbonate film capacitor. Avoid using ceramic capacitors in the low-pass filter. Ceramic capacitors experience large changes in capacitance as the voltage across them changes, which can result in distortion.
Single-Ended Outputs
Many designs u amplifiers with single-ended outputs becau they only require half as many transistors as a full-bridge output, and integrated amplifiers with single-ended outputs only require one output pin instead of two.
Single-ended amplifiers also have a few disadvantages compared to amplifiers with bridge-tied load (BTL) outputs. First, single-ended amplifiers require either split positive and negative power supplies
or DC blocking capacitors. If DC blocking caps are ud, they need to be large in order to prevent them from affecting the low-frequency performance of the amplifier. For example, an amplifier with an 8Ω speaker needs a 1000µf cap in order to achieve a -3dB point of 20Hz.
DC blocking caps can also cau audible pops as they charge up to Vcc/2 when the amplifier is turned on. A resistor divider from Vcc to ground can be ud to charge the capacitor up to Vcc/2 at a relatively slow rate when the power is turned on, minimizing or eliminating the pop.
If the amplifier does not have feedback then PSRR might be a problem. Two DC blocking capacitors can be ud to create a low-impedance AC voltage divider to improve the PSRR. If two DC blocking capacitors are ud then each cap only needs to have ½ of the capacitance becau the circuit es the parallel impedance of the two capacitors.
Figure 2a: Single-ended amplifier with a DC blocking capacitor
Figure 2b: Single-ended amplifier with a resistor divider to minimize pop at turn on
Figure 2c: Single-ended amplifier with two DC blocking caps to improve PSRR
Figure 2: Single-ended Class-D amplifier filters with DC blocking capacitors
manatees
BTL Outputs中国人体摄影
大象和蚂蚁的故事Amplifiers with BTL outputs are popular becau they do not require DC blocking caps even when operating with a single positive power supply. DC blocking caps limit the low-frequency respon of the amplifier and can be quite large.
BTL amplifiers have another advantage over amplifiers with single-ended outputs - the maximum peak-to-peak voltage that a amplifier with BTL outputs can apply to the speaker is twice the power supply voltage, which in turn means that up to four times as much output power can be delivered to the load compared to a single-ended amplifier. This can be a big advantage in applications where the power supply voltage is limited, especially in portable applications where the amplifier is operating off of a battery.
Common-Mode Filters
Common-mode filters are L-C filters with one side of the capacitor grounded. The inductor is placed in ries with the amplifier output(s) and the capacitor is connected from the speaker terminal to ground. Since single-ended amplifier filters already have one side of the capacitor grounded the low-pass filter is a common-mode filter.
When ud with an amplifier with BTL outputs, the filter shown in figure 1 would be a differential filter since it filters the signal between the two outputs. Common-mode filters for amplifiers with BTL outputs are different than the differential low-pass filter. The filter inductance for BTL amplifier outputs is normally split into two parate inductors, with one inductor is placed in ries with each of the amplifier outputs. Each inductor has half of the total inductance required for the low-pass filter.
Becau the capacitance across the load is now the ries combination of the two capacitors for BTL outputs, each capacitor needs to be twice the value calculated for the low-pass filter so that the total capacitance
will be correct. Note that becau the inductance for each BTL output is cut in half but the capacitance to ground is twice the normal value, the resonant frequency of the common-mode filter is the same as the resonant frequency of the differential filter. f C = 1/(2π√(L T C T )) = 1/(2π√(½L T小野鸡炖蘑菇
• 2C T )) The simplest common-mode L-C filter is just an inductor and a capacitor to ground (figure 3). This produces excellent attenuation at high frequencies but this filter has an underdamped common-mode respon that can cau unwanted ringing on the speaker leads. It can also cau very high ripple current through the inductor and capacitor. The impedances of the inductor and capacitor cancel at the resonant frequency so the current at the resonant frequency is only limited by the stray resistance in the circuit (primarily the output impedance of the amplifier and the DC resistance of the
inductor).
Figure 3: A simple common-mode filter and its respon
叶用芥菜In order to damp the common-mode respon of the filter it is necessary to add some resistance to the filter. Normally a resistor is added between the capacitor and ground. However, adding a resistor between the capacitor and ground creates a zero in the filter respon which can greatly reduce the
filter's effectiveness at higher frequencies. The effects of adding a resistor in ries with the capacitor can be en in the following plot. Note that the amplitude of the resonant peak is greatly reduced but the attenuation at high frequencies is also reduced.
Figure 4: Common-mode filter respon with damping resistors in ries with the capacitors
