Analysis and Solution of High-order Harmonic Overcurrent Protection of Class D Power Amplifier

Summary

High-order harmonic overcurrent protection is a special overcurrent and overpower phenomenon. Usually the user’s circuit design is completely correct, and the conventional power test does not exceed the rated power. It is difficult to locate and solve this kind of protection. This paper combines theoretical analysis and practical experience to analyze the causes of high-order harmonic overcurrent protection, and provides solutions.

1,Class D High harmonic overcurrent protection phenomenon

Usually Class D power amplifier chips are designed with overcurrent protection function. When the output current exceeds the current limit threshold, the chip automatically turns off the driving signal and stops outputting. The general overcurrent protection is caused by the output power exceeding the rated value or the output short circuit. There is also a special overcurrent protection phenomenon caused by excessive high-order harmonic energy. High harmonic overcurrent protection is a special overpower phenomenon. Usually the user’s circuit design is completely correct, and the conventional power test does not exceed the rated power. This protection has the following characteristics:

. The output power of the problem machine does not exceed the maximum output power during the 1KHz standard audio signal test.

. Protection is more likely to occur when playing songs with more high frequency components.

. Use cement resistance instead of horn as load, the protection phenomenon disappears.

. Reduce or remove the capacitor of the output LC filter, the protection phenomenon disappears.

If the above phenomenon occurs, it can be suspected that it is due to overcurrent and overpower protection caused by high-order harmonic energy. The reasons for the overcurrent protection of higher harmonics are more complicated. First, analyze the frequency response characteristics of the LC filter network and the speaker impedance.

2,LC Filter Frequency Response

Figure 1 is a typical Class D output filter network (BTL output mode). The LC filter consists of L, C and load R.

Analysis and Solution of High-order Harmonic Overcurrent Protection of Class D Power Amplifier

The cutoff frequency and Q value of the LC filter are calculated as:

Generally, the output LC filter cutoff frequency of Class D is set in the range of 30kHz – 50kHz, in order to provide sufficient high frequency attenuation without affecting the gain flatness in the audio frequency band. The Q value of the LC filter increases with the increase of the load impedance, that is, the output gain has a certain increase at the cutoff frequency. The following figure is a filter frequency response curve:

In this example L=15uH, C=1uF, cutoff frequency FnAbout 29kHz, it provides -40dB of attenuation at the Class D switching frequency (about 300kHz). At the cutoff frequency, different load impedances exhibit different gains. Theoretically, in the extreme case of no-load, the gain is infinite, and the LC enters the resonance state.

In general, since FnDesigned above 20kHz, gain changes at the cutoff frequency do not affect the amplitude-frequency response in the audio frequency band. But out-of-band signals are affected by this part and output to the load to generate power. If the output signal happens to exist at Fc When the Q value of the LC filter is very high, the power of the higher harmonic will be amplified. It is possible to exceed the current limit threshold and cause overcurrent protection.

Since the Q value of the LC filter is related to the load R, only when R is very large will the higher harmonics be amplified. In practice the load R is a moving coil horn. The relationship between the horn impedance R and frequency is given in the next section.

3Impedance analysis of moving coil speakers

An ordinary moving coil speaker is composed of a paper disc (Paper Cone), a coil (Voice Coil) and a permanent magnet (permanent magnet). The nominal impedance of the speaker is DC impedance, generally 4, 6 or 8. However, due to the inductance characteristics of the coil and other parasitic parameters, the actual impedance curve (vs frequency) reflected by the speaker is shown in the figure:

As can be seen from the curve, the horn is a 4 horn. There is a resonance point around 110Hz. From 500Hz, the speaker has obvious inductive characteristics, and the impedance continues to increase with the increase of frequency. It can be seen that the nominal value of the horn impedance is its DC characteristic, and the horn impedance will change greatly with the frequency. At the cut-off frequency of LC around 30kHz, the speaker impedance is already much larger than its nominal DC impedance. In the example of Figure 3, the impedance at 30kHz is around 40Ω.

3.1 Impedance Model of Moving Coil Speaker

The impedance characteristic of the moving coil horn can be simulated using the equivalent circuit model in Figure 4 (for a detailed analysis of the equivalent circuit model, please refer to Reference 2). in:

According to the frequency response curve of the Inductor given in Figure 3, the parameters of the equivalent circuit model can be fitted as follows:

The frequency response curve of the equivalent model drawn by Mathcad is shown below, and the result is in good agreement with the actual test curve.

3.2 ZOBEL compensation network

The high frequency impedance of an actual speaker tends to increase with frequency due to the coil inductance, resulting in a high Q value of the LC filter network. ZOBEL is a RC network in parallel with the speaker, it can be used to compensate the inductance of the speaker and suppress the rise of the speaker impedance. As shown in Figure 7, the ZOBEL network consists of resistors and capacitors. The calculation formula is (see Reference 3):

The sum is calculated using the horn parameters in Section 3 as an example. Figure 8 is a comparison of the speaker impedance curves before and after adding the ZOBEL network. It can be seen that the role of the ZOBEL network is obvious, suppressing the impedance boost of the high-frequency part and keeping it nearby. This limits the Q value around the cutoff frequency of the LC filter network. Therefore, the overcurrent protection problem of higher harmonics will not be generated.

4 Phenomenon Analysis and Solutions

Based on the above theoretical analysis, the phenomenon analysis and solution for high-order harmonic overcurrent protection are as follows:

Usually, the frequency response of the electrical signal within the 20Hz-20KHz audio frequency bandwidth is considered in the design of the Class D power amplifier. It is guaranteed that the output power of each frequency point within 20Hz-20KHz will not exceed the rated value. Generally, the standard sine wave of 1KHz is used in the aging test. At this time, the speaker works near the rated impedance (about 4.2ohm in this example).

But if the frequency of the output signal exceeds 20kHz, the output contains a lot of harmonics. There will be high frequency signals located near the cutoff frequency (resonant frequency) of the LC filter. If the Q value of the LC filter is very high, the high frequency harmonics will be amplified and cause the problem of overcurrent protection.

The Q value of the LC filter network is related to the load impedance. It is known from the third section that the impedance of the speaker near the cutoff frequency is usually very high, so the Q value of the filter is very large. Figure 6 is the result of combining the actual speaker impedance curve with the frequency response curve of the LC filter.

It can be seen that when the load is pure resistance 4, the Q value of the LC filter network is low at the cut-off frequency, and there is no amplification effect. After connecting to the speaker, the LC filter network produces a gain greater than 20dB at the cutoff frequency. This is the root cause of high harmonic overcurrent protection.

In summary, the supplementary analysis of the phenomenon of high-order harmonic overcurrent protection given in Section 1 is as follows:

. The output power of the problem machine does not exceed the maximum output power at 1KHz standard audio signal.

Analysis: Because the protection phenomenon occurs near the cut-off frequency of the LC filter network, the power output in the range of 20Hz~20kHz is normal, and the overcurrent protection will not be triggered.

. Protection is more likely to occur when playing songs with more high frequency components.

Analysis: Songs with many high-frequency components are prone to generate harmonic energy in the range of 20kHz~40kHz, which just triggers the high-order harmonic overcurrent protection at the cut-off frequency of the LC filter network.

. Use cement resistance instead of horn as load, the protection phenomenon disappears.

Analysis: This type of high-order harmonic overcurrent protection is related to the high impedance presented by the speaker at high frequencies. If pure resistance is used to replace the speaker, this type of protection will not appear.

. Reduce or remove the capacitor of the output LC filter, the protection phenomenon disappears.

Analysis: The position of the cut-off frequency of the LC filter is changed, reducing the capacitance pushes the cut-off frequency above 40kHz. Generally, the harmonic component at this position is very small, which is not enough to cause the overcurrent protection phenomenon. Removing the capacitor LC filter does not exist and does not create protection issues.

4.1 solution

1. reduceLC Capacitance of filter networkC value:

Decreasing the value of the capacitor C of the LC filter network can increase the cutoff frequency of the LC filter. Make the cutoff frequency much larger than the possible frequency of higher harmonics. Usually, reducing the capacitance value by more than 5 times can effectively suppress the problem of high-order harmonic overcurrent protection.

Advantages: No need to modify the circuit, only need to modify the parameter value.

Disadvantages: LC network filtering effect becomes poor, switching ripple increases, and EMI may deteriorate.

Note: It is not recommended to directly remove the filter capacitor. Failure to do so will result in Class D switching ripple input into the speaker, increasing losses and deteriorating EMI.

2. Add toZOBEL network:

Advantages: Effectively suppress the high-frequency impedance rise of the speaker and solve the problem of high-order harmonic overcurrent. At the same time, it can homogenize the mid-high frequency response and improve the high-frequency hearing.

Disadvantages: need to add peripheral components, the capacitance value is large, it is recommended to use non-polar film capacitors.

Note: If it is only to solve the high-order harmonic overcurrent problem, the capacitance of the ZOBEL network can be smaller than the calculated value, generally as long as the effect of impedance suppression can be achieved.

5 ,Summarize

High-order harmonic overcurrent protection is a special overpower phenomenon. Under the premise that the circuit design is completely correct and the conventional power test does not exceed the rated power, the protection problem is relatively hidden. Combined with the frequency response of the LC filter circuit and the impedance frequency characteristics of the moving coil speaker, this paper analyzes the reasons for the Class D harmonic overcurrent protection and provides solutions.

6,references

[1] Leach, WM, Jr., Impedance Compensation Networks for the Lossy Voice-Coil Inductance of Loudspeaker Drivers, Georgia Institute of Technology, School of Electrical and Computer Engineering, J. Audio Eng. Soc., Vol. 52, No. 4, April 2004.

[2] Speaker Impedance, http://www.epanorama.net/documents/audio/speaker_impedance.html

[3] Speaker Zobel Impedance Equalization Circuit Calculator,

http://diyaudioprojects.com/Technical/Speaker-Zobel/

The Links:   LC260W01-A5KA A3PN030-ZVQG100