KR102320834B1 - multi-level Class D amplifier of low voltage-stress - Google Patents

Abstract

The present invention generally relates to a technique for improving reverse recovery characteristics of a switching transistor in a digital amplifier (Class D amplifier) and lowering a voltage stress applied to a switching transistor. In particular, the present invention can reduce the voltage stress applied to the switching transistor during the switching operation by the PWM carrier signal by configuring the circuit of the digital amplifier in a multi-level method, thereby reducing the switching loss that occurs during switching and freewheeling. It relates to a multi-level digital amplifier with low voltage stress that improves reverse recovery characteristics through a diode. According to the present invention, a digital amplifier can be applied to a high voltage application field by lowering the voltage stress of a switching transistor, and a Si-FET component device having a body diode with slow reverse recovery characteristic can be applied to the digital amplifier, thereby lowering the product price and reducing the product price. It has the advantage of securing flexibility in product design. In addition, since the voltage stress of the switching transistor is low, it is possible to reduce the switching loss occurring during switching, so it has advantageous advantages in terms of efficiency and heat generation.

Description

{multi-level Class D amplifier of low voltage-stress}

The present invention generally relates to a technique for improving reverse recovery characteristics of a switching transistor in a digital amplifier (Class D amplifier) and lowering a voltage stress applied to a switching transistor.

In particular, the present invention can reduce the voltage stress applied to the switching transistor during the switching operation by the PWM carrier signal by configuring the circuit of the digital amplifier in a multi-level method, thereby reducing the switching loss that occurs during switching and freewheeling. It relates to a multi-level digital amplifier with low voltage stress that improves reverse recovery characteristics through a diode.

In general, analog amplifiers have many advantages in terms of sound quality because they directly amplify analog signals with power transistors. However, analog amplifiers matching high-quality speakers generate a lot of heat from the power transistor, so a large heat sink must be mounted to dissipate the heat, resulting in an increase in size, weight, and price. There is this.

In response to these shortcomings of the analog amplifier, a digital amplifier, also called a Class D amplifier, has been proposed. A digital amplifier synthesizes an analog input signal with a high-frequency carrier signal and modulates it in a PWM (Pulse Width Modulation) method. A method of amplifying a small-signal analog input signal into a large-signal analog signal having a large amplitude was adopted by passing it through (Low Pass Filter).

[Figure 1] is a schematic configuration diagram of a general digital amplifier, [Figure 2] is a waveform graph when there is no input signal in this digital amplifier, and [Figure 3] is a waveform graph when there is an input signal in the digital amplifier am. The waveform graphs of [Fig. 2] and [Fig. 3] are conceptually shown to explain the operation method of the digital amplifier, and in reality, the output capacitances of the switching transistors 11 and 12 or the LC low-pass filters 13 and 14 It is quite different from this due to the characteristics of

Referring to FIG. 1 , the amplifier control unit 15 receives an input signal (audio signal) of a low frequency (eg, 20 Hz to 20 kHz) and a carrier signal having a high frequency (eg, 200 kHz to 550 kHz). These input signals and carrier signals are small signals (eg 3.3 V). In this case, the carrier signal is also called a sampling signal, and a triangular wave, a sawtooth wave, or the like may be used. The amplifier control unit 15 generates a synthesized PWM signal having a high frequency (eg, 200 kHz to 550 kHz) by PWM modulation by combining an input signal and a carrier signal. In this process, the signal amplitude of the input signal is reflected in the duty ratio of the composite PWM signal.

In accordance with the synthetic PWM signal, the upper switching transistor 11 and the lower switching transistor 12 are controlled to be switched in opposite directions. Generally, MOSFET power transistors are used as these switching transistors 11 and 12 . A PWM signal amplified to a high voltage (±HV) (eg, ±70 V) is obtained through the switching control of these transistors 11 and 12, and the amplified PWM signal is passed through the LC low-pass filters 13 and 14 get the output signal. This output signal is transmitted to the speaker to drive the speaker unit.

An internal mechanism by which the digital amplifier 10 operates will be described with additional reference to [FIG. 2] and [FIG. 3].

First, referring to [Fig. 1] and [Fig. 2], when there is no input signal, the amplifier control unit 15 outputs a carrier signal having a duty ratio of 50% as a synthetic PWM signal as it is, and accordingly, the upper and lower switching transistors (11, 12) is alternately turned on and off for the same time in response to a duty ratio of 50%. At this time, while the upper switching transistor 11 is in an ON state, current is supplied to the low-pass filters 13 and 14 from the +HV power supply, and while the lower switching transistor 12 is in an ON state, the low-pass power is supplied. Current flows from the filters 13 and 14 to the -HV power supply. Since the duty ratio of the synthesized PWM signal is 50%, no energy is accumulated in the low-pass filter capacitor (Cf; 14).

On the other hand, referring to [Fig. 1] and [Fig. 3], for example, when there is an input signal of a rising type, the amplifier control unit 15 outputs a composite PWM signal of a type of increasing the duty ratio. Accordingly, the upper switching transistor While the percentage of time when (11) is in the ON state gradually increases, the percentage of time when the lower switching transistor 12 is in the ON state gradually decreases. For this reason, each time the duty cycle elapses, more energy is gradually accumulated in the low-pass filter capacitor Cf 14, whereby the output signal gradually increases to follow the input signal.

The digital amplifier 10 operates by this mechanism, and the upper and lower switching transistors 11 and 12 constituting the digital amplifier 10 only go back and forth between the cut-off region and the saturation region, and the linear ( Since it does not operate in the linear region, the loss is definitely smaller than that of an analog amplifier.

However, the digital amplifier 10 is not suitable for a high voltage application field despite the above advantages due to problems described later.

[Fig. 4] is a diagram showing a general equivalent circuit configuration and a current signal of a digital amplifier (Class D amplifier). [Fig. 4] is a simplified representation of the circuit configuration of [Fig. 1] so that the electrical circuit analysis can be performed intuitively. Referring to FIG. 4 , the digital amplifier has a configuration in which the two switching transistors 11 and 12 are switched on/off in opposite directions by the voltage Vin of the synthesized PWM signal. In [Fig. 4], the voltage stress applied to the switching transistors 11 and 12 is twice Vin.

In a digital amplifier, MOSFET power transistors are generally used as the switching transistors 11 and 12, but the reverse recovery characteristics of the body diodes of the switching transistors 11 and 12 must be fast. According to the manufacturing method of the field effect transistor (FET), when the transistor is made, the diode characteristic is inherent even if it is not intended, and this is called a body diode. In FIG. 4 , diodes D1 and D2 connected in a direction from a source to a drain of the switching transistors 11 and 12 are body diodes of the switching transistors 11 and 12 .

When a reverse voltage is applied to the diode device, the current is not actually cut off immediately, but is cut off after a certain time has elapsed due to physical properties. This elapsed time is called reverse recovery time (Trr). Conventional diodes do not guarantee a fast reverse recovery time (Trr), and if a short reverse recovery time is required, a specially designed device must be used. The digital amplifier must use the switching transistors 11 and 12 with body diodes D1 and D2 having a high-speed reverse recovery characteristic, otherwise permanent damage to the circuit of the digital amplifier occurs. Referring to (b) of [Fig. 4], at the time tA when the upper switching transistor 11 is turned on, the body diode D2 of the lower switching transistor 12 is still open due to the slow reverse recovery characteristic. current flows and the component is damaged.

However, the Si material field effect transistor in which the body diode D2 having a high-speed reverse recovery characteristic is embedded has a breakdown voltage of about 200V or less. For this reason, most digital amplifiers are used only in applications where the driving voltage is 200V or less, and digital amplifiers cannot be used in high voltage applications, for example, applications in which the driving voltage is about 400V. Of course, by changing the material of the transistor, for example, a transistor made of SiC material (SiC-FET) or transistor made of GaN material (GaN-FET) has high withstand voltage and excellent reverse recovery characteristics of the body diode. However, they have a problem that they are very expensive.

In addition, the digital amplifier maintained low heat generation by using the switching transistor as a switching device instead of an amplifying device. Since the switching loss occurs only in the on-off transition period, there is no problem even if the switching transistor is switched at a high frequency, for example, several hundred kHz. However, as the driving voltage increases, the switching loss also increases in proportion to the increase in the amount of heat generated by the digital amplifier. Accordingly, additional heat dissipation means are required, thereby eliminating the advantage of the digital amplifier.

Accordingly, there is a need for a technique capable of solving the problems of the prior art by improving the circuit configuration of the digital amplifier.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a technique for improving reverse recovery characteristics of a switching transistor and lowering a voltage stress applied to the switching transistor in a general digital amplifier (Class D amplifier).

In particular, an object of the present invention is to reduce the voltage stress applied to the switching transistor during the switching operation by the PWM carrier signal by configuring the circuit of the digital amplifier in a multi-level method, thereby reducing the switching loss that occurs during switching. It is to provide a multi-level digital amplifier with low voltage stress that improves reverse recovery characteristics through a freewheeling diode.

In order to achieve the above object, the multi-level digital amplifier of low voltage stress according to the present invention is sequentially connected in series to connect both ends of the digital amplifier driving power source, the upper first switching transistor 111 and the upper second switching transistor 112 ) and a lower first switching transistor 121 and a lower second switching transistor 122; an inner loop capacitor 150 connecting the contacts of the upper first and second switching transistors 111 and 112 and the lower first and second switching transistors 121 and 122; first and second inner loop diodes 161 and 162 connected in forward and reverse directions with respect to both ends of the inner loop capacitor 150 from the common potential of the digital amplifier driving power supply; The low-pass filter inductor 130 and the low-pass filter capacitor 140 are connected to the contacts of the upper second switching transistor 112 and the lower first switching transistor 121 to form a low-pass filter.

In addition, the multi-level digital amplifier of low voltage stress according to the present invention, a first freewheeling diode 171 that reversely connects the final both ends of the upper first and second switching transistors 111 and 112 connected in series; A second freewheeling diode 172 that reversely connects the final ends of the lower first and second switching transistors 121 and 122 connected in series above; may be configured to further include.

At this time, the first inner loop diode 161 is forwardly connected from the common potential of the digital amplifier driving power source to the contacts of the upper first and second switching transistors 111 and 112 , and the second inner loop diode 162 is digital Preferably, it is configured to be connected in reverse with respect to the contact points of the lower first and second switching transistors 121 and 122 from the common potential of the amplifier driving power supply.

In addition, the upper first and second switching transistors 111 and 112 and the lower first and second switching transistors 121 and 122 are Si-material field effect transistors (Si-FET) having a body diode having a slow reverse recovery characteristic. can be composed of

According to the present invention, a digital amplifier can be applied to a high voltage application field by lowering the voltage stress of a switching transistor, and a Si-FET component device having a body diode with slow reverse recovery characteristic can be applied to the digital amplifier, thereby lowering the product price and reducing the product price. It has the advantage of securing flexibility in product design. In addition, since the voltage stress of the switching transistor is low, it is possible to reduce the switching loss occurring during switching, so it has advantageous advantages in terms of efficiency and heat generation.

[FIG. 1] is a schematic configuration diagram of a general digital amplifier.
[Figure 2] is a waveform graph when there is no input signal in the digital amplifier.
[Figure 3] is a waveform graph when there is an input signal from a digital amplifier.
4 is a diagram showing an equivalent circuit configuration and a current signal of a digital amplifier according to the prior art.
5 is a diagram showing the circuit configuration of a multi-level digital amplifier according to the present invention.
[Fig. 6] is a diagram showing an internal signal loop corresponding to PWM level switching of the driving power in the multi-level digital amplifier of the present invention.
[Fig. 7] is a view showing a concept of reducing the switching loss of the digital amplifier in the present invention.
8 is a diagram conceptually illustrating the role of a freewheeling diode in the multi-level digital amplifier of the present invention.
9 is a diagram showing the conduction path of the operating current constituted by the freewheeling diode during the reverse recovery process in the multi-level digital amplifier of the present invention.
[Fig. 10] is a view showing a general method of externally connecting a diode to a switching transistor in the prior art.
11 is an operation waveform showing the role of a freewheeling diode in the multi-level digital amplifier according to the present invention.
[Fig. 12] is a diagram showing the operation waveform through the simulation of the multi-level digital amplifier according to the present invention.

Hereinafter, the present invention will be described in detail with reference to the drawings.

[Fig. 5] is a diagram showing the circuit configuration of the multi-level digital amplifier 100 according to the present invention, and [Fig. 6] is a diagram showing an internal signal loop corresponding to PWM level switching of the driving power in the present invention.

The circuit configuration of [Fig. 5] is made to have the following advantages over the prior art in implementing a digital amplifier. First, by lowering the voltage stress applied to the switching transistor, the digital amplifier can be used even in high voltage applications, and the switching loss of the switching transistor is also reduced, thereby reducing the heat generation of the digital amplifier. And, by specially connecting a separate freewheeling diode with excellent reverse recovery characteristics to the switching transistor, it is possible to construct a digital amplifier using a Si-material field effect transistor (Si-FET), which has a body diode with a generally slow reverse recovery characteristic. is to make it possible

First, referring to [Fig. 5], four switching transistors 111, 112, 121, and 122 are connected in series to connect both ends of the digital amplifier driving power. Among them, the upper switching transistors based on the output terminals connected to the LC low-pass filters 130 and 140 are referred to as the upper first switching transistor 111 and the upper second switching transistor 112 , and the lower switching transistor is referred to as the upper switching transistor. is called a lower first switching transistor 121 and a lower second switching transistor 122 .

The inner loop capacitor Ccl 150 connects between the contact point where the upper first and second switching transistors 111 and 112 meet and the contact point where the lower first and second switching transistors 121 and 122 meet.

In addition, the first and second inner loop diodes Dcl1 and Dcl2 161 and 162 are connected in forward and reverse directions from the common potential of the digital amplifier driving power source to both ends of the inner loop capacitor Ccl 150 , respectively. As shown in FIG. 5 , the first inner loop diode Dcl1 161 is forwardly connected from the common potential of the digital amplifier driving power source to the contacts of the upper first and second switching transistors 111 and 112 , and the second inner loop diode Dcl1 . The loop diode (Dcl2) 162 is connected in reverse from the common potential of the digital amplifier driving power source to the contact points of the lower first and second switching transistors 121 and 122 .

In this case, a contact point where the source terminal of the upper second switching transistor 112 and the drain terminal of the lower first switching transistor 121 meet constitutes an output terminal of the switching transistor circuits 111 to 122 .

As in the prior art, a low-pass filter inductor (Lf; 130) constituting a low-pass filter (LPF) in order to remove the switching frequency component and pass only the frequency component of the analog input signal to the output terminal of the switching transistor circuits 111 to 122 and A low-pass filter capacitor (Cf) 140 is connected.

In addition, first and second freewheeling diodes 171 and 172 are provided from the output terminal to the final ends of the upper switching transistors 111 and 112 and the lower switching transistors 121 and 122 . That is, the first freewheeling diode 171 reversely connects both ends of the upper first and second switching transistors 111 and 112 connected in series. Similarly, the second freewheeling diode 172 reversely connects the final ends of the lower first and second switching transistors 121 and 122 connected in series.

These first and second freewheeling diodes 171 and 172 neutralize the body diodes D11 to D22 built in the switching transistors 111 to 122 as described later with reference to FIGS. 8 and 9 . installed to do so. That is, the body diodes D11 to D22 do not function due to these freewheeling diodes 171 and 172 . Accordingly, the switching transistors 111 to 122 do not need to have fast reverse recovery characteristics, and even if a device composed of, for example, a Si-material field effect transistor (Si-FET) having a body diode having a slow reverse recovery characteristic is adopted. free of charge

6 shows an inner loop of the digital amplifier circuit according to the present invention in response to PWM level switching of the driving power. (a) and (b) of [Fig. 6] respectively show the charging path of the inner loop capacitor Cc1 according to the operation of the switching transistor. A loop is formed. In the present invention, since the high voltage application field of 100V or more is the main interest, the voltage drop of Vds in the turn-on state of the switching transistors 111 and 122 and the forward voltage of the inner loop diodes Dcl1, Dcl2; 161, 162 having a value within 1.0V The drop Vf is ignored for convenience.

If we follow the inner loop indicated by the thick line in (a) and (b) of [Fig. 6], it can be seen that the voltage Vcl across the inner loop capacitor Ccl becomes equal to the driving voltage Vin in both cases. That is, the driving voltage Vin is always applied to the two switching transistors 112 and 121 located inside among the four switching transistors 111 to 122 connected in series. Through the process, the voltage stress applied to the internal switching transistors 112 and 121 becomes Vin.

As such, in the present invention, a series of switching transistors 111 to 122 connected in series are configured to share the driving voltage. Such a characteristic is called 'multilevel' in this specification.

The voltage stress applied to the switching transistor in the prior art was Vin, whereas in the present invention, the voltage stress was reduced by half. Accordingly, the circuit configuration of the present invention can be applied to a high voltage application field having twice the driving voltage by utilizing the same switching transistor components compared to the prior art.

In addition, according to the present invention, the advantage of lowering the switching loss of the digital amplifier can also be obtained, and [Fig. 7] conceptually shows this. (a) and (b) of [Fig. 7] represent the switching loss occurring in one of the switching transistors 11 and 12 in the prior art of [Fig. 4], and (c) and (d) of [Fig. 7] are In the present invention of FIG. 5 , a switching loss occurring in one of the switching transistors 111 to 122 is shown. As the voltage stress is reduced by half, it can be expected that the switching loss of one switching transistor is intuitively reduced to (1/4). 1/2) can be expected.

Even if it is calculated mathematically, the same result can be obtained. The switching loss occurring in the digital amplifier 10 of the prior art of FIG. 4 can be expressed by [Equation 1], and the switching loss occurring in the digital amplifier 100 of the present invention of FIG. 5 is [Equation 1] It can be expressed by Equation 2]. In this case, Cds represents the capacitance of the output capacitor inherent in the switching transistor. Comparing [Equation 1] and [Equation 2], according to the present invention, it can be confirmed that the switching loss is reduced to (1/2).

[Fig. 8] is a diagram conceptually illustrating the role of a freewheeling diode in the multi-level digital amplifier 100 of the present invention, and [Fig. 9] is a diagram illustrating freewheeling in the reverse recovery process in the multi-level digital amplifier 100 of the present invention. It is a diagram showing the conduction path of the operating current constituted by the diodes Dfw1 and Dfw2; 171 and 172.

Referring to the circuit diagram of FIG. 5 , in the present invention, the freewheeling diode reversely connects the final ends of the upper switching transistors 111 and 112 and the lower switching transistors 121 and 122, each of which is configured by connecting two transistors in series. (freewheeling diode) (Dfw1, Dfw2; 171, 172) is preferably installed. These freewheeling diodes 171 and 172 are installed to disable the body diodes D11 to D22 built in the switching transistors 111 to 122 .

Referring to FIG. 8 , the reverse recovery current should flow through any one of these diodes D11, D12, and Dfw1 in a dead time section where the reverse recovery current should flow. In the path on the body diode (D11, D12) side, since two diodes are connected in series, there is twice the forward voltage drop (Vf), that is, a voltage drop of 2Vf, whereas the path on the freewheeling diode (Dfw1; 171) side The voltage drop is only Vf. Accordingly, the reverse recovery current flows through the path of the freewheeling diode Dfw1 171 to which a relatively low voltage drop is applied, and does not flow toward the body diodes D11 and D12 of the switching transistor.

9 is a diagram illustrating a signal loop constituted by the freewheeling diodes (Dfw1, Dfw2; 171, 172) in the reverse recovery process in the multi-level digital amplifier 100 of the present invention. In the reverse recovery process, no current flows through the body diodes D11 to D22 and current flows through the freewheeling diodes Dfw1 and Dfw2; 171 and 172.

As a result, as shown in FIG. 5 , if the diode components 171 and 172 having excellent reverse recovery characteristics are externally connected, the switching transistors 111 to 122 do not need to have fast reverse recovery characteristics. Therefore, although the reverse recovery characteristics of the body diodes present in the switching transistors 111 to 122 are bad, it is ok to adopt a device having a high withstand voltage and a low price, for example, a device composed of a Si-material field effect transistor (Si-FET).

Meanwhile, even in the prior art, when the body diode characteristics of the switching transistor are unsatisfactory, there has been an approach to solve the problem by connecting a separate diode to the outside. [Fig. 10] is a diagram showing a general approach of externally connecting a diode to a switching transistor in the prior art.

Referring to [Fig. 10], in the prior art, when the body diode Da of the switching transistor is unsatisfactory, the blocking diode Db is connected to the drain terminal of the switching transistor in the reverse direction to block the function of the body diode Da, , and then an external diode (Dext) was connected to the outside.

In the prior art, not only there is an inconvenience of having to connect the blocking diode Db, but also a forward voltage drop due to the blocking diode Db is added when driving power is applied in the forward direction, and there is a continuity loss due to this. There is this. On the other hand, in the present invention, the reverse recovery current path is set to the external freewheeling diode by the forward voltage drop inherent in the multi-level switching transistors to share the voltage stress, so unlike the prior art, the blocking diode Db is used. There is no need to connect, so there is no such problem.

[Fig. 11] is an operation waveform showing the role of the freewheeling diode in the multi-level digital amplifier 100 according to the present invention. Referring to FIG. 11 , the current flow for reverse recovery does not go through the body diodes D11 to D22 inside the switching transistors Q11 to Q22, but to the freewheeling diodes Dfw1 and Dfw2 connected externally. It is shown that this is done through

[Fig. 12] is a diagram showing the operation waveform through the simulation of the multi-level digital amplifier 100 according to the present invention.

As the simulation conditions, the driving voltage Vin was ±70V, the output voltage Vout was 60V, and the total power capacity was designed to be 360W. Since the driving voltage Vin is ±70V, a voltage of 140V, which is effectively peak-to-peak (Vpp), is applied. From the simulation results of [Fig. 12], it can be confirmed that the voltage stress applied to the switching transistors 111 and 122 is about 70V, which is only 1/2 of that of the prior art. According to the prior art of FIG. 4 , a voltage twice Vin, that is, a peak-to-peak voltage (Vpp) of 140V, would have been applied to the switching transistor as a voltage stress. In addition, from the simulation results of [Fig. 12], it can also be confirmed that current flows through the freewheeling diode in the dead time section.

Meanwhile, the present invention can be implemented in the form of computer-readable codes on a computer-readable non-volatile recording medium. Various types of storage devices exist as such non-volatile recording media. For example, hard disks, SSDs, CD-ROMs, NAS, magnetic tapes, web disks, cloud disks, etc. A form in which it can be implemented and executed can also be implemented. In addition, the present invention may be implemented in the form of a computer program stored in a medium to execute a specific procedure in combination with hardware.

100: digital amplifier
111, 112: high-side switching transistors (Q11, Q12)
121, 122: lower side switching transistors (Q21, Q22)
130: low-pass filter inductor (Lf)
140: low-pass filter capacitor (Cf)
150: inner loop capacitor (Ccl)
161, 162: inner loop diode (Dcl1, Dcl2)
171, 172: Freewheeling diodes (Dfw1, Dfw2)

Claims (4)

an upper first switching transistor 111, an upper second switching transistor 112, a lower first switching transistor 121, and a lower second switching transistor 122 connected in series to connect both ends of the digital amplifier driving power supply;
an inner loop capacitor 150 connecting the contacts of the upper first and second switching transistors 111 and 112 and the lower first and second switching transistors 121 and 122;
a first inner loop diode (161) having an anode terminal connected to a common potential of the digital amplifier driving power source and a cathode terminal connected to a contact point of the upper first and second switching transistors (111, 112);
a second inner loop diode 162 having a cathode connected to a common potential of the digital amplifier driving power and an anode connected to a contact point of the lower first and second switching transistors 121 and 122;
a low-pass filter inductor 130 and a low-pass filter capacitor 140 connected to a contact point between the upper second switching transistor 112 and the lower first switching transistor 121 to form a low-pass filter;
The anode terminal is connected to the contact point between the upper second switching transistor 112 and the lower first switching transistor 121 and the cathode terminal is connected to the contact point between the upper first switching transistor 111 and the digital amplifier driving power source. 1 freewheeling diode 171;
A second electrode in which a cathode is connected to a contact between the upper second switching transistor 112 and the lower first switching transistor 121 and an anode is connected to a contact between the lower second switching transistor 122 and the digital amplifier driving power source 2 freewheeling diodes 172;
A multi-level digital amplifier with low voltage stress, comprising:
delete delete The method according to claim 1,
The upper first and second switching transistors 111 and 112 and the lower first and second switching transistors 121 and 122 are Si-material field effect transistors (Si-FET) having a body diode having a slow reverse recovery characteristic. Multi-level digital amplifier of low voltage stress, characterized in that consisting of.

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