Transconductance error amplifier and switching power supply

Abstract

The application discloses a transconductance type error amplifier for a switching power supply and a corresponding switching power supply. The switching power supply has a feedback circuit and provides a feedback voltage, a reference voltage and an output voltage. The error amplifier comprises an output current generating circuit, an input stage, two first type current mirrors and a second type current mirror. The output current generating circuit is connected to the input stage composed of a triode pair. The emitter stages of the two triodes of the triode pair are connected to the output end of the output current generating circuit, and the collector stages are connected to the input ends of the two first type current mirrors respectively. The output ends of the two first type current mirrors are connected to the input end and the output end of the second type current mirror respectively. The output current of the output current generating circuit is controlled by the output voltage of the switching power supply, so that the transconductance of the error amplifier is proportional to the output voltage, thereby realizing that the bandwidth of the system does not change when the output voltage changes under the same compensation parameters, and the maximum bandwidth is obtained for different output voltages.

Description

Technical Field

[0001] This invention belongs to the field of switching power supply technology, specifically relating to a transconductance error amplifier and a switching power supply. Background Technology

[0002] Switching power supplies are widely used in power supply systems due to their high conversion efficiency. Common topologies in current-mode switching power supplies include buck, boost, and buck-boost types. Among these, the structure using a transconductance operational amplifier as the error amplifier is the most common. Generally, the transconductance of the error amplifier is fixed. This leads to a problem: for different output voltages, if the same external compensation parameters are used, the bandwidth of the switching power supply varies with the output voltage. To accommodate a wide output voltage range, a smaller bandwidth is needed to ensure stability across all output voltages.

[0003] For structures such as buck, boost, and buck-boost, their unified small-signal model is as follows: Figure 1 As shown in the diagram. ref is the reference voltage, A1 is a transconductance-type error amplifier with a transconductance value of gmea. The compensation resistor Rcomp and the compensation capacitor Ccomp constitute the compensation network. The power stage is a power conversion circuit, generally composed of a switching transistor and an inductor. For a current-controlled power stage, its equivalent model is a voltage-controlled current source with a transconductance value of gmps. The transconductance values ​​gmea and gmps are generally set internally by the chip and are fixed values. The first feedback resistor Rfb1 and the second feedback resistor Rfb2 constitute the output voltage feedback network. Cout is the output capacitor, and Rload is the output equivalent load resistance.

[0004] According to this small-signal model, the system loop bandwidth fc can be expressed as:

[0005]

[0006] To facilitate use and reduce the number of external components, thereby lowering system costs, an increasing number of power chips or modules integrate compensation components Rcomp, Ccomp, and output capacitor Cout within the chip or module itself. Users only need to adjust the first feedback resistor Rfb1 and the second feedback resistor Rfb2 to obtain the desired output voltage Vout.

[0007] In switching power supply systems, to maintain system stability, the loop bandwidth is generally not too wide; for example, fc is typically set to around 1 / 10 to 1 / 6 of the switching frequency. However, to achieve faster response speeds, a bandwidth as large as possible is generally desirable. As can be seen from the formula above, with the external components Rcomp and Cout fixed, the system loop bandwidth fc increases as the output voltage decreases. Therefore, if Rcomp and Cout are fixed, the user must ensure that the system bandwidth meets the stability requirements in the worst-case scenario, i.e., the system bandwidth is around 1 / 10 to 1 / 6 of the switching frequency when the output voltage is at its minimum. In this case, when the output voltage is high, the system bandwidth will be relatively small, resulting in poor transient response to the load.

[0008] In existing technical solutions, the transconductance gmea of ​​the error amplifier is a fixed value. Figure 2 This paper illustrates a transconductance error amplifier with a PNP or PMOS input stage. Iea is a fixed current bias, and Q1 and Q2 are differential input pairs, which can be PNP or PMOS.

[0009] Figure 3 The image shows a typical N-type current mirror formed by NMOS or NPN. Figure 4 The image shows a typical P-type current mirror formed by PMOS or PNP. Figure 5 The diagram shows a transconductance error amplifier using NPN or NMOS transistors as the input differential pair. Q1 and Q2 are the differential input pair transistors, which can be either NPN or NMOS transistors.

[0010] Figure 6 The diagram shows a typical level shifting circuit. Level shifting circuits are used to shift the common-mode level to meet the requirements of the error amplifier's common-mode input range.

[0011] As can be seen from the system loop bandwidth formula of the small-signal model, when the peripheral components, including the compensation resistor Rcomp and the output capacitor Cout, are fixed, the system loop bandwidth fc increases as the output voltage decreases.

[0012] Figure 7 The gain curves of a certain type of existing switching power supply are shown at output voltages of 5V and 1V. From... Figure 7 As can be seen from this, the output voltage V O The bandwidth at 1V is approximately equal to the output voltage V. O =5 times the bandwidth at 5V. Summary of the Invention

[0013] (a) Technical problems to be solved

[0014] The present invention aims to solve the technical problem that when a switching power supply outputs a high voltage, the system bandwidth is relatively small, resulting in poor load transient response.

[0015] (II) Technical Solution

[0016] To address the aforementioned technical problems, the first aspect of this invention proposes a transconductance error amplifier for a switching power supply. The switching power supply has a feedback circuit and provides a feedback voltage, a reference voltage, and an output voltage. The transconductance error amplifier includes an output current generation circuit, an input stage, two first-type current mirrors, and one second-type current mirror. The output current generation circuit is connected to the input stage, which is composed of transistor pairs. The emitters of both transistors in the transistor pairs are connected to the output terminals of the output current generation circuit, and their collectors are respectively connected to the input terminals of the two first-type current mirrors. The output terminals of the two first-type current mirrors are respectively connected to the input and output terminals of the second-type current mirror. The output current of the output current generation circuit is controlled by the output voltage of the switching power supply.

[0017] According to a preferred embodiment of the present invention, the output current generating circuit includes a first current generating circuit, a second current generating circuit, and a current selection circuit; the first current generating circuit generates a first current, the second current generating circuit generates a second current, and the current selection circuit selects either the first current or the second current as the output current based on the output voltage of the error amplifier; the first current is a constant current; the second current is proportional to the output voltage of the switching power supply; the current selection circuit controls the output current output by the output current generating circuit so that when the output voltage of the switching power supply is less than or equal to the minimum reference voltage, the first current is output, and when the output voltage of the switching power supply is greater than the minimum reference voltage, the second current is output.

[0018] According to a preferred embodiment of the present invention, the second current generating circuit includes an operational amplifier, a first MOS transistor, a first resistor, and a P-type current mirror; the first MOS transistor is a PMOS transistor, its emitter is connected to the negative input terminal of the operational amplifier and grounded through the first resistor, the base of the first MOS transistor is connected to the output terminal of the operational amplifier, and its collector is connected to the input terminal of the P-type current mirror; the positive input terminal of the operational amplifier is connected to the output voltage of the error amplifier, and the output terminal of the P-type current mirror is used to output the second current.

[0019] According to a preferred embodiment of the present invention, the current selection circuit includes a first MOS transistor pair and a second MOS transistor pair, each MOS transistor pair including two MOS transistors. The emitters of the two MOS transistors in the first MOS transistor pair are connected to the first current, and the emitters of the two MOS transistors in the second MOS transistor pair are connected to the second current.

[0020] According to a preferred embodiment of the present invention, the MOS transistors of the first MOS transistor pair and the second MOS transistor pair are both NMOS transistors; the first MOS transistor pair includes a second MOS transistor and a third MOS transistor, and the second MOS transistor pair includes a fourth MOS transistor and a fifth MOS transistor; the bases of the second MOS transistor and the fifth MOS transistor are connected to a minimum reference voltage, the bases of the third MOS transistor and the fourth MOS transistor are connected to the output voltage of the switching power supply, and the collectors of the third MOS transistor and the fifth MOS transistor are both connected to the output terminal of the output current generating circuit for outputting the output current.

[0021] According to a preferred embodiment of the present invention, the input stage is a PNP type input stage, the first type is N type, and the second type is P type.

[0022] According to a preferred embodiment of the present invention, the PNP input stage is composed of a pair of PNP transistors, and the bases of the two transistors in the PNP transistor pair are respectively connected to the feedback voltage and the reference voltage.

[0023] According to a preferred embodiment of the present invention, the input stage is an NPN type input stage, the first type is P type, and the second type is N type.

[0024] According to a preferred embodiment of the present invention, the NPN input stage is composed of an NPN transistor pair, wherein the bases of the two transistors in the NPN transistor pair are respectively connected to the shifted voltages of the feedback voltage and the reference voltage.

[0025] A second aspect of the present invention provides a switching power supply including the transconductance error amplifier for switching power supplies described above.

[0026] (III) Beneficial Effects

[0027] In this invention, the transconductance of the error amplifier is proportional to the output voltage, thereby achieving a constant bandwidth of the system when the output voltage changes under the same compensation parameters, and obtaining the maximum bandwidth for different output voltages. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of a small-signal model circuit for an existing switching power supply.

[0029] Figure 2 A circuit diagram of an existing transconductance error amplifier with a PNP or PMOS input stage is shown.

[0030] Figure 3 A typical N-type current mirror circuit diagram formed by existing NMOS or NPN is shown.

[0031] Figure 4A typical P-type current mirror circuit diagram formed by existing PMOS or PNP is shown.

[0032] Figure 5 A circuit diagram of an existing transconductance error amplifier with an input stage of NPN or NMOS is shown.

[0033] Figure 6 An existing level shifting circuit is shown.

[0034] Figure 7 The gain curves of a certain type of existing switching power supply are shown at output voltages of 5V and 1V.

[0035] Figure 8 A circuit diagram of a PNP transconductance error amplifier according to an embodiment of the present invention is shown.

[0036] Figure 9 A circuit diagram of an NPN transconductance error amplifier according to an embodiment of the present invention is shown.

[0037] Figure 10 The diagram shows the relationship between the output current Iea of ​​the output current generation circuit Is of the present invention and the output voltage vout of the switching power supply.

[0038] Figure 11 A circuit diagram of an output current generation circuit according to an embodiment of the present invention is shown.

[0039] Figure 12 The gain curves of a switching power supply according to an embodiment of the present invention at output voltages of 5V and 1V are shown.

[0040] Figure 13 A circuit diagram of an NMOS transconductance error amplifier according to an embodiment of the present invention is shown.

[0041] Figure 14 A circuit diagram of an NMOS transconductance error amplifier according to an embodiment of the present invention is shown. Detailed Implementation

[0042] This invention proposes to use the tail current of the transconductance error amplifier as the current controlled by the output voltage vout of the switching power supply, so that the transconductance of the error amplifier is proportional to vout when the output voltage is stable. This achieves that the bandwidth of the system remains unchanged when the output voltage changes under the same compensation parameters, and obtains the maximum bandwidth for different output voltages.

[0043] Specifically, the transconductance error amplifier for switching power supplies of the present invention includes an output current generation circuit, an input stage, two first-type current mirrors, and one second-type current mirror. The output current generation circuit is connected to the input stage, which is composed of transistor pairs. The emitters of the two transistors in each transistor pair are connected to the output terminal of the output current generation circuit, and their collectors are respectively connected to the input terminals of the two first-type current mirrors; the output terminals of the two first-type current mirrors are respectively connected to the input and output terminals of the second-type current mirror.

[0044] The output current of the output current generating circuit is controlled by the output voltage of the switching power supply, such that when the output voltage of the switching power supply is less than or equal to the minimum reference voltage, the first current is output, and when the output voltage of the switching power supply is greater than the minimum reference voltage, the second current is output. The first current is a constant current, and the second current is proportional to the transmission voltage of the switching power supply.

[0045] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0046] Figure 8 A circuit diagram of a PNP transconductance error amplifier according to an embodiment of the present invention is shown. Figure 8 As shown, the transconductance error amplifier in this embodiment includes an output current generation circuit Is, a PNP input stage, two N-type current mirrors, and one P-type current mirror. The output current generation circuit Is is connected to the PNP input stage, which is composed of a PNP transistor pair consisting of a first transistor Q1 and a second transistor Q2. The emitters of both transistors Q1 and Q2 are connected to the output terminal of the output current generation circuit Is, and their collectors are connected to the input terminals of the two N-type current mirrors (the first current mirror Mr1 and the second current mirror Mr2). The output terminals of the first and second current mirrors Mr1 and Mr2 are connected to the input and output terminals of a third current mirror Mr3, which serves as a P-type current mirror, respectively. The base of the first transistor Q1 is connected to the feedback voltage of the feedback circuit of the switching power supply, and the base of the second transistor Q2 is connected to a reference voltage. The tail current of the error amplifier is the current generated by the output current generation circuit Is. The output current generation circuit Is includes a current source and a control circuit, so that its output current Iea is controlled by the output voltage vout of the switching power supply. The specific implementation of the output current generation circuit Is will be described in detail later.

[0047] Figure 9 A circuit diagram of an NPN transconductance error amplifier according to another embodiment of the present invention is shown. Figure 9As shown, the transconductance error amplifier in this embodiment includes an output current generation circuit Is, an NPN input stage, two P-type current mirrors, and one N-type current mirror. The current generation circuit Is is connected to the NPN input stage, which is composed of an NPN transistor pair consisting of a third transistor Q3 and a fourth transistor Q4. The emitters of both transistors are connected to the output terminals of the output current generation circuit Is, and their collectors are connected to the input terminals of the two P-type current mirrors (the fourth current mirror Mr4 and the fifth current mirror Mr5). The output terminals of the fourth current mirror Mr4 and the fifth current mirror Mr5 are connected to the output and input terminals of the sixth current mirror Mr6, which serves as an N-type current mirror, respectively. The base of the third transistor Q3 is connected to the reference shift voltage ref_shift of the switching power supply, and the base of the fourth transistor Q4 is connected to the feedback shift voltage fb_shift. The reference shift voltage and the feedback shift voltage are voltages generated by shifting the reference voltage ref and the feedback voltage fb through a level shifting circuit LSC. Similarly, the tail current Iea of ​​the error amplifier, which is the current generated by the output current generation circuit Is, is controlled by the output voltage vout of the switching power supply. The specific implementation of the output current generation circuit Is will be described in detail later.

[0048] The level shifting circuit LSC mentioned above can be adopted as follows: Figure 6 The level shifter circuit shown is an LSC (Level Shifter) used to shift the common-mode level to meet the common-mode input range requirements of the error amplifier. Figure 6 In the circuit, the relationship between the reference shift voltage ref_shift and the reference voltage ref, and the relationship between the feedback shift voltage fb_shift and the feedback voltage are as follows:

[0049] ref_shift = ref + Vgs1

[0050] fb_shift = fb + Vgs2

[0051] Vgs1 and Vgs2 are the source-gate voltages of the two MOSFETs, respectively.

[0052] Figure 10 The diagram shows the relationship between the output current Iea of ​​the output current generation circuit Is and the output voltage vout of the switching power supply. Figure 10 As shown, the minimum reference voltage ref1 is a preset value, which is equal to or less than the minimum output voltage. When the output voltage vout of the switching power supply is less than the minimum reference voltage ref, an appropriate value is given to the output current Iea to ensure normal operation before the output voltage is established. When the output voltage reaches a value greater than the minimum reference voltage ref1, Iea is proportional to the output voltage vout of the switching power supply.

[0053] When vout > ref1, Iea = k * vout, where k is the scaling factor.

[0054] According to the transconductance calculation formula of a transistor,

[0055]

[0056] Where gm is the transconductance of the transistor, IC is the collector current of the transistor (IC = Iea), and Vt is the thermal voltage. Since Vt = KT / q, K is the Boltzmann constant (K = 1.38 * 10⁻⁶). -23 J / K), T is the absolute temperature, and q is the electron charge (q = 1.6 * 10⁻⁶). - 19 C), therefore Vt is a coefficient that is proportional to the absolute temperature.

[0057] Therefore:

[0058]

[0059] Where gmea is the transconductance of the error amplifier, and n is the scaling factor introduced by the N-type current mirror and the P-type current mirror.

[0060] Substituting this into the system loop bandwidth formula, we can obtain:

[0061]

[0062] Therefore, it can be seen that the system loop bandwidth fc of the switching power supply is independent of the output voltage. Meanwhile, Vt has a positive temperature coefficient, increasing with increasing temperature, while Cout generally has a negative temperature coefficient, decreasing with increasing temperature. Vt and Cout have a certain compensating effect at different temperatures, reducing the variation of loop bandwidth with Vout and temperature.

[0063] Figure 11 A circuit diagram of an output current generating circuit according to an embodiment of the present invention is shown. Figure 11 As shown, the output current generation circuit includes a first current generation circuit, a second current generation circuit, and a current selection circuit. The first current generation circuit generates a first current I1, the second current generation circuit generates a second current I2, and the current selection circuit selects either the first current I1 or the second current I2 as the output current based on the output voltage of the error amplifier.

[0064] The first current generating circuit generates and outputs a first current I1 from the first current source Is1.

[0065] The second current generating circuit includes an operational amplifier U1, a first MOSFET M1, a first resistor R1, and a P-type current mirror. In this embodiment, the first MOSFET M1 is a PMOS transistor, with its emitter connected to the negative input terminal of the operational amplifier U1 and grounded via the first resistor R1. The base of the first MOSFET M1 is connected to the output terminal of the operational amplifier U1, and its collector is connected to the input terminal of the P-type current mirror Mr0. The positive input terminal of the operational amplifier U1 is connected to the output voltage of the error amplifier. The output terminal of the P-type current mirror Mr0 is used to output the second current I2. It can be seen that the second current I2 generated by the second current generating circuit satisfies I2 = vout / R1, that is, it is proportional to the output voltage vout of the switching power supply. R1 here also represents the resistance value of the first resistor R1.

[0066] The current selection circuit includes two MOSFET pairs: a first MOSFET pair and a second MOSFET pair. Each MOSFET pair consists of two MOSFETs. The emitters of the two MOSFETs in the first MOSFET pair are connected to a first current I1, and the emitters of the two MOSFETs in the second MOSFET pair are connected to a second current I2. Figure 11 In the illustrated embodiment, each MOSFET pair consists of NMOS transistors. Specifically, the first MOSFET pair includes a second MOSFET M2 and a third MOSFET M3, the second MOSFET pair includes a fourth MOSFET M4 and a fifth MOSFET M5, the bases of the second MOSFET M2 and the fifth MOSFET M5 are connected to the minimum reference voltage ref1, the bases of the third MOSFET M3 and the fourth MOSFET M4 are connected to the output voltage vout of the switching power supply, and the collectors of the third MOSFET M3 and the fifth MOSFET M5 are both connected to the output terminal of the output current generation circuit for outputting the output current Iea.

[0067] When the above-mentioned output current generating circuit is working, when the output voltage vout of the switching power supply is less than the minimum reference voltage ref1, the second MOSFET M2 and the fifth MOSFET M5 are turned off, and the third MOSFET M3 and the fourth MOSFET M4 are turned on. Therefore, the output current Iea is the first current I1.

[0068] When the output voltage vout of the switching power supply is greater than the minimum reference voltage ref1, the second MOSFET M2 and the fifth MOSFET M5 are turned on, while the third MOSFET M3 and the fourth MOSFET M4 are turned off. Therefore, the output current Iea is the second current I2, which satisfies Iea=I2=vout / R1.

[0069] Figure 12 The figure shows the gain curves of a switching power supply according to an embodiment of the present invention at output voltages of 5V and 1V. It can be seen that the system loop bandwidth is essentially the same under both extreme cases of output voltages of 5V and 1V.

[0070] According to the present invention, Figure 8 and Figure 9 The differential input NPN and PNP transistor pairs in the illustrated embodiment can also be replaced with NMOS and PMOS transistors. Their circuit diagrams are shown below. Figure 13 and Figure 14 As shown, the first and second PNP transistors Q1 and Q2 are replaced by the sixth NMOS transistor M6 and the seventh NMOS transistor M7, respectively, and the third and fourth NPN transistors Q3 and Q4 are replaced by the eighth PMOS transistor M8 and the ninth PMOS transistor M9, respectively.

[0071] For MOSFETs (NMOS and PMOS), to reduce threshold voltage offset, the device size is generally large, and the device operates in the subthreshold region. The relationship between its transconductance and tail current is as follows:

[0072]

[0073] Among them, I D The tail current (I) of the MOSFET D =Iea), where m is a process-related constant, and similarly, Vt = kT / q is a coefficient proportional to temperature. Therefore,

[0074]

[0075] Substituting into the system loop bandwidth formula, we get:

[0076]

[0077]

[0078] Therefore, the system loop bandwidth does not change with the output voltage. At the same time, Vt has a certain compensating effect on the temperature change of Cout, thereby reducing the change of loop bandwidth with vout and temperature.

[0079] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A transconductance error amplifier for a switching power supply, the switching power supply having a feedback circuit and providing a feedback voltage, a reference voltage, and an output voltage, characterized in that: The transconductance error amplifier includes an output current generation circuit, an input stage, two first-type current mirrors and one second-type current mirror; The output current generating circuit is connected to the input stage, which is composed of transistor pairs; The emitters of both transistors in the transistor pair are connected to the output terminal of the output current generating circuit, and the collectors are respectively connected to the input terminals of the two type I current mirrors. The output terminals of the two Type I current mirrors are respectively connected to the input and output terminals of the Type II current mirror; The output current of the output current generating circuit is controlled by the output voltage of the switching power supply. The output current generating circuit includes a first current generating circuit, a second current generating circuit, and a current selection circuit. The first current generating circuit is used to generate a first current, the second current generating circuit is used to generate a second current, and the current selection circuit is used to select the first current or the second current as the output current according to the output voltage of the error amplifier. The first current is a constant current; The second current is proportional to the transmission voltage of the switching power supply; The current selection circuit controls the output current generated by the output current generation circuit so that when the output voltage of the switching power supply is less than or equal to the minimum reference voltage, the first current is output, and when the output voltage of the switching power supply is greater than the minimum reference voltage, the second current is output.

2. The transconductance error amplifier for switching power supplies as described in claim 1, characterized in that: The second current generating circuit includes an operational amplifier, a first MOSFET, a first resistor, and a P-type current mirror; The first MOSFET is a PMOS transistor, whose emitter is connected to the negative input terminal of the operational amplifier and grounded through the first resistor. The base of the first MOSFET is connected to the output terminal of the operational amplifier, and the collector is connected to the input terminal of the P-type current mirror. The positive input terminal of the operational amplifier is connected to the output voltage of the error amplifier, and the output terminal of the P-type current mirror is used to output the second current.

3. The transconductance error amplifier for switching power supplies as described in claim 2, characterized in that: The current selection circuit includes a first MOSFET pair and a second MOSFET pair. Each MOSFET pair includes two MOSFETs. The emitters of the two MOSFETs in the first MOSFET pair are connected to the first current, and the emitters of the two MOSFETs in the second MOSFET pair are connected to the second current.

4. The transconductance error amplifier for switching power supplies as described in claim 3, characterized in that: Both the first and second MOS transistor pairs use NMOS transistors. The first MOSFET pair includes the second MOSFET and the third MOSFET, and the second MOSFET pair includes the fourth MOSFET and the fifth MOSFET; The bases of the second and fifth MOSFETs are connected to the minimum reference voltage, and the bases of the third and fourth MOSFETs are connected to the output voltage of the switching power supply. The collectors of the third and fifth MOSFETs are both connected to the output terminal of the output current generation circuit to output the output current.

5. The transconductance error amplifier for a switching power supply as described in any one of claims 1 to 4, characterized in that: The input stage is a PNP type input stage, with the first type being N type and the second type being P type.

6. The transconductance error amplifier for a switching power supply as described in claim 5, characterized in that: The PNP input stage is composed of a pair of PNP transistors, with the bases of the two transistors in the PNP transistor pair connected to the feedback voltage and the reference voltage, respectively.

7. The transconductance error amplifier for a switching power supply as described in any one of claims 1 to 4, characterized in that: The input stage is an NPN type input stage, with the first type being P type and the second type being N type.

8. The transconductance error amplifier for a switching power supply as described in claim 7, characterized in that: The NPN input stage is composed of NPN transistor pairs, with the bases of the two transistors in the NPN transistor pair connected to the shifted voltages of the feedback voltage and the reference voltage, respectively.

9. A switching power supply, characterized in that: Includes the transconductance error amplifier for a switching power supply as described in any one of claims 1 to 4.

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