A wide range operation interleaved parallel LLC resonant converter and control method

Abstract

The present invention discloses a wide-range operation staggered parallel LLC resonant converter and control method, which relates to the field of power electronic conversion technology, including: N LLC resonant conversion units, the LLC resonant conversion units include inverters, resonant networks, transformers and secondary side rectifier circuits connected in series in sequence; the input ends of the N inverters are connected in parallel to the DC input end; the output ends of the N secondary side rectifier circuits are connected in series; wherein the first output end of the first secondary side rectifier circuit serves as the first DC output end, and the second output end is connected to the first output end of the next secondary side rectifier circuit until it is connected to the first output end of the Nth secondary side rectifier circuit, and the second output end of the Nth secondary side rectifier circuit serves as the second DC output end. The power conversion with wide range operation, high power transmission and high step-up ratio is realized, while taking into account the conversion efficiency and inter-phase current sharing.

Description

Technical Field

[0001] The invention relates to the technical field of power electronic conversion, and in particular to a wide-range operation staggered parallel LLC resonant converter and a control method. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] With the rapid growth of offshore wind power generation capacity and the development of flexible direct current transmission, offshore wind turbines are required to provide stable and efficient power conversion. At present, many offshore wind farms use resonant DC-DC converters, and resonant converters only use variable frequency control strategies. Taking LLC resonant converters as an example, on the one hand, since offshore wind farms require a wider operating range, when the wind turbine input is low, the gain of the LLC resonant network may be less than 1. At this time, if voltage regulation is achieved, a large range of frequency adjustment is required, and excessive switching frequency causes higher losses. On the other hand, when the wind farm load is light, the excitation current loss of the resonant converter is relatively large, and the light load operation efficiency is relatively low. Using only variable frequency control strategies is not conducive to improving efficiency.

[0004] Since offshore wind farms require high-power DC-DC conversion, multiplexing such as interleaving and paralleling has become an important means to solve large-scale power conversion. However, one of the problems brought about by interleaving and paralleling technology is the phase-to-phase current imbalance caused by phase parameter mismatch. Therefore, how to achieve phase-to-phase current sharing through appropriate control means is a problem that needs to be solved at present. Summary of the invention

[0005] In order to solve the above problems, the present invention proposes a wide-range operation interleaved parallel LLC resonant converter and a control method, which adopts an interleaved parallel LLC resonant topology structure to achieve wide-range operation, high power transmission and high step-up ratio DC-DC power conversion, and at the same time, by adopting a composite control strategy of frequency conversion and phase shifting, it takes into account both conversion efficiency and inter-phase current sharing.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a wide-range operation interleaved parallel LLC resonant converter, comprising: N LLC resonant conversion units with input ends connected in parallel and output ends connected in series, N being an integer greater than or equal to 2, each LLC resonant conversion unit comprising an inverter, a resonant network, a transformer, and a secondary-side rectifier circuit connected in series in sequence;

[0008] The input terminals of the N inverters are connected in parallel to the DC input terminal;

[0009] The output ends of N secondary-side rectifier circuits are connected in series; wherein the first output end of the first secondary-side rectifier circuit serves as the first DC output end, and the second output end is connected to the first output end of the next secondary-side rectifier circuit until it is connected to the first output end of the Nth secondary-side rectifier circuit, and the second output end of the Nth secondary-side rectifier circuit serves as the second DC output end.

[0010] As an optional implementation, the inverter adopts a two-level inverter circuit or a three-level inverter circuit, and the control time of the N inverters is different, and the control time of each inverter is staggered by T. s / N time, T s is the switching cycle.

[0011] As an optional implementation, the resonant network includes a first inductor, a second inductor and a capacitor; the first end point of the first inductor is connected to the first input end of the resonant network, and the second end point of the first inductor is connected to the first output end of the resonant network; the first end point of the second inductor is connected to the first output end of the resonant network, and the second end point of the second inductor is connected to the second output end of the resonant network; the first end point of the capacitor is connected to the second input end of the resonant network, and the second end point of the capacitor is connected to the second output end of the resonant network.

[0012] As an optional implementation, the two input ends of the primary side of the transformer are connected to the two output ends of the resonant network, and the two output ends of the secondary side of the transformer are connected to the two input ends of the secondary side rectifier circuit.

[0013] As an optional implementation, the secondary-side rectifier circuit is used to convert the AC voltage output by the transformer into a DC voltage, and adopts a rectifier circuit in which multiple diodes are connected in series.

[0014] The wide-range operation interleaved parallel LLC resonant converter proposed in the present invention adopts N-phase interleaved parallel, that is, the input is parallel and the output is series. Through this topology, wide-range operation (that is, wide-range input and output capabilities), high power transmission and high step-up ratio DC-DC power conversion are achieved, the load capacity is improved, and at the same time, the cost, power loss and volume are reduced by adopting a resonant network.

[0015] In a second aspect, the present invention provides a control method for a wide-range operation interleaved parallel LLC resonant converter, using the wide-range operation interleaved parallel LLC resonant converter described in the first aspect, comprising:

[0016] Obtaining a DC input voltage at a DC input terminal, a DC output voltage at a DC output terminal, and a DC output current of each phase;

[0017] Obtaining a first unitized control amount according to the output reference voltage and the DC terminal output voltage;

[0018] A second unitized control quantity is obtained according to the output reference current and the DC output current of each phase, and a phase-shift control strategy is adopted to adjust the phase current according to the second unitized control quantity;

[0019] According to the comparison result of N times DC input voltage and n times DC output voltage, n is the transformation ratio of the transformer, a phase shift control strategy or a frequency conversion control strategy is selected, and the voltage is adjusted by adjusting the phase shift angle according to the first unitized control amount or using a voltage-controlled oscillator to control the switching frequency.

[0020] As an optional implementation, a first unitized control quantity is obtained according to the difference between the output reference voltage and the DC output voltage; a second unitized control quantity is obtained according to the difference between the output reference current and the DC output current of each phase; wherein the absolute values ​​of the first unitized control quantity and the second unitized control quantity are both less than 1.

[0021] As an optional implementation, if N times the DC input voltage is greater than or equal to n times the DC output voltage, the output gain of the resonant network is less than 1, and a phase shift control strategy is adopted to adjust the phase shift angle according to the first unitized control amount, thereby adjusting the voltage.

[0022] As an optional implementation, if N times the DC input voltage is less than n times the DC output voltage, it is determined whether the output current of each phase is less than 0.5 times the rated current, and based on the determination result, the phase shift angle is adjusted according to the first unitized control amount or the voltage is adjusted by using a voltage-controlled oscillator to control the switching frequency.

[0023] As an optional implementation method, if it is less than, the load condition is less than half-load operation, and a phase shift control strategy is adopted to adjust the phase shift angle according to the first unitized control quantity to adjust the voltage; otherwise, the load condition is greater than half-load operation, and a variable frequency control strategy is adopted to control the switching frequency using a voltage-controlled oscillator to adjust the voltage.

[0024] The control method of the wide-range operation interleaved parallel LLC resonant converter proposed in the present invention adopts a composite control strategy of variable frequency phase shifting, and adopts different control strategies according to different boost ratios, different load conditions and the voltage gain of the resonant network, so as to improve the power conversion efficiency and realize phase-to-phase current sharing.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The present invention proposes a wide-range operation interleaved parallel LLC resonant converter, which adopts N-phase interleaved parallel, that is, input parallel and output series connection mode. Through this topology structure, wide-range operation (that is, wide-range input and output and load capacity), high power transmission and high step-up ratio DC-DC power conversion are realized, and the load capacity is improved. At the same time, the cost, power loss and volume are reduced by adopting a resonant network.

[0027] The present invention proposes a control method for a wide-range operation interleaved parallel LLC resonant converter, which adopts a composite control strategy of variable frequency phase shifting, adopts different control strategies according to different boost ratios, different load conditions and the voltage gain of the resonant network, improves the power conversion efficiency, and realizes phase-to-phase current sharing.

[0028] Advantages of additional aspects of the present invention will be given in part in the following description, and in part will become obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0030] Figure 1 A structural block diagram of a wide range operation interleaved parallel LLC resonant converter provided in Example 1 of the present invention;

[0031] Figure 2 A topological diagram of a two-level inverter circuit structure provided in Embodiment 1 of the present invention;

[0032] Figure 3 A topological diagram of a three-level inverter circuit structure provided in Embodiment 1 of the present invention;

[0033] Figure 4 A topological diagram of a resonant network structure provided in Embodiment 1 of the present invention;

[0034] Figure 5 A transformer structure topology diagram provided in Example 1 of the present invention;

[0035] Figure 6 A topological diagram of the secondary side rectifier circuit structure provided in Example 1 of the present invention;

[0036] Figure 7 This is a schematic diagram of a control method for a wide range operating interleaved parallel LLC resonant converter provided in Example 2 of the present invention. DETAILED DESCRIPTION

[0037] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0038] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0039] It should be noted that the terms used herein are only for describing specific embodiments, and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that the terms "include" and "comprise" and any of their variations are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device comprising a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0040] In the absence of conflict, the embodiments of the present invention and the features of the embodiments may be combined with each other.

[0041] Example 1

[0042] like Figure 1 As shown, this embodiment provides a wide range operation staggered parallel LLC resonant converter, which can be applied to the offshore wind power field, including: N LLC resonant conversion units with input ends connected in parallel and output ends connected in series, N is an integer greater than or equal to 2, and each LLC resonant conversion unit includes an inverter, a resonant network, a transformer and a secondary side rectifier circuit connected in series in sequence;

[0043] The input terminals of the N inverters are connected in parallel to the DC input terminal;

[0044] The output ends of N secondary-side rectifier circuits are connected in series; wherein the first output end of the first secondary-side rectifier circuit serves as the first DC output end, and the second output end is connected to the first output end of the next secondary-side rectifier circuit until it is connected to the first output end of the Nth secondary-side rectifier circuit, and the second output end of the Nth secondary-side rectifier circuit serves as the second DC output end.

[0045] In this embodiment, the inverter has two input terminals and two output terminals, the first input terminals of the N inverters are all connected to the first DC input terminal, and the second input terminals are all connected to the second DC input terminal, thereby connecting the input terminals of the N inverters in parallel to the DC input terminals.

[0046] The first output end of the inverter is connected to the first input end of the resonant network, the second output end of the inverter is connected to the second input end of the resonant network, the first output end of the resonant network is connected to the first input end of the transformer, the second output end of the resonant network is connected to the second input end of the transformer, the first output end of the transformer is connected to the first input end of the secondary-side rectifier circuit, and the second output end of the transformer is connected to the second input end of the secondary-side rectifier circuit, thereby realizing the inverter, the resonant network, the transformer and the secondary-side rectifier circuit are connected in series in sequence.

[0047] The secondary-side rectifier circuit also includes two output terminals. The first output terminal of the first secondary-side rectifier circuit serves as a first DC output terminal, and the second output terminal is connected to the first output terminal of the next secondary-side rectifier circuit, and so on, until it is connected to the first output terminal of the Nth secondary-side rectifier circuit, and the second output terminal of the Nth secondary-side rectifier circuit serves as a second DC output terminal, thereby realizing the series connection of the output terminals of the N secondary-side rectifier circuits.

[0048] In this embodiment, the LLC resonant converter has N phases, that is, there are N groups of fully controlled inverters that need to be controlled (eg Figure 2 or Figure 3 The control method of N groups of fully controlled inverters is the same, but not simultaneous. Taking the first switch S1 as an example, assuming that the first phase is turned on at t = 0, its switching period is T s , then the second phase switch will be at t = T s / N, the third phase switch will be turned on at t = 2T s / N time is open, and so on, the N-1th phase switch will be opened at t = (N-1)T s In other words, the control time of each phase is not different, but the two phases are staggered by T s / N moments, and take turns controlling, which is helpful to reduce harmonics.

[0049] In summary, the LLC resonant converter of this embodiment adopts N-phase staggered parallel connection, that is, the input is parallel and the output is series connection. Through this topology, a wide range of operation (that is, a wide range of input and output and load capacity), high power transmission and high step-up ratio DC-DC power conversion are achieved, the load capacity is improved, and at the same time, the cost, power loss and volume are reduced by adopting a resonant network.

[0050] In this embodiment, the inverter is a fully controlled inverter, which is used to convert a DC input voltage into an AC voltage. The inverter adopts a two-level inverter circuit or a three-level inverter circuit. The structure of the two-level inverter circuit is as follows: Figure 2 As shown, the three-level inverter circuit structure is as follows Figure 3 As shown, the three-level inverter circuit can achieve greater power conversion and can be applied to a larger voltage level. Conventional inverter circuits can be used and will not be described in detail.

[0051] In this embodiment, if Figure 4 As shown, the resonant network is an LLC resonant network, using a first inductor L r , the second inductor L m It is used together with capacitor C as a resonant element to realize soft switching of fully controlled devices.

[0052] Specifically:

[0053] The first inductor L r The first end point of the LLC resonant network is connected to the first input end of the LLC resonant network, and the first inductor L r The second end of the resonant network is connected to the first output end of the LLC;

[0054] The second inductor L m The first end of the LLC resonant network is connected to the first output end, and the second inductor L m The second end of the resonant network is connected to the second output end of the LLC;

[0055] The first end of the capacitor C is connected to the second input end of the LLC resonant network, and the second end of the capacitor C is connected to the second output end of the LLC resonant network.

[0056] The LLC resonant network has the following characteristics: 1. It realizes soft switching, greatly reduces switching losses, and improves the efficiency of the converter. 2. Both inductors of LLC can be integrated into the transformer. The transformer has an isolation function and must be used by all isolated DC-DC converters. Therefore, if the LLC converter can reasonably design the transformer, it only needs to add an additional resonant capacitor, which greatly reduces the cost and volume and improves the power density. 3. From the frequency characteristics of the LLC resonant network, it can be seen that its load range is very wide, which is suitable for power conversion in occasions with large power fluctuations such as offshore wind power, and its input and output voltage changes can also be large, which is also in line with the output characteristics of the wind turbine. If the wind speed is low, the wind turbine voltage is low. At this time, the wide range of input and output characteristics is conducive to maintaining the output voltage of the converter.

[0057] In this embodiment, the transformer is used for isolation and to provide a suitable transformation ratio, and adopts a single winding connection mode. Its circuit structure is as follows: Figure 5 As shown, the two input ends of the primary side of the transformer are connected to the two output ends of the resonant network, and the two output ends of the secondary side of the transformer are connected to the two input ends of the secondary side rectifier circuit.

[0058] In this embodiment, the secondary side rectifier circuit is used to convert the AC voltage output by the transformer into a DC voltage. A rectifier circuit with multiple diodes connected in series is used to increase the withstand voltage. The circuit structure is as follows: Figure 6 As shown, a conventional rectifier circuit can be used and will not be described in detail.

[0059] Example 2

[0060] like Figure 7 As shown, this embodiment provides a control method for a wide-range operation interleaved parallel LLC resonant converter, which adopts a composite control strategy of variable frequency and phase shifting to achieve voltage control and phase-to-phase current sharing under a wide range of operation.

[0061] Specifically include:

[0062] Step (1): Obtain the DC input voltage u at the DC input terminal dc1 , DC output voltage u at the DC output end dc2 And each phase DC output current i dc2 .

[0063] Step (2): Set the output reference voltage u dc2 *With DC output voltage u dc2 After taking the difference, it is input into the first voltage controller W1 to obtain the first unitized control quantity U1.

[0064] Step (3): Set the output reference current i dc2 *With each phase DC output current i dc2 After the difference is made, it is input into the second voltage controller W2 to obtain the second unitized control quantity U2. The second unitized control quantity U2 reflects the relative size of the DC output current of each phase. The phase-to-phase current can be adjusted according to the second unitized control quantity U2 by adopting the phase shift control strategy to achieve the purpose of staggered parallel current sharing.

[0065] If the DC output current of a phase is relatively large, it means that there is a problem of power imbalance between phases, and current sharing is required to balance the power between phases. Then the output reference current i dc2 *With each phase DC output current i dc2 The value after subtraction will become smaller, that is, the second unitized control quantity U2 becomes smaller. The size of the phase shift angle is adjusted by the second unitized control quantity U2. If the phase shift degree is large, the converter transmission power becomes smaller, which reduces the DC output current of this phase and finally achieves current equalization.

[0066] The absolute values ​​of the first unitized control amount U1 and the second unitized control amount U2 are both less than 1.

[0067] Step (4): Compare N times the DC input voltage u dc1 and n times the DC output voltage u dc2 The size of n is the transformation ratio of the transformer.

[0068] (4-1) If Nu dc1 Less than nu dc2 , then judge u dc2 >2i dc2 R L , that is, to determine the output current i of each phase dc2 Whether it is less than 0.5 times of the rated current, and according to the judgment result, the phase shift angle is adjusted according to the value of the first unitized control quantity U1, or the switching frequency is controlled after being processed by the voltage-controlled oscillator VCO; wherein, R L is the load resistance.

[0069] Specifically, if it is less than, it means that the load condition of the LLC resonant converter is less than half-load operation at this time. At this time, the phase shift control strategy is adopted to adjust the phase shift angle according to the value of the first unitized control quantity U1 to adjust the voltage;

[0070] Otherwise, it means that the load condition of the LLC resonant converter is greater than half-load operation. At this time, a variable frequency control strategy is adopted, and a voltage-controlled oscillator VCO is used to control the switching frequency to adjust the voltage.

[0071] (4-2) If Nu dc1 Greater than or equal to nu dc2 , it means that the output gain of the resonant network is less than 1 at this time, and the LLC resonant converter works in a heavier load mode. At this time, the phase shift control strategy is adopted to adjust the phase shift angle according to the value of the first unitized control quantity U1, thereby adjusting the voltage.

[0072] (5) The power switch control unit outputs the frequency and phase shift angle control signals of the power switch, thereby adjusting the voltage and phase current.

[0073] It should be noted that the algorithm for obtaining the unitized control amount in the first voltage controller W1 and the second voltage controller W2 can be implemented by using a PI adjustment algorithm or other existing algorithms, and those skilled in the art can make a specific selection according to actual conditions.

[0074] It can be understood that the phase shift control strategy and the frequency conversion control strategy are both existing control strategies and will not be described in detail.

[0075] The control method of this embodiment adopts a composite control strategy of variable frequency phase shifting, and adopts different control strategies according to different boost ratios, different load conditions and the voltage gain of the resonant network to improve the power conversion efficiency and achieve phase-to-phase current sharing.

[0076] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. A control method for a wide range operation interleaved parallel LLC resonant converter, characterized in that: The wide range operation interleaved parallel LLC resonant converter comprises: N LLC resonant conversion units with input ends connected in parallel and output ends connected in series, N being an integer greater than or equal to 2, and each LLC resonant conversion unit comprising an inverter, a resonant network, a transformer and a secondary side rectifier circuit connected in series in sequence; The input terminals of the N inverters are connected in parallel to the DC input terminal; The output ends of N secondary-side rectifier circuits are connected in series; wherein the first output end of the first secondary-side rectifier circuit serves as the first DC output end, and the second output end is connected to the first output end of the next secondary-side rectifier circuit until it is connected to the first output end of the Nth secondary-side rectifier circuit, and the second output end of the Nth secondary-side rectifier circuit serves as the second DC output end; A composite control strategy of variable frequency phase shift is adopted, and different control strategies are adopted according to different boost ratios, different load conditions and the voltage gain of the resonant network; The control method comprises: Obtaining a DC input voltage at a DC input terminal, a DC output voltage at a DC output terminal, and a DC output current of each phase; Obtaining a first unitized control amount according to the output reference voltage and the DC terminal output voltage; A second unitized control quantity is obtained according to the output reference current and the DC output current of each phase, and a phase-shift control strategy is adopted to adjust the phase current according to the second unitized control quantity; According to the comparison result of the N-times DC input voltage and the n-times DC output voltage, where n is the transformation ratio of the transformer, a phase shift control strategy or a frequency conversion control strategy is selected, and the voltage is adjusted by adjusting the phase shift angle according to the first unitized control amount or by using a voltage controlled oscillator to control the switching frequency; If the N-times DC input voltage is greater than or equal to the n-times DC output voltage, the output gain of the resonant network is less than 1, and a phase shift control strategy is adopted to adjust the phase shift angle according to the first unitized control amount, thereby adjusting the voltage; If the N-times DC input voltage is less than the n-times DC output voltage, it is determined whether the output current of each phase is less than 0.5 times the rated current, and based on the determination result, the phase shift angle is adjusted according to the first unitized control amount or the switching frequency is controlled by a voltage-controlled oscillator to adjust the voltage; If it is less than, the load condition is less than half-load operation at this time, and the phase shift control strategy is adopted to adjust the phase shift angle according to the first unitized control quantity to adjust the voltage; otherwise, the load condition is greater than half-load operation at this time, and the frequency conversion control strategy is adopted to use a voltage-controlled oscillator to control the switching frequency to adjust the voltage.

2. A control method for a wide range operation interleaved parallel LLC resonant converter as claimed in claim 1, characterized in that: Obtaining a first unitized control amount according to a difference between an output reference voltage and a DC terminal output voltage; A second unitized control quantity is obtained according to the difference between the output reference current and the DC output current of each phase; The absolute values ​​of the first unitized control amount and the second unitized control amount are both less than 1.

3. A wide range operation interleaved parallel LLC resonant converter, characterized in that: Executing a control method for a wide-range operation interleaved parallel LLC resonant converter as claimed in any one of claims 1 to 2, comprising: connecting a first output end of an inverter to a first input end of a resonant network, connecting a second output end of an inverter to a second input end of a resonant network, connecting a first output end of the resonant network to a first input end of a transformer, connecting a second output end of the resonant network to a second input end of the transformer, connecting a first output end of the transformer to a first input end of a secondary-side rectifier circuit, and connecting a second output end of the transformer to a second input end of the secondary-side rectifier circuit; The inverter is a fully controlled inverter that converts a DC input voltage into an AC voltage. The inverter adopts a two-level inverter circuit or a three-level inverter circuit. The inverter has two input terminals and two output terminals. The first input terminals of the N inverters are all connected to the first DC input terminal, and the second input terminals are all connected to the second DC input terminal.

4. A wide range operation interleaved parallel LLC resonant converter as claimed in claim 3, characterized in that: The control time of N inverters is different, and they are staggered between each other. T s / N time, T s is the switching cycle.

5. A wide range operation interleaved parallel LLC resonant converter as claimed in claim 3, characterized in that: The resonant network includes a first inductor, a second inductor and a capacitor; a first end point of the first inductor is connected to a first input end of the resonant network, and a second end point of the first inductor is connected to a first output end of the resonant network; a first end point of the second inductor is connected to a first output end of the resonant network, and a second end point of the second inductor is connected to a second output end of the resonant network; a first end point of the capacitor is connected to a second input end of the resonant network, and a second end point of the capacitor is connected to a second output end of the resonant network.

6. A wide range operation interleaved parallel LLC resonant converter as claimed in claim 3, characterized in that: The two input ends of the primary side of the transformer are connected to the two output ends of the resonant network, and the two output ends of the secondary side of the transformer are connected to the two input ends of the secondary side rectifier circuit.

7. A wide range operation interleaved parallel LLC resonant converter as claimed in claim 3, characterized in that: The secondary side rectifier circuit is used to convert the AC voltage output by the transformer into a DC voltage, and adopts a rectifier circuit with multiple diodes connected in series.

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