LLC converter rectification output current estimation method

Abstract

The invention discloses an LLC converter rectification output current estimation method. According to the method, the rectification output current is estimated based on a model by sampling the output voltage of the LLC converter and the primary side alternating current of the isolation transformer. According to the method, a rectification output current sensor with a large size can be prevented from being adopted, the situation that the performance of the converter is reduced due to the fact that parasitic parameters are introduced is avoided, the power density of the converter is improved, meanwhile, universality is achieved, and the method can be suitable for various LLC converter topologies including two-level or multi-level full bridges, half bridges and the like.

Description

Technical Field

[0001] The invention relates to the field of power electronic converter control, and in particular to a method for estimating rectifier output current of an LLC converter. Background Art

[0002] The typical two-stage medium voltage solid-state transformer structure is an AC / DC converter + an isolated DC / DC converter. In order to improve the efficiency and power density of the medium voltage solid-state transformer, the LLC converter is usually used as the subsequent isolated DC / DC converter.

[0003] For LLC converters, traditional single voltage loop control is usually difficult to meet the dynamic response requirements. In order to improve the dynamic performance of LLC converters, predecessors have conducted in-depth research on LLC control strategies and made significant progress. Among them, the most widely used control strategy is voltage and current dual loop control.

[0004] Voltage-current dual-loop control generally samples the rectifier output current as the controlled signal of the current loop. In high-power applications, a larger current sensor is required, which reduces the power density of the converter. In addition, the current sensor will introduce parasitic parameters into the rectifier side loop, affecting the performance of the converter. The method of sampling the primary AC current of the isolation transformer as the controlled signal of the current loop does not require a larger current sensor, but because the primary AC current of the isolation transformer contains the component of the excitation current, it cannot directly reflect the change in output power. Summary of the invention

[0005] The purpose of the present invention is to provide a method for estimating the rectified output current of an LLC converter, which avoids using a current sensor with a large volume to directly sample the rectified output current and improves the power density of the converter.

[0006] The technical solution adopted by the present invention is as follows:

[0007] The present invention provides a method for estimating the rectifier output current of an LLC converter, including the following two cases:

[0008] For the sub-resonant region or the resonant region, that is, the case where the switching frequency is less than or equal to the resonant frequency, the estimation formula for the switching cycle average value of the rectified output current is:

[0009]

[0010] For the over-resonance region, that is, when the switching frequency is greater than the resonant frequency, the estimation formula for the switching cycle average value of the rectified output current is:

[0011]

[0012] In the above two equations, n is the primary-to-secondary turns ratio of the isolation transformer in the LLC converter, and f is s is the switching frequency, T s is the switching period, i p (t) is the primary AC current of the isolation transformer, V o is the output voltage, L m is the magnetizing inductance of the isolation transformer, f r is the resonant inductor L in the LLC converter r , resonant capacitor C r Series resonant frequency, at time 0 is the carrier signal v c The zero-crossing moment, t0, is the primary AC current i of the isolation transformer in the first half of the switching cycle. p (t) and the excitation current i Lm Intersection moment.

[0013] Further, the current loop control based on the LLC converter rectifier output current estimation method comprises the following steps:

[0014] With carrier signal v c The zero-crossing moment is the switching cycle 0 moment, and the LLC converter output voltage V o The primary AC current i of the isolation transformer in half a switching cycle p (t) is sampled according to the current switching frequency f s With the resonant frequency f r The LLC converter rectifier output current estimation method proposed in the present invention is used to estimate the switching period average value of the rectifier output current, which is used as the controlled signal of the current loop.

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] This method can avoid the use of a large rectifier output current sensor, thereby avoiding the introduction of parasitic parameters that reduce the performance of the converter, improving the power density of the converter, and at the same time has versatility and can be applied to various LLC converter topologies including two-level or multi-level full-bridge, half-bridge, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be noted that, for ordinary technicians in this field, other drawings can be obtained by making several modifications and improvements based on the provided drawings without departing from the concept of the present invention.

[0018] Figure 1 A control block diagram of a three-level LLC converter including the LLC converter rectifier output current estimation method proposed by the present invention;

[0019] Figure 2 It is the typical waveform of the switching cycle of the three-level LLC converter in the sub-resonance region;

[0020] Figure 3 It is the typical waveform of the switching period of the three-level LLC converter in the over-resonance region; DETAILED DESCRIPTION

[0021] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0022] refer to Figure 1 , is a specific example of applying the method of the present invention, which is a three-level LLC converter, and its control block diagram includes a power part 101, a rectifier output current i rec The estimation module 102 , the driving circuit 103 , the voltage controlled oscillator VCO 104 , the current controller 105 and the voltage controller 106 are 6 parts.

[0023] The main power part is a typical three-level LLC converter topology, including the DC bus capacitor C dcp With C dcn 、Flying capacitor C f , switch tubes S1~S4, clamping diodes D1 and D2, resonant inductor L r , resonant capacitor C r , an isolation transformer with a turns ratio of n and its magnetizing inductance L m , rectifier diodes D3~D6, output filter capacitor C o , load resistance R L .

[0024] Rectified output current i rec The estimation module 102 receives the synchronization SYN signal from the driving circuit, that is, the triangular carrier v c The zero-crossing signal is based on the primary AC current i of the isolation transformer. p Sampling value, output voltage v o The sampling value and the current switching frequency f s Estimate the average value of the rectified output current switching cycle rec (t)> Ts , this estimated value is used to replace the actual rectified output current i rec , as the controlled signal of the current loop.

[0025] Reference Figure 2 The typical waveform of the switching cycle of the three-level LLC converter in the sub-resonance region mainly includes the carrier signal v c , switch tube S1~S4 drive signal v gs1 ~v gs4 , primary bridge arm output voltage v p , secondary bridge arm output voltage v​s , isolation transformer primary AC current i p , excitation current i Lm With the rectified output current i rec The first half of the switching cycle is divided into phase 1 (0~T r / 2) and stage 2 (T r / 2~T s / 2).

[0026] Assume that the output voltage remains constant during a switching cycle, that is, v o =V o , the excitation current in stage 2 (T r / 2~T s / 2) remains unchanged, that is, i Lm =I Lm , the peak value of the excitation current is

[0027]

[0028] The integral value of the rectifier output current in half a switching cycle, that is, Figure 2 The shaded area is

[0029]

[0030] The formula for estimating the switching period average value of the rectified output current in the sub-resonance region and the resonant region is:

[0031]

[0032] in

[0033]

[0034] Reference Figure 3 The typical waveform of the switching cycle of the three-level LLC converter in the over-resonance region is divided into stage 1 (0~t0) and stage 2 (t0~T s / 2).

[0035] The peak value of the excitation current is

[0036]

[0037] The integral value of the rectifier output current in half a switching cycle, that is, Figure 3 The shaded area is

[0038]

[0039] The formula for estimating the average value of the switching cycle of the rectified output current in the over-resonance region is:

[0040]

[0041] The time t0 is the primary AC current i of the isolation transformer in the first half of the switching cycle. p With the excitation current i Lm The intersection moment of p with -I Lm The t0 value can be obtained.

Claims

1. A method for estimating rectifier output current of an LLC converter, characterized in that: For the sub-resonant region or resonant region, that is, when the switching frequency is less than or equal to the resonant frequency, the average value of the rectified output current switching cycle is The estimation formula is: For the over-resonance region, that is, when the switching frequency is greater than the resonant frequency, the average value of the rectifier output current switching cycle is The estimation formula is: Where n is the primary-to-secondary turns ratio of the isolation transformer in the LLC converter, and f s is the switching frequency, T s is the switching period, i p (t) is the primary AC current of the isolation transformer, V o is the output voltage, L m is the magnetizing inductance of the isolation transformer, f r is the resonant inductor L in the LLC converter r , resonant capacitor C r Series resonant frequency, at time 0 is the carrier signal v c The zero-crossing moment, t0, is the primary AC current i of the isolation transformer in the first half of the switching cycle. p (t) and the excitation current i Lm Intersection moment.

2. The current loop control method of LLC converter is characterized by: With carrier signal v c The zero-crossing moment is the switching cycle 0 moment, and the LLC converter output voltage V o The primary AC current i of the isolation transformer in half a switching cycle p (t) is sampled according to the current switching frequency f s With the resonant frequency f r The LLC converter rectifier output current estimation method according to claim 1 is used to estimate the switching period average value of the rectifier output current as the controlled signal of the current loop.

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