Research on the technology of transmitting low frequency electric power by high frequency transformer

Abstract: A new DC/AC power transmission circuit topology is proposed. By using pulse-by-pulse magnetic reset technology, the high-frequency transformer can withstand the high-frequency SPWM pulse train modulated by low-frequency AC or audio signals, and complete the task of transmitting low-frequency electric power. Experimental and simulation results prove its feasibility. Keywords: high-frequency transformer; transmission; low-frequency power


1 Introduction

The development of high-frequency switching technology has made industrial frequency transformers withdraw from many fields. However, in many fields such as uninterruptible power supplies that require isolation, digital linear power amplifiers, and DC/AC converters that require output of low-frequency sine waves, low-frequency transformers have to be retained for the purpose of isolation or voltage conversion. In order to overcome the disadvantages of low-frequency transformers, such as bulky and large size, as high-frequency switching technology continues to mature, it has become possible to remove low-frequency transformers. Figure 1 shows a relatively typical circuit structure [1][2].

As shown in Figure 1, the inverter is used twice in this circuit structure. The first time is to obtain high frequency so that high-frequency transformers can be used for voltage transformation and isolation, and the second time is to obtain industrial frequency sinusoidal AC voltage. Since an additional power inverter is used, the power loss is increased. This paper proposes a new method for transmitting low-frequency power using a high-frequency transformer. The high-frequency transformer can be directly used to simultaneously complete the tasks of voltage transformation, isolation, and power transmission without adding a power inverter. This simplifies the structure, reduces the volume and weight, improves the efficiency, and creates conditions for realizing high frequency, high efficiency, and high power density of power electronic equipment. The circuit structure is shown in Figure 2.

2 Circuit working principle

2.1 System composition

As shown in Figure 3, the system consists of a dual-combination single-ended flyback converter, a bidirectional high-frequency rectifier, a high-frequency filter and a control part. The dual-combination single-ended flyback converter is essentially two single-ended flyback converters that share a transformer core and a secondary side.





The negative half cycle is controlled by vg1 and vg2 for chopping operation to complete the tasks of voltage transformation, isolation, and power transfer. The bidirectional high-frequency rectifier uses two field-effect transistors to replace the secondary diode in the general flyback converter. The two field-effect transistors are controlled by vg3 and vg4 to conduct in the positive and negative half cycles of the low-frequency signal, and form a path with the parasitic diodes in each other's body to realize bidirectional high-frequency rectification. After bidirectional high-frequency rectification, a column of bidirectional pulses is obtained. The envelope of the pulse column is similar to the waveform of the control signal vc, with the same frequency and different amplitude. After high-frequency filtering, an output voltage with the same frequency as vc is obtained. The control part generates dual-column unipolar SPWM high-frequency pulses vg1, vg2 and dual-column low-frequency switching pulses vg3, vg4 with the same frequency and phase difference (Tc is the period of the vc waveform) as the low-frequency control signal vc, respectively controlling the dual-combination single-ended flyback converter and the bidirectional high-frequency rectifier, and changing the amplitude modulation depth ma of the SPWM high-frequency pulse column through real-time feedback of the output voltage to achieve the converter's regulation of the output voltage.

2.2 Working principle of the control part

The control principle block diagram and the voltage waveforms at each point are shown in Figure 4. vc is the low-frequency modulation signal to be transmitted and amplified (such as a 50Hz sine wave signal), and vt is a unipolar isosceles triangle high-frequency carrier signal (such as a 20kHz high-frequency triangle wave). To achieve the waveforms at points vg1 to vg4, the following control strategy is adopted.

1) Compare the low-frequency modulation signal vc with the high-frequency carrier triangular wave signal vt to obtain a unipolar SPWM signal vg1 with the same frequency as vc;

2) Compare the low-frequency modulation signal vc through a zero comparator to obtain a low-frequency switching pulse signal vg3 with the same frequency as vc; 3) Invert the low-frequency signal vc to obtain a modulation signal -vc with the same frequency as vc, and then compare -vc with the carrier signal vt to obtain a unipolar SPWM signal vg2 with the same frequency and phase difference as vg1; 4) Compare the modulation signal -vc through a zero comparator to obtain a low-frequency switching pulse signal vg4 with the same frequency and phase difference as vg3.

2.3 Main circuit topology

Figure 5 shows a traditional single-ended flyback converter with a reset winding. The number of turns of the reset winding N2 is equal to the number of turns of the winding N1. When the switch tube V is turned on, D3 is reversely blocked and the transformer stores energy. When V is turned off, D3 is turned on, and the energy storage of the transformer is output to the load Zl and the filter capacitor Cf; D2 is turned on, and N2 acts as a reset winding to feed back the leakage inductance energy storage of the converter to the power supply U, and clamps Uds on V to 2U.

Figure 6 shows the topology of the new DC/AC power transmission circuit. N1, V1, and N3 form a single-ended flyback converter, which together with another single-ended flyback converter composed of N2, V2, and N3 form a dual-combination single-ended flyback converter, and are controlled by the vg1 and vg2 high-frequency SPWM pulses to chopped and turned on in the positive and negative half-cycles of the control signal cycle. V3 and V4 form a bidirectional high-frequency rectifier, which is turned on in the positive and negative half-cycles of the control signal cycle, and forms a rectifier circuit with each other and the parasitic parallel diodes in each other's bodies.

When the circuit is in the low-frequency AC positive half-cycle (vg1~vg4 signal waveform)



See Figure 4), vg2=0, V2 is in the off state, vg3 is high level, and V3 is in the on state. During the high-frequency pulse period, when vg1 high level is applied to the gate of V1, its equivalent circuit is shown in Figure 7(a). On the primary side of the transformer, V1 is turned on with the high level applied to the gate, and the power supply U, winding N1 and power switch tube V1 form a loop. On the secondary side of the converter, the polarity of winding N3 is negative at the top and positive at the bottom. V3 is turned on with vg3 being high level. V4 is turned off with vg4=0, and its parasitic diode is reversely turned off. No current loop is formed on the secondary side, and no current flows. The transformer is in the energy storage stage. Therefore, the current i1=t increases linearly until I1p=ton, and the energy storage of the transformer core also increases to (where L1 is the inductance of winding N1).

When V1 is turned off with vg1=0, its equivalent circuit is shown in Figure 7(b). On the primary side of the transformer, due to the shutdown of V1, the leakage inductance energy storage causes a large reverse voltage to be applied to both ends of V1. Since the number of turns of N1 is equal to the number of turns of N2, when UN2=U, the parasitic diode D2 in V2 is turned on, and the Uds on the clamp V1 is 2U. At this time, N2 forms a path with D2 as a reset winding, feeding back the leakage inductance energy storage in the transformer to the power supply U; on the secondary side of the transformer, the voltage polarity of winding N3 is positive at the top and negative at the bottom, and the parasitic diode D4 in N3, V3, Cf, Zl and V4 forms a loop. At this time, D4 ​​undertakes the high-frequency rectification task, obtains a high-frequency DC pulse, and after filtering by Cf, outputs low-frequency electric power to the load Zl, completing the energy transfer of the converter in the single pulse. From the SPWM modulation principle, it can be seen that when the frequency modulation ratio mf= is large enough, the system phase shift can be ignored, and the output voltage vo=Vosinω1t is obtained on the high-frequency filter capacitor Cf, which is the same frequency and phase as vc.

2.4 Requirements for magnetic reset technology

On the primary side of the high-frequency transformer, when V1 or V2 receives the SPWM pulse train and turns on, the modulation frequency is very low, far less than the frequency of the high-frequency carrier. In the positive or negative half-cycle of the low-frequency modulation signal, the voltage applied to the transformer winding is in the same direction. The magnetic flux in the transformer core will gradually increase step by step, eventually leading to core saturation, bias magnetization or unidirectional magnetization, resulting in a large magnetizing current and making the circuit unable to work properly. This article proposes a pulse-by-pulse magnetic reset technology, which is to take timely measures after each high-frequency pulse to restore the increase in magnetic flux caused by each high-frequency pulse to zero, thereby avoiding core saturation. The triangle method generates a unipolar SPWM wave as shown in Figure 8 (taking the control signal as a low-frequency AC as an example). In the figure, the control signal voltage (modulation wave) vc=Vsinsinω1t (where: ω1=2πf1, f1 is the fundamental frequency required by the inverter output voltage, also the modulation frequency; Vsin is the peak value of the control signal voltage), vt is the isosceles triangle carrier voltage, Vtri is the peak value of the carrier voltage, the carrier frequency is fs, and the period is =Ts. Then the amplitude modulation ratio ma=, and the frequency modulation ratio mf=.


When fs f1 and mf are even numbers, and the starting phases of vc and vt are equal, the waveforms of vt and vc have the relationship shown in Figure 8. This situation is discussed below.

From time tn-1 to tn is the nth carrier cycle of vt.

tn-1=(n-1)Ts

tn=nTs. Its vertex=(n-)Ts.

So we have the equations of two straight lines of isosceles triangle wave vt: When (n-1)Ts
vt1=2Vtrifs[t-(n-1)Ts] When (n-)Ts
vt2=-2Vtrifs(t-nTs)

Assume that the intersection points of vt1, vt2 and vc are at t=t1 and t=t2 respectively, then

Vsinsinω1t1=2Vtrifs[t1-(n-1)Ts] (1)

Vsinsinω1t2=-2Vtrifs[t2-nTs] (2)

From equations (1) and (2), we can get Doff=1-masin (3) Don=masin (4)

Figure 7 Equivalent circuit diagram

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Research on the technology of high-frequency transformer transmitting low-frequency electric power

Figure 9 Voltage waveforms on V1 and V2 power tubes

Figure 10 Current waveforms on V1 and V2 power tubes

Figure 11 Experimental output voltage waveform

Where: Doff = is the off duty cycle, toff = t2-t1 is the off time; Don = is the on duty cycle.
Equation (4) shows that when the amplitude modulation ratio ma is kept constant, the duty cycle Don of the SPWM high-frequency pulse changes sinusoidally at the fundamental frequency (modulation frequency) and without phase difference. To reset the magnetic core, the volt-second balance law of the transformer magnetic core requires (ignoring the tube voltage drop) [3] VccDon?voDoff(5 )

Where: Vcc is the input DC voltage applied to the primary winding of the transformer;

vo is the output voltage of the secondary side of the transformer.

Substituting equations (3), (4) and vo = Vosinω1t into equation (5), we get ma?(6) From equation (4), when sin = 1, the pulse has the largest duty cycle Don in this SPWM pulse train. If Doff meets the magnetic reset requirement at this time, then the SPWM pulse train meets the magnetic reset requirement pulse by pulse. Therefore, from formula (6), when ma?=(7)

, the transformer core can achieve pulse-by-pulse magnetic reset.

3 Experimental and simulation results

To verify the principle of this circuit, the following simulations and experiments were performed: input DC voltage 36V; output AC voltage 24V; transformer ratio 1:1; low-frequency signal 50Hz sine wave; carrier signal 15kHz triangle wave; amplitude modulation ratio ma=0.5; power switch tube IRF460; switching frequency 15kHz; output high-frequency filter capacitor Cf=5μF; load Zl=200Ω.

At this time, the maximum duty cycle of the circuit is 0.5. When V1 is turned off, the diode D2 in V2 is turned on, forming a path with N2, and there is current Id(V2), completing the feedback of leakage inductance energy storage, and clamping Vds(V1) to 2U. When a single high-level pulse is applied to the switch tube V1 in the low-frequency positive half cycle, its current Id(V1) starts to rise from zero current and the waveform is smooth, indicating that the magnetic flux of the transformer core has returned to zero and the excitation current has not reached the saturation current. The output voltage waveform shown in Figure 11 is obtained by performing an experiment with the same parameters as the simulation.

4 Conclusion

A novel DC/AC power transmission circuit topology is proposed, its working principle is introduced, and the requirement for high-frequency transformers to achieve pulse-by-pulse magnetic reset is mathematically proved. The experimental and simulation results show that this circuit topology can better complete the transmission and amplification of low-frequency power, and has the advantages of simple structure, small size and light weight. It can be widely used in engineering and technical fields such as UPS, aviation power supply, sine wave inverter, digital linear power amplifier, etc.

References

[1] Zhou Daiwen, Hou Zhencheng. Discussion on a new type of uninterruptible power supply (UPS) circuit [J]. Power Electronics Technology, 1998 (4): 49-59.

[2] Xiong Yahong, Chen Daolian. Novel bidirectional power flow high frequency link DC/AC inverter [J]. Power Electronics Technology, 2000 (4): 10-12.

[3] Xu Degao, Jin Gang. Pulse width modulation converter type voltage stabilized power supply [M]. Science Press, 1983