Application of Current Transformer in Switching Power Supply

Abstract: Current transformers can be used to detect unipolar current pulses in high-frequency switching power supplies. The working process of the current detection circuit composed of current transformers is analyzed. The characteristics of self-reset and forced reset of the magnetic core are compared. The experimental results are given.

Keywords: current transformer; core reset; switching power supply

1 Introduction

In high-frequency switching power supplies, it is necessary to detect the current of components such as switch tubes and inductors and provide them to the control and protection circuits. Current detection methods include current transformers, Hall elements, and direct resistance sampling. When using Hall element sampling, the control and main power circuits are isolated, DC signals can be detected, and the signal restoration is good, but there is a μs-level delay and the price is relatively expensive; the price of resistance sampling is very cheap and the signal restoration is good, but the control circuit and the main power circuit are not isolated, and the power consumption is relatively large.

Current transformers have many advantages, such as low energy consumption, wide bandwidth, good signal restoration, low price, and isolation between control and main power circuits. In double-ended converters such as Push-Pull and Bridge, the primary side of the power transformer flows through positive and negative symmetrical bipolar current pulses without DC components, so current transformers can be well used. However, in single-ended applications such as Buck and Boost, unipolar current pulses flow through the switching device; the DC component contained in the primary side cannot be reflected in the secondary detection signal, and may also cause unidirectional saturation of the current transformer core; for this reason, it is necessary to make some improvements to the detection circuit composed of the current transformer.

2 Analysis of the application circuit of current transformer to detect unipolar current pulse

According to the different methods of resetting the core of the current transformer, there are two circuit forms: self-reset and forced reset. After the current pulse on the primary side of the current transformer disappears, the self-reset circuit uses the negative voltage generated by the excitation current through the open-circuit impedance of the secondary side of the current transformer to achieve reset. The reset voltage is related to the excitation current and the open-circuit impedance of the current transformer. During the disappearance of the DC pulse on the primary side of the current transformer, a large reset voltage is applied to the forced reset circuit to achieve rapid reset of the core in a short time.

2.1 Current transformer detection circuit

The commonly used current transformer detection circuit is shown in Figure 1(a).

FIG1( b ) shows the equivalent circuit when there is a current pulse on the primary side. The current transformer is simplified to an ideal transformer and an excitation inductor Lm model, and Rs is a sampling resistor.

When the duty cycle D<0.5, after the primary current pulse of the current transformer disappears, the magnetic core relies on the excitation current to flow through the sampling resistor Rs to generate a negative volt-second value, thereby achieving self-reset [as shown in Figure 1 (d1)~(i1)]. Since the sampling resistor Rs is very small, the negative reset voltage is small. When the duty cycle of the current pulse is very large (D>0.5), the reset time is very short, and there is not enough reset volt-second value, which increases the DC component Id in the magnetic core, and may cause the magnetic core to gradually become positively biased and saturated [as shown in Figure 1 (d2)~(i2)], losing its detection function. Therefore, self-reset can only be applied to situations where the current pulse duty cycle D<0.5.

(a) Detection circuit

(b) Equivalent circuit when there is a pulse on the primary side

(c) Equivalent circuit when the core is reset

Figure 1 Analysis of commonly used current transformer detection circuits

It can be seen that this circuit has many disadvantages for detecting unipolar DC pulses. There is a DC component Id in the excitation inductor current im, which easily leads to core saturation. The output voltage signal uR is bipolar, which is not convenient for subsequent circuit processing.

2.2 Improved self-resetting current transformer

In order to realize the unipolar output of the output voltage uR, a diode is added to the current transformer end. According to the different phases of the primary input current i1 and the output voltage uR and the different positions of the signal ground, there are four circuit structures, as shown in Figure 2.

Figure 2 Improved current transformer detection circuit

The working process of the circuit in FIG2(c) is analyzed, and the working waveform of the circuit in one pulse cycle is shown in FIG3.

(a) Detection circuit

(b) Equivalent circuit when there is a pulse on the primary side

(c) Equivalent circuit when the core is reset

Figure 3 Analysis of improved current transformer detection circuit

Figure 3(c) shows the equivalent circuit of the current transformer core reset, CT is the secondary distributed capacitance of the current transformer, and CD is the diode junction capacitance. Figures 3(d) to (i) show the waveforms of various parameters when the duty cycle is small and the core is fully reset.

After the primary current pulse of the current transformer disappears, the reset of the magnetic core relies on the resonance of the excitation current in Lm, CT, and CD to generate a negative reset voltage value to achieve self-reset, as shown in Figure 3(g). The characteristic impedance of the resonant circuit composed of Lm and CT is much larger than Rs, so the reset effect is better than the circuit in Figure 1. However, the reset voltage generated by resonance is not very large. When the pulse duty cycle is large, the reset time is very short, which may still cause the magnetic core to gradually saturate in the forward bias, so it can only be applied to occasions where the current pulse duty cycle D<0.5.

Since the transformer secondary coil has many turns and large distributed capacitance, the resonant current mainly flows through the current transformer; the current flowing through the Rs and CD branches is very small, and Rs is very small, so the negative voltage generated by the resonant current of the reset current through the CD branch on Rs can be ignored, and the sampled output voltage uR waveform is shown in Figure 3(h). Due to the effect of the diode, the output voltage signal uR is unipolar, and its amplitude is proportional to the ripple of the primary current signal, which is convenient for the subsequent circuit to process.

2.3 Forced Reset

In single-ended applications, especially in boost circuits, it is necessary to accurately reproduce high duty cycle unipolar pulses. Self-reset cannot detect high duty cycle current pulses, and the magnetic core must be forced to reset. There are many forced reset circuits. Here we analyze the simplest and most feasible forced reset circuit. As shown in Figure 4, they correspond to the four circuits in Figure 2.

The working process of the circuit in Figure 4(c) is analyzed. Figure 5(b) shows the equivalent circuit when there is a current pulse on the primary side. Due to the isolation effect of the diode, the reset voltage +Vr has no effect on the current detection. Figure 5(c) shows the equivalent circuit when the magnetic core is reset. The working waveform of the circuit in a pulse cycle is shown in Figure 5(d)~(i). There is a DC pulse on the primary side during the time 0~t1, the time t1~t2 is the magnetic core reset process, and t2~T is the waveform after the reset is completed.

Figure 4 Forced reset current transformer detection circuit

(a) Detection circuit

(b) Equivalent circuit when there is a pulse on the primary side

(c) Equivalent circuit when the core is reset

(j) Impact of reset voltage on sampling

Figure 5 Analysis of the current transformer detection circuit for forced reset

After the current pulse of the primary side of the current transformer disappears, the magnetic core begins to reset, the diode is reversely blocked, and the reset voltage Vr is added to the excitation inductor, forcing the magnetic core to reset quickly. Figure 5(g) plots the voltage on the excitation inductor. Since the reset voltage is much larger than the forward voltage of the magnetic core, the magnetic core can be fully reset in a very short time, which can be applied to the occasions where the duty cycle of the current pulse D>0.9 is detected.

Figure 5(j) shows the error caused by the reset voltage Vr to the detection signal. After the core is reset, the secondary side of the current transformer is equivalent to a wire, and Vr has a voltage divider on the sampling resistor, which causes an error of

VR(error)=·Vr（1）

Since Rr is much larger than Rs, VR(error) is very small and its influence can be ignored. During the time from t2 to T, there is also a very small DC component in the magnetic core.

im=－（2）

Since Rr is very large, its influence can also be ignored.

2.4 Combination of multiple current transformers

Multiple current transformers can be combined to detect unipolar high-frequency DC pulsations containing low-frequency components. For example, the commonly used single-phase PFC circuit composed of a Boost circuit works in a CCM state and needs to detect the inductor current for use in the control circuit. The inductor current contains both industrial frequency sinusoidal current and high-frequency pulsating current. For this reason, current transformers can be used to detect unipolar current pulses in the switch tube and diode respectively, and then superimposed to obtain the inductor current. The detection circuit is shown in Figure 6. The duty cycle may exceed 0.5, so the magnetic core needs to be forced to reset.

Figure 6 Synthetic inductor current using current transformer combination

3 Design method of current transformer

The current transformer ratio n is determined according to the requirements of the primary current i1 and the secondary output voltage Um; the magnetic core can be made of ferrite material with a large initial magnetic permeability, and the size is determined according to the effective area of ​​the magnetic circuit. It can be selected according to formula (3)

Ae = (3)

Where: Ae is the effective cross-sectional area of ​​the magnetic circuit;

Um is the maximum output voltage of the secondary side of the current transformer;

N is the number of turns of the secondary coil;

B is the maximum working magnetic flux of the magnetic core, which is generally taken as 1/2 to 1/3 of the saturation magnetic flux;

fs is the primary pulse current frequency.

Rs is calculated based on the maximum voltage Um and current IR of the secondary side. The value of Rr should be much larger than Rs, which can be 50 to 100 times of Rs. The specific size can be adjusted according to experimental results.

4 Experimental Results

1) In the Boost circuit, a self-resetting current transformer is used to detect the switch tube current and provide it to the current feedback loop of the control circuit. The magnetic core is made of manganese-zinc ferrite, the effective cross-sectional area of ​​the magnetic circuit is 0.25mm2, the transformation ratio is 100, and the sampling resistance is 3.9Ω. The switching frequency is 20kHz, the primary current peak of the current transformer is 3.6A, and the maximum duty cycle is 0.45. The experimental waveform is shown in Figure 7.

(a) Secondary voltage um waveform

(b) Sampling resistor voltage uR waveform

Figure 7 Experimental waveform

2) In the bipolar SPWM inverter circuit, the DC bus flows through asymmetric bipolar current pulses, which are detected by a forced reset current transformer and provided to the overcurrent and direct-through protection circuit. The core is made of manganese-zinc ferrite, with a core cross-sectional area of ​​0.35mm2, a transformation ratio of 100, and a sampling resistor of 16Ω. The switching frequency is 20kHz, the inverter output sine wave frequency is 50Hz, the primary current peak of the current transformer is 3A, and the maximum duty cycle is 0.95. The experimental waveforms within a switching cycle and a power frequency cycle are shown in Figures 8 and 9 respectively.

(a) Secondary voltage um waveform

(b) Sampling resistor voltage uR waveform

Figure 8 Waveform in one switching cycle

(a) Secondary voltage um waveform

(b) Sampling resistor voltage uR waveform

Figure 9 Waveform within one power frequency cycle

5 Conclusion

This paper analyzes the working process of self-reset and forced reset current transformers when detecting unipolar DC pulses, draws the waveform of the circuit in a pulse cycle, compares the characteristics of magnetic core reset of various circuits, and briefly introduces the design method of current transformers. The experimental results verify the correctness of the analysis.