1. Introduction
With the rapid development of information and technology, there are more and more electronic devices for industry, business, and even the home. Cell phones, personal computers, home appliances, and other electronic products are also undergoing rapid and continuous innovation. The power supply used by these electronic devices is DC voltage, but most of the current power systems provide AC power, so it is necessary to convert the AC power to DC voltage. The simplest way to do this is to use a bridge rectifier consisting of diodes to rectify the AC power supply, which is then filtered by a bulk capacitor to obtain the DC voltage [
1,
2,
3]. However, the input current is a steep pulse due to the charging current in the capacitor. Consequently, the current harmonics are large, causing serious harmonic pollution to the power grid and resulting in interference with the normal operation of other electrical equipment connected to the power grid. Accordingly, it is necessary to take measures to limit the current harmonics generated by these electronic devices, and the International Electrotechnical Commission (IEC) has issued the IEC 61000-3-2 current harmonic standard, which formally regulates the current harmonics created by electronic devices in detail [
4,
5,
6].
In order to comply with the limit values of the IEC 61000-3-2 current harmonic standard, the power factor correction (PFC) must be added to make the input current sinusoidal and in phase with the input voltage of the power supply to increase the effective power so that the harmonic components of the input current will be reduced. The power factor correction (PFC) rectifier can be categorized as passive or active [
7]. The traditional active PFC rectifier has a bridge rectifier at the front end, which accounts for a significant portion of the power loss. Therefore, if the power loss of this bridge rectifier can be reduced, the overall efficiency will be improved.
Figure 1 shows the basic, widely used semi-bridgeless PFC rectifier [
8,
9,
10], which improves the overall efficiency by replacing two diodes with two MOSFETs. The disadvantages of this structure are the inconvenience of detecting the input voltage and inductor current [
11,
12,
13] and the problem of electromagnetic interference (EMI) between the input and output due to the common-mode noise generated by high-switching power semiconductor devices [
14,
15].
Figure 2 shows the semi-bridgeless PFC rectifier [
16,
17,
18,
19,
20,
21,
22,
23,
24,
25,
26,
27,
28,
29], which is a modified version of the basic, widely used semi-bridgeless PFC rectifier. In this modified circuit, two additional slow diodes,
D3 and
D4, are connected between the negative and positive terminals of the input AC voltage
vin and the negative terminal of the output capacitor
Co, respectively, to reduce the common-mode noise and, hence, to solve the EMI problem [
14,
15]. Furthermore, due to these two diodes, the input voltage can be detected directly by the resistive voltage divider. However, the inductor current will flow through the body diode of the inactivated MOSFET main switch, leading to an error in detecting the inductor current, causing a more complex current-detecting circuit to be required.
In order to solve the problem of detecting the inductor current, [
30] suggests three methods to obtain the detected inductor current signal
CS: Hall sensor detection, differential amplifier detection, and current transformer detection. These three methods are described below:
(1) Hall effect sensor detection
This method only requires the Hall effect sensor to be placed directly on the input to detect the inductor current, as shown in
Figure 3. This method is simple, and the detected value is accurate and reliable, but the Hall sensor is more expensive.
(2) Differential amplifier detection
This method is to connect a current-detecting resistor
RS in series with the input, and then to use a differential amplifier
DA to detect the inductor current signal on
RS, as shown in
Figure 4. This method is simple and relatively inexpensive. Since the current-sampling resistor is placed at the negative terminal of the AC input voltage
vin, the detected
CS is easily interfered with by the common-mode noise, resulting in a relatively low power factor.
(3) Current transformer detection
As shown in
Figure 5, this method requires the use of three current transformers; the main switch
S1 and the main switch
S2 are each connected in series with the current transformers
CT1 and
CT2, the diodes
D5 and
D6, and the resistors
R1 and
R2, respectively, whereas the output is also connected in series with the current transformer
CT3, the diode
D7, and the resistor
R3. The following is a brief description of the corresponding operating principle:
(a) When vin > 0, the main switch S1 is turned on, the inductor L1 stores energy, the inductor current flows through the current transformer CT1, and CS is detected. When vin > 0, the main switch S1 is cut off, the inductor L1 releases energy through diode D1, the inductor current flows through current transformer CT3, and CS is detected.
(b) When vin < 0, the main switch S2 is turned on, the inductor L2 stores energy, the inductor current flows through the comparator CT2, and CS is detected. When vin < 0, the main switch S2 is cut off, the inductor L2 releases energy through diode D2, the inductor current flows through current transformer CT3, and CS is detected.
By integrating the detected inductor current signals from the above three current transformers, these inductor current signals are then converted into voltage signals that can be used by the controller via the peripheral circuits. Therefore, this method uses a large number of current transformers and complex peripheral circuits.
In this paper, our new method is to replace the metal–oxide–semiconductor field-effect transistor (MOSFET) with a gallium nitride field-effect transistor (GaNFET) in the selection of the main switch. The reverse-biased conduction voltage of the third quadrant of the GaNFET is much higher than the forward-biased voltage of the diode, which solves the problem of detecting the inductor current, reduces the number of current sensors, and simplifies the corresponding peripheral circuits and components.
2. Operating Principle of the Semi-Bridgeless PFC Rectifier Using MOSFET Main Switches
Figure 6 shows the waveforms relevant to the circuit operation during the positive and negative half-cycles of the input AC voltage, as well as these waveforms under high-frequency switching corresponding to the peak value of the sine-wave voltage. From this figure, it can be seen that the circuit has four operating states. When the input AC voltage
vin is under the positive half-cycle, the main switch
S1 is turned on/off under high-frequency switching and the main switch
S2 is always cut off. When the input AC voltage
vin is under the negative half-cycle, the main switch
S1 is cut off and the main switch
S2 is turned on/off under a high switching frequency. There are four operating states for this semi-bridge rectifier, and they are described in the following text.
As shown in
Figure 7, when the input AC voltage
vin is under the positive half-cycle, i.e.,
vin > 0, the main switch
S1 conducts, the main switch
S2 is cut off, the diode
D3 conducts, and the diodes
D1,
D2, and
D4 are all cut off. The input current
iin flows through the inductor
L1, the main switch
S1, and the diode
D3, and another small portion of the current flows through the body diode of the main switch
S2 and the inductor
L2. At the same time, the voltage
vL1 across the inductor
L1 is the input voltage
vin, and the inductor
L1 is in a state of magnetization due to the positive voltage across the inductor
L1. During this state, the inductor current
iL1 rises linearly, and the inductor
L1 stores energy. The energy required for the output resistor
Ro is supplied by the output capacitor
Co.
To speak more lucidly, in
Figure 7, the main switch
S1 is on, so the body diode of
S2 and the diode
D3 are both forward-biased; hence, there are two inductor current paths returning to the input voltage
vin. One path has the diode
D3 connected in series with the inductor
L1, whereas the other path has the body diode of
S2 connected in series with the inductors
L1 and
L2. Accordingly, the impedance of the path with the body diode of
S2 is higher than that of the path with diode
D3, so a small portion of the inductor current will flow through the body diode of
S2, and the majority of the inductor current will flow through the diode
D3. Note that the switching frequency is much higher than the line frequency. Therefore, the voltage drop due to the line frequency is quite low, so the forward-biased voltage of the body diode of the main switch
S1 is similar to the voltage forward-biased voltage of the diode
D3, causing the small portion to seem constant.
As shown in
Figure 8, when the input voltage
vin is under the positive half-cycle, i.e.,
vin > 0, the main switches
S1 and
S2 are cut off, the diodes
D1 and
D3 are turned on, and the diodes
D2 and
D4 are cut off. The input current
iin flows through the inductor
L1, the diodes
D1 and
D3, the output capacitor
Co, and the output resistor
Ro, and another small portion of the current flows through the body diode of the main switch
S2 and the inductor
L2. At the same time, the voltage
vL1 across the inductor
L1 is the input voltage
vin minus the output voltage
Vo, and the inductor
L1 is in a state of demagnetization due to the negative voltage across the inductor
L1. During this state, the inductor current
iL1 decreases linearly and the inductor
L1 releases energy. The energy required for the output resistor
Ro is supplied by the input voltage
vin and the inductor
L1, which also charge the output capacitor
Co.
As shown in
Figure 9, when the input AC voltage
vin is under the negative half-cycle, i.e.,
vin < 0, the main switch
S1 is cut off, the main switch
S2 conducts, the diode
D4 is turned on, and the diodes
D1,
D2, and
D3 are all cut off. The input current
iin flows through the inductor
L2, the main switch
S2, and the diode
D4, and another small portion of the current flows through the body diode of the main switch
S1 and the inductor
L1. At the same time, the voltage
vL2 across the inductor
L2 is the input voltage
vin, and the inductor
L2 is in a state of magnetization due to the positive voltage across the inductor
L2. During this state, the inductor current
iL2 rises linearly, and the inductor
L2 stores energy. The energy required for the output resistor
Ro is supplied by the output capacitor
Co.
As shown in
Figure 10, when the input AC voltage
vin is under the negative half-cycle, i.e.,
vin < 0, the main switches
S1 and
S2 are cut off, the diodes
D2 and
D4 are turned on, and the diodes
D1 and
D3 are cut off. The input current
iin flows through the inductor
L2, the diodes
D2 and
D4, the output capacitor
Co, and the output resistor
Ro, and a small portion of the current flows through the body diode of the main switch
S1 and the inductor
L1. At the same time, the voltage
vL2 across the inductor
L2 is the input voltage
vin minus the output voltage
Vo, and the inductor
L2 is in a state of demagnetization due to the negative voltage across the inductor
L2. During this state, the inductor current
iL2 decreases linearly, and the inductor
L2 releases energy. The energy required for the output resistor
Ro is supplied by the input voltage
vin and the inductor
L2, which charge the output capacitor
Co.
6. Conclusions
In this paper, the problem of detecting the inductor current of a semi-bridgeless PFC rectifier was improved. Firstly, we introduced the GaNFET feature that the reverse-biased conduction voltage VSD in the third quadrant is higher than the forward-biased conduction voltage of the diode. This feature was used to improve the current-detecting circuit. Accordingly, a semi-bridgeless PFC rectifier prototype was implemented to demonstrate the effectiveness of the proposed strategy.
From the experimental results, it can be seen that under different input voltages and output power ratings, the inductor current path has only one loop and does not flow through the inactivated main switch and the inductor. It was confirmed that the proposed strategy is effective, thereby reducing the number of current sensors and simplifying the number of peripheral circuits and components.
Furthermore, under an input voltage of AC 115 V and the rated output power, the PF is 0.994 and the efficiency is 96.42%, whereas under an input voltage of AC 230 V and the rated output power, the PF is 0.996 and the efficiency is 98.45%. This confirms that the semi-bridgeless PFC rectifier prototype with GaNFET main switches possesses high PF and high efficiency. Moreover, the IEC61000-3-2 Class D current harmonic standard can be met under both of these two cases.