issue: October 2005 APPLIANCE Magazine
Partial Switching Power Factor Correction Module
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by Ki-Young Jang, Yo-Chan Son, Man-Kee Kim, Sung-Il Yong, and Bum-Seok Suh, Fairchild Semiconductor
This paper introduces an integrated power module for a partial switching power factor correction circuit (PSP) used in air-conditioning systems. The proposed module incorporates two insulated gate bipolar transistors (IGBTs) and rectifier diodes, along with a gate-driving low-voltage integrated circuit (LVIC) and a thermistor for temperature monitoring, all of which are integrated into a direct-bonded-copper (DBC) based transfer-molded package. It is intended for use in inverter systems of low-power air-conditioners and is expected to provide compactness, reliability, and electrical and thermal performance.
Harmonic current generated by a non-linear load deteriorates the power quality of a utility, and is a source of electrical interference for equipment connected to the point of common coupling. In the case of a diode rectifier, the input current flows only if the input voltage is higher than the output voltage of the rectifier, which generates large harmonic current especially if the load current is large.
Today this is regulated by domestic or international regulations, such as IEC61000-3-2, and compliance is typically mandatory. To meet this requirement, it is necessary to have a pre-regulator instead of a diode rectifier to reduce the distortion of input current. Air-conditioners, with power ratings ranging from 1 to 4 kW, are Class A equipment according to IEC61000-3-2 (input current is less than 16 Arms), and most of them are adopting power factor correction (PFC) circuits as the pre-regulator to mitigate harmonic distortion.
There are three common methods of PFC in air-conditioning systems. The first calls for installing the AC reactor in front of the diode rectifier. This method is effective for harmonic mitigation air-conditioners below 1 kW. However, as system power rating increases, it is difficult to apply this method because the input reactor causes a lag in input current and reduction of the input voltage.
The second method calls for using a boost-converter-based, full-switching PFC circuit  as shown in Figure 1(a). With this method, it is possible to meet the harmonic regulation with
99 percent of the input power factor, regardless of the power rating of the air-conditioner. This method relies on high frequency switching (generally 20 ~ 100 kHz) of a PFC switch to generate sinusoidal input current. This greatly increases the switching loss of a switching device as well as the electromagnetic interference (EMI) due to the recovery current of the fast-recovery diode and high-speed switching of the PFC switch.
The third method calls for adopting a PSP . The circuit topology is the same as that of the full-switching PFC, as shown in Figure 1(a), while the switching frequency is twice the utility frequency. Compared with the full-switching PFC, the performance of a PSP is limited and unsuitable for high-power air-conditioners, over 3 kW capacity, due to limited harmonic regulation. However, for most room air-conditioners below 3 kW it provides moderate performance as well as low EMI, due to low switching frequency. Moreover, due to its simplicity, it can be controlled by a microprocessor used for inverter control, without a dedicated control IC. For this reason, PSP is popular for 1 to 3 kW room air-conditioning systems, and its variants are widely adopted with approximately 97 percent of input power factor.
The module employs the circuit topology shown in Figure 1(b), which is equivalent to Figure 1(a).
Concept of PSP
Figure 2(a) is the external view of the package and Figure 2(b) shows the cross section diagram of PSP. It is a DBC-based transfer-molded package. In the case of a lead-frame-based package , it is hard to change the circuit topology because the lead-frame has to be changed for that purpose. The greatest advantage of a DBC substrate is that it allows for easily creating a new topology without significant cost. Moreover, due to the thickness (0.68 mm) of the isolation layer (Al2O3), it is possible to have a low thermal resistance while maintaining the isolation voltage of 2.5 kV for 1 minute. By virtue of this DBC substrate, PSP can share the same package with SPM3 , which has a three-phase inverter topology.
Using the same package with SPM3 can be an advantage in the assembly of an inverter printed circuit board (PCB). Figure 3 shows SPM3 and PSP mounted on the same PCB. Unlike discrete packages, module packages are not standardized or have their own packages. Especially in the case of a power module, the heat sink mounting can be troublesome when different modules are placed on a single PCB, which can lead to decreased assembly productivity. The adoption of a PSP in the inverter PCB using SPM3 can increase productivity.
Figure 2(b) is the internal block diagram of a PSP. It integrates two IGBTs for boost converter operation and four rectifier diodes as shown in Figure 1(b) as well as an optional thermistor and a gate-driving LVIC that provides a protection function. In a PFC circuit in Figure 1(a), the boost inductor and switch are installed at the DC-bus, whereas in the proposed PSP they are installed at the AC input of the rectifier as shown in Figure 1(b) and 2(b). The proposed PSP module can be controlled exactly in the same way as the conventional PSP circuit in Figure 1(a). That is, input signals IN(R) and IN(S) for IGBT Q1 and Q2 can be tied and controlled together. In Figure 4(a), when the input signal is applied to PSP, the inductor current flows through Q1 and D4 as indicated in Figure 4(b), if Vin is positive. If Vin is negative, Q2 and D3 will conduct. At zero crossing of the input voltage, Q1 or Q2 (depending on the Vin sign) is turned on, and the inductor is charged. After turning off the IGBT, the inductor is discharged, and the current will flow to the load and the DC capacitor, as shown in Figure 4(b). Since the IGBT is turned on alternately at every period, the power dissipation of each IGBT will be half that of Figure 1(a). For this reason, the IGBT current rating can be half.
Simulation and Experiment Result
In Figure 6(a) and (b), the PSP operation is simulated under the conditions outlined in Table 1. Based on the simulation results, the power dissipation and allowable maximum case temperature (showing the PSP power rating) have been calculated. In the simulation, the output voltage (DC-bus voltage) was controlled at 280 V. The turn-on delay and turn-off time can be used to control the output DC-bus voltage and the input current distortion. In this simulation, the PSP turn-on delay time was fixed at 400us and the turn-off delay time was controlled by a simple controller that regulates the average DC bus voltage.
Figure 6(a) is the waveform of the proposed PSP, and Figure 6(b) shows the harmonic distortion of the input current compared with the IEC61000-3-2 limit of Class A equipment. Using this simple scheme, it was possible to meet the regulation limit in most of the low frequency regions (f < 500 Hz) with 97 percent of power factor. Harmonic contents above 500Hz can be suppressed by the additional EMI filter installed at the front-end of the application circuit.
In Figure 6(c) and (d), the current waveform from Figure 6(a) is scaled and decomposed as shown in Figure 4(a) to calculate the loss of IGBT and diode. As seen from Figure 6(c), the power dissipation by the rectifier diodes (especially by the lower-side diode) is much larger than that by the IGBT. Since the junction temperature of chips inside PSP should be lower than 125°C, the allowable maximum case temperature is limited by the power dissipation of the low-side rectifier diodes, which have the largest power dissipation. Based on this, the allowable maximum case temperature is calculated as shown in Figure 6(d), where the junction temperature of the low-side diodes is 125°C. Figure 6(d) shows that, if the case temperature is 110°C, then the input current can be up to 15 Arms.
To check the feasibility of the proposed PSP, it is applied to the outdoor unit of a 1.8-kW air-conditioner with a maximum power of 2.4 kW. Figure 7 shows the experiment results. Here Vsh is the voltage across the shunt resistor (Rsh = 25mW) in Figure 5 used for current sensing. Under the operation conditions shown in Table 1, the case temperature was maintained at 48°C where the ambient temperature is about 30°C.
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