The design of conducted electromagnetic interference (EMI) filters for PFC circuits is more and more important for the fast development of power electronic product and the accompanied EMI reduction standards. The design procedure is not an exact science. It usually likes a try-error process for a designer to come up with a proper filter design. So, designing an EMI filter is a time-consuming process not only for junior engineers but also for experienced engineers when they meet new design.
The following three major requirements are important for the EMI filter design:
1. Requirement for switching noise attenuation,
2. Low input displacement angle (IDF) between input voltage and input current, 3. Overall system is stability.
The first requirement is defined by the EMI control standards, such as the frequency range from 10 kHz to 30MHz for VDE 0871/B limits and 450 kHz to 30MHz for FCC limits by definition the conducted EMI.
The second requirement exists only for the PFC circuit input EMI filter design. Fig. 2.16 [25] shows a simplified diagram of a PFC converter with a standard L-C input EMI filter. The voltage Va causes the reactive current, ic. The IA has the phase shifted relative to the input voltage, VA by the angle θ are given by following equations (2-12)~(2-15).
t
The capacitor size has to be minimized and the limit value is given in equation (2-16) [25]-[26]
)
Fig. 2.17 shows the phasor diagram of the input currents and voltages. It is important to design a minimum displacement angle between the input current and voltage and keep low phase shift after the input EMI filter is added.
The third requirement amounts to controlling the impedance interaction between the input filter and the PFC converter. The filter output impedance should be small than the converter input impedance to keep the stability of system [22]-[24]. The impedance interaction constraint will practically determine the lower bound on the filter capacitor value.
Additionally, proper filter pole damping is very important to achieve low filter output impedance for all frequencies and keep the overall system stability.
In order to keep the filter component values and size small, the filter corner frequency has
18
to close to the switching frequency. So, the high-order filters can have a reasonable size and meet all the requirements in the PFC circuit.
Fig. 2.16. PFC Circuit with simply input EMI filter diagram [25].
Fig. 2.17. Input voltage and current phasor diagram [25].
Usually, the total conducted EMI noise is caused by two parts, the common-mode (CM) noise and the differential-mode (DM). Normally the DM noise is related to switching current the source is a switch component such as MOSFET, and the CM noise is related to capacitive coupling of switching voltage into the input. Fig.2.18 shows the typical setup for conducted EMI measurement [27]. The LISN contains the components: inductors, capacitors and 50 W resistors. The inductors are shorted; the capacitors are open for a 60 Hz line
frequency. For EMI noise frequency, the inductors are essentially open, the capacitors are shorted and the noise sees 50 W resistors.
The noise voltage measured across the 50 W input impedance of a spectrum analyzer is defined as the conducted EMI emission.
Fig. 2.18. Test setup for conducted EM1 measurement.
Fig. 2.19 shown a typical EMI filter topology, and the Fig. 2.20 (a) and (b) shows the equivalent circuit of CM section and the DM section. It also indicates that some components of filter affect the CM or DM noise only and some components of filter affect both of CM and DM noise [27].
Fig. 2.19. A typical EMI filter topology.
20
Fig. 2.20. CM noise equivalent circuit.
Fig. 2.21. DM noise equivalent circuit.
Chapter 3
O p e r a t i o n o f M C 3 3 2 6 0 a n d H a r d w a r e D e s i g n
3.1 I
NTRODUCTION OFMC33260
MC33260 is a power factor correction control IC of ON Semiconductor to meet the international standard requirements in electronic equipments. Fig. 3.1 shows the typical boost PFC converter using a CRM PFC controller MC33260. The CRM control scheme features a constant ON time and has a variable switching frequency. The MC33260 is optimized to just as well drive a free running as a synchronized discontinuous voltage control mode. It also provides features protections such as under-voltage, over-voltage protection, over-current limitation to make the PFC pre-regulator works in a safe condition. It is also able to safely face any uncontrolled direct charges of the output capacitor from the mains which occur when the output voltage is lower than the input voltage [28].
The MC33260 can also work in an innovative mode named “Follower Boost” that offers to significantly reduce the size of the inductor and power MOSFET and improving the efficiency [28] [29]. When MC33260 works in Follower mode, the output voltage is not forced to a constant value, it can be changed according to the AC input voltage. The gap between the output voltage and the AC input voltage becomes lower and causing the inductor and power MOSFET of PFC size reduction. Finally, this method brings a significant cost reduction.
Fig. 3.1. CRM PFC circuit with MC33260.
Fig. 3.2 shows a typical schematic of a CRM PFC controller without multiplier [13]. The comparator generates a constant on-tome PWM signal by comparing the output of a programmable on-shot timer with a constant voltage reference. The zero current detector turn on the power MOSFET again when the inductor current drops to zero. The inductor current with a constant rising slope and is switched up and down between a sinusoidal reference in proportional to line voltage and a zero current.
A novel approach to the CRM PFC controller is available in an ON Semiconductor chip, MC33260. This chip provides the same function as the controllers described above. And it accomplishes this without the use of a multiplier.
Fig. 3.2. Basic schematic for a CRM PFC controller without multiplier [5].
Fig. 3.3 is the waveform as explained in the previous section, the current waveform for a CRM controller ramps from zero to the reference signal and then slopes back down to zero.
The reference signal is a scaled version of the rectified input voltage, and be referred to as k
× Vin, where k is a scaling constant from the ac voltage divider and multiplier in a classic circuit. The turn-on time is equal to k × L.
Fig. 3.3. Inductor current waveform [13].
Above equation shows that ton is a constant with a given reference signal (k × Vin). Toff
vary throughout the cycle, which is the cause of the variable frequency. The on time is constant for a given line and load condition is the basis for this control circuit.
3.2 O
PERATION OFMC33260
3.2.1 Oscillator Section
The oscillator consists of three states: 1.Charge State: The oscillator capacitor voltage grows up from ground until it exceeds Vcontrol (regulation block output voltage) linearly. At that moment, the PWM latch output is low and the oscillator discharge sequence is set. 2.
Discharge State: The oscillator capacitor discharged down to its valley value (0 V) shortly.
3.Waiting State: The oscillator voltage is kept in a low until the PWM latch is set again during the end of the discharge sequence.
The charge current of oscillator is dependent on the feedback current where Icharge is the
Pin 3 is the oscillator terminal includes an internal capacitance (Cint) that varies versus the pin 3 voltage. The average value is 15 pF.
3.2.2 Regulation Section
The feedback current is obtained by connecting a resistor between the output and pin 1.
The value is given by following equation:
o outputs a signal following the future as Fig. 3.4
Fig. 3.4. Regulation characteristic [17].
3.2.3 Current Sense and Zero current detection Section
A ground reference resistor (Rcs) inserts in series with the input rectifier and input filtering capacitor to convert the inductor current into a negative voltage as following equation:
) ( cs L
cs R I
V =− ⋅ (3-3)
The IL is the inductor current, Rcs is the current of sense resistor, Vcs is the voltage on Rcs and a negative voltage to the inductor current proportionally.
The zero current detection function controls the power MOSFET off as long as the inductor current does not reach zero during the off time. The pin 4 voltage compared to the threshold (-60 mV) to kept the gate drive signal in low state when Vcs is small than the threshold. The MOSFET turn on until the Vcs is smaller than 60 mv when the inductor current is close to zero. The pin 4 signal is used for the over-current limitation during on time, and it serves the zero current detection during the off time. The Fig. 3.4 shows the current sense block includes the main components, PWM Latch, Rcs, and comparator for the operation of zero current detection. The Fig. 3.5 shows the waveforms of power switch drive, inductor current, and pin 4 voltage where simulated with PSIM and the results are the same with the application notes.
Fig. 3.5. Current sense block [17].
Fig. 3.6. Zero current sensing circuit.