Zero Ripple Current AC-DC Converters to. Minimize Input Filter. Frank CHEN*, Bruce LU**, Eric CHOU** and Claudio Adragna***. * STMicroelectronics/ Power ...
Zero Ripple Current AC-DC Converters to Minimize Input Filter Frank CHEN*, Bruce LU**, Eric CHOU** and Claudio Adragna*** *
STMicroelectronics/ Power Competence Center, Shanghai, China STMicroelectronics/ Power Competence Center, Taipei, Taiwan *** STMicroelectronics/I &PC Division, Agrate Brianza, Italy
**
Abstract— In this paper, a boost topology with a coupled inductor to achieve zero-ripple input current for power factor correction (PFC) application is proposed. By using a coupled inductor as the PFC transformer, this topology not only can comply with any standards or regulations, but it also provides clean power in a universal ac line (90 Vac to 264 Vac). The zero ripple current PFC topology is designed to work in critical conduction mode (DCM boundary) to eliminate diode reverse recovery loss and provide automatic control. This is done so that the complexity of the controller is reduced; the control strategy of PFC stage is similar to that of a conventional boost converter. By using a coupled inductor boost topology, one can remove electromagnetic interference (EMI) filter without sacrificing the input current harmonics. Consequently, smaller passive components can be used, and a lighter weight and lower cost can be achieved. Experimental results are given to confirm the theoretically predicted behavior. I.
Figure 1. Ripple Current Cancellation Circuit
there are at least two reasons why it is desirable to minimize inductor ripple currents. First, it reduces the stress on converter capacitors, resulting in either lower associated power loss or more relaxed filtering requirements. Second, and often more important, most converter topologies have a pulsating current at either input or output, or both, and most applications require low conducted noise at both ports for EMC or load requirements. In addition, a conventional boost converter with TM (Transition mode) operation has been widely used by the power supply industry; the TM of control strategy is the same as conventional boost PFC. Moreover, it eliminates most of the differential mode conducted noise, which will enable the reduction of the EMI filter size and capacitor, especially in the differential filtering section (CX capacitors and differential mode inductor). Therefore, removing the CX capacitors will bring an additional benefit to applications with tight specifications on standby power consumption: CX capacitors cause a considerable reactive current to flow through the filter during light load, which is a source of additional and unwanted loss (0.1W or more at high input voltage); furthermore, the discharge resistor that must be placed in parallel to CX for safety can be higher. As a result, both losses will also be decreased. For that reason, this paper presents a 200W board reference design for an LCD TV application with zero-ripple input current using a coupled inductor in PFC stage and resonant LLC topology in DC/DC stage. As shown in Fig.2, the proposed zero ripple PFC topology is composed of two windings of coupled inductor (LAC and LDC) and one capacitor CS as smooth capacitor. Experimental results show that the proposed PFC boost topology not only can produce zero-ripple input current,
INTRODUCTION
Recently, power supplies with high efficiency power factor correction are highly recommended because a reduction of the input current harmonics can optimize the capability of the line voltage to supply energy and avoid power losses in transmission lines. Harmonic Regulation IEC 1000-3-2 defines that unity power factor is not critical in some applications, such as portable tools (class B), lighting equipment (class C), computers (class D), etc. However, power factor correction with zero-ripple input current and sinusoidal line current is very attractive because an EMI filter in power supplies can be eliminated and clean power in the ac line can be obtained. The concept of zero-ripple or ripple-free input current is not new. It was originally used to reduce weight and increase power density of the converter. Generally, a zero-ripple phenomenon is achieved by using the coupled inductor technique, such as Cuk converter, Sepic converter, modified boost converter, etc. Based on the zero-ripple input current concept, various PFC converters with separate inductors and EMI filter requirements can be found in reference papers [2-5]. As previously studied, a modified boost converter with a coupled inductor can provide a smaller converter size compared with conventional boost converter. The application of the zero-ripple current phenomenon is of considerable interest in switching converters, where
1
but also can provide unity power factor operation with clean power in the line current. II.
inductor: L1-M is regarded as differential mode filter, M is regarded as the equivalent boost inductor, Rac is regarded as effective resistance of AC winding, L2-M is regarded as the leakage inductor series with Cs of differential mode capacitor, all of which will impact the performance of the zero-ripple input current. So the key for attaining zero-ripple current is to meet the condition that the value of L2 is equal to the mutual inductor M. In addition, to select the power semi-conductor component, the current stress and voltage stress of the power MOSFET and fast diode must also be the same as conventional PFC topology at the same output power, switching frequency. Moreover, because of input filter concerns for higher power applications (over 200W), it is a major limitation of ripple current for transition mode control of conventional PFC topology. In order to meet the EMI standard, the size of the input filter needs to be increased, in particular, the differential mode inductor. Based on this consideration, the ripple steering PFC topology is also an integrated differential mode filter that makes a reactive current (ac current) flow AC winding to Ground through the capacitor Cs. It is simple to achieve in higher powers because of the zero-ripple current of the input line.
ZERO-RIPPLE CURRENT PFC CONVERTER
As mentioned in section 1, the idea of zero-ripple or ripple-free input current is not new for buck topology applications. Here, the concept will be illuminated in boost topology for PFC applications. A. Ripple Steering PFC Topology The major difference that is shown in Fig.1 is a boost inductor compared with conventional PFC topology; the boost inductor of ripple steering PFC includes AC winding and DC winding where the high frequency current value and line frequency current value flow through if the topology is controlled properly. The original idea is from the concept of a coupled inductor. The equivalent circuit of ripple steering PFC topology is deducted according to the concept of a coupled inductor.
C. Design Criteria And Application Tips for Ripple Steering PFC Fig. 2 Zero Ripple Current PFC Topology
i1
B. Coupled Inductors Concept Figure 3 shows how an equivalent circuit decouples a coupled inductor.
Ls1 Lm
Ls2
i’1 M u ’1
i2
u’2 V2(t)
V1(t)
1:N
S1 Fig. 5 Modified Topology of Ripple Steering PFC Where, Ls1: the leakage inductor of the primary side; L1: the value of the primary side Ls2: the leakage inductor of the secondary side; L2: the value of the secondary side Lm: magnetizing inductor; V1(t)&V2(t): the source of the primary and secondary of transformer N: turns ratio of Primary Vs secondary. Eq.1 shows the relationship of these parameters according to Electrical-Magnetics theory.
Fig. 3 decouple equivalent circuit of coupled inductor Where, La=L1-M; Lb=M; Lc= L2-M; M=Squart(L1*L2) L1-M
M
L2-M
L1 = Ls1 + LM
M = nLM 2 L2 = n LM + Ls 2
Rac
(eq.1) Figure 5 shows the modified equivalent circuit of ripple steering PFC topology. Where, i1: input current through DC winding, i2: AC winding Current through Cs to Ground. Eq.2 shows the
Fig. 4 the Equivalent Circuit of Ripple Steering PFC Topology From Fig.4, one can see that this topology is the same as the conventional PFC topology after modified ripple steering PFC topology using the concept of coupled
2
Where, Vinmin=90Vac; Tonmax=0.682*20us=13.63us; ∆Bac=0.2T; Ae: section area of Core The turns ratio of DC winding Vs AC winding, K=0.75 (this is experienced coefficient) So, the turns number of DC Winding:
mode while S1 is turned on. To achieve no ripple current of input line that means: di1/dt=0
di 1 + u 1' dt ' u1 ∆I M = * T on LM V 2 (t ) i2 = * T on L2
V1 ( t ) = L s 1
N AC K = N DC N = N AC DC 0.75
(eq.2)
V1 (t ) = Ls1
(eq.7) 3), calculating the wire diameter of AC winding and DC winding (followed fig.7),
di1 + u1' dt
2 2 2 2 2 I Drms + I Mrms = I DCrms + I ACrms = 3 * I in I DCrms = I in 1 I ACrms = * I in 3
V1 (t ) = u1' (eq.3)
nLM V1 (t ) = 2 *V2 (t ) n LM + Ls 2 (eq.4) Where, V1 (t) =V2 (t) according to the theory of the inductor balance between voltage and seconds The results to achieve zero ripple input current are listed as the below: N LM = n2 LM +Ls2; according to the eq.1, So, M=L2 that means mutual inductor equal to the value of AC winding. The equation is consistent with the explanation of equivalent circuit in fig.4.
(eq.8) Where, Iin=200/(90*0.88)=2.52A The RMS of DC winding: IDCrms=2.52A The RMS of AC winding: IACrms=1.45A Current density: J=5.1A/mm2 AC winding should use lize wire, because the high frequency ripple current flows through it; DC winding should use single wire because 100Hz frequency current flows through it.
D. Design procedure of ripple steering PFC transformer The design steps of a ripple steering PFC transformer are similar to those of a conventional PFC. In particular, the required inductance value of AC winding L depends on the maximum peak current under the condition of full load, minimum input voltage, the RMS, DC and AC winding currents. The equations are listed as below: 1), calculating the required inductance value of AC winding
Vin2min
L=
2 * f s min *
Pout
* (1 −
η
The diameter of AC winding (Lize the diameter of DC winding (using single wire):
wire)
I 0 .1 2 ) * n = ACms 2 J I ACrms 400 n= * J π n = 36
π *(
(eq.9)
φ
I π * ( ) 2 = DCrms 2 J I 22 φ 2 = DCrms * J π φ = 0.6
2 *Vin min ) Vout (eq.5)
L=243uH
(eq.10) Where, Vinmin=90; fsmin=50 KHz; Pout=200W; Vout=400Vdc; η=0.88 2), calculating the numbers of AC winding and DC winding
N AC =
IDC IAC
Iin 1uF
2 *Vin min * t on max ∆Bac * Ae
ID IM
1uF
Fig.6 Reference Circuit of RMS Current Calculation
(eq.6)
3
4) Checking zero-ripple (Lac=M) i. ii. iii. iv. v. vi.
current PFC transformer
IV.
According to the previous analysis and derivation, the ripple steering PFC topology is verified by a 200W prototype with the following specifications: Output voltage: 24V*6A,12V*4A for main power Universal input voltage: 90Vac-264Vac Maximum output power: 200W D/D stage: LLC half bridge topology Prototype Size:22mm*218mm*27mm The main power stage parameters are as follows: PFC Inductor: LP-3320 LLC Transformer: LP-3925 Core provided by Yujing, Taiwan manufactory. ST Device: PFC stage: STTH8R06, STP25NM50; L6563 (Controller) D/D stage: L6599 (Resonant Controller), STP14NK50Z, STPS20H100CF, STPS20L40CF; Standby power (5V-SB): Viper-17
First, recording the value of DC winding and AC winding(LDC&LAC) Following Fig.7 as shown winding connection, Recording LA and LO data, Using Equation: LA-Lo=4M, get M data. Comparing M with Lac, confirm M-Lac
