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Multi-Phase Adaptive On-Time PFC for Better Light

Load Efficiency and EMI Performance

Qian Li and Fred C. Lee

ECE Department of Virginia TechCenter for Power Electronics Systems

655 Whittemore Hall, Blacksburg, VA

[email protected]

Abstract —With the fast growing information technologies, high

efficiency and high power density become the two major

challenges for AC-DC front-end power supplies in all kinds of

distributed power system applications. For the power factor

correction (PFC) stage, the adaptive on-time control was

proposed to achieve high efficiency over the whole load

range[1]. The EMI performance of single phase adaptive on

time controlled PFC is tested in this paper, and multi-phase

approach is used to improve the EMI performance. Since theswitching frequency of the adaptive on time control is a

variable, the novel adaptive phase angle control is proposed to

achieve the best noise cancellation effect, leading to significant

EMI filter size reduction. The experiment results show that

both high efficiency and high power density can be achieved by

the multi-phase adaptive on-time PFC approach.

I. I NTRODUCTION

With the fast growing information technologies, high

efficiency and high power density become the two major

challenges for AC-DC front-end power supplies in all kinds

of distributed power system. Driven strongly by economic

and environmental concerns, the efficiency requirement is being pushed by various organizations and programs, such

as the 80 PLUS [2], U.S. Energy Star [3] and Climate

Savers [4]. 80 plus is a basic efficiency requirement for the

front-end converter, as shown in Figure 1. Other than the 80

plus requirement, Climate Savers is targeting at higher

efficiency. They even target to achieve 4% or 3% efficiency

improvement every year in these coming two years.

Moreover, the industry customer is targeting even more

aggressive efficiencies, as shown in Figure 1. Not only the

efficiency target is set higher, but also 10% and 5% load

efficiency is required. To achieve the most stringent

efficiency requirement of the whole front-end converter, the

corresponding PFC and DC-DC efficiency targets are also

shown in Figure 1. Comparing efficiency of today’s PFC

product which uses the conventional constant frequency

average current mode control with the efficiency target, as

shown in Figure 2, the light load range efficiency needs to

be improved.

Another challenge of the of AC-DC front-end converter is

the power density. Figure 3 shows the power density

roadmap for server/telecom front end converter [5]. The

EMI filter, boost inductor and bulk capacitors represent the

major portion of the PFC stage size. In a typical 1 kilowatt

PFC design, the EMI filter can occupy about 35% ~ 40%

size of the whole PFC circuit.

In the following sections, the adaptive on-time control to

improve light load efficiency for PFC will be illustrated

93%

94%

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96%

97%

98%

0% 20% 40% 60% 80% 100%

Load

E f f i c i e n c y

Measured

V in = 220Vrms

PFC Target

Efficiency

Figure 2. PFC light load efficiency challenge

70%

75%

80%

85%

90%

95%

100%

0% 20% 40% 60% 80% 100%

E f f i c i e n c y

Load

80 Plus Bronze82%

85%

82%

80 Plus Silver 85% 85%

88%80 Plus Gold

90%

87%87%

50%5%10%

85%

Target Efficiency92%

75%

94%

92%

DC-DC Target Efficiency

PFC Target Efficiency

Figure 1. Efficiency requirements for AC-DC front-end converters

978-1-4244-8085-2/11/$26.00 ©2011 IEEE 529

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briefly first. The efficiency and EMI performance of single

phase adaptive on time PFC are tested to compare with the

conventional constant frequency PFC. To reduce the EMI

noise of the adaptive on time PFC, multi-phase approach is

used and the novel adaptive phase shift control is proposed

for the adaptive on time PFC to achieve the best noise

cancellation effect. At last, the experiment results show the

improvement on both efficiency and EMI filter size

reduction.

II. ADAPTIVE O N-TIME PFC

The light load efficiency of the conventional constant

frequency PFC drops very fast because of high switching

frequency related loss, such as turn-on loss, turn-off loss and

inductor core loss. The adaptive on-time control [1] can

effectively reduce the switching frequency at light load

condition and therefore reduce the switching frequency

related loss. The on time profile of the adaptive on time PFC

is controlled the same as that of the constant frequency PFC

in continuous conduction mode (CCM), and it remains

unchanged in different load conditions. Based on the boost

converter voltage gain in CCM, the on time in a half line

cycle can be expressed as equation (1) below,

const o

inoon

f t V

t V t V t T

)(

)()()(

−

=(1)

where f const is the switching frequency in CCM.

The on time and frequency profiles of the adaptive on

time control within on half line cycle at 90V input in

different load conditions are shown in Figure 4. From

Figure 4, we can see that by keeping the same on time profile at different load, the switching frequency of adaptive

on-time control is constant in heavy load, and it is reduced

at light load to enter DCM operation. The two sides of the

half line cycle will enter DCM prior to the middle of the

half line cycle when load is decreasing, so there are three

operation modes in total:

1) CCM for the whole half line cycle;

2) Part CCM, part DCM in the half line cycle;

3) DCM for the whole half line cycle.

The efficiency comparison of single phase adaptive on

time PFC and constant frequency PFC is shown in Figure 5.

Since the circuit operation condition under the two control

methods are the same in CCM, the heavy load efficiency arethe same. However, in light load range, the adaptive on time

control shows significant efficiency improvement.

93.5%

94.0%

94.5%

95.0%

95.5%

96.0%

96.5%

97.0%

97.5%

98.0%

98.5%

0% 20% 40% 60% 80% 100%

Fs=70kHzAdaptive Ton

Vin=220V, Vo=400V

Load

E f f i c i e n c y

Figure 5. Efficiency comparison of single phase adaptive on time

PFC and constant frequency PFC

Tline /2Tline /40

S w i t c h i n g f r e q u e n c y

( k H z )

0

20

40

60

80 Heavy load, CCM

Medium load,

part CCM part DCM

Light load, DCM

(b). Frequency profile

Figure 4. On time and frequency profile of adaptive on-time

control at 90V input in different load conditions

0

4

8

12

16

Tline /2Tline /40

O n T i m e ( u s )

Same Ton at

different load

(a). On time profile

Figure 3. Power density roadmap of sever/telecom front end power

supply

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The average DM noise comparison of single phase

adaptive on time PFC and constant frequency PFC are

shown in Figure 6. The noise peak of the constant frequency

PFC at each order of switching harmonic can be clearly

identified. The EMI noise of the adaptive on time PFC is a

continuously changed curve because the switching

frequency varies in a half line cycle in DCM, leading to a

wider frequency band.

Based on the tested EMI noise magnitude, in order tomeet the noise standard, the corresponding attenuation

requirement can be identified. For DM noise, the single

phase adaptive on time PFC needs 64dB attenuation at

150kHz, while the single phase constant frequency PFC

needs 79dB attenuation at 210kHz. Once we get the

attenuation requirements, the filter corner frequency can be

calculated, and the results are shown in table I. For the CM

noise, if the balance technique [6] is applied, the CM noise

of the PFC is very small and can be neglected. So the CM

noise is mainly determined by the downstream DC/DC

converter. In the filter design process, we just use the CM

noise of the LLC resonant converter with 1MHz switching

frequency. Based on the measured DM and CM noise, theEMI filter is design and prototyped, as shown in Figure 7.

TABLE I. DM CORNER FREQUENCY COMPARISON

Control Method DM Corner Frequency

Adaptive On Time 33.5kHz

Constant Frequency 33.4kHz

III. ADAPTIVE PHASE A NGLE CONTROL FOR MULTI-

PHASE ADAPTIVE O N TIME PFC

The multi-phase interleaving is an effect way to reduce

the EMI noise due to the ripple cancellation. In paper [7], themulti-phase PFC with a properly selected phase angle is presented to greatly reduce the EMI filter size. In this paper,as long as the switching frequency and phase number areselected, the phase shift angle is a fixed value. Nevertheless,the situation is different for multi-phase adaptive on timePFC since the switching frequency is a variable. Theremaining part of this section will use a four-phaseinterleaved adaptive on time PFC as an example to illustratethe benefit of interleaving.

From Figure 4, we can see that the switching frequency ischanging with load variation. Figure 6 clearly shows that theEMI spectrum is a continuous curve so the filter will always be designed based on the noise magnitude at 150kHz. In

order to reduced the noise magnitude at 150kHz, proper phase shift angle of the multi-phase PFC should be chosen.There are many sources of the 150kHz harmonic, such as the3rd harmonic of 50kHz ripple, 4th order of 37.5kHz ripple,5th harmonic of 30kHz ripple, etc. Among these sources, the3rd harmonic of 50kHz ripple is dominant. If the phase shiftangle is chosen to 30 degrees, the 3rd harmonic of 50kHzripple will be totally cancelled with 4 phase interleaving. TheEMI noise comparison between single phase adaptive ontime PFC and 4 phase adaptive on time PFC with 30 degrees phase shift is shown in Figure 8. It can be clearly seen thatthe DM noise at 150kHz is reduced by about 10dB.

Although the 3rd order harmonic of 50kHz ripple is

totally cancelled, there are still 150kHz noise contributed by

high order harmonics of lower frequency ripple (37.5kHz,

30kHz, etc). The fixed phase shift angle is not able to cancel

these harmonics. In order to get the best cancellation effect

Figure 7. Filter prototype for single phase constant frequency PFC

and adaptive on time PFC

Figure 8. EMI noise comparison of single phase adaptive on timePFC and 4 phase adaptive on time PFC with 30 degree phase shift

Figure 6. DM noise comparison of single phase adaptive on timePFC and constant frequency PFC

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of the 150kHz noise, the novel adaptive phase angle control

is proposed. The basic concept can be illustrated in Figure 9.

When the switching frequency is between 50kHz to 70kHz,

the 3rd harmonic of switching frequency ripple is dominant

for DM noise, so 30 degrees phase shift is used to totally

cancel the 3rd order harmonic. As the switching frequency

is changing, the same principle applies. For example, if the

switching frequency is between 37.5kHz and 50kHz, the 4th

harmonic of switching frequency ripple is dominant for DMnoise, so 22.5 degrees phase shift is used to totally cancel

the 4th order harmonic. By following this phase angle

control method, the DM noise at 150kHz can be cancelled.

The proper phase shift angle at different switching

frequency is summarized in Figure 10.

In Figure 10, the blue solid line the phase angle selection

criterion of the adaptive phase angle control. This is a stair case curve and the steps get smaller when the frequency isreduced. It is difficult to implement this method because the phase-angle vs. frequency curve is not continuous. Becausethe inductor current ripple is lower at lower frequency, thelow frequency current ripple has very small contribution tothe EMI noise, which means the phase angle selection is notthat critical at low frequency range. Based on the analysis,

the phase angle at low frequency range (less than 25kHz) can be reasonably approximated by a simple straight line, asshown as the red dashed line. In this way, the phase anglealgorithm can be greatly simplified.

IV. EXPERIMENT VERIFICATION

The 4 phase interleaved adaptive on-time PFC prototypewas built for experiment verification. The 4 600uH boost

inductors are implemented by using CoolMu 77894 toroidcore from Magnetics Inc. Two 220uF electrolytic capacitorsare in parallel for the output bulk capacitor. The boostinductor waveforms at heavy load and light load are shownin Figure 11 and Figure 12, respectively.

From Figure 11 and Figure 12, we can see that the phase

shift angle is automatically adjusted based on the present

cycle switching frequency. In DCM operation condition,

since the switching in one half line cycle changes, the phase

shift angle changes accordingly.

The tested efficiencies at low line input and high line

input of the multi-phase adaptive on time PFC are shown inFigure 13 and Figure 14, respectively. In both figures, the

switching frequency of 1 phase CCM PFC is 70kHz. It

indicates that the light load efficiency is greatly improved

over the conventional constant frequency PFC.

2A/div

70kHz, 14.3us 30°phase shift,

1.2us

Figure 11. Inductor current at heavy loadVin=110V, Vo=400V, Po=960W

10 20 30 40 50 60 70 80

0

5

10

15

20

25

30

35

P h a s e s h i f t a n g l e ( d

e g r e e )

Frequency (kHz)

Exact Phase Angle

Simplified Phase Angle

Figure 10. Phase angle selection at different switching frequency

30

37.5

70

50

0

20

40

60

Typical Frequency Profile

0 Tline/2Tline/4

80

F r e q u e n c y (

k H z )

4-phase with 30°shift to cancel 3rd

order harmonic.

4-phase with 22.5°shift to cancel 4th

order harmonic.

4-phase with 18°shift to cancel 5th

order harmonic.

100% load

30% load

15% load

Figure 9. Adaptive phase angle selection for adaptive on time PFC

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The tested DM noise is shown in Figure 15 and it is

compared with the noise of multi-phase adaptive on time

PFC with fixed 30 degrees phase shift. Since better ripple

cancellation can be achieved by the adaptive phase angle

control, the noise magnitude at 150kHz is further reduced

by 6dB. Based on the tested noise magnitude, the DM filter

corner frequency of the adaptive phase angle controlled PFC

can be calculated as 49kHz. Comparing with the single

phase adaptive on time PFC, there is about 16kHz increase.

By using the same CM filter as the single phase PFC and

redesigned DM filter, the total EMI filter size can be

reduced by 50%. The filter prototype for the 4 phase

adaptive on time PFC is shown in Figure 16.

V. CONCLUSION

In this paper, the multi-phase adaptive on time control

PFC with adaptive phase angle control is proposed.

Improved light load efficiency is achieved by reducing

switching frequency. The novel adaptive phase angle

control can achieve the best noise cancellation effect,

therefore greatly reduce the EMI filter size. The analysis is

verified by experiments results.

ACKNOWLEDGMENT

The author would like to thank the Power Manage

Consortium (PMC) for the support.

Figure 16. Filter prototype for 4 phase adaptive on time PFC

with adaptive phase angle control

Figure 15. EMI noise comparison of 4 phase adaptive on time

PFC with 30 degree phase shift and 4 phase adaptive on time

PFC with adaptive phase shift

93.5%

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98.5%

0% 20% 40% 60% 80% 100%

1 phase const-fs

4 phase adaptive Ton

Vin=220V, Vo=400V

Power

E f f i c i e n c y

Figure 14. Efficiency comparison at high line input

Vin=110V, Vo=400V

0% 20% 40% 60% 80% 100%93.0%

93.5%

94.0%

94.5%

95.0%

95.5%

96.0%

96.5%

97.0%

1 phase const-fs

4 phase adaptive Ton

Power

E f f i c i e n c y

Figure 13. Efficiency comparison at low line input

1A/div

40kHz, 25us

22.5°,1.56us 29kHz,

34.5us15°,

1.44us

Figure 12. Inductor current at light load

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R EFERENCES

[1] Qian Li; Lee, F. C.; Ming Xu; Chuanyun Wang; "Light LoadEfficiency Improvement for PFC", Energy Conversion Congress andExposition, 2009, pp. 3755 - 3760

[2] http://www.80plus.org/

[3] http://www.energystar.gov/

[4] http://www.climatesaverscomputing.org/

[5] Lee, F.C.; Ming Xu; Shou Wang; Bing Lu; "Design Challenges for Distributed Power Systems", International Power Electronics andMotion Control Conference, 2006, Vol. 1, pp. 1 - 15.

[6] Pengju Kong; Shuo Wang; Chuanyun Wang; Lee, F. C.; "ReductionTechnique for the Interleaved Multichannel PFC Converter", AppliedPower Electronics Conference and Exposition, 2008, pp. 729 - 735

[7] Chuanyun Wang; Ming Xu; Lee. F.C. Bing Lu; "EMI Study for theInterleaved Multi-Channel PFC", Power Electronics SpecialistsConference, 2007, pp. 1336 - 1342

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