digital power factor correction - handling the corner cases

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Digital Power Factor Correction

Handling the corner cases

Superior THD over entire operating range

www.controltrix.com


Power Factor primer

Re

act

ive

Po

we

rReal Power

Ind

uct

ive

- L

ag

gin

gC

ap

aci

tive

- L

ea

din

g

Φ

cosΦ = Real Power Apparent Power

= Power Factor

Applies for ideal sinusoidal waveforms for both voltage and current


Calculating Power FactorPower factor = Real power / Apparent power = (Vrms * I1rms * CosΦ) / (Vrms * Irms)

= cosΦ * ( I1rms / Irms)

Power factor = KΦ * Kd

Kd = distortion factor (THD) KΦ = displacement factor (D.F)Vrms = AC input rms voltage Irms = AC input rms currentI1rms = Fundamental component of Irms

cos Φ = Phase angle between input AC voltage and the fundamental current

Irms = Sqrt (I12 + I2

2 + I32+ ………….+In

2)


PF Degradation

Sinusoidal Current with phase shift

Current with harmonic content

Voltage

Resulting

Current

Voltage

Resulting

Current


Power factor correction• Reduce energy loss in transmission lines• Improve power quality • Cost• Regulatory needs


Useful Power

Φ Φ

Negative Power Region Applied Voltage

Resulting Current

Without PFC

Applied Voltage

Resulting Current

Useful Power

With Active PFC

Useful Power Region


Digital PFC system

AC Supply

Basic Components of the PFC Converter

Rectifier

DSP/DSC

Load

Vac Iac VdcPWM

Switch

Inductor

Capacitor

Diode

Boost

PFC


Boost Topology

IL

IS

ID

PFC Boost Converter

S

DL

C

-

+

+

-vIN

IL IDIS

tON

VOUT > VIN

IL

Average Inductor Current

Average Current Mode ControlThe average current through the inductor is made to follow the input voltage

Ref: AN1274 Interleaved PFC app note from microchip


Challenges Ideally ……• Low THD & high PF over entire 90 -265 Vac input• Low THD & high PF over entire 10 -100 % load range

Low line and high load meeting specifications is EASY !!!!Low load ( < 50%) & Hi Line (> 220 V) spec is HARD !!!!

Cause : Change of system dynamics


Typical specsLoad(%) THD(%)

10 <15

20 <10

30 <6

50 <5

70 <3

80 <3

100 <3

• (Typical Desired) State of the artspec.

• 2.4 KW bridgeless PFC spec. forpower supplies for server farms

• Digital (DSP) control• Fixed switching frequency

operation

Gets harder @ Hi line


State of the art reviewApproach 1

• Determine Discontinuous/continuous conduction modeoperation

• Change the control laws

Challenge• Computation• If-else ladder• Parameter sensitivity• Non linearity of discontinuous mode of operation is hard• Fixed point implementation is challenging


State of the art reviewApproach 2

• Harmonic injection

Challenge• Trial and error• Not plug and play / System specific• If-else ladder (discontinuities in code execution and dynamics)• Limited Code size Memory/MIPS


State of the art summary• Computationally complex (MIPS, code size)• Fixed point implementation hard !!• Physical models sensitive to parameter estimates (e.g. inductor

saturation )• Poor convergence• Strange artefacts• Jumps/spikes/kinks/oscillations due to if-else ladder


Proposed solution features• Good THD at all operating conditions• Plug and play• Just enter parameter dependent coefficients• Low parameter and feedback sensitivity• Fast convergence


Proposed solution features .• No if-else ladder• Small extra code size• Low MIPS requirement ~ 12-14 MIPS (25% of 40 MIPS) @ 50 KHz

interrupt frequency

(Compares favorably with traditional methods)


Proposed solution features ..• System independent /scalable to any rating• Relevant for Interleaved PFC and bridgeless PFC topologies• Guaranteed convergence/no large scale oscillations• No if -else ladder• Patent pending technology


Switched mode Simulation Results• Fixed frequency operation ~100 KHz• Vac = 220V rms ac, Vdc = 400 V

( High line is hardest to handle !!! )• 330 W boost PFC system• 700 uH inductance • 300 uF output capacitance


Simulation Results:• Left plot:

Average inductor current• Right plot:

Switched mode inductor current (continuous and discontinuous conduction mode)


100 % load • THD ~ 3%


50 % load• THD ~ 5%


10 % load • THD ~10%


IPFC reference design from microchip 2.4 KW server power supply from delta

Switching frequency : 100 KHzOne side max load : 180 WInductance : 700 uH(much smaller value than equivalent server supply for that rating)

Switching Frequency : 65KhzInductance : 200uH

Comparison of Specifications

• A system for similar specification as server supply for 700 uH, 100 KHz the power rating would be, 2400 * 200 / 700 * 65 / 100 = 445 W

• Thus 87 W is effectively 19.5 % load• The results are thus very convincing


Experimental results (IPFC board 89 W only single phase enabled)

Voltage 90 110 160 220 RemarksMethodClassical PI controller (over damped) modeled on linear dynamics of continuous conduction mode system

5.98 7.2 17.0 18.4 PF and THD rapidly degrades at low loads

Classical PI controller modeled on linear (critically damped)

3.5 6.5 13.5 15.5 Easily ends up becoming unstable / sub harmonic oscillations due to parameter changes

Classical P I controller with voltage feedforward 3.35 5.35 7.65 17.75 Works great in CCM. But rapidly degrades in DCM /low load conditions

Proposed method 2.9 5.2 6.21 8.5 Works equally well in all regions of operation

89W load when corrected for inductance values and switching frequency is equivalent to 19.5 % load for comparable 2.4 KW system used in server power supply.


Scope shots (87 W load , 400 Vdc output)

110 V


Scope shots (87 W load , 400 Vdc output)

220 V


110 V

Classical PI control w/o FF


220 V

Classical PI control w/o FF


Results with AN1278 from microchipInput voltage: 220 V, Load: 180 W (50%) dual phase

180 W for IPFC is equivalent to 90 W with only one phase of IPFC operational.


Thank [email protected]

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