Digital Power Factor Correction - Handling the corner cases

Digital Power Factor Correction

Handling the corner cases

Superior THD over entire operating range

www.controltrix.com

copyright 2011 controltrix corp www. controltrix.com

Power Factor primer

Inductive - Lagging
we
r cosΦ = Real Power
Po
re n t Apparent Power
pa
Ap

Reactive Power
= Power Factor
Φ
Real Power

Capacitive - Leading
Applies for ideal sinusoidal
waveforms for both voltage
and current


Calculating Power Factor
Power 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 current
I1rms = Fundamental component of Irms
cos Φ = Phase angle between input AC voltage
and the fundamental current

Irms = Sqrt (I12 + I22 + I32+ ………….+In2)

PF Degradation
Voltage

Resulting
Current

Sinusoidal Current with phase
shift

Voltage

Resulting
Current

Current with harmonic
content


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


Useful Power
Useful Power Negative Power
Applied Voltage
Region Region

Resulting Current

Without PFC

Φ Φ
Applied Voltage
Resulting Current

With Active PFC

copyright 2011 controltrix corp Useful Power www. controltrix.com

Digital PFC system
Boost
AC Supply Rectifier Load
PFC

Vac Iac PWM Vdc

DSP/DSC

Basic Components of the PFC Converter
Switch Capacitor

Inductor Diode


Boost Topology
Average Inductor Current

IL IL
PFC Boost Converter
L D
tON +
IL ID
IS IS
+
vIN S C
-

ID -

VOUT > VIN
Average Current Mode Control
The 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 specs
Load(%) THD(%) • (Typical Desired) State of the art
spec.
10 <15
• 2.4 KW bridgeless PFC spec. for
20 <10
power supplies for server
30 <6 farms
50 <5 • Digital (DSP) control
70 <3 • Fixed switching frequency
operation
80 <3
100 <3

Gets harder @ Hi line


State of the art review
Approach 1
• Determine Discontinuous/continuous conduction mode
operation
• 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 review
Approach 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%


Comparison of Specifications
IPFC reference design from microchip 2.4 KW server power supply from delta
Switching frequency : 100 KHz Switching Frequency : 65Khz
One side max load : 180 W Inductance : 200uH
Inductance : 700 uH(much
smaller value than
equivalent server
supply for that
rating)

• 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 Remarks
Method
Classical PI controller (over damped) modeled 5.98 7.2 17.0 18.4 PF and THD rapidly degrades
on linear dynamics of continuous conduction at low loads
mode system

Classical PI controller modeled on linear 3.5 6.5 13.5 15.5 Easily ends up becoming
(critically damped) 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


Classical PI control w/o FF
110 V


Classical PI control w/o FF
220 V


Results with AN1278 from microchip
Input 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 You
consulting@controltrix.com


Digital Power Factor Correction - Handling the corner cases

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