Digital Power Factor Correction - Handling the corner cases
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Transcript of Digital Power Factor Correction - Handling the corner cases
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copyright 2011 controltrix corp www. controltrix.com
Digital Power Factor Correction
Handling the corner cases
Superior THD over entire operating range
www.controltrix.com
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copyright 2011 controltrix corp 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
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copyright 2011 controltrix corp www. controltrix.com
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)
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copyright 2011 controltrix corp www. controltrix.com
PF Degradation
Sinusoidal Current with phase shift
Current with harmonic content
Voltage
Resulting
Current
Voltage
Resulting
Current
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Power factor correction• Reduce energy loss in transmission lines• Improve power quality • Cost• Regulatory needs
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copyright 2011 controltrix corp www. controltrix.com
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
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copyright 2011 controltrix corp www. controltrix.com
Digital PFC system
AC Supply
Basic Components of the PFC Converter
Rectifier
DSP/DSC
Load
Vac Iac VdcPWM
Switch
Inductor
Capacitor
Diode
Boost
PFC
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copyright 2011 controltrix corp www. controltrix.com
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
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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
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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
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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
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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
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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
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Proposed solution features• Good THD at all operating conditions• Plug and play• Just enter parameter dependent coefficients• Low parameter and feedback sensitivity• Fast convergence
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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)
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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
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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
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Simulation Results:• Left plot:
Average inductor current• Right plot:
Switched mode inductor current (continuous and discontinuous conduction mode)
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100 % load • THD ~ 3%
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50 % load• THD ~ 5%
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10 % load • THD ~10%
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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
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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.
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Scope shots (87 W load , 400 Vdc output)
110 V
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Scope shots (87 W load , 400 Vdc output)
220 V
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110 V
Classical PI control w/o FF
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220 V
Classical PI control w/o FF
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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.