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Output Voltage Ripple Correctionwith Spread Spectrum
Using Frequency Modulationfor Switching Converters
Yasunori Kobori*, Natsuko Miki, Yifei Sun,
Nobukazu Tsukiji and Haruo Kobayashi
( Gunma University )
INVITED
International Conference on Technology and Social Science 2018
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1
Outline
1. Background1-1 Switching Converter1-2 EMI Reduction Method
2. EMI Reduction with Clock Modulation 2-1 Frequency modulation method 2-2 Relationship with modulation level
3. Output Voltage Ripple Correction 3-1 Analyze the correction method
3-2 Simulation results
4. Conclusion
EMI: Electro-Magnetic Interference
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2
100V AC 1.5V DC
12V DC
Switching
Converters
Supply many kinds of voltage
by switching power
EMISwitching Noise
◆ Important to reduce SW noise & conductive noise.
◆ Decrease the spectrum level of switching noise.
2
3
Conductive Noise
1. Background
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● Need to reduce noise spectrum level
below the Standard Level
★ By modulating the clock frequency,
spread the clock noise around
the clock frequency or its harmonics.
research process
CISPR 22
● Decrease the peak level of spread spectrum
without increase of the output ripple
by compensating the clock modulation.
◆ The more clock modulation increases,
the lower noise spectrum decreases.
⇒ Output ripple increases. ⇒ No Good!
※ Our Objective
3
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4Fig.1-2 Circuit of the Buck converter Fig.1-3 Waveforms
Clock
SAW
Tooth
PWM
I L
* Output Voltage Vo is compared with reference voltage Vref
and amplified to get ⊿Vo.
*⊿Vo is compared with SAW-tooth signal
to generate Pulse Width Modulation (PWM) pulse.
* PWM controls power SW & IL changes to control Vo stable.
Amp.
VrefSAW
Switching
Pulse
⊿Vo
Comp.
Saw-tooth
Generator
PWM
Clock
Power Stage Vo
Load
Reg.
IIN
1-1 Buck-type Switching Converter
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5
Table 1 Parameter of converter
● Output ripple : 2.5mV
● Spectrum of PWM pulse:
Peak level @ Fck: 3.1V
★Important to reduce
the spectrum level at Fck.
Input Voltage 12 [V]
Output Voltage 5.0 [V]
Clock Frequency 200 [kHz]
Inductance 200 [uH]
Capacitance 220 [uF]
Fig.1-5 Noise Spectrum
3.1V
Fck=200kHz
Frequency [MHz]
Sp
ectr
um
Le
ve
l
[V]
Fig.1-4 Stable output ripple
⊿Vo = 2.5 mV
Time [ms]
Ou
tpu
t V
olta
ge
[V
]
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Amp.
VrefSAW
PWM ⊿Vo
Comp.
Saw-tooth
Generator
Mod.
SignalMod.
Clock
Gen.
Modulated
Clock
6Fig. 1-5 Converter with EMI reduction
1-2 Spectrum Reduction Method
* To reduce EMI noise, clock frequency is modulated.⇒ Clock spectrum is spread and reduced.
Fig.1-6 Waveforms
Original
Clock
SAW
Tooth
PWM
Modulated
Clock
*Clock to SAW generator is modulated
by shaking phase or frequency.
*PWM pulse is modulated and
the clock frequency is spread.
Power
Stage
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7
● Modulation signal: SIN
・Frequency: Fm =500 Hz
・Amplitude : Vm=2 Vpp
*Spectrum level is reduced
from 3.1V to 0.5V (ー15.8dB).
But output voltage ripple is ・・・
Fig.1-7 Spectrum with modulation
Fig.1-6 Modulation signal
Sp
ectr
um
Le
ve
l [
V]
Frequency [MHz]
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Time [ms]
Ou
tpu
t V
olta
ge
[V
]
Fig.1-7 Spectrum with modulation
Fig.1-6 Modulation signals
Fig.1-8 Spectrum with modulation
Need to suppress the ripple
keeping spectrum level low.
Sp
ectr
um
Le
ve
l [
V]
Frequency [MHz]
8
● Modulation signal: SIN
・Frequency: Fm =500 Hz
・Amplitude : Vm=2 Vpp
*Spectrum level is reduced
from 3.1V to 0.5V (ー15.8dB).
★Output ripple increased 20mV.
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9
Outline
1. Background1-1 Switching Converter1-2 EMI Reduction Method
2. EMI Reduction with Clock Modulation 2-1 Frequency modulation method 2-2 Relationship with modulation level
3. Output Voltage Ripple Correction 3-1 Analyze the correction method
3-2 Simulation results
4. Conclusion
EMI: Electro-Magnetic Interference
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10Fig. 2-2 Spectrum with Linear Sweep
2-1 Frequency modulation method*Clock pulse is generated using VCO.
・VCO is modulated by Triangular (or Sine) signal.*Spectrum of PWM pulse has flat top shapes.
・Peak level is reduced from 3.1V to 0.55V. (-15dB)
VCOModulated
Clock
+
>
TriangularSaw-tooth
Generator
SAW
⊿VoPWMComp.
Fig. 2-1 Frequency modulation circuit
2. EMI Reduction with Frequency Modulation
200kHz
0.55VS
pe
ctr
um
Le
ve
l [
V]
Frequency [MHz]
VCO: Voltage ControlledOscillator
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● Comparison of spectrum
・(a) Without modulation
(Vm=0 V)
・(b) with modulation 1
(Vm=0.5V)
・(c) with modulation 2
(Vm=2.0V)
● Spectrum level at Fck
・3.1V ⇒ 0.9V ⇒ 0.5V
[-10.4dB] [-5.1dB]
[V]
[V]
Fig.2-5 Spread Spectrum
(c)
Check the relationship
between Vm & spectrum level
12Frequency [MHz]
Frequency [MHz]
Spectr
um
Level [V
]S
pectr
um
Level [V
]S
pectr
um
Level [V
]
(a)
(b)
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(1) modulation level vs. Peak spectrum level
*Spectrum level is in proportion to logarithm of Vm.
・Vpeak = 0.55-0.65・LOG10(Vm) [V] (2-1)
Here, Vpeak is the peak level of Fck spectrum,
Vm is the amplitude of modulation signal.
Fig.2-6 Modulation level vs. Spectrum level
2-2 Relationship with modulation level
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0.1 10.2 0.4 0.7 2
Modulation Level Vm [V]
Spe
ctr
um
Leve
l of
Fck
[V]
13
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13Fig.2-7 Modulation level vs. Output ripple
(2) Relationship: modulation level vs. output ripple
*Output voltage ripple is changed by modulation signal level.
・Modulated ripple: caused by modulation signal
・Static ripple : caused by switching pulse
*Modulated output ripple is in proportion to modulation level.
・ ⊿Vom = 12.5・Vm [mV] (2-2)
Fig.2-6 Modulated output ripple
Time [ms]
[Static Ripple]
[Modulated
Ripple]
Ou
tpu
t ri
pp
le [V
]
Modulation signal [V]
[mV]
[V]
0
5
10
15
20
25
30
0 0.5 1 1.5 2
Mo
du
late
d o
utp
ut ri
pp
le [V
]
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14
Outline
1. Background1-1 Switching Converter1-2 EMI Reduction Method
2. EMI Reduction with Clock Modulation 2-1 Frequency modulation method 2-2 Relationship with modulation level
3. Output Voltage Ripple Correction 3-1 Analyze the correction method
3-2 Simulation results
4. Conclusion
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3. Output Voltage Ripple Correction
3-1 Analysis of ripple correction method
● Consider the duty ratio of modulated SAW-tooth signal ・Period of SAW signal is changed by modulation signal.
But amplified output voltage keeps the level in a short time.
⇒ Pulse width W is not changed. ⇒ Duty ratio is changed.
D’=W/(To-⊿T)=(W/To)/(1-α)≒Do(1+α) (3-1)
∵ α =⊿T/To <0.01 (=Fm/Fck)
⊿D=αDo= 2(Vm/Vb)/(Fck/Fm) (3-2)
Here, Fck=K・(Vb+Vm)
K [kHz/V] is the sensitivity of VCO
*⊿D is the cause of the modulated ripple.Fig. 3-1 Modulated SAW
15
W
To
To-⊿T
⊿Vo
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*Cancel ⊿D in the SAW-tooth generator
by compensating the slope of SAW-tooth signal
⇒ Pulse width W is changed into modulated width W’.
⇒ Compensated circuit: additional current source.
● Investigate conductance G which is controlled by Vm.
Fig.3-3 Compensation of duty 16
⊿T=(TMAX-TMIN)/{Fo/(Fm/2)}
≒2(Vm/Vb)/(Fo/Fm)=2(Vm/Vb)To/Tm (3-3)
∴ α=⊿T/To≒2(Vm/Vb)/Tm (3-4)
W
To
To-⊿T
⊿Vo
W’
⊿W
Fig.3-2 Compensated SAW generator
Compensated current source SAW-tooth generator
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*To design conductance G, check the relationship Fm & Fck.
*The number of clock pulses in the half period of Fm
is derived as Eq. (3-5).
*Set the additional current ⊿ISAW, derive the current ratio β.
⇒ Ratio β is equal to ratio α. ⇒ Set conductance G.
Fig.3-3 Compensation of duty
17
α=⊿T/To≒2(Vm/Vb)/Tm (3-4)
N=(Tm/2)/Tck=0.5・Fck/Fm (3-5)
⊿ISAW =G・Vm (3-6)
β=(⊿ISAW /ISAW)/N=G・Vm/(ISAWN) (3-7)
W
To
To-⊿T
⊿Vo
W’
⊿W
*From Eq.(4)=Eq.(7),
2(Vm/Vb)/Tm= G・Vm/(ISAWN) (3-8)
・The conductance G is set bellow.
G = ISAW/Vb [S] (3-9)
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18
*Theoretical conductance:
G= ISAW/Vb [S] = 500 [uS]
Table 2-1 Parameters of converter
1 Sensitivity S 50 [kHz/V]
2 Base Voltage Vb 4.0 [V]
3 Modulation V. Vm 0.5~2.0 [V]
4 Modulation Freq. Fm 0.5~2.1 [kHz]
5 SAW Current ISAW 2.0 [mA]
6 SAW Capacitance CSAW 1.2 [nF]
1 Input Voltage Vin 12 [V]
2 Output Voltage Vo 5.0 [V]
3 Output Current Io 0.5 [A]
4 Inductance Lo 200 [uH]
5 Capacitance Co 220 [uF]
6 Clock Frequency Fck 200 [kHz]
Table 2-2 Parameters of VCO & SAW gen.
SAW
Fig.3-4 Circuit of new SAW generator
Additional Saw-tooth
current generator
3-2 Simulation results
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● Relationship between G & Vom
*Set modulation signal: Fm=2 kHz, Vm=2 V.
*Modulated ripple with various G. Modulated ripple is decreased
when the conductance increase.But G is larger than 500uS,
phase of the ripple is reversed.*Show relationship in Fig. 6.
Fig. 3-6 Conductance vs. modulated ripple
[ms]
[V]0 uS 200uS
800uS
400uS
600uS
Fig.3-5 Ripples with various G
*Best conductance isG= 516 uS
VR=20-(31mV/800uS)・G [mV]
-15
-10
-5
0
5
10
15
20
25
0 100 200 300 400 500 600 700 800
[uS]
G=516 uS
Conductance G
Mo
du
late
d o
utp
ut ri
pp
le [m
V]
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● Simulation result (Fm=2.0kHz, Vm=2.0V, Sine Wave)
*Best conductance is aboutG=510 uS
・Modulated ripple is reducedfrom 20mV to 0.8mV (-28dB).
Fig.3-8 Ripples with various conductance
Vm=2Vpp
Vo
[ms]
1.6 mV0.8 mV
Fig. 3-7 Ripple with G=510 us
20
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● Modulation signal : Triangular, Fm=2.0kHz, Vm=2.0V
*Output ripple @ G=510 uSModulated ripple = 0.8mV, Static ripple = 2.7mV
*Peak spectrum level: Vp = 0.65 V. (@ Fck = 200 kHz)
It is almost same as that of no ripple correction.
Fig. 3-9 Ripple with correction Fig. 3-10 Spectrum with correction
21
0.65VVm=2Vpp
Vo=1mV
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1) Transfer function: modulation signal to the output ripple (Modulation signal : Triangular, Vm=2.0 V)
*Without correction: similar to closed-loop transfer function*With correction : very low and constant
2) Spectrum level vs. Modulation Frequency Fm;*Peak spectrum level is proportional to Modulation Frequency.*Spectrum level is no concern with ripple correction.
Fig. 3-11 Fm vs. Ripple22
Fig. 3-12 Fm vs. Peak spectrum
0.1
1
10
100
1 102 4 [kHz] 20
[mV]
W/O correction
With correction
0.1
1
10
1 102 4 [kHz] 20
[V]
2
0.5
W/O correction
With correction
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Conclusion1. Developed ripple correction method for SW converters
in which spectrum level is reduced with frequency modulation.
2. Shown the relationship
between modulation level and compensation level.
・Conductance G for the additional current level is
G= ISAW/Vb [S] : Here, ISAW is current of SAW generator.
Vb is the DC voltage of VCO.
3. Simulation results:
Modulated ripple is corrected less than 1mV
Peak spectrum level at clock Fck is reduced -28dB.
● Future work is to apply this correction method
to another SW converters.
(ex. current mode switching converter) 23
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Thank you for your kind attention!
Is there any question?