Effects of High Switching
Frequency on Buck Regulators
1
Abstract
The switching frequency is an operating parameter which affects nearly
every performance characteristic of the supply, as well as the cost.
Determining the proper switching frequency for a particular design
requires that the designer knows the application sensitivity to each of
these characteristics, in order to simultaneously minimize cost and
satisfy all application requirements.
This presentation presents the effects of switching frequency on buck
switching regulator operating characteristics, and how switching
frequency affects the cost of the supply.
2
Benefits of High Switching Frequency
1.
Smaller converter
Smaller can be cheaper – up to a certain power output
Beyond that power level small size might be worth some added cost
2.
Transient response can improve with higher switching frequency.
3.
Avoids frequency bands in which noise would be disruptive
AM Broadcast
Vehicle motion/position monitors
Drawbacks of High Switching Frequency
Efficiency is worse
Switching loss is proportional to switching frequency
FET Switch drive power is also proportional to frequency, and is
usually provided by a Linear Regulator!
Maximum conversion ratio (maximum VIN) is lower
Dropout voltage (minimum VIN) is higher
Current limit accuracy is likely to be worse
3
Output Filter Size & Cost
With higher ripple frequency, output filter inductor and/or capacitor
values could be smaller, reducing total converter size and cost.
For competing designs differing only in switching frequency:
Inductor value is inversely proportional to switching frequency for equal
peak-to-peak ripple current. But di/dt is then proportional to frequency,
with impacts discussed later.
Capacitor value is inversely proportional to switching frequency for
equal output ripple voltage (also assuming equal peak-to-peak ripple
current). But capacitor ESR increases with decreasing capacitor value,
with impacts discussed later.
4
Inductor Size or Dissipation Reduction?
The lower inductor value permitted by higher frequency has lower I2R loss for
the same core since the number of turns decreased.
But if thinner wire is also used, such that the DC resistance is not changed, using a
core with a shorter mean magnetic path will result in a smaller volume inductor,
with less core material experiencing the same loss per unit volume.
Example (Wurth WE-PD series)
Induct Induct Isat
Volume
(μH)
Ratio
(A)
(mm2)
4.7
1.00
3.9A
170.5
22
4.68
3.8A
864
Volume
Ratio
1.00
5.07
DCR
(mΩ)
35
36
Freq
(kHz)
2000
400
Freq
Ratio
5.00
1.00
S-type
L-type
Powdered iron core loss is significant, and is proportional to frequency for the same
peak flux level (same % saturation), so the above examples would have identical
I2R and core losses, with inductor volume inversely proportional to frequency.
So inductor loss can be traded off for inductor volume, but the product of loss
and volume reductions cannot exceed the frequency increase factor.
5
Capacitor Size Reduction – Value Impact
The smaller capacitor value permitted by higher switching frequency increases
the converter high frequency output impedance.
Output Impedance vs Frequency
Lo
Zc w
C
H
Zc i C
Higher Impedance
H
ES i C
L
Hi C
ESR
Frequency
6
L
ES ow
L C
Low C
ESR
Capacitor Size Reduction – ESR Impact
The smaller capacitor value permitted by higher switching frequency increases the
converter high frequency output impedance – including at the switching frequency.
Output Impedance vs Frequency
Lo
Zc w
C
H
Zc i C
Higher Impedance
at Fsw
H
ES i C
L
Hi C
ESR
Frequency
7
L
ES ow
L C
Low C
ESR
Output Transient Performance
If the increase in high frequency output impedance shown in the previous slides is
not a concern, or only the inductor value is decreased, the higher resonant
frequency resulting from lower value output filter components does permit more of
the Error Amplifier gain to be used to control output voltage at middle frequencies.
This improves mid-frequency transient response.
Output Impedance vs Frequency
Hi C
ESR
Hi C
ESR
Frequency
8
L
ES ow
L C
Low C
ESR
H
ES i C
L
Low C
ESR
C
Lower Peak
Impedance
H
ES i C
L
H
Zc i C
Lo
Zc w
Frequency
L
ES ow
L C
Output Impedance vs Frequency
Lo
Zc w
C
H
Zc i C
Capacitor Size Reduction – ESR Impact
The smaller capacitor value permitted by higher switching frequency will
typically have proportionally higher ESR for equally smaller case volume.
But the relationship of ESR to case volume is highly variable, so must be
carefully examined for the particular application.
Assuming an inversely proportional scaling of ESR with case volume,
reaping the volume reduction afforded by higher switching frequency
affects the output ripple voltage of the converter.
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Component Volume
ernal
t
x
E
2
hes
Switc
Component Volume
e
Diod
g
n
i
l
uencyr
hee
q
w
e
r
e
F
e
r
ard
ulato
rnal F
Stancdhing Reg
Exte
Swit
Mono
lithic
lator
itching Regu
w
S
y
c
n
e
u
q
de
High Fre
heeling Dio
w
e
re
F
l
a
rn
Exte
or
at
l
gu
Re
r
ea
n
Li
Monolithic
0
1
2
3
4
5
6
7
Maximum Converter Power Output
8
9
10
5:1 frequency ratio assumed for this graph.
Linear Regulator component volume reflects case size needed for cooling.
10
Lin
ea
rR
eg
ula
to
r
Cooling Volume
Cooling Volume – Heatsinks, etc.
gh
Hi
F
c
en
u
q
re
ing
h
tc
wi
S
y
nda
Sta
0
1
2
3
4
or
t
a
l
gu
e
R
u
req
F
d
r
5
en
6
in
itch
w
cy S
7
tor
a
l
u
eg
gR
8
9
10
The same upper cooling volume limit is assumed for all regulators
11
eg
ul
at
R
ar
Li
ne
F
High
0
1
2
3
itch
w
S
y
enc
requ
4
5
ing
6
r
lato
u
g
Re
7
Maximum Converter Power Output
Sum of previous 2 graphs
12
y
encr
u
q
re
o
rd F egulat
a
d
Stantching R
Swi
or
Total Converter Volume
How Frequency Affects Total Converter Size
8
9
10
Efficiency
Switching loss increases with increasing switching frequency due to the
greater number or constant energy switching events per time.
Besides this, if switching frequency is sufficiently high, Synchronous
Rectification cannot be implemented – further increasing conduction loss
for high conversion ratio applications.
Gate drive current also hurts efficiency – especially at high battery – unless
bootstrapped from the output.
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Regulator Characteristics Determine Impacts
The impacts of high frequency switching depend on the answers to several
questions:
1. Is the regulator using Voltage-mode or Current-mode to control output
voltage?
Conversion ratio is limited for Current-mode control
2. Does the application need the efficiency of Synchronous Rectification?
If yes, then switching frequency or input voltage range will be
restricted.
3. Is the regulator in Continuous Conduction mode (CCM) or
Discontinuous Conduction mode (DCM)?
With DCM, light load switching frequency will decrease.
14
Reduced Conversion Ratio (lower max VIN)
Current-mode converters typically have a blanking time, which sets the
minimum D for a given Vout, and therefore max VIN, that will be regulated
without reducing switching frequency (pulse skipping).
Max Switching Frequency versus Input Voltage
Vout=0.5
Vout versus Input Voltage
Vout=1.5
3400
3200
3000
2800
2600
2400
2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0
6
Vout=2.0
Vout=2.5
5
Vout=3.0
Vout=3.5
Vout=4.0
Vout=4.5
Vout=5.0
4
Vout (V)
Max Switching Frequency (kHz)
Vout=1.0
3
2
Min Vout@1800kHz
Min Vout@2100kHz
1
Min Vout@2400kHz
0
2
4
6
8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50
Input Voltage (V)
Graphs reflect 70ns blanking time.
15
2
4
6
8
10
12
14
16
18
20
22
24
Input Voltage (V)
26
28
30
32
34
36
38
40
42
Reduced Conversion Ratio (lower max VIN)
As the load on a non-synchronous, current-mode buck converter approaches zero,
the switching frequency will decrease in an irregular manner (pulse skipping) since
a VIN-dependent charge must be delivered to the output during the blanking time
at each turnon. For a given load current, FSWAVERAGE = Load current/(VIN – Vout)
Why be concerned with performance at loads <10% of max converter rating?
Loads will be in sleep mode some of the time
Bench evaluation should not exhibit pulse skipping over a wide load range
Even for output voltages that do not produce pulse skipping at high VIN, dissipation
caused by Gate Drive current is high if the current is derived directly from VIN.
Bootstrapping from the output can sometimes be used to avoid this excessive
dissipation.
16
Dropout Voltage (minimum VIN)
Dropout voltage = the minimum (VIN – Vout) that allows the switcher to maintain
output voltage regulation
For a buck regulator, Vout = VIN * D (Duty cycle = ton/(ton + toff))
Therefore, dropout voltage = VIN - VIN * D
Achieving D close to 1.0 minimizes dropout voltage, and minimizes the VIN needed
for regulation
Above light load (all buck regulators)
toff needed to recharge bootstrap capacitor is the sum of both non-overlap
times + a short time to actually charge the bootstrap capacitor
17
Dropout Voltage (minimum VIN)
Light load (non-synchronous buck, only)
For very light loads, the power switch ‘OFF’ time (toff) is required to be greater
than some minimum needed to recharge the bootstrap capacitor. This
minimum toff varies inversely with load current. Since D = ton/(ton + toff), this
minimum toff determines the maximum D, and thereby the minimum VIN at a
particular switching frequency. Expressed another way: for a given minimum
load, the minimum VIN is determined by the switching frequency.
dv/dt = Io/CPAR
VIN
VD
VSW
VBST
DBST
VDRV
BOOTSTRAP
Q SOURCE
VIN
POWER
SWITCH
FLOATING
GATE
DRIVE
DFW
dv/dt =
Io/(CPAR+CBST)
GND
VBST
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VDRV
- VD
- VBST
CBST
VOUT
L
CPAR VSW
COUT
Looser Current Limit
Lower inductor values increase the current limit variation due to the current sense
delay. This is because V = L di/dt, or di = (VIN-VOUT)/L x dt:
1.
The bandwidth of current monitoring circuitry causes delay
Delay is highly dependent upon overdrive, and consequently nearly
independent on VIN (higher di/dt is compensated by shorter delay)
2.
Noise blanking time following switch turnon determines the shutoff delay
during a dead short. The peak current limit is then proportional to di/dt.
VIN variation causes di/dt variation, causing average current to vary
Vout droops when in current limit, increasing di/dt & raising average
current
19
EMC
There can be value in pushing the switching frequency above sensitive bands
Radiation Limit
Measured Radiation
Energy
Energy
Radiation Limit
Measured Radiation
Fsw1
20
Frequency
Fsw2
Frequency
EMC
Too high a switching frequency can be worse
The energy at the fundamental frequency, and at each harmonic, does not
change with switching frequency. But emission limits get more restrictive at
higher frequency.
Radiated Energy vs Frequency
Radiation Limit
Measured Radiation
Energy
Energy
Radiation Limit
Measured Radiation
Fsw2
Frequency
Frequency
Fsw3
Less capacitance is needed to attenuate conducted noise at higher switching
frequency, but the ESR of the capacitors must remain low – eroding some cost
savings that might otherwise result from lower capacitance.
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EMC
Synchronization of the regulator to an external clock can help the fundamental
and harmonics avoid a narrow frequency band of sensitivity.
Tuned Radiation Limit
Measured Radiation
Energy
Energy
Radiation Limit
Measured Radiation
Fsw1
22
Frequency
Fsw tuned
Frequency
References
ON Semiconductor Switchmode Power Supply Reference Manual Rev 2
Apr-2000
Unitrode Application note U-68A
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•
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