switcher efficiency & snubber...
TRANSCRIPT
1
Switcher Efficiency & Snubber Design
2
Agenda• SMPS Basics• Control Methods• Losses• Example: Buck• Example: Boost• BJTs vs. MOSFETs• Snubber Design
3
SMPS Basics
4
Pin Pout
It ’s gettinghot in here!
The conversion mechanism generates heat...
The goal of a converter is to deliver powerpower
5
η=Pout/Pin is called the efficiency
A 50% efficiency means Ploss = Poute.g. Pout = 100 W Ploss = 100 WPin = 150 W, Pout = 100 W η = 66%
⎟⎟⎠
⎞⎜⎜⎝
⎛−⋅=−=−= 11
ηηPoutPoutPoutPoutPinPloss
Heat means that the energy transfer is notnotperfect
6
The linearlinear approach:efficiency is poorgood noise performanceacceptable when (Vout-Vin) is smallcan only decrease the input level
The switchingswitching approach:efficiency is highnoise performance is poorworks with large (Vout-Vin)increase/decrease/invert the input level
R
ON or OFF
0 to ∞
Two different options exist to build a converter:
7
50 V
Vout
100 V
10 A10 A
η=50%
Help!I am dissipating500 W!! ( )
The linearlinear approach:
8
Hey!I feel much coolernow...
50 V
6 A10 A
Vout
Vin
0Vout
η=83%
The switchingswitching approach:
Vout
100 V
9
ControllingControlling the power flow
VinDVinTstondttVout
TsVout
Ts
avg ⋅=⋅=⋅⋅= ∫ )(1
0
Ts TstonD =
ton
toff
PPulse WWidth MModulation (PWM) control
(Duty-cycle)
10
Control Methods
11
Regulation, keeping an outputoutput signal constant by…
• adjusting the duty-cycle via the PWM block• regulating the inductor peak current• regulating the inductor average current• adjusting the switching frequency• off time adjustment• …
Current-mode control…Voltage-mode control…
Two most popularmethods!
12
The voltagevoltage--modemode method
I > Imax?reset
The error level sets the duty-cycle
13
The dutyduty--cyclecycle factory…
+
-
-1.50
497m
2.50
4.50
6.50
verr,
ram
p in
vol
tspl
ot1
12
10.0u 30.0u 50.0u 70.0u 90.0utime in seconds
-1.00
1.00
3.00
5.00
7.00
out i
n vo
ltspl
ot2
3
100%
0%
A ramp is compared to a DC level, the error voltage
PWMmodulator
Verror
14
The currentcurrent--modemode method
The error level sets the current setpoint
15
The peakpeak follows the error voltage
1 idrain 2 vfb 3 vfb 4 idrain
1.31m 1.65m 1.99m 2.33m 2.67mtime in seconds
-500m
500m
1.50
2.50
3.50
vfb
in v
olts
-1.00
0
1.00
2.00
3.00
idra
in in
am
pere
spl
ot1
12
2.50m 2.57m 2.65m 2.72m 2.80mtime in seconds
-1.00
0
1.00
2.00
3.00
idra
in in
am
pere
s
-500m
500m
1.50
2.50
3.50
vfb
in v
olts
Plo
t2
34
S
R
Q
Q
Error voltage
Inductor peak current
MOSFET
16
Losses
17
Losses
ICConSWinout PPPPP −−−=PSW: Switching Losses
PCon: Conduction Losses
PIC: Power consumed by the chip
18
IC Losses
qinIC IVP =Vin: IC input voltage
Iq: Quiescent current (read from the data sheet)
19
Switching Losses• Losses which occur when the
power switch is turned on or off.• During this transition the voltage
and current on the FET are both high.
• Different for Buck and Boost configurations.
Y-A
xis
Time
Current(DS)
Voltage(DS)
ton
20
Switching Losses (Buck)
SWoffonoutinSW FttIVP )(21
+=
Vin: Input Voltage
Iout: Average inductor current
ton: Turn on time of high side switch
toff: Turn off time of high side switch
FSW: Switching frequency
21
Switching Losses (Boost)
SWoffonout
outSW FttD
IVP )(12
1+
−=
Vout: Output Voltage
D: Duty Cycle
Iout: Average inductor current
ton: Turn on time of high side switch
toff: Turn off time of high side switch
FSW: Switching frequency
22
Reducing Switching Losses• Increase gate drive strength
– Increases cost (die area)– Increases EM emissions
• Decrease frequency– Requires a larger value inductor
• Use a smaller FET– Increases conduction losses
23
Conduction Losses• Losses which occur when current flows through a resistive
path (I2*R), such as a FET, or a diode (V*I).• Major contributors include:
– Power Switch Rds,on
– Freewheeling Diode– Inductor DCR
• Different for synchronous and non-synchronous mode designs.
24
Conduction Losses (Non-Synchronous)
DCRLdiodeLondsLCon RIDVIDRIP 2,
2 )1( +−+=
IL: RMS current through the inductor
Rds,on: On resistance of the power switch
D: Duty cycle
Vdiode: Forward voltage of the diode
RDCR: Winding resistance of the inductor
25
Conduction Losses (Synchronous)
DCRLondsLondsLCon RIDRIDRIP 22,
21,
2 )1( +−+=
IL: RMS current through the inductor
Rds,on1: On resistance of the high side switch
D: Duty cycle
Rds,on2: On resistance of the low side switch
RDCR: Winding resistance of the inductor
26
Example: Buck
27
Example: NCV8851 Evaluation Board• Synchronous buck converter• Vin = 13.2 V• Vout = 5 V• Iout,max = 4 A• FSW = 170 kHz• Inductor: Wurth Electronics 7447709150 15 uH• Power switch (both): ON Semiconductor NTD5407N
28
Example: NCV8851 Evaluation Board• D = Vin / Vout = 5 V / 13.2 V = 0.379• ton = 5 ns (empirical)• toff = 7 ns (empirical)• Rds,on1 = Rds,on2 = 50 mΩ (including self heating temperature
effects)• RDCR = 26 mΩ• Iq = 15 mA• IL = Sqrt(Iout
2 + (0.609 A)2 / 3)
29
Example: NCV8851 Evaluation Board• PIC = 13.2 V * 15 mA
= 0.198 W• PSW = (1/2) * 13.2 V * Iout * (5 ns
+ 7 ns) * 170 kHz = 0.01346 * Iout W
• PCON = (Iout2 + 0.12378 A2) * (50
mΩ) * (0.379) + (Iout2 + 0.12378
A2) * (50 mΩ) * (0.621) + (Iout2 +
0.12378 A2) * 26 mΩ = (0.076 * Iout
2 + 0.009407) W
Sources of Power Dissipation
0
0.2
0.4
0.60.8
1
1.2
1.4
0 1 2 3 4
Iout (A)Po
wer
(W)
PiqPswPcon
30
Example: NCV8851 Results
8851 Efficiency
0.00%10.00%20.00%30.00%40.00%50.00%60.00%70.00%80.00%90.00%
100.00%
0.000 1.000 2.000 3.000 4.000
Iout (A)
Effic
ienc
y
ActualCalculated
31
Example: Boost
32
Example: NCV8871 Sample Application• Non-synchronous boost converter• Vin = 13.2 V• Vout = 18 V• Iout,max = 9 A• FSW = 170 kHz• Inductor: Vishay IHLP6767GZER330M11 33 uH• Power Switch: ON Semiconductor NTD5803 x 2• Diode: ON Semiconductor MBRB1645T4G
33
Example: NCV8871 Sample Application• D = 1 – (Vin / Vout) = 0.267• ton = 30 ns• toff = 20 ns• Rds,on = (12 mΩ) / 2 = 6 mΩ (including temperature effects)• RDCR = 37 mΩ• Iq = 10 mA• IL = Sqrt ((Iout / (1 – D)) ^ 2 + (0.3137)^2 / 3)
34
Example: NCV8871 Sample Application• PIC = 13.2 V * 10 mA = 0.132 W• PSW = (1/2) * 18 V * Iout / (1-0.267)
* (20 ns + 30 ns) * 170 kHz = 0.10436 * Iout W
• PCON = (Iout2 + 0.0328 A2) * (12
mΩ) * (0.267) + Sqrt(Iout2 +
0.0328 A2) * (0.5V) * (0.733) + (Iout
2 + 0.0328 A2) * 37 mΩ = (0.0402 * Iout
2 + Sqrt(Iout2 +
0.0328)*0.3665 + 0.00131869 W
Power Dissipation
0
1
2
3
4
5
6
0 3 6 9
Iout(A)Po
wer
(W)
PicPswPcon
35
Example: NCV8871 Results
Efficency
0.00%
20.00%
40.00%
60.00%
80.00%
100.00%
0 3 6 9
Iout(A)
Effic
ienc
y
Calculated
36
BJTs vs. MOSFETs
37
Switches and converters…
The bipolar transistor is often used:1. In high voltage - high current applications 2. In low-cost converters
The bipolar transistor
=
Ic
Vce
When saturated…
sat avgPcond Vce Ic= ⋅
38
Switches and converters…
The bipolar transistor
The bipolar transistor switching losses:1. Depend on temperature (storage time, current tail) 2. Watch-out for hot spots!3. Often needs proportional drive (shallow saturation)
Vce Ic
( )3
Vce Ic ton toffPswTsw
⋅ ⋅ +=
⋅
losses
ton toff
If ton = toff…
39
Switches and converters…
The MOSFET transistor is the most popular:1. Ease of drive (capacitive input)2. Avalanche rugged3. BVdss of 600 V for SMPS, 500 V for PFCs…
)(² ONdRMS RdsIPcond ⋅==
d
s
g
d
s
g
Rds(ON)
body diode
The MOSFET transistor
Often used infreewheel function
40
To enhanceenhance a MOSFET, bring it charge
VT
Plateau levelMiller effect
How many coulombs to turn on the MOSFET: Q = i x t…
d
s
g
Crss
Coss
Ciss
41
Snubber Design
42
When is it needed?• Parasitic inductances
and capacitances from the power devices form a RLC filter that resonates
• Excessive ringing can cause damage to the devices
43
Snubber Design• Measure the frequency of the ringing (fc) at maximum input
voltage– Use a low capacitance probe
• Find out either the L or C of the circuit– L is dominated by the top power switch– C is dominated by the body diode of the bottom power switch or the
capacitance of the freewheeling diode
• Calculate the characteristic impedance of the circuit– If L is known: Z = 2πfc L– If C is known: Z = 1 / ( 2 πfc C )
44
Snubber Design• Choose RSNUB = Z• Choose CSNUB = 1 / (2 πf R)• Power dissipation in RSNUB is CV2fs• Put RSNUB and CSNUB in series across the device causing
ringing.• Test in circuit. RSNUB can be fine turned further to reduce
ringing if it is found to be insufficient
45
For More Information
• View the extensive portfolio of power management products from ON Semiconductor at www.onsemi.com
• View reference designs, design notes, and other material supporting automotive applications at www.onsemi.com/automotive