Download SMPS BASIC CONVERTERS INHERENT PROTECTION

SMPS BASIC CONVERTERS
INHERENT PROTECTION CAPABILITY
1. BUCK CONVERTER INHERENT PROTECTION FEATURES
1.1 BUCK Output Short Circuit
L
Iin
Ei
C
s/c
dI in Ein

dt
L
Vout
=0
n
0
t
FIGURE 1
An output short circuit, as shown in Figure 1, translates into an input current problem. The zero
output voltage demands an increased duty cycle and the input current will continue to rise as a
function of Ein, L and Dsw. If not controlled the increasing current would tend to saturate the
inductor with the result that the current would become even larger and damage could occur to the
source and the circuit components.
Fortunately it will take several switching cycles before such a problem can occur and it is therefore
possible to sense the circuit current and modify the drive to the transistor switch to either a current
limiting or a current shut down mode.
SOLUTION
PROBLEM
output short circuit  input current problem
effect needs several switch cycles to be a problem

sense the circuit current

modify the drive to the transistor switch (Dsw)

either current limiting or current shut down mode
closed loop system zero output voltage
 increased duty cycle demand

rising input current as a function of Ein, L and Dsw

tends to saturate the inductor (if not controlled)

current becomes even larger
8.2 Output Open Circuit

source and the circuit component damage
EET423 POWER ELECTRONICS – 2
Basic Converters Inherent Protection
Buck and Buck derived converters
inherently self protecting
due to transistor switch at input
1
Prof R T Kennedy
2007-2008
1.2 BUCK Output Short Circuit
L
C
Ei
o/c Vout
n
FIGURE 2
In the absence of a load, as shown in Figure 2, the capacitor will retain charge dependent on the
high value of the capacitor's parallel leakage resistance and the output voltage will rise towards the
maximum possible value Ein. The output capacitor voltage must therefore be rated accordingly.
The increasing output voltage results in transistor switch turn-off due to insufficient forward bias
and it turns off. The switch can also be turned off in Buck converters by sensing the output voltage
build up and modifying the transistor switch drive.
no load

capacitor retains charge

output voltage  towards maximum possible
Ein

output capacitor  higher voltage rating
eventual lack of forward bias

switch inherently turns odd
aliter:
switch turn off by sensing Vout

modify transistor Dsw
Buck and Buck derived converters
inherently self protecting
due to transistor switch at input
1.3 BUCK Start (Power) -Up
High start-up currents are possible due to the zero capacitor voltage at power-on. Start-up currents
can be limited. by the use of a controlled soft-start; designing the transistor switch duty cycle to
increase from a small initial value to that required for normal operation.
1.4 Shut (Power)- Down
Controlled shut down is also possible in buck converters by reducing the transistor switch duty
cycle provided that the control circuit time constant >CR, otherwise the turn-off characteristic
depends on R, L and C.
EET423 POWER ELECTRONICS – 2
Basic Converters Inherent Protection
2
Prof R T Kennedy
2007-2008
2. BOOST CONVERTER INHERENT PROTECTION FEATURES
2.1 BOOST Output Short Circuit
L
Ei n
Vout = 0
C
PROBLEM
output short circuit

uncontrolled input-output path via L & rectifier

tends to saturate the inductor
SOLUTION
sense output  Dsw  1

source and L and r damage
Boost and Boost derived converters
inherently NON self protecting

need additional transistor
either an input or output
sense output  Dsw  0

source and rectifier damage
2.1 BOOST Output Open Circuit
closed loop system zero outputLvoltage
 increased duty E
cycle
C
i n demand

rising input current as a function of Ein, L and
Dsw

no load
tends to saturate the inductor (if not controlled)


continual
capacitor energy build up
current becomes
even larger


eventual
some
component
source and the circuit component damage breaks down
sense output

Dsw  0

Vout  Ein

output capacitor  higher voltage rating
Boost and Boost derived converters
O/C output not recommended.
3
EET423 POWER ELECTRONICS – 2
Basic Converters Inherent Protection
inherently self
Prof R T Kennedy
2007-2008
In addition to the better inherent protection Buck converters have other advantages over the Boost
Buck converters   less output ripple current  smaller capacitor
Boost capacitor needs to provide full output when switch on  Boost capacitor larger 
Buck derived topologies peak device current approximately half the of Boost derived. 

Limits Boost to about 150 W 
Boost converters usually have duty cycle limited to < 0.8 
to allow the inductor current to fall to zero each cycle to avoid core saturation.
Buck duty cycle  1 ok. 
Boost converter typical operation is DCM and favoured for Power Factor Correction
Buck Converter normally CCM: less current stress on all other components than DCM
better line and load regulation CCM
EET423 POWER ELECTRONICS – 2
Basic Converters Inherent Protection
4
Prof R T Kennedy
2007-2008