BuckBoost SEPIC CV CC 5-30V 5A 150W TL494 - 0 1 4- STABLE

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CURRENT MODE BUCK BOOST DESIGN DATA and TL494 -
SEPIC Converter Design Procedure
We are going to use the TL494 to design a SEPIC converter that converts and input voltage of 12V-24V to an output voltage of
5V-30V supplying a load of 5A 150W with a switching frequency of 100kHz. The outline schematic is shown in FIG. 1
V12Vin_min V0.5VD
V24Vin_max
V4Vout_min
V30Vout_max
A5Iout_max
%95η
Assuming 95% efficiency we know the output power is:
I
out_max
V
out_max
P
out_max W150Pout_max
η
P
out_max
P
in_max A13.1579V
in_min
P
in_max
I
in_max
W157.8947Pin_max
I
out_max
V
out_max
R
load_min
Ω6Rload_min
The duty cycle of the converter when operating in continuous conduction mode is given by
V
in_max
V
D
V
out_min
V
D
V
out_min
D
min
0.1579Dmin
V
in_min
V
D
V
out_max
V
D
V
out_max
D
max
0.7176Dmax
Operation Frequency Single-ended applications:
kΩ10RT
nF1CT
kHz100
C
T
R
T
1
f
sw ns7176.4706
f
sw
D
max
T
on
this equates to a FET ON time of
ns10000
f
sw
1
At this point it is worth checking that we are not violating the minimum ON time of the controller
ns7176.4706TonFET ON time IRFZ44N (60ns) < Controller ON time (150ns) <<
Inductor Choice
To design the SEPIC regulator we need to define the maximum allowed ripple in the inductors. A good rule of thumb is to use
20% to 40% of the input current. Assuming the allowed ripple through both inductors is 40% we arrive at the following
expression:
I
in_max%40ΔIL A5.2632ΔIL
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Average input current: Individual inductors
f
sw
D
max
ΔI
L
V
in_min
L
1
μH16.3624L1L2
If inductors L1 and L2 are coupled inductors, the value of the inductance required is half of what would be needed should there
be two separate inductors. Therefore, for coupled inductors the value of inductance required is given by:
f
sw
D
max
ΔI
L2
V
in_min
L
1couple
μH8.1812L1coupleL2couple
Since we have calculated a desired (individual) inductance of 20uH, if we use a coupled inductor we need the inductance of each coil to be
10uH. A coupled inductor often works out smaller in size than 2 separate inductors. Also, note the winding phase of the transformer. A useful
way to remember the phasing of the SEPIC is to consider removing the coupling cap - the SEPIC then becomes a flyback converter and indeed
the circuit operation of a coupled inductor based SEPIC and a flyback are very similar.
Maximum inductor peak current is for minimum input voltage:
f
sw
D
max
L
12
V
in_min
ηV
in_min
P
out_max
I
L1_peak 2
ΔI
L
I
in_max
I
L1_peak
or A15.7895IL1_peak
Must make sure that the inductor never
saturates.
2
ΔI
L
I
out_max
I
L2_peak
A7.6316IL2_peak
MOSFET ChoiceMOSFET Choice
A23.4211IL2_peakIL1_peak
V54.4V0.4Vout_maxVin_max
The MOSFET needs to be able to handle the peak current from both inductors so in this design a drain source current rating (Id)
of is more than sufficient. The Drain–Source voltage rating (Vds) needs to be in excess of the
(Vin + Vout + Vdiode) .
The Gate-Source turn on voltage of the MOSFET (Vgs) needs to be less than the input voltage, to ensure that the Gate drive
voltage can actually activate the MOSFET. Logic level MOSFETs have a low turn on voltage, are widely available and usually
perfect for low voltage dc/dc converters.
I
L2_peak
I
L1_peak
I
FET_peak A23.4211IFET_peak
D
max
I
in_max
I
FET_rms A15.5321IFET_rms
Diode Selection
To ensure proper operation and avoid damaging the diode,the diode selected must be able to withstand reverse voltages
equal to:
V
out_max
V
in_max
V
R V54VR
A23.4211IFET_peakThe peak current through the diode is equal to the peak current of transistor .
Coupling Capacitor Selection
The coupling capacitor must be able to handle voltages equal to: Vin_max.
mV240Vin_max%1ΔVcp - The peak−to−peak voltage across the capacitor.
if
else
I
L2_peak
max_I
Lpeak
I
L1_peak
max_I
Lpeak
I
L2_peak
I
L1_peak
f
sw
D
max
ΔV
cp
I
out_max
C
cp
μF149.5098Ccp
It is recommended to use a ceramic capacitor to keep the ESR losses as small as possible.
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max_I
Lpeak
ΔV
cp
ESR mΩ15.2ESR
Output Capacitor Choice
The ripple caused by the discharge of the output capacitor while the inductor is charging is dictated by:
mV300Vout_max%1ΔVout
f
sw
D
max
ΔV
out
I
out_max
C
out
μF119.6078Cout
I
L2_peak
I
L1_peak
ΔV
out
ESR mΩ12.809ESR
Input Capacitor
The input capacitor sees moderately low RMS current thanks to the input inductor. The RMS current in the input capacitor is
given by:
A1.5193
12
ΔI
L
I
Cin_rms
UC3843 PARAMETERS
Transistor INRFZ44N
ns100tr nC63Qt V12VG Ω0Rout_DriverIC
t
r
Q
t
I
G
A0.63IG
mA6.3fswQtIGG
R
out_DriverICI
G
V
G
R
G
Ω19.0476RG
Transistor Drive
V12VCC VCCVout_TL494
A0.63IGIC2 choose -> 2N2222, 2N2907
mA63
10
I
C2
I
B2
for V0.21VCE2 must be ---> mA63IB2IC1 choose -> 2N2222
mA6.3
10
I
C1
I
B1 ---> V0.07VCE1
I
C1
V
CE1
V
CC
R
C
Ω189.3651RC
I
B1
V
CE1
V
out_TL494
R
B
Ω1893.6508RB
Rsense Calculation
2
ΔI
L
I
FET_peak
V
1R
mΩ38.3838R
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Feedback Resistor Calculator
2
V
ref
V
R2V5Vref kΩ5.6R2 kΩ5.6R1fix
Voltage
Positive Output Voltage
R
21V
R2
V
out_max
R
1
kΩ61.6R1 if kΩ50R1
Negative Output Voltage
R
2V
R2
V
out_max
R
1N
kΩ67.2R1N
Ponteciometer calculation
if kΩ0Rpot kΩ5.6RpotR2R2var
kΩ55.6R1fixRpotR1R1var
V27.3214
R
2var
R
1var
1VR2Vout_min_max
R
1fixIn FIG.36 R8 is ( + Rpot) in serie.
Current
kΩ1R1
R
1V1
V1Vref
R
2
kΩ4R2
Soft Start and Dead Time
The soft-start time generally is in the
range of 25 to 100 clock cycles.
If 50 clock cycles at a f-kHz switching
rate is selected, the soft-start time is: t
f
sw
1
t
kΩ1
10
R
T
R
6
V0.4545VrefR
6
R
T
R
6
V
r6
μF0.25
R
6
t
25C2 ms0.25C2R6
ms0.25
f
sw
1
25
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The outline schematic is shown in FIG. 1
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