We do not need an auxiliary power supply to run the control IC. The battery pack provides
an instant-on power supply for the control IC. A single battery will not provide enough
voltage to drive the switch when the battery is fully discharged, and two batteries supply
too much voltage when the batteries are fully charged. We will use two batteries and
reduce the voltage to provide the IC control voltage. We do not need soft start operation,
since the load will be supplied by the battery until the output reaches the battery voltage.
11.4 Output Converter Detailed Design
Push-pull operation is a reasonable choice for the output converter, since it will derive its
input from a relatively constant input voltage. The frequency doubling allows using much
smaller inductors and fi lter capacitors. The National LM5030 is an excellent choice for
this application. It meets all of our requirements: external synchronization, internal slope
compensation, and large gate drive capability. We will set the maximum duty cycle (input
side) to 40% to leave room for control and to stay away from the need to ensure that the
devices do not conduct simultaneously. The lowest voltage is 35.1 V battery voltage minus
the 0.7 V voltage drop of the switching diode, or 34.4 V. The maximum voltage is the 50 V
preregulator voltage minus the 0.7 V drop of the switching diode, or 49.3 V. This sets the
minimum duty cycle to 28%. The duty cycle on the output side will be double or a range of
56–80%.
The RMS AC output current is 2.5 A. However, the average current for the 170 V supply
will be 900 mA and the 120 V supply will be 1.25 A. The peak currents will be 3.5 A and
2.5 A, respectively. The maximum rectifi er diode voltage will occur at minimum duty
cycle. The maximum input voltage for a 170 V supply will be 303 V plus the rectifi er
voltage drop. The maximum 120 V supply will be 214 V. PRV for both supplies will
equal double the input voltage. One way to reduce the stress on the diodes for the 170 V
supply is to put a 50 V supply in series with the 120 V supply instead of a single 170 V
supply. This arrangement will force the two supplies to track more closely. Increasing
the maximum duty cycle to 45% and putting two supplies in series gives 190 V and 79 V
for the two PRV values. The minimum duty cycle changes to 63%. The HFA08TA60 is a
reasonable diode for the 120 V supply, but the HFA16TA60 is actually less expensive and
allows us to use the same part in two positions. The MURD620CT is a reasonable diode
for the 50 V supply. The forward voltage drop at 1 A forward current is 1.2 V for both
diode types, so the transformer voltages need to be 80 V and 191 V. Figure 11.5 shows the
circuit for the power conversion circuit.
The output current is low even at full load, so we can start with 600 mA ripple current.
The supply will transition to discontinuous mode at 300 mA output current, which
corresponds to about 25 VA in the load. The 50 V inductor needs to be:
Ldtdi   V/ V V s/A H()(..).79 50 0 63 3 27 0 60 100
(11-6)
356
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Chapter 11
U1
LM5030
Vin
Vcc
SW A
Comp
Rt
FB
SS
GND
SW B
Csense
IRFB33N15D
IRFB33N15D
D1A
D1B
MURD620CT
D2A
HFA08TA60
D2B
R2
10
R3
10
L2
240
µh
150
µF/63 WV
C1
100 µF/200 WV
C2
T1
T
170 V
120 V
To 15V Battery Supply
R4
21 K
R5
1
K
R6
4.68
K
R7
187
K
D5A
MBR2080CT
D5B
To 50V Pre-regulator
To 36V Battery Pack
To Battery Return
and Pre-regulator Return
C3
330
nF
T2
T
R1
10
K
R8
4.7
D7
10BQ060
C4
1
nF
R9
301
C5
100 pF
To Sync
L1
100
µH
To output return
100:1
Figure 11.5 : Circuit for the power conversion circuit
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