Datasheet

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SLUS419C − AUGUST 1999 − REVISED NOVEMBER 2001
15
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TYPICAL APPLICATION
power switch selection (Q3 in Figure 9)
As in any power supply design, tradeoffs between performance, cost and size are necessary. When selecting
a power switch, it is useful to calculate the total power dissipation in the switch for several different devices at
the switching frequencies being considered for the converter. Total power dissipation in the switch is the sum
of switching loss and conduction loss. Switching losses are the combination of the gate charge loss, drain
source capacitance of the MOSFET loss and turnon and turnoff losses:
P
GATE
+ Q
GATE
V
GATE
f
S
P
COSS
+
1
2
C
OSS
ǒ
V
OFF
Ǔ
2
f
S
P
SW
+
1
2
V
OFF
I
L
ǒ
t
ON
) t
OFF
Ǔ
f
S
Where Q
GATE
is the total gate charge, V
GATE
is the gate drive voltage, f
s
is the switching frequency, C
OSS
is
the drain source capacitance of the MOSFET, t
ON
and t
OFF
are the switching times (estimated using device
parameters R
GATE
, Q
GD
and V
TH
) and V
OFF
is the voltage across the switch during the off time, in this case V
OFF
= V
BOOST
.
Conduction loss is calculated as the product of the R
DS(on)
of the switch (at the worst case junction temperature)
and the square of RMS current:
P
COND
+ R
DS(on)
K
ǒ
I
RMS
Ǔ
2
where K is the temperature factor found in the manufacturer’s R
DS(on)
vs junction temperature curves.
Calculating these losses and plotting against frequency gives a curve that enables the designer to determine
either which manufacturer’s device has the best performance at the desired switching frequency, or which
switching frequency has the least total loss for a particular power switch. For this design example an IRFP450
HEXFET from International Rectifier is chosen because of its low R
DS(on)
and its V
DSS
rating. The IRFP450’s
R
DS(on)
of 400 mΩ and the maximum V
DSS
of 500 V makes it an ideal choice. A comprehensive review of this
procedure can be found in the Unitrode Power Supply Design Seminar SEM−1200, Topic 6, TI Literature No.
SLUP117.
More recently, faster switching insulated gate bipolar transistors (IGBTs) have become widely available.
Depending on the system power level (and the switching frequency), use of IGBTs may make sense for the
power switch.
boost diode selection (D3 in Figure 9)
In order to keep the switching losses to a minimum and meet the voltage and current requirements, a
HFA08TB60 fast recovery diode from International Rectifier is selected for the design. This diode is rated for
a maximum reverse voltage of 600 V and a maximum forward current of 8 A. The typical reverse recovery of
18 ns made this diode ideal for this design.
(5)
(6)
(7)
(8)