HV9906 Supertex Inc, HV9906 Datasheet - Page 7

• Manufacturer Part Number: HV9906
• Description: Simple Off-Line/PFC & >9V DC/DC Switcher
• Manufacturer: Supertex Inc

Design Information -
Managing Power Dissipation
The maximum I
current plus the current required to drive the gate of the external
MOSFET at the maximum operating frequency of the particular
application. Depending on the available data on the MOSFET the
current can be calculated by one of the following methods.
or
Where f is the maximum operating frequency for the application,
Q
capacitance and V
approximately equal to V
The input regulator supplies all the current and the worst-case total
regulator current may be calculated as follows.
or
As an example for a particular application where C
and the maximum operating frequency is f = 200KHz the regulator
input current
If the application is operating in an open-air environment with a
known maximum ambient temperature, then the maximum
allowable input voltage may be calculated using the following
equation.
Where T
maximum ambient temperature, R
the particular package from junction to ambient and I
required input current.
Using the I
maximum ambient and a plastic DIP package the maximum
allowable input voltage is as follows.
GATE
is the total gate charge, C
j
Supertex, Inc. 1235 Bordeaux Drive, Sunnyvale, CA 94089 TEL: [phone] FAX: [phone] www.supertex.com
is the maximum operating junction temperature, T
I
I
I
I
I
V
V
GATE
GATE
IN
IN
IN
IN
IN
IN
(max)
(max)
1
1
1
DD
calculated in the previous example in a 50 C
5 .
5 .
5 .
f
f
current required is the sum of the chip operating
GATE
10
10
10
R
110
Q
C
T
j
GATE
GATE
ja
150
is the maximum gate drive voltage, which is
3
3
3
DD
T
I
3
a
IN
I
I
200
.
GATE
GATE
50
V
10
GATE
3
10
1
1
continued
5 .
5 .
3
ja
303
750
is the thermal resistance for
10
10
GATE
V
3
3
DC or RMS
10
is the effective gate
f
f
12
Q
C
GATE
GATE
10
GATE
3
V
mA
GATE
IN
= 750pF
a
is the
is the
7
In the event that this maximum allowable input voltage is less than
what is required by the application, then the following means may
be considered to reduce the dissipation in the regulator.
Bootstrapping V
Forcing V
voltage of the internal regulator (i.e. 13V) will force the regulator to
turn off and all the required operating current will be provided by
the forcing source of power. If this power source is derived from
the output of the converter, possibly by means of a secondary
winding on one of the inductors or an additional winding on a
transformer, then the internal regulator will provide the required
current during startup only. Care must be taken to assure that the
absolute maximum voltage rating of the V
After initial startup, bootstrapping will reduce the power dissipated,
even at the absolute maximum V
negligible level (V
Operating from a DC input
For DC applications there is usually some minimum operating
voltage. A resistor may be added in series with +V
reduce the effective input voltage to +V
some of the power dissipation to the series resistor.
Using the input current of 3mA previously calculated and assuming
an operating input voltage range (V
the application, the maximum value of the series resistor can be
calculated as follows.
The maximum power dissipation in the resistor will be
and the maximum power dissipation in the HV9906 will be
which for an SOIC packaged device will result in junction to
ambient temperature difference of 159 C/W x 0.48W = 76.32 C,
thereby allowing operation up to an ambient temperature of
73.68 C for the absolute maximum junction temperature of 150 C.
W
W
R
IC
1. Bootstrapping V
2. If the input is DC then a resistor can be added in series
3. If the input is AC then a depletion MOSFET may be added
4. Encapsulating the circuit with a high thermal conductivity
5. Boostrapping V
R
V
series
IN
with V
in series with V
material
or from an auxiliary transformer winding.
(max)
DD
R
series
to a voltage greater than the regulation set point
I
IN
2
IN
I
IN
DD
DD(max)
30
V
W
S
(min)
R
10
IN
DD
x I
DD
I
3
250
IN
IN
from an auxiliary bifilar inductor winding
from an output of the converter
V
=15V x 3mA = 45mW).
3 (
IN
(min)
3
10
10
3
100
DD
3
)
S
2
3
) of 100VDC to 250VDC for
10
voltage, to an essentially
IN(min)
. 0
. 0
DD
10
27
27
3
pin is not exceeded.
W
, thereby transferring
30
. 0
48
k
W
IN
HV9906
07/23/02
which can