HV9906LG データシートの表示（PDF） - Supertex Inc
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HV9906LG Datasheet PDF : 10 Pages
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Design Information - continued
Managing Power Dissipation
The maximum IDD current required is the sum of the chip operating
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.
IGATE = f × QGATE
or
IGATE = f × CGATE × VGATE
Where f is the maximum operating frequency for the application,
QGATE is the total gate charge, CGATE is the effective gate
capacitance and VGATE is the maximum gate drive voltage, which is
approximately equal to VDD.
The input regulator supplies all the current and the worst-case total
regulator current may be calculated as follows.
IIN = 1.5 × 10−3 + IGATE = 1.5 × 10−3 + f × QGATE
or
IIN = 1.5 × 10−3 + IGATE = 1.5 × 10−3 + f × CGATE × VGATE
As an example for a particular application where CGATE = 750pF
and the maximum operating frequency is f = 200KHz the regulator
input current
IIN = 1.5 × 10−3 + 200 × 103 × 750 × 10−12 × 10 = 3mA
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.
VIN(max)
=
Tj − Ta
Rθja × IIN
Where Tj is the maximum operating junction temperature, Ta is the
maximum ambient temperature, Rθja is the thermal resistance for
the particular package from junction to ambient and IIN is the
required input current.
Using the IIN calculated in the previous example in a 50°C
maximum ambient and a plastic DIP package the maximum
allowable input voltage is as follows.
VIN(max)
=
150 − 50
110 × 3 × 10−3
= 303V
DC or RMS
HV9906
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.
1. Bootstrapping VDD from an output of the converter
2. If the input is DC then a resistor can be added in series
with VIN
3. If the input is AC then a depletion MOSFET may be added
in series with VIN
4. Encapsulating the circuit with a high thermal conductivity
material
5. Boostrapping VDD from an auxiliary bifilar inductor winding
or from an auxiliary transformer winding.
Bootstrapping VDD
Forcing VDD to a voltage greater than the regulation set point
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 VDD pin is not exceeded.
After initial startup, bootstrapping will reduce the power dissipated,
even at the absolute maximum VDD voltage, to an essentially
negligible level (VDD(max) x IIN =15V x 3mA = 45mW).
Operating from a DC input
For DC applications there is usually some minimum operating
voltage. A resistor may be added in series with +VIN which can
reduce the effective input voltage to +VIN(min) , thereby transferring
some of the power dissipation to the series resistor.
Using the input current of 3mA previously calculated and assuming
an operating input voltage range (VS) of 100VDC to 250VDC for
the application, the maximum value of the series resistor can be
calculated as follows.
R series
=
VS(min) − VIN(min)
IIN
=
100 − 10
3 × 10−3
= 30kΩ
The maximum power dissipation in the resistor will be
WR = Rseries × II2N = 30 × 103 × (3 × 10−3 )2 = 0.27W
and the maximum power dissipation in the HV9906 will be
WIC = VIN(max) × IIN − WR = 250 × 3 × 10−3 − 0.27 = 0.48W
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.
7
07/23/02
Supertex, Inc. 1235 Bordeaux Drive, Sunnyvale, CA 94089 TEL: (408) 744-0100 FAX: (408) 222-4895 www.supertex.com
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