HV9906

Managing Power Dissipation

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.

GATE

GATE

Q

f

I

×

=

or

GATE

GATE

GATE

V

C

f

I

×

×

=

Where f is the maximum operating frequency for the application,

The input regulator supplies all the current and the worst-case total

regulator current may be calculated as follows.

GATE

3

GATE

3

IN

Q

f

10

5

.

1

I

10

5

.

1

I

×

+

×

=

+

×

=

−

−

or

GATE

GATE

3

GATE

3

IN

V

C

f

10

5

.

1

I

10

5

.

1

I

×

×

+

×

=

+

×

=

−

−

and the maximum operating frequency is f = 200KHz the regulator

input current

mA

3

10

10

750

10

200

10

5

.

1

I

12

3

3

IN

=

×

×

×

×

+

×

=

−

−

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.

IN

ja

a

j

(max)

IN

I

R

T

T

V

×

−

=

θ

required input current.

°C

maximum ambient and a plastic DIP package the maximum

allowable input voltage is as follows.

V

303

10

3

110

50

150

V

3

(max)

IN

=

×

×

−

=

−

DC or RMS

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.

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

material

or from an auxiliary transformer winding.

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

After initial startup, bootstrapping will reduce the power dissipated,

Operating from a DC input

For DC applications there is usually some minimum operating

some of the power dissipation to the series resistor.

Using the input current of 3mA previously calculated and assuming

the application, the maximum value of the series resistor can be

calculated as follows.

Ω

=

×

−

=

−

=

−

k

30

10

3

10

100

I

V

V

R

3

IN

(min)

IN

(min)

S

series

The maximum power dissipation in the resistor will be

W

27

.

0

)

10

3

(

10

30

I

R

W

2

3

3

2

IN

series

R

=

×

×

×

=

×

=

−

and the maximum power dissipation in the HV9906 will be

W

48

.

0

27

.

0

10

3

250

W

I

V

W

3

R

IN

(max)

IN

IC

=

−

×

×

=

−

×

=

−

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