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HV9906X Datasheet(PDF) 7 Page - Supertex, Inc |
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HV9906X Datasheet(HTML) 7 Page - Supertex, Inc |
7 / 10 page HV9906 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. GATE GATE Q f I × = or GATE GATE GATE V C f I × × = 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. 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 × × + × = + × = − − As an example for a particular application where CGATE = 750pF 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 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. 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. 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. 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 |
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