Depletion-Mode Power MOSFETs and Applications

Transcription

1 Application Note DepletionMode Power MOSFETs and Applications R3 1

2 1 Introduction Applications like constant current sources, solid state relays, and high voltage DC lines in power systems require Nchannel depletionmode power MOSFETs that operate as normallyon switches when the gatetosource voltage is zero ( =V). This paper will describe IXYS IC Division s latest Nchannel, depletionmode, power MOSFETs and their application advantages to help designers to select these devices in many industrial applications. Figure 1 NChannel DepletionMode MOSFET D G I G I S V DS S A circuit symbol for an Nchannel depletionmode power MOSFET is given in Figure 1. The terminals are labeled as G (gate), S (source) and D (drain). IXYS IC Division depletionmode power MOSFETs are built with a structure called vertical doublediffused MOSFET, or DMOSFET, and have better performance characteristics when compared to other depletionmode power MOSFETs on the market such as high VDSX, high current, and high forward biased safe operating area (FBSOA). Figure 2 shows a typical drain current characteristic,, versus the draintosource voltage, V DS, which is called the output characteristic. It s a similar plot to that of an Nchannel enhancement mode power MOSFET except that it has current lines at equal to 2V, 1.5V, 1V, and V. Figure 2 CPC371 MOSFET Output Characteristics (ma) Output Characteristics (T A =25ºC) =.V =1.V =1.5V =2.V V DS 6 The onstate drain current, SS, a parameter defined in the datasheet, is the current that flows between the drain and the source at a particular draintosource voltage (V DS ), when the gatetosource voltage ( ) is zero (or shortcircuited). By applying positive gatetosource ( ) voltage, the device increases the current conduction level. On the other hand, negative gatetosource ( ) voltage reduces the drain current. The CPC371 stops conducting drain current at = 3.9V. This 3.9V is called the gatetosource cutoff voltage or threshold voltage ((off) ) of the device. In order to ensure proper turnon, the applied gatetosource ( ) voltage should be close 2 R3

3 to V, and to properly turn off, a more negative voltage than the cutoff voltage ((off) ) should be applied. Theoretically, the onstate drain current, (on), can be defined as: Note that Equation (1) is a theoretical formula that, in most cases, would not yield an accurate value of the drain current. (off) has a range of 3.9V to.8v and (on) depends both on (off) and the temperature. A list of IXYS IC Division Nchannel discrete depletionmode power MOSFETs is given in Table 1. The table shows the device s four main parameters: the draintosource breakdown voltage (BV DSX ), the onstate resistance (R DS(on) ), the minimum and maximum gatetosource cutoff voltage ((off) ), and the onstate drain current (SS ) along with standard discrete package options such as SOT89 and SOT223. Table 1: off IXYS IC Division NChannel DepletionMode MOSFETs = SS 1 2 Equation (1) BV DSX R DS(on) Part No. () Min V Max V Min ma Package CPC SOT89 CPC SOT89 CPC SOT89, SOT223 CPC SOT89 CPC SOT89 CPC SOT89 CPC SOT89 CPC SOT223 CPC SOT223 CPC SOT223 CPC SOT223 CPC SOT23 CPC SOT223 CPC SOT223 2 Selecting a DepletionMode MOSFET (off) (off) Depletionmode power MOSFETs will function in those applications requiring a normallyon switch. The main selection criteria for a depletionmode MOSFET, based on the application, are as follows: 1. Select the breakdown voltage meeting the margin for reliable operation ~ BV DSX, the draintosource breakdown voltage. The application voltage must be lower than the draintosource breakdown voltage of the device. BV DSX needs to be selected to accommodate the voltage swing between the positive bus and the negative bus as well as any voltage peaks caused by voltage ringing due to transients. 2. Identify the current requirement, and pick a package capable of handling that current ~ SS, the onstate drain current. The application current must be lower than the onstate drain current (SS )of the device. It is the maximum current that can flow between the drain and source, which occurs at a particular draintosource voltage (V DS ) and when the gatetosource voltage ( ) is zero. 3. (off), the gatetosource cutoff voltage Nchannel depletionmode MOSFETs have a negative channel cutoff voltage, which is designated as (off). A designer has to know the magnitude of the negative cutoff voltage (or threshold voltage). A negative gatetosource voltage ( ) will reduce the drain current until the device s cutoff voltage level is reached and conduction ceases. R3 3 SS

4 3 Applications 3.1 Current Source #1 Figure 3 shows a very precise current source to the load, RL1. TL431 is a programmable voltage reference IC. The feedback voltage from the sense resistor RS is controlled to be 2.5V. The circuit will operate as a current source at any current level below the CPC371 s rated current rating, SS. Note that at 2V power dissipation will be 1W. Figure 3 DepletionMode MOSFET Current Source and the Current Waveform U1 TL431 M1 CPC371 RB 1kΩ RL1 5Ω RS 5Ω V1 2 (ma) V DS The theoretical sense resistor value is given by: V REF RS Equation (2) Where: V REF = 2.5V (TL431) = 5mA (Desired Current) Note that Equation (2) is a theoretical formula that would probably not estimate the practical values of RS. In most cases, it s convenient to use a potentiometer to set the desired current level. 3.2 Current Source #2 Figure 4 shows a current source example with a voltage reference IC and a depletionmode MOSFET,, which compensates for supply voltage fluctuations. The current source provides a total current to the load comprising the set current through the resistor, RS, and the IC quiescent current, I Q. This circuit provides precision current and ultrahigh output impedance. Figure 4 NChannel DepletionMode MOSFET with a Voltage Reference to provide a Precise Current Source V R SET 1kΩ.1% 1ppm/ºC 1 I SET V REF GND DepletionMode MOSFET I Q C.1μF I SET = R SET I OUT = I SET I Q Output Impedance > 1MΩ I OUT 4 R3

5 3.3 NMOS Inverter Circuit Figure 5 shows an NMOS inverter circuit that uses a depletionmode MOSFET as a load. The depletionmode MOSFET,, acts as a load for the enhancementmode MOSFET, Q2, which acts as a switch. Figure 5 NMOS Inverter with DepletionMode Device used as a Load V DepletionMode MOSFET =V RG1 Q2 EnhancementMode MOSFET 3.4 OffLine SwitchMode Power Supply Many applications in industrial and consumer electronics require offline switchmode power supplies that operate from wide voltage variations of 11VAC to 26VAC. Figure 6 shows such a power supply that uses a depletionmode MOSFET,, to kickstart the offline operation by providing initial power to the IC (U1) through the source of. Figure 6 Power Supply StartUp Circuit with DepletionMode MOSFET D1 C3 L1 D4 DepletionMode MOSFET V O VAC In Input Filter Rectifier D3 D2 R1 C1 R2 PFC IC C2 V CC GATE GND RG1 Q2 DZ1 R3 15V Q3 C4 R4 provides initial power from the output, V O. R3 and R4 set up a working point to obtain the minimum required current from. The Zener diode, DZ1, limits the voltage across the IC (U1) to 15V. After the startup, the secondary winding of boost inductor L1 generates the supply voltage for the IC through D1, D2 and C3, and enough current through D3 and R1 for the base of Q3 that turns on and clamps the gate of to ground. R3 5

6 3.5 Voltage Ramp Generator Applications such as high voltage sweep circuits and automatic test equipment require high voltage ramps with a linear relationship between output voltage and time. The circuit shown in Figure 7 utilizes one depletionmode MOSFET to design a voltageramp generator circuit. Figure 7 High Voltage Ramp Generator with DepletionMode and EnhancementMode NChannel MOSFETs V DD D CPC G 4 4 S R1 17kΩ C1 1nF R2 25kΩ Q2 IXTP2N12P Time (ms) is configured as a constant current source charging a capacitor, C1; R1 provides negative feedback to regulate and set the desired current value. The constant current source charges the capacitor C1, and generates a voltage ramp,, across the capacitor. Q2 can be turned on with a TTL or a CMOS control signal to reset the ramp voltage by discharging the capacitor to ground through R2. Resistor R2 is used to limit the discharge current for Q2 to operate within its SOA rating. Assume the ramp voltage: The value of capacitor C1 should be small enough to reduce excessive charging and discharging of energy, but large enough that output loads and stray capacitances will not introduce significant errors. C1 is chosen to be 1nF. The charging current is defined as: dv =.1V/s dt I = C1 dv dt I = 1nF.1V/s = 1A The value of R1 for a 1A current source can be approximated: Equation (3) R1 I D 1 SS Where: = Pinchoff voltage = desired S(on) SS = Saturation current = 1mA, typical = 1A 6 R3

7 R1 1.75V 1A V = = = 1.695V = 16.9 k 1A 1mA 1A 1A Assume the switching frequency for Q2 is swf=2 Hz and the discharge time is: t Dischg = 1s Power loss in the output capacitor, C1: P 1 = C1 V 2 f 2 sw Equation (4) Using equation (4), Discharging time: P 1 = 1nF 5 2 2Hz = 125J 2Hz = 25mJ/s = 25mW 2 t Dischg = 4 R2C1 Equation (5) Using equation (5): R2 = 1s = 4 1nF 25k R3 7

8 3.6 Linear Voltage Regulator Many applications require a linear voltage regulator that operates from high input voltage that is sourced from a wide voltage range of 12 VAC to 24 VAC with a maximum peak voltage of / 34V. Applications like CMOS ICs and small analog circuits require a 5V to 15V DC power supply that provides protection from very fast high voltage transients, and that has low quiescent current requirements. Figure 8 shows a high voltage offline linear voltage regulator using a Depletionmode MOSFET that can meet the above requirement of low transient voltage and low quiescent current. Figure 8 High Voltage Offline Linear Voltage Regulator H C1 1μF CPC563 VIN IN SHDN LT395 GND OUT C2 1μF R L 5kΩ and I OUT H I OUT (ma) High voltage transients are generated in telecommunication circuits because of lightning and spurious radiation, and in automotive and avionics circuits because of inductive loads. Low quiescent current is required to minimize power dissipation in these linear regulators. H Calculation: V = SS 1 GS 2 Solving for : off = off 1 I D SS Where: = = off 1 I D SS = mA 1mA = = 6.38V 8 R3

9 3.7 CurrentMonitor Circuit A simple current monitor circuit using an opamp and a depletionmode MOSFET is shown in Figure 9. R 1 monitors the current to the load and the MOSFET,, provides an output voltage proportional to the current being monitored. R S R 2 = I LOAD Equation (7) Resistor, R 1, should have a tolerance of.1% with an appropriate wattage rating. R 1 Figure 9 Current Monitor using DepletionMode MOSFET and a SingleSupply OpAmp R S I LOAD R LOAD C 1 C 2 D R 1 G S R 2 For example: R S =.1 R 1 =1 R 2 =1k Using equation (7): V OUT R S R = 2 =.1 1 = 1V/A 1 I LOAD R 1 R3 9

10 3.8 Normally Closed Solid State Relay Depletionmode FETs can be used to create normally closed solid state relays using IXYS IC Division s optical driver, FDA217. Figure 1 shows a typical connection of two external CPC398 depletion FETs arranged in backtoback configuration to make an AC/DC switch. FDA217 has internal turnoff circuitry so that no external bleed resistors are required. Figure 1 FDA217 used with CPC398 FETs to create Normally Closed Solid State Relay V1 5V R FDA217 PV PV CPC398 CPC398 R2 V2 22VAC For additional information please visit our website at: IXYS Integrated Circuits Division makes no representations or warranties with respect to the accuracy or completeness of the contents of this publication and reserves the right to make changes to specifications and product descriptions at any time without notice. Neither circuit patent licenses nor indemnity are expressed or implied. Except as set forth in IXYS Integrated Circuits Division s Standard Terms and Conditions of Sale, IXYS Integrated Circuits Division assumes no liability whatsoever, and disclaims any express or implied warranty, relating to its products including, but not limited to, the implied warranty of merchantability, fitness for a particular purpose, or infringement of any intellectual property right. The products described in this document are not designed, intended, authorized or warranted for use as components in systems intended for surgical implant into the body, or in other applications intended to support or sustain life, or where malfunction of IXYS Integrated Circuits Division s product may result in direct physical harm, injury, or death to a person or severe property or environmental damage. IXYS Integrated Circuits Division reserves the right to discontinue or make changes to its products at any time without notice. Specification: R3 Copyright 214, IXYS Integrated Circuits Division All rights reserved. Printed in USA. 1/3/ R3