Application Note AN-500 INTEGRATED CIRCUITS DIVISION Depletion-Mode Power MOSFETs and Applications AN-500-R03 www.ixysic.com 1 AN-500 INTEGRATED CIRCUITS DIVISION 1 Introduction Applications like constant current sources, solid state relays, and high voltage DC lines in power systems require N-channel depletion-mode power MOSFETs that operate as normally-on switches when the gate-to-source voltage is zero (VGS=0V). This paper will describe IXYS IC Division’s latest N-channel, depletion-mode, power MOSFETs and their application advantages to help designers to select these devices in many industrial applications. Figure 1 N-Channel Depletion-Mode MOSFET D ID + G VDS + IG VGS IS - - S A circuit symbol for an N-channel depletion-mode power MOSFET is given in Figure 1. The terminals are labeled as G (gate), S (source) and D (drain). IXYS IC Division depletion-mode power MOSFETs are built with a structure called vertical double-diffused MOSFET, or DMOSFET, and have better performance characteristics when compared to other depletion-mode 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, ID, versus the drain-to-source voltage, VDS, which is called the output characteristic. It’s a similar plot to that of an N-channel enhancement mode power MOSFET except that it has current lines at VGS equal to -2V, -1.5V, -1V, and 0V. CPC3710 - MOSFET Output Characteristics ID (mA) Figure 2 Output Characteristics (TA=25ºC) 300 270 240 210 180 150 120 90 60 30 0 VGS=0.0V VGS=-1.0V VGS=-1.5V VGS=-2.0V 0 1 2 3 VDS (V) 4 5 6 The on-state drain current, IDSS, a parameter defined in the datasheet, is the current that flows between the drain and the source at a particular drain-to-source voltage (VDS), when the gate-to-source voltage (VGS) is zero (or short-circuited). By applying positive gate-to-source (VGS) voltage, the device increases the current conduction level. On the other hand, negative gate-to-source (VGS) voltage reduces the drain current. The CPC3710 stops conducting drain current at VGS= -3.9V. This -3.9V is called the gate-to-source cutoff voltage or threshold voltage (VGS(off)) of the device. In order to ensure proper turn-on, the applied gate-to-source (VGS) voltage should be close 2 www.ixysic.com R03 AN-500 INTEGRATED CIRCUITS DIVISION to 0V, and to properly turn off, a more negative VGS voltage than the cutoff voltage (VGS(off)) should be applied. Theoretically, the on-state drain current, ID(on), can be defined as: V GS 2 I D = I DSS 1 – -----------------V GS off Equation (1) Note that Equation (1) is a theoretical formula that, in most cases, would not yield an accurate value of the drain current. VGS(off) has a range of -3.9V to -0.8V and ID(on) depends both on VGS(off) and the temperature. A list of IXYS IC Division N-channel discrete depletion-mode power MOSFETs is given in Table 1. The table shows the device’s four main parameters: the drain-to-source breakdown voltage (BVDSX), the on-state resistance (RDS(on)), the minimum and maximum gate-to-source cutoff voltage (VGS(off)), and the on-state drain current (IDSS) along with standard discrete package options such as SOT-89 and SOT-223. Table 1: 2 IXYS IC Division N-Channel Depletion-Mode MOSFETs Part No. BVDSX (V) RDS(on) ( ) VGS(off) Min V VGS(off) Max V IDSS Min mA Package CPC3701 CPC3703 CPC3708 CPC3710 CPC3714 CPC3720 CPC3730 CPC3902 CPC3909 CPC3960 CPC3980 CPC3982 CPC5602 CPC5603 60 250 350 250 350 350 350 250 400 600 800 800 350 415 1 4 14 10 14 22 30 2.5 6 44 45 380 14 14 -0.8 -1.6 -2 -1.6 -1.6 -1.6 -1.6 -1.4 -1.4 -1.4 -1.4 -1.4 -2 -2 -2.9 -3.9 -3.6 -3.9 -3.9 -3.9 -3.9 -3.1 -3.1 -3.1 -3.1 -3.1 -3.6 -3.6 600 300 130 220 240 130 140 400 300 100 100 20 130 130 SOT-89 SOT-89 SOT-89, SOT-223 SOT-89 SOT-89 SOT-89 SOT-89 SOT-223 SOT-223 SOT-223 SOT-223 SOT-23 SOT-223 SOT-223 Selecting a Depletion-Mode MOSFET Depletion-mode power MOSFETs will function in those applications requiring a normally-on switch. The main selection criteria for a depletion-mode MOSFET, based on the application, are as follows: 1. Select the breakdown voltage meeting the margin for reliable operation ~ BVDSX, the drain-to-source breakdown voltage. The application voltage must be lower than the drain-to-source breakdown voltage of the device. BVDSX 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 ~ IDSS, the on-state drain current. The application current must be lower than the on-state drain current (IDSS)of the device. It is the maximum current that can flow between the drain and source, which occurs at a particular drain-to-source voltage (VDS) and when the gate-to-source voltage (VGS) is zero. 3. VGS(off), the gate-to-source cutoff voltage N-channel depletion-mode MOSFETs have a negative channel cutoff voltage, which is designated as VGS(off). A designer has to know the magnitude of the negative cutoff voltage (or threshold voltage). A negative gate-to-source voltage (VGS) will reduce the drain current until the device’s cutoff voltage level is reached and conduction ceases. R03 www.ixysic.com 3 AN-500 INTEGRATED CIRCUITS DIVISION 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 CPC3710’s rated current rating, IDSS. Note that at 200V power dissipation will be 1W. Figure 3 Depletion-Mode MOSFET Current Source and the Current Waveform M1 CPC3710 6 V1 5 + RL1 500Ω 4 200 ID (mA) RB 10kΩ 3 2 U1 TL431 1 RS 500Ω 0 0 20 40 60 80 100 120 140 160 180 200 VDS (V) The theoretical sense resistor value is given by: V REF RS ------------ID Equation (2) Where: • VREF = 2.5V (TL431) • ID = 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 depletion-mode MOSFET, Q1, 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, IQ. This circuit provides precision current and ultra-high output impedance. Figure 4 N-Channel Depletion-Mode MOSFET with a Voltage Reference to provide a Precise Current Source +V Q1 + VGS1 Depletion-Mode MOSFET - ISET = VIN VOUT RSET 10kΩ 0.1% 10ppm/ºC C+ VREF 0.01μF GND ISET VOUT RSET IOUT = ISET+IQ Output Impedance > 100MΩ IQ IOUT 4 www.ixysic.com R03 AN-500 INTEGRATED CIRCUITS DIVISION 3.3 NMOS Inverter Circuit Figure 5 shows an NMOS inverter circuit that uses a depletion-mode MOSFET as a load. The depletion-mode MOSFET, Q1, acts as a load for the enhancement-mode MOSFET, Q2, which acts as a switch. Figure 5 NMOS Inverter with Depletion-Mode Device used as a Load +V Q1 Depletion-Mode MOSFET VGS=0V VOUT Q2 Enhancement-Mode MOSFET VIN RG1 3.4 Off-Line Switch-Mode Power Supply Many applications in industrial and consumer electronics require off-line switch-mode power supplies that operate from wide voltage variations of 110VAC to 260VAC. Figure 6 shows such a power supply that uses a depletion-mode MOSFET, Q1, to kick-start the off-line operation by providing initial power to the IC (U1) through the source of Q1. Figure 6 Power Supply Start-Up Circuit with Depletion-Mode MOSFET Depletion-Mode MOSFET L1 D1 VAC In Input Filter Rectifier D3 +VO C3 D2 VCC R1 + PFC GATE IC GND C1 R2 Q1 D4 C2 RG1 Q2 R3 +15V + C4 DZ1 Q3 R4 Q1 provides initial power from the output, +VO. R3 and R4 set up a working point to obtain the minimum required current from Q1. The Zener diode, DZ1, limits the voltage across the IC (U1) to +15V. After the start-up, 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 Q1 to ground. R03 www.ixysic.com 5 AN-500 INTEGRATED CIRCUITS DIVISION 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 depletion-mode MOSFET to design a voltage-ramp generator circuit. Figure 7 High Voltage Ramp Generator with Depletion-Mode and Enhancement-Mode N-Channel MOSFETs +VDD 600 Q1 CPC3980 500 VOUT (V) G ID S VOUT C1 1nF R1 17kΩ 60 VOUT 400 40 300 30 200 R2 25kΩ 50 20 VIN 100 Q2 IXTP02N120P 10 0 VIN VIN (V) D 0 0 1 2 3 4 5 6 Time (ms) 7 8 9 10 Q1 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, VOUT, 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: dV ------- = 0.1V/s dt 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 I = C1 ------dt I = 1nF 0.1V/s = 100A Equation (3) The value of R1 for a 100A current source can be approximated: V GS I D R1 --------- ---------– 1 I D I DSS Where: • VGS = Pinch-off voltage = -1.75V @ desired IDS(on) • IDSS = Saturation current = 100mA, typical • ID = 100A 6 www.ixysic.com R03 AN-500 INTEGRATED CIRCUITS DIVISION – 1.75V 1.75V100A- – 1 = –---------------R1 = ----------------- --------------- 0.03162 – 1 = 1.695V ----------------- = 16.9k 100A 100A 100mA 100A Assume the switching frequency for Q2 is swf=200 Hz and the discharge time is: t Dischg = 100s Power loss in the output capacitor, C1: 2 1 P = --- C1 V f sw 2 Equation (4) Using equation (4), 2 1 P = --- 1nF 500 200Hz = 125J 200Hz = 25mJ/s = 25mW 2 Discharging time: t Dischg = 4 R2C1 Equation (5) Using equation (5): 100s- = 25k R2 = -----------------4 1nF R03 www.ixysic.com 7 AN-500 INTEGRATED CIRCUITS DIVISION 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 120 VAC to 240 VAC with a maximum peak voltage of +/- 340V. 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 off-line linear voltage regulator using a Depletion-mode MOSFET that can meet the above requirement of low transient voltage and low quiescent current. High Voltage Off-line Linear Voltage Regulator 1.2 10 +HVIN IOUT LT3009-5 VIN C1 1μF IN SHDN VOUT (V) and VIN (V) Q1 CPC5603 VOUT OUT GND C2 1μF RL 5kΩ 8 VIN 6 VOUT 4 1.0 0.8 0.6 IOUT (mA) Figure 8 0.4 2 0.2 0 0 30 60 0 90 120 150 180 210 240 270 300 HVIN (V) 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. HVIN Calculation: V GS 2 I D = I DSS 1 – -----------------V GS off Solving for VGS: ID V GS = V GS off 1 – --------- I DSS Where: • V GS = V OUT – V IN ID V IN = V OUT – V GS off 1 – --------- I DSS 1mA- = 5 + 2 1 – 0.3162 = 6.38V V IN = 5 + 2 1 – ------------10mA 8 www.ixysic.com R03 AN-500 INTEGRATED CIRCUITS DIVISION 3.7 Current-Monitor Circuit A simple current monitor circuit using an op-amp and a depletion-mode MOSFET is shown in Figure 9. R1 monitors the current to the load and the MOSFET, Q1, provides an output voltage proportional to the current being monitored. RS R2 V OUT = I LOAD ------------------ R1 Equation (7) Resistor, R1, should have a tolerance of 0.1% with an appropriate wattage rating. Figure 9 Current Monitor using Depletion-Mode MOSFET and a Single-Supply Op-Amp ILOAD RS +VIN RLOAD C1 C2 R1 + D Q1 VOUT G S R2 For example: • RS=0.1 • R1=100 • R2=1k Using equation (7): RS R2 V OUT 1000- = 1V/A -------------- = ----------------- = 0.1 -----------------------R1 I LOAD 100 R03 www.ixysic.com 9 AN-500 INTEGRATED CIRCUITS DIVISION 3.8 Normally Closed Solid State Relay Depletion-mode FETs can be used to create normally closed solid state relays using IXYS IC Division’s optical driver, FDA217. Figure 10 shows a typical connection of two external CPC3980 depletion FETs arranged in back-to-back configuration to make an AC/DC switch. FDA217 has internal turn-off circuitry so that no external bleed resistors are required. Figure 10 FDA217 used with CPC3980 FETs to create Normally Closed Solid State Relay FDA217 R2 + 8 + 6 PV 2 3 PV R1 V1 5V CPC3980 V2 7 + 1 220VAC 5 4 CPC3980 For additional information please visit our website at: www.ixysic.com 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: AN-500-R03 ©Copyright 2014, IXYS Integrated Circuits Division All rights reserved. Printed in USA. 10/3/2014 10 www.ixysic.com R03

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