Appli ca ti o ns for D ep l eti on MO S F ET Pradeep Kumar Tamma Application Note About this document Scope and purpose Depletion MOSFETs, unlike Enhancement MOSFETs, are in an On-state even at 0 V of gate to source voltage (VGS). This feature makes them suitable for using as a constant current source as well as in other ways. This application note explains how Depletion MOSFETs can be used in different applications. Intended audience This document is intended for SMPS or LED driver designers, as an introduction to Depletion MOSFET and highlighting the advantages. Table of Contents 1 What is a Depletion MOSFET?......................................................................................................... 2 2 Linear mode operation of a Depletion MOSFET ............................................................................. 3 3 3.1 3.2 3.2.1 3.3 Application Examples..................................................................................................................... 5 Start-up circuit for SMPS ..................................................................................................................... 5 Linear regulators ................................................................................................................................. 8 Examples ....................................................................................................................................... 9 Other examples ................................................................................................................................. 10 4 Depletion MOSFETs from Infineon ............................................................................................... 11 1 Revision 1.0, 2015-02-03 Applications for Depletion MOSFET What is a Depletion MOSFET? 1 What is a Depletion MOSFET? As Depletion MOSFETs are in the on-state, they operate as an ON switch even when the gate to source voltage (VGS) is zero. This can be best shown by comparing the transfer characteristics of both Enhancement and Depletion MOSFETs. Enhancement MOSFET Figure 1 Depletion MOSFET Transfer Charateristics of Enhancement and Depletion MOSFETs Figure 1 above illustrates an example of the transfer characteristics of both devices. For a MOSFET, the gate to source voltage (VGS) should be greater than the gate to source threshold voltage (VGS(th)) in order to conduct current through it. For an N-channel Enhancement MOSFET VGS(th) is greater than 0 V. Therefore, even at VGS of 0 V, a Depletion MOSFET conducts current. To turn off a Depletion MOSFET the VGS should be less than the VGS(th) which is a negative. This is clearly shown in schematic symbols of both. Figure 2 below shows the schematic symbols for Enhancement and Depletion MOSFETs respectively. (a) Enhancemnet MOSFET Figure 2 (b) Depletion MOSFET Schematic symbols In the symbols there is a clear difference in the second vertical line from the left. For the Enhancement MOSFET this line is discontinuous which mean that the MOSFET is in an off state at zero gate voltage. With the Depletion MOSFET, as it is in an on-state at zero gate voltage, the second vertical line is a solid continuous one. Application Note 2 Revision 1.0, 2015-02-03 Applications for Depletion MOSFET Linear mode operation of a Depletion MOSFET 2 Linear mode operation of a Depletion MOSFET In the previous section, how a Depletion MOSFET conducts at zero gate to source voltage and how it can be turned off was described. Now it will be discussed how a Depletion MOSFET can be used in a circuit. Figure 3 below illustrates a simple Depletion MOSFET circuit. Initially the voltage across the resistor R1 is 0 V, therefore, the gate to source voltage of Q1 is 0 V. As stated in the previous section, at zero gate to source voltage there will be current flowing through the MOSFET. Thus, when VIN is applied there will be a current (ID) flowing in the circuit. Due to this current, there will be a voltage drop across the resistor R1 which in turn reduces the gate to source voltage of Q1. The value is given by equation 1 below: GS = −D ∗ 1 (1) Q1 ID D R1 R2 VIN Figure 3 S G Simple circuit using a Depletion MOSFET and a resistor As the gate to source voltage (VGS) is reduced to a negative, the current through Q1 decreases (from Figure 1). The current through Q1 is given by equation 2 below: D = DSS (1 − GS GS(th) 2 ) (2) With: IDSS = on-state drain current at VGS = 0 V VGS(th) = threshold voltage of the MOSFET The drain to source voltage is: = − ( ∗ (1 + 2)) (3) At lower VDS, Q1 will be in the resistive mode of operation so the drain current ID of Q1 depends upon the RDS(on) and VDS of Q1. Thus the equation (2) will no longer be valid. But at higher VDS the MOSFET will be in linear mode region. In this mode the drain current gets saturated and does not depend on VDS which means equation (2) is valid here. For a MOSFET to be in linear mode VDS should be greater than or equal to 2*ID*RDS(on). Therefore in the circuit (shown in Figure 3), when Q1 is in linear mode the current ID is independent of the VDS. Thus, a Depletion MOSFET in series with a resistor can be used as a constant current source whose current value is independent of the drain to source voltage. As mentioned previously, the VGS of Q1 in the circuit is Application Note 3 Revision 1.0, 2015-02-03 Applications for Depletion MOSFET Linear mode operation of a Depletion MOSFET less than 0 V. For example, Figure 4 below shows the typical output charateristics of BSP179 and the red dotted line is the constant current operating region for a respective VDS. The current through MOSFET Q1 depends on the gate to source voltage and this in turn depends upon the series resistance R1. Thus for a required current through Q1, R1 can be calculated using the formula below. 1 = GS(th) (√ D − 1) D DSS (4) VGS(th) is the gate threshold voltage of the MOSFET, IDSS is the on current at VGS = 0 V and ID is the required current. Figure 4 Output charateristics of a Depletion MOSFET Application Note 4 Revision 1.0, 2015-02-03 Applications for Depletion MOSFET Application Examples 3 Application Examples 3.1 Start-up circuit for SMPS A major application where a Depletion MOSFET can be used is the start-up circuit for switch mode power supplies (SMPS). Vin ID VAUX Vout Q1 R1 VZ Figure 5 Z IIC VCC PWM IC C1 Start-up circuit for SMPS Commonly used low voltage bipolar or CMOS PWM ICs usually operate from supply voltages of up to 18 V. When the input power for the converter is available at voltages higher than the maximum voltage rating of the IC, the voltage has to be reduced with a start-up circuit. A frequent requirement is for operation directly from a rectified 120 VAC or 230 V AC line without the use of tap changing switches for the selection of different voltages. Figure 5 shows a starting circuit using a Depletion MOSFET, a resistor and a zener diode. Depletion MOSFET Q1 is configured as a source follower. Being a Depletion MOSFET, Q1 is in the on-state when there is 0 V VGS. When power is available at the input, Q1 supplies current to charge up the capacitor C1 through the external source resistor R1. Therefore, the voltage across the capacitor (VCC) starts increasing. Figure 6 shows the start-up current (Ist) and voltage wave forms of a typical PWM IC. As VCC reaches VCC,ON, it allows the PWM IC to start PWM pulses to primary MOSFET Q2. As a result the auxiliary voltage (VAUX) from the auxiliary winding starts building up. During this time the power required by the IC is supplied by the capacitor C1. Once VAUX is up, it turns off Q1 and the IC’s power is delivered only by the auxiliary winding. The condition for VAUX to turn off Q1 is AUX ≥ (ℎ), + Z + 0.7 (5) The time taken to reach VCC,ON across the capacitor is given by equation (6) below. = C1 ∗ , − (6) ID is the required current though Q1. From section 2 it is clear that when the Depletion MOSFET has high VDS and the VGS is ≤ 0V, the MOSFET operates in linear mode. Here current ID does not depend on VDS and remains constant, therefore tstc and the start-up time for the SMPS are always constant. Application Note 5 Revision 1.0, 2015-02-03 Applications for Depletion MOSFET Application Examples IIC IOP Ist VCC VCC,ON VCC,OFF VAUX tstc Figure 6 VCC,ON PWM IC turn on threshold voltage Ist start-up current (drawn by IC before Vcc,ON reached) VCC,OFFPWM IC turn off threshold voltage IOP Operating current (drawn by IC after Vcc,ON reached tsto PWM IC start-up waveforms For example, consider a PWM IC with electrical parameters as stated in Table 1 below: Table 1 PWM IC electrical parameters PWM IC Ist [µA] Iop [µA] VIN,on [V] VIN,off [V] 300 7000 15 8 Assume that C1 is equal to 10 µF, VIN = 340 V. A reasonable start-up time tstc is 500 ms, so by reconfiguring the equation (5) the drain current ID is = C1 ∗ , 15 + = 10µ ∗ + 300 µ = 600 µ 500 (7) This current is constant irrespective of VIN. Therefore, the start-up time tstc is constant even for a wide range of input (90 to 240 VAC). This is not the case with resistive solutions. For the above same example with a startup time of 500 ms the current in the resistor will be the same as the drain current 600 µA. So at 340 V of VIN the start-up resistor value can be calculated. − = 340 = ≈ 570 Ω 600 µ (8) But for a wide input range the start-up time will no longer be a constant in the resistor solution. Figure 7 shows the variations of start-up time with input voltage. Application Note 6 Revision 1.0, 2015-02-03 Applications for Depletion MOSFET Application Examples Figure 7 Start-up time vs input voltage in the resistor solution Therefore, using a Depletion MOSFET as a start-up circuit has the greater benefit of keeping the start-up time constant irrespective of the input voltage. Table 2 shows the basic electrical parameters of the Infineon Depletion MOSFET BSS126. Table 2 BSS126 Electical parameters Part Number BVDSS [V] VGS(th),min [V] VGS(th),typ [V] VGS(th),max IDSS,min @VGS = 0 V [mA] BSS126 600 -2.7 -2.0 -1.6 7 After the PWM IC is up and running, the only current drawn by Q1 is a small amount of drain to source leakage current which is typically less than 100µA. The power dissipation of Q1 during start-up time is given by: = ( − , ) ∗ ( ) (9) Since Q1 conducts only for a short duration, the maximum chip temperature rise is minimal. The power dissipation in BSS126 is: = ( − , ) ∗ ( ) = (340 − 15 ) ∗ 600µ = 0.195 Thus the chip temperature rise is given by: ∆ = ∗ ℎ@ ,@ (10) ZthJA is the maximum transient thermal impedance of BSS126. Since the MOSFET conducts only at the start-up time of the SMPS, it is considered that the power dissipation is a single pulse event. Figure 8 shows ZthJA for a single pulse of 500ms duration as in our example is equal to 130 K/W on a standard PCB. Therefore, the maximum temperature rise of BSS126 in the application example is: Application Note 7 Revision 1.0, 2015-02-03 Applications for Depletion MOSFET Application Examples ∆ = 0.195 ∗ 130 Figure 8 BSS126 max. transient thermal impedance 3.2 Linear regulators ≈ 25 A Depletion MOSFET can also be used as a pass transistor for a linear regulator. Figure 9 shows a linear regulator circuit using a Depletion MOSFET and a zener diode. In the circuit, Q1 acts as a source follower where the source voltage (Vcc) follows the gate voltage (VG) minus the gate to source voltage. VGS increases with increasing drain current. Thus, with a fixed gate voltage, the source voltage will drop with increasing load current. For design purposes, VGS under saturation and cut-off conditions (0 V and VGS(th), respectively) can be used. These values can be readily obtained from the data sheet, for example Figure 10 shows the cut-off and saturation conditions of the Infineon BSS169. Bias current for the zener diode is determined by VGS/R1.The output capacitor C1 is to compensate for the ripple at the input side. The zener diode (Z) sets the gate voltage and should be selected to provide a source voltage within the range determined, taking into account the variances of VGS with load. The source VCC generated is approximately equal to: cc = Z + (11) Substituting VGS value (from equation (2)) in the above equation we get: cc = Z − (GS(th) (1 − √ Application Note 8 D DSS (12) )) Revision 1.0, 2015-02-03 Applications for Depletion MOSFET Application Examples Q1 ID ILoad VCC S D R1 Vin G Load VGS IBIAS C1 Z VG Figure 9 Using a Depletion MOSFET as a voltage regulator The power dissipation for Q1 can be calculated from the voltage drop across it times the current through it: (7) 1 = ( − )( + ) The point to be noted here, is that the MOSFET’s saturation current (IDSS) must be greater than the load and bias currents otherwise the device can be stressed due to over current which can destroy it. Figure 10 Cutt-off conditions of BSS169 3.2.1 Examples The above application can be explained more clearly with an example. Consider a regulator with 5 V output at 5 mA of output current from 24 V input using Infineon’s Depletion MOSFET BSS169. The important features of BSS169 are shown in the table below: Table 3 BSS169 Depletion MOSFET parameters Part Number BVDSS [V] VGS(th),min [V] VGS(th),typ [V] VGS(th),max IDSS,min @VGS = 0 V [mA] BSS169 100 -2.9 -2.2 -1.8 90 In the circuit, output current is equal to the current through the MOSFET denoted here by ID. The equation to calculate the zener voltage can be calculated from equation (12). Application Note 9 Revision 1.0, 2015-02-03 Applications for Depletion MOSFET Application Examples z = cc,typ + (GS(th),typ (1 − √ z = 5 + (−2.2 ∗ (1 − √ 5∗10−3 90∗10−3 )) (8) )) ≅ 3.3 Due to VGS(th) spreading, the output voltage of the regulator varies from 4.7 V to 5.5 V. The power dissipation in BSS169 can be calculated from equation (12). (95) 1 = ( − )( + ) 1 = (24 − 4.7) ∗ (5 ∗ 10−3 + 30 ∗ 10−6 ) ≈ 0.1 This is well below the specified maximum power dissipation of BSS169. 3.3 Other examples A constant current source can be used in many different applications. A few examples are listed below: − A constant current source is useful to generate a bias current that is independent of the voltage across it. It can be used to charge a capacitor at a constant rate, generating a linear ramp for timing purposes. − A constant current source can be used as a trickle charger to maintain battery charge state. − A constant current source can be used as a current limiter when the current is below the limit. − A constant current source can also be used as a linear LED driver for driving LED strings typically up to 20mA. Figure 12 below shows a linear LED driver circuit using a Depletion MOSFET and a series resistor. Q1 R1 ~20 mA VLED LED string Vin Figure 11 Linear LED driver Application Note 10 Revision 1.0, 2015-02-03 Applications for Depletion MOSFET Depletion MOSFETs from Infineon 4 Depletion MOSFETs from Infineon Infineon offers Depletion MOSFETs in SOT-223 and SOT-23 packages with breakdown voltages ranging from 60 V to 600 V. Table 4 shows Infineon’s Depletion MOSFET portfolio: Table 4 Depletion MOSFET portfolio Part Number Package BVDSS [V] RDS(on) [Ω] VGS(th),max [V] VGS(th),min [V] IDSS,min @VGS = 0V [mA] BSP149 SOT-223 200 1.8 -1 -2.1 140 BSP129 SOT-223 240 6 -1 -2.1 55 BSP179 SOT-223 400 24 -1 -2.1 20 BSP135 SOT-223 600 45 -1 -2.1 20 BSS159N SOT-23 60 3.5 -2.4 -3.5 130 BSS169 SOT-23 100 6 -1.8 -2.9 90 BSS139 SOT-23 250 14 -1 -2.1 30 BSS126 SOT-23 600 7 -1.6 -2.7 7 For Infineon’s Depletion MOSFETs VGS(th) is spread across 5 bands. Each band has a distribution of 0.2 V and is assigned with an identification letter. Limits for each device vary from VGSth,min to VGSth,max. Figure 12 shows an example of VGS(th) bands of BSP149. Figure 12 Threshold voltage bands Please note - a single specific band cannot be ordered separately. However, it can be requested that each reel contains only one band of VGS(th). 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