AN-1001 Understanding Power MOSFET Parameters

Transcription

1 AN-1001 Understanding Power MOSFET Parameters

2 Content 1. Absolute Maximum Ratings rain-source Voltage (VS) Gate-Source Voltage (VGS) Continuous rain Current (I) Pulsed rain Current (IM) Single Pulse Avalanche Current (IAS) Single Pulse Avalanche Energy (EAS) Total Power issipation (P) Operating Junction and Storage Temperature Range (TJ,TSTG) Thermal Performance Junction To Case Thermal Resistance (RθJC) Junction To Ambient Thermal Resistance (RθJA) Electrical Specifications rain-source Breakdown Voltage (BVSS) Gate Threshold Voltage (VGS(TH)) Gate-Source Leakage Current (IGSS) rain-source Leakage Current (ISS) rain-source On-State Resistance (RS(ON)) Forward Transconductance (gfs) ynamic Total Gate Charge (Qg) Capactance (Ciss, Coss, Crss) Gate Resistance (Rg) Switching Time Switching Time (Td(on),Tr,Td(off),Tf) Source-rain iode Characteristics Forward Voltage (VS) Body iode Reverse Recovery (trr,qrr) Characteristics Curves Output Characteristics Transfer Characteristics Gate-Source Voltage vs. Gate Charge On-Resistance vs. Junction Temperature Version: A1611

3 Source-rain iode Forward Current vs. Voltage BVSS vs. Junction Temperature Capacitance vs. rain-source Voltage SOA Normalized Thermal Transient Impedance, Junction-to -Case Version: A1611

4 Introduction MOSFET datasheet parameters introduction When choosing a MOSFET, parameters that are focused on by most engineers intuitively are V S, R S(on), I. However, in power systems, it is significant to pick up a suitable MOSFET based on different applications. In this application note, Taiwan Semiconductor (TSC) introduces the definition of every single parameter of a MOSFET, and from chapter 3, TSC also explains how each parameter is realized, hoping this would help designers on the power projects. 1. Absolute Maximum Ratings ABSOLUTE MAXIMUM RATINGS (T A = 25 C unless otherwise noted) PARAMETER SYMBOL LIMIT UNIT Notes rain-source Voltage V S 30 V 1.1 Gate-Source Voltage V GS ±20 V 1.2 Continuous rain Current (Note 1) T C = 25 C 39 I T A = 25 C 11 A 1.3 Pulsed rain Current I M 156 A 1.4 Single Pulse Avalanche Current (Note 2) I AS 15.6 A 1.5 Single Pulse Avalanche Energy (Note 2) E AS 36.5 mj 1.6 Total Power issipation Total Power issipation T C = 25 C P 33 T C = 125 C 6.6 T A = 25 C P 2.6 T A = 125 C 0.5 Operating Junction and Storage Temperature range T J, T STG - 55 to +150 C rain-source Voltage (V S ) V S represents MOSFET absolute maximum voltage between rain and Source. In operations, voltage stress of rain-source should not exceed maximum rated value. 1.2 Gate-Source Voltage ( V GS ) V GS represents operating driver voltage between Gate and Source. In operations, voltage stress of Gate-Source should not exceed maximum rated value. 1.3 Continuous rain Current ( I ) I represents MOSFET s continuous conduction current and could be calculated by below equation. I T J = Junction Temperature R T C = Case Temperature R S(ON) = rain-source On-State Resistance R θjc = Junction to Case Thermal Resistance K = On-Resistance vs. Junction Temperature JC TJ T R C S ( ON ) K W W Version: A1611

5 1.4 Pulsed rain Current ( I M ) I M represents maximum limit current in MOSFET SOA (Safe Operating Area ). A MOSFET could be well operated within SOA to make sure the stability and safety of a power system. 1.5 Single Pulse Avalanche Current ( I AS ) When power MOSFET enters the avalanche mode, the current transformed into the form of voltage across rain and Source of a MOSFET is called avalanche current ( I AS ). 1.6 Single Pulse Avalanche Energy ( E AS ) UIL (unclamped inductive load) or UIS (unclamped inductive switching) tests are important to check the degree of robustness of a power MOSFET. E AS represents allowed maximum energy in either one pulse that is over rated V S. When the operating voltage exceeds the specified V S at off state of a MOSFET, it will enter the avalanche mode and cause high power dissipation. The parameter is often taken into consideration when inductive load or transient loading is operated. L VS I Switch BVS VGS + - UT G S V V + - I VGS IAS tav EAS V 1 2 BVS UIS Test Circuit E AS L I AS ( ) 2 BV V The UIS test circuit: When applying a VGS signal to a UT and the UT is turned on, the current will start to charge Inductor (L), phenomenon of the I current rising rate behaves linearly as above picture. When the required I is reached to IAS, the UT is then turned off, causing the Inductor to dissipate all of its stored energy and enforcing the UT to reach its breakdown voltage rating. The UT will remain in breakdown (BVS) until all of the energy is dissipated. The blue area is E AS (Energy Avalanche Single). S 4 Version: A1611

6 1.7 Total Power issipation ( P ) P represents the capability of maximum power dissipation that a MOSFET can handle. Moreover, capability of power dissipation varies by different temperature conditions. When case temperature (T C ) is considered, equation would be: P TJ T R JC As for ambient temperature (T A ), equation turns into: C P TJ T R JA A T J = Junction Temperature T C = Case Temperature R θjc = Junction to Case Thermal Resistance R θja = Junction to Ambient Thermal Resistance 1.8 Operating Junction and Storage Temperature Range ( T J, T STG ) T J represents maximum operating temperature of a MOSFET. A MOSFET should be avoided to be operated over the rated temperature limit. T STG represents the range of temperature for storage or transportation of a MOSFET. It must be storage in specified temperature values. 5 Version: A1611

7 2. Thermal Performance THERMAL PERFORMANCE PARAMETER SYMBOL LIMIT UNIT Notes Junction to Case Thermal Resistance R ӨJC 3.8 C/W 2.1 Junction to Ambient Thermal Resistance R ӨJA 48 C/W Junction To Case Thermal Resistance ( R ӨJC ) R ӨJC is the sum of the junction to case thermal resistances. The case thermal reference is defined at the solder mounting surface of the drain pins. 2.2 Junction To Ambient Thermal Resistance ( R ӨJA ) R ӨJA is the sum of the junction to case and case to ambient thermal resistances. The case thermal reference is defined at the solder mounting surface of the drain pins. R ӨJA is guaranteed by design while R ӨCA is determined by the user s board design. 6 Version: A1611

8 3. Electrical Specifications ELECTRICAL SPECIFICATIONS (T A = 25 C unless otherwise noted) PARAMETER CONITIONS SYMBOL MIN TYP MAX UNIT Notes Static rain-source Breakdown Voltage V GS = 0V, I = 250µA BV SS V 3.1 Gate Threshold Voltage V GS = V S, I = 250µA V GS(TH) V 3.2 Gate-Source Leakage Current V GS = ±20V, V S = 0V I GSS ±100 na 3.3 rain-source Leakage Current rain-source On-State Resistance (Note 3) V GS = 0V, V S = 30V V GS = 0V, V S = 30V T J = 125 C V GS = 10V, I = 11A I SS R S(on) V GS = 4.5V, I = 11A µa 3.4 mω 3.5 Forward Transconductance (Note 3) V S = 5V, I = 11A g fs S rain-source Breakdown Voltage ( BV SS ) To measure breakdown voltage of a MOSFET, at first, short Gate pin and Source pin, and then, supply the I =250μA, and monitor the reading of V S. G S + VS - V I BV SS is determined when I reaches 250μA. Gate pin is shorted to Source pin. 3.2 Gate Threshold Voltage ( V GS(TH) ) To measure gate threshold voltage of a MOSFET, at first, short Gate pin and rain pin, and then, with a given I =250μA, and monitor the voltage difference between Gate-Source. One significant characteristics of V GS(TH) is its negative temperature coefficient. If power system has to be operated at a certain minus degree, to avoid unpredicted being turned on, V GS(TH) needs to be taken into consideration. VGS(TH) + - V G S I V GS(TH) is determined when I reaches 250μA. Gate pin is shorted to rain pin. 7 Version: A1611

9 3.3 Gate-Source Leakage Current ( I GSS ) To measure Gate-Source leakage current of a MOSFET, at first, short rain pin and Source pin, and then, apply maximum allowable voltage on Gate-Source and monitor the leakage current of Gate- Source. I GSS is dependent on the structure and design of the gate oxide. IGSS A G S I GSS is determined when maximum V GS voltage is applied. rain pin is shorted to source pin 3.4 rain-source Leakage Current ( I SS ) To measure rain-source leakage current of a MOSFET, at first, short Gate pin and Source pin, and then, apply maximum allowable voltage on rain-source and monitor the leakage current of rain- Source. A ISS G S I SS is determined when maximum V S voltage is applied. Gate pin is shorted to Source pin 3.5 rain-source On-State Resistance ( R S(on) ) To measure rain-source on resistance, R S(on), at first, apply a voltage across Gate-Source, which is specified to be higher than V GS(TH). With a given current source, I, measure the voltage drop across rain-source, V S. And after that, through the equation, R S(on) = V S / I, R S(on) is observed. In TSC MOSFET datasheet, two additional figures are introduced as well. One is R S(on) vs V GS graph since R S(on) varies by different amplitude of V GS. The other one is R S(on) vs T J. Characteristics of R S(on) is positive temperature coefficient. It is important to consider the surrounding temperature of selecting a MOSFET in a power system. G S + VS - V I Apply specified V GS, and given I. measured V S of a MOSFET, then R S(on) is obtained 8 Version: A1611

10 3.6 Forward Transconductance ( g fs ) Forward transconductance, g fs, represents the signal gain (drain current divided by gate voltage) of a MOSFET. Higher g fs indicates the high current (I S ) handling capability can be gained from the low gate voltage (V GS ). It is also expressed as below equation. IS VGS(TH) VGS 9 Version: A1611

11 4. ynamic (Note 4) ynamic Total Gate Charge V GS = 10V, V S = 15V, I = 11A Q g Total Gate Charge Q g V GS = 4.5V, V S = 15V, I Gate-Source Charge Q gs = 11A Gate-rain Charge Q gd Input Capacitance C iss V GS = 0V, V S = 15V Output Capacitance C oss f = 1.0MHz Reverse Transfer Capacitance C rss nc 4.1 pf 4.2 Gate Resistance f = 1.0MHz, open drain R g Ω Total Gate Charge ( Q g ) Gate charge plays prominently in high frequency switching applications. Total gate charge Q g include Q gs and Q gd. Q gs represents the accumulation of Gate-Source capacitance while Q gd is the accumulation of Gate-rain capacitance, also called miller capacitor. In high frequency operations, Q g should be selected as small as possible. Another tip of Gate charge for picking up a MOSFET in H- bridge power system design is that ratio of Q gd /Q gs be lower than 1 to prevent the circuit from shoot through. 4.2 Capacitances ( C iss, C oss, C rss ) C iss, C oss, and C rss, like gate charge, also influence on switching performance. In TSC MOSFET atasheet, either one would be tested at various rain-source voltages and under 1MHz frequency conditions. Relationships of MOSFET capacitances are listed below. C iss = C gs +C gd C oss = C ds +C gd C rss = C gd. 4.3 Gate Resistance ( R g ) R g is designed and implemented inside gate area. And with the help of MOSFET embedded Rg, the external gate drive circuit could be simplified. 10 Version: A1611

12 5. Switching time (Note 4) Switching Turn-On elay Time t d(on) V GS = 10V, V S = 15V, Turn-On Rise Time t r I = 7A, R G = 10Ω, Turn-Off elay Time t d(off) Turn-Off Fall Time t f ns Switching Time ( t d(on), t r, t d(off), t f ) Switching time includes t d(on), t r, t d(off), and t f four main parameters and also represent important impact on switching loss of MOSFET. Each one is described as below. t d(on) Turn-on elay Time The turn-on delay time is defined as the time interval measured between 10% of V GS rising from zero and 90% of V S falling from rated voltage. t r Rise Time t r represents the time interval between V S falling from 90% to 10% of rated voltage. I starts to rise and is considered to be the major turn-on losses during this period. t d(off) Turn-off elay Time The turn-off delay time is defined as the time interval measured between 10% of V S rising from zero and 90% of V GS falling from rated voltage. t f Fall Time - t f represents the time interval between V S rising from 10% to 90% of rated voltage. I starts to fall and is considered to be the major turn-off losses during this period. VS 90% VGS 10% td(on) tr td(off) tf 11 Version: A1611

13 6. Source-rain iode Characteristics Source-rain iode Forward Voltage (Note 3) V GS = 0V, I S = 11A V S V 6.1 Reverse Recovery Time I S = 11A, t rr ns Reverse Recovery Charge di/dt = 100A/μs Q rr nc 6.1 Forward Voltage ( VS ) To measure Source-rain voltage, VS, at first, short Gate pin and Source pin. Apply a specified reverse current, I S, and V S is measured. 6.2 S VS - V + IS The V S is measured by applying I S current. Gate pin is shorted to Source pin. 6.2 Body iode Reverse Recovery ( t rr, Q rr ) To measure reverse recovery time, t rr and reverse recovery charge, Q rr, at first, short Gate pin and Source pin of UT MOSFET. Moreover, a gate drive circuit, a given voltage source, V dd, and an inductor are required to form a diode recovery test circuit. When MOSFET of gate drive circuit is turned on, L is charged; when it is turned off, energy of inductor will be discharged through intrinsic body diode of UT MOSFET. When MOSFET of gate drive circuit is turned on again, the body diode of UT will be reversed biased and reverse recovery starts to behave. t rr and Q rr are measured. UT G S Gate Signal Gate Signal + - river VS + - UT UT IS VS di/dt trr Qrr t rr and Q rr are measured through above test circuit. Short Gate pin and Source pin of UT MOSFET In applications, when I S reaches back to zero, the voltage spike happens on rain-source due to inductor energy released to C oss of MOSFET. Higher Q rr would cause severe voltage spike and vice versa. 12 Version: A1611

14 7. Characteristics Curves (T A = 25 C unless otherwise noted) Output Characteristics Ohmic Region VS =VGS-VGS(TH) Linear Region Cutoff Region In output characteristics, three major regions are introduced, Ohmic region, Linear region, and Cut off region. The figure also shows curve defined by I vs V S at various gate to source voltage conditions. When MOSFET is turned on with V GS < V GS(TH), it is operated in cut off region, when V S >V GS -V GS(TH), it is operated at linear region. Finally, when V S < V GS -V GS(TH), it is operated in Ohmic region. The blue dash line is defined by VS = VGS VGS(TH). Transfer Characteristics ZTC Negative Temperature Coefficient Positive Temperature Coefficient The transfer characteristics curve represents both V GS and I relationship. From above, there is a ZTC point (Zero Temperature Coefficient). If V GS is under ZTC point, V GS becomes a negative temperature coefficient and vice versa. 13 Version: A1611

15 Gate-Source Voltage vs. Gate Charge Qg Taiwan Semiconductor Qgd Qgs Gate charge include 3 specified values, Qgs, Qgd,and Qg. Qgs denotes Gate to Source charge. Qgd denotes Gate to rain charge, which is also called the miller effect charge. Qg denotes the total gate charge. VGS VP VGS(TH) VS I T0 T1 T2 T3 T4 Operations of Gate Charge are presented as below from above diagram. 1. T0 ~ T1 When V GS rises and reaches to V GS(TH ), C GS is the capacitor to be charged. At the time, both I and V S present unchanged. 2. T1 ~ T2 At this time interval, MOSFET is working in linear region. V GS continues stepping up to the plateau voltage, V P. C GS is completely charged and I is raised up to full load current. 3. T2 ~ T3 At this time interval, MOSFET is still working in linear region. And, V S voltage starts to decrease and finally drops to zero. So the C G * (dv S /dt) becomes big and thus, creates current flowing through C G, which blocks V GS keeps rising and maintains constant. uring this time, R S(on) changes from high impedance to low impedance, entering Ohmic region. 4. T3 ~ T4 In this timing, V GS is raised up to rated value and MOSFET is operated in Ohmic region. 14 Version: A1611

16 On-Resistance vs. Junction Temperature 1.85 Taiwan Semiconductor From the curve of R S(on) vs. T J, it s seen the normalized R S(on) is 1 at T J =25 C, and 1.85 at T J =150 C. This indicates if the RS(on) is 12mohm at T J =25 C, it is expected to be 22.2 mohm at T J =150 C. K =( K C / K - 25 C ) = 1.85 / 1 => 1.85 R S(on) = 12mohm R S(on) C = ( R S(0n) - 25 C x K ) = 12mohm * 1.85 => 22.2 mohm 15 Version: A1611

17 Source-rain iode Forward Current vs. Voltage From above curve, it is easily seen VS has negative temperature coefficient. So, it is expected to obtain lower V S voltage at higher temperature. From curve remarked, V S is 0.5V at T J =150 C; V S is 0.72V at T J =25 C; V S is 0.85V at T J =-55 C. BV SS vs. Junction Temperature 1.09 Above curve is BVSS vs. T J. BV SS is positive temperature coefficient. Based on above curve, It is seen the normalized BV SS is 1 at T J =25 C, and 1.09 at T J =150 C. That is, if the BVSS is 30V at T J =25 C, it is expected BV SS is 32.7V at T J =150 C. K =( K C / K - 25 C ) = 1.09 / 1 => 1.09 BV SS = 30V BV SS C = ( BV SS - 25 C x K ) = 30V * 1.09 => 32.7V 16 Version: A1611

18 Capacitance vs. rain-source Voltage MOSFET have three types capacitances, Ciss, Coss and Crss. Regarding to the relationship of these capacitances, VS voltage and frequency, it is introduced from above curve with a fixed frequency to observe to variations of C iss, C oss, and C rss. These three capacitances will affect the switching loss of power system. Below shows defined three types capacitances. rain () Cgd Gate (G) Cds Cgs Source (S) MOSFET Equivalent Circuit 17 Version: A1611

19 SOA Line 2 Line 1 Line 3 Line 4 SOA is defined the maximum value of V S, I and time envelope of operation which guarantees safe operation when MOSFET works is actively biased. Four lines are used to determine the maximum operating limits. Line 1 R S(on) limits. For example, take TSC 30V MOSFET as example, whose maximum R S(on) is 11.7 mohm. According to the ohm s law, when V S is 1.0V and R S(on) is 11.7 mohm. I = 1.0V / 11.7mohm = 85.47A. Line 2 The I M is defined by Pulsed rain Current of datasheet. It is based on the C 25 C value and generally speaking, I M = 4 I (@T C 25 C) I R JC Line 3 This line is limited by the breakdown voltage. TJ TC K R C Line 4 This line is limited by the a lot of parameters, including T J, T C, P, R θjc, Z θjc, and K factor (R S(on) vs. T J ). The normalized effective transient thermal impedance (Z θjc ) depends on different square wave pulse duration(t) then got the both P and I from below equation. For example, take TSC 30V MOSFET as example: V S = 30V, R θjc is 3.8 C/W, Z θjc is 0.31@1mS 18 Version: A1611

20 P T R J JC T Z C JC P I V S P = (150 C - 25 C)/(3.8 C/W x 0.31) = W I = W/30V = 3.54A pulse and 30V condition, suggested operating current will not be exceeding 3.54A. K is 150 C - ( Please refer to spec) R S(on) is 11.7mohm I R JC TJ T K R V P C S ds( on) Z JC I 150 C 25 C I 70.02A@ 1ms 3.8 C / W mohm V S = W / 70.02A = 1.515V ( Corner condition ) Normalized Thermal Transient Impedance, Junction-to-Case 0.31 Above picture provides the normalized effective transient thermal impedance. These curves can be Junction to Ambient or Case based. From above, TSC 30V MOSFET is based on junction to case condition. As the duty cycle and the pulse duration increase, the transient thermal impedance gets close to 1. In the other words, It approaches to steady state thermal resistance. T J T C P M Z JC R JC For example, the TSM120NA03CR run in single pulse condition, the Z θjc based on curve is 0.31 at 1ms. The R θjc is 3.8 C/W and P M is 1.5W. So the junction temperature of TSM120NA03CR at T C =100 C can be got as below list. T J = 100 C + 1.5W x 0.31 x 3.8 C/W = C 19 Version: A1611

21 References 1. TSM120NA03CR V Power MOSFET atasheet 2. Power FETs and their applications Edwin S. Oxner 3. POWER MOSFETS Theory and Applications uncan A. Grant / John Gowar Authors HL Lin : Product Application Manager : hl_lin@mail.ts.com.tw Childs Chung : Product Marketing Manager : Childs@mail.ts.com.tw Jerry Chen : Vice President of Technical Marketing : Jerry_chen@mail.ts.com.tw Taiwan Semiconductor Co., Ltd. Headquarters Address: 11F. No. 25 Sec. 3, Beishin Rd, Shindian istrict, New Taipei City, Taiwan R.O.C. Telephone: marketing@mail.ts.com.tw Website: 20 Version: A1611