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Power MOSFET Basics: Understanding Gate Charge and Using It to Assess Switching Performance

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Power MOSFET Basics: Understanding Gate Charge and Using It to Assess Switching Performance VISHAY SILICONIX www.vishay.com Power MOSFETs Device Application Note AN608A Power MOSFET Basics: Understanding Gate Charge and Using it to Assess Switching Performance INTRODUCTION This note is part of a series of application notes that define the fundamental behavior of MOSFETs, both as standalone VDS devices and as switching devices implemented in a Switch Cgd Mode Power Supply (SMPS). Vishay Application Note Igd AN-605 [1] provides a basic description of the MOSFET and Ig Rg the terminology behind the device, including definitions and Vgs physical structure. AN-850 [2] provides a broad, physical VGS I description of the switching process. This application note gs goes into more detail on the switching behavior of the Cgs MOSFET when used in a practical application circuit and attempts to enable the reader / designer to choose the right device for the application using the minimum available information from the datasheet. The note goes through Fig. 1 - An Equivalent MOSFET Gate Circuit Showing Just C , C , and R several methods of assessing the switching performance of gs gd g the MOSFET and compares these methods against practical results. Several definitions used within the text are VGS- Vgs drawn from application note AN-605. Ig = ------------------------ (1) Rg SWITCHING THE MOSFET IN ISOLATION I = I + I (2) Using Capacitance g gs gd To get a fundamental understanding of the switching dV I = C x ------------gs- (3) behavior of a MOSFET, it is best first to consider the device gs gs dt in isolation and without any external influences. Under these conditions, an equivalent circuit of the MOSFET gate is Since VDS is fixed illustrated in Fig. 1, where the gate consists of an internal dV– V gate resistance (R ), and two input capacitors (C and C ). gs DS g gs gd Igd = Cgd x ---------------------------------- With this simple equivalent circuit it is possible to obtain the dt output voltage response for a step gate voltage. dVgs = Cgd x ------------- (4) The voltage VGS is the actual voltage at the gate of the dt device, and it is this point that should be considered when V - V I ==------------------------GS gs I + I analyzing the switching behavior. Throughout this note g R gs gd upper case subscripts will refer to rated or applied voltages g and currents, while measured or time varying quantities will therefore be designated by lower case subscripts. For example the APPLICATION NOTE dV dV variable gate drain capacitance is designated as Cgd while gs gs Ig = Cgs------------- + Cgd------------- its corresponding charge will be designated as QGD. dt dt If a step input is applied at VGS, then the following holds true: dVgs = C + C ------------- (5) gs gd dt and dVgs dt -------------------------- = ----------------------------------------------- (6) VGS - Vgs Rg x Cgs + Cgd Revision: 16-Feb-16 1 Document Number: 73217 For technical questions, contact: [email protected] THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Device Application Note AN608A www.vishay.com Vishay Siliconix Power MOSFET Basics: Understanding Gate Charge and Using it to Assess Switching Performance giving t Ids -In VGS - Vgs = ----------------------------------------------- + K (7) Rg x Cgs + Cgd Vgs -t ---------------------------------------------- Rg x Cgs + Cgd (8) Vgs = VGS - K x e at t = 0 s, VGS = 0 V, solving for K, Vgp VTH -t ---------------------------------------------- Rg x Cgs + Cgd (9) Vgs = VGS1 - e Vds Time t1 t3 We define two parameters RG and Ciss to simplify the t2 equations. RG is the effective total gate resistance defined as the sum of internal gate resistance Rg of the MOSFET and Fig. 2 - Turn-On Transient of the MOSFET any external resistance Rgext that is part of the gate drive circuitry. C is the effective input capacitance of the iss MOSFET as seen by the gate drive circuit. 1 t1 = RGCiss x ln-------------------- (11) VTH 1 - ---------- RG = Rg + Rgext and Ciss = Cgs + Cgd VGS Rewriting equation (9) with effective values of gate 1 t2 = RGCiss x ln-------------------- (12) resistance and capacitance Vgp 1 - ---------- V t GS ------------------- R C V e G iss = 1 - ----------gs and VGS VDS t3 = RGCgd x -------------------------- (13) In most cases the parameter of importance is not the actual VGS - Vgp gate voltage but the time taken to reach it. This gives accurate t1 and t2 when using datasheet values, but the time period t3 is difficult to calculate since Cgd 1 tR= GCiss x ln-------------------- (10) changes with Vds. During t3, gate voltage Vgs is constant at Vgs V and all of the gate current goes to discharge C from 1 - ---------- gp gd VGS VDS to almost zero. The drain source voltage across the MOSFET when conducting full load current is considered While the RC circuit of Fig. 1 is rather simple, when the negligible compared to VDS voltage across the MOSFET MOSFET is considered with additional parasitics, it when it is off. becomes increasingly difficult to manipulate these Using the same principles for turn-off, the formulas for the equations manually. Therefore a method of analyzing a switching transients are given below: APPLICATION NOTE practical circuit is required. If the second order or parasitic V components are ignored, then it is possible to come up with t = R C x ln----------GS 4 G iss (14) analytical solutions for formulas for the turn-on and turn-off Vgp time periods of the MOSFET. These are given in equations V (11) through to (16) and the resulting waveforms are shown t = R C x ----------DS (15) 5 G gd V in Fig. 2 and Fig. 3. These equations are based on those gp developed in [3], VTH is the MOSFET threshold voltage, and V t = R C ln---------gp- (16) Vgp is the gate plateau voltage. 6 G iss VTH Revision: 16-Feb-16 2 Document Number: 73217 For technical questions, contact: [email protected] THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Device Application Note AN608A www.vishay.com Vishay Siliconix Power MOSFET Basics: Understanding Gate Charge and Using it to Assess Switching Performance In this instance, t4 and t6 can be calculated accurately, but it is the formula for t5 which is more difficult to solve, since during this time period VDS will change, causing Cgs to also QG change. Therefore some method is required to calculate t3 VGS and t5 without using the dynamic Cgd. QGD QGS Ids ource Voltage (V) S Miller Vgp V Plateau gs Vds ate G Vgp Gate Charge (nC) Fig. 4 - Sketch Showing Breakdown of Gate Charge VTH The slope of the Miller Plateau is generally shown to have a zero, or a near-zero slope, but this gradient depends on the Time division of drive current between Cgd and Cgs. If the slope is t4 t5 t6 non-zero then some of the drive current is flowing into Cgs. If the slope is zero then all the drive current is flowing into Fig. 3 - Turn-Off Transient of the MOSFET Cgd. This happens if the Cgd x Vgd product increases very quickly and all the drive current is being used to Using Gate Charge to Determine Switching Time accommodate the change in voltage across Cgd. As such, Looking at the gate charge waveform in Fig. 4, QGS is QGD is the charge injected into the gate during the time the defined as the charge from the origin to the start of the Miller device is in the Miller Plateau. Plateau Vgp; QGD is defined as the charge from Vgp to the It should be noted that once the plateau is finished (when end of the plateau; and Q is defined as the charge from the G Vds reaches its on-state value), Cgd becomes constant again origin to the point on the curve at which the driving voltage and the bulk of the current flows into Cgs. The gradient is not V equals the actual gate voltage of the device. [4] GS as steep as it was in the first period t2, because Cgd is much The rise in Vgs during t2 (Fig. 2) is brought about by charging larger and closer in magnitude to that of Cgs. C and C . During this time V does not change and as gs gd ds Combination of Gate Charge and Capacitance to Obtain such C and C stay relatively constant, since they vary as gd ds Switching Times a function of Vds. At this time Cgs is generally larger than Cgd The objective of this note is to use datasheet values to and therefore the majority of drive current flows into Cgs predict the switching times of the MOSFET and hence rather than into Cgd. This current, through Cgd and Cds, depends on the time derivative of the product of the allow the estimation of switching losses. Since it is the time capacitance and its voltage. The gate charge can therefore from the end of t1 to the end of t3 that causes the turn-on loss, it is necessary to obtain this time (Fig. 2). Combining be assumed to be QGS. The next part of the waveform is the Miller Plateau. It is generally accepted that the point at 11 and 12 it is possible to obtain the rise time of the current which the gate charge figure goes into the plateau region (tir = t2 - t1) and because Vds stays constant during this time APPLICATION NOTE coincides with the peak value of the drain current. However, then it is possible to use the specified datasheet value of Ciss the knee in the gate charge actually depends on the product at the appropriate VDS value.
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