Application Note AN 2012-12 V2.0 December 2012 Small Signal OptiMOS™ 606 MOSFET in Low Power DC/DC converters IFAT PMM APS SE DS Pradeep Kumar Tamma Small Signal OptiMOS™ 606 MOSFET in Low Power DC/DC converters Application Note AN 2012-12 V2.0 December 2012 Edition 2012-12-06 Published by Infineon Technologies Austria AG 9500 Villach, Austria © Infineon Technologies Austria AG 2011. All Rights Reserved. Attention please! THE INFORMATION GIVEN IN THIS APPLICATION NOTE IS GIVEN AS A HINT FOR THE IMPLEMENTATION OF THE INFINEON TECHNOLOGIES COMPONENT ONLY AND SHALL NOT BE REGARDED AS ANY DESCRIPTION OR WARRANTY OF A CERTAIN FUNCTIONALITY, CONDITION OR QUALITY OF THE INFINEON TECHNOLOGIES COMPONENT. THE RECIPIENT OF THIS APPLICATION NOTE MUST VERIFY ANY FUNCTION DESCRIBED HEREIN IN THE REAL APPLICATION. INFINEON TECHNOLOGIES HEREBY DISCLAIMS ANY AND ALL WARRANTIES AND LIABILITIES OF ANY KIND (INCLUDING WITHOUT LIMITATION WARRANTIES OF NON-INFRINGEMENT OF INTELLECTUAL PROPERTY RIGHTS OF ANY THIRD PARTY) WITH RESPECT TO ANY AND ALL INFORMATION GIVEN IN THIS APPLICATION NOTE. Information For further information on technology, delivery terms and conditions and prices please contact your nearest Infineon Technologies Office (www.infineon.com). Warnings Due to technical requirements components may contain dangerous substances. For information on the types in question please contact your nearest Infineon Technologies Office. Infineon Technologies Components may only be used in life-support devices or systems with the express written approval of Infineon Technologies, if a failure of such components can reasonably be expected to cause the failure of that life-support device or system, or to affect the safety or effectiveness of that device or system. Life support devices or systems are intended to be implanted in the human body, or to support and/or maintain and sustain and/or protect human life. If they fail, it is reasonable to assume that the health of the user or other persons may be endangered. 2 Small Signal OptiMOS™ 606 MOSFET in Low Power DC/DC converters Application Note AN 2012-12 V2.0 December 2012 Table of contents 1 Introduction .................................................................................................................................................. 4 2 Application Example ................................................................................................................................... 4 2.1 Application Boundary conditions ........................................................................................................ 5 2.2 Application circuit configuration ......................................................................................................... 5 3 Review of Losses in a DC/DC converter ................................................................................................... 6 3.1 MOSFET Switching Losses ............................................................................................................... 6 3.2 MOSFET Gate Losses ....................................................................................................................... 8 3.3 MOSFET Output Losses .................................................................................................................... 8 3.4 MOSFET Conduction Losses ............................................................................................................ 8 4 Calculation of MOSFET Losses in the application................................................................................... 9 5 Comparison and Summary .......................................................................................................................11 3 Small Signal OptiMOS™ 606 MOSFET in Low Power DC/DC converters 1 Application Note AN 2012-12 V2.0 December 2012 Introduction ™ Infineon’s new 60V class Small Signal OptiMOS 606 family will be available in TSOP-6, SOT-89 and SC59 packages. The low Qg and low RDS (on) makes the OptiMOS™ 606 suitable for low power DC/DC converters and cell balancing in Battery Energy Control Modules (BECM). Also the logic level gate enables it to be easily interfaced directly with MCUs / Digital circuits. As all products are qualified to AEC Q101, they are ideally suitable for automotive and high quality demanding applications. This application note illustrates the benefits of BSL606SN (TSOP-6 package) in low power DC/DC applications. The main features of BSL606SN are shown in the table below. Parameter Symbol Conditions Value Unit Continuous drain current ID TA = 25 °C 4.5 Drain-source breakdown voltage V(BR)DSS VGS =0 V, ID = 250 µA 60 - - Gate threshold voltage VGS(th) VDS = 0 V, ID = 15 µA 1.3 1.8 2.3 Drain-source on-state resistance RDS(on) VGS =4.5 V, ID = 3.6A 69 95 mΩ Gate to source charge Qgs 1.9 2.5 nC Gate to drain charge Qgd VDD = 48 V, ID = 4.5 A, VGS = 0 to 5 V 1.0 1.5 Gate charge total Qg 4.1 6.1 A V Table 1: BSL606SN Main Features 2 Application Example A good example of a low power DC/DC converter is an LED power supply. In Automotive, LED lighting is now common. A typical automotive lighting application is the Daytime Running Light (DRL) function such as the one shown in the picture below. Figure 1: DRL function example 4 Small Signal OptiMOS™ 606 MOSFET in Low Power DC/DC converters 2.1 Application Note AN 2012-12 V2.0 December 2012 Application Boundary conditions An example of the application boundary conditions for a DRL function is given below. The sum of the LED forward voltage (DC/DC converter output voltage VOUT) is approximately 25 V. The supply input voltage VIN is specified in the range of 8 V to 16 V. The nominal value is 12 V. The LED current or the output current of the DC/DC converter I OUT should be 400 mA. Boost configuration is used with a switching frequency of around 400 kHz operated in continuous conduction mode. 2.2 Application circuit configuration The figure below illustrates the Boost configuration of a DRL application. Figure 2: Boost circuit configuration A summary of the application boundary conditions, which should be the basis for the calculation of the losses in the application, are presented in the table below Symbol Value Unit Name VIN 12 V Nominal input Voltage VOUT 25 V LED forward voltage IOUT 0.4 A LED Current Fs 400 kHz Switching Frequency ΔVOUT 100 mV Max. ripple voltage on VOUT ΔIL% 20 % Pk-Pk inductor Ripple current. Table 2: Application Boundary conditions 5 Small Signal OptiMOS™ 606 MOSFET in Low Power DC/DC converters 3 Application Note AN 2012-12 V2.0 December 2012 Review of Losses in a DC/DC Converter The efficiency of a DC/DC converter is a measure of the ratio of the output power supplied to the load with respect to the input power. The input power is equal to the load power plus the converter losses. A DC/DC converter has its losses in its control circuit and magnetics, out of which switching losses are the greatest contributor. These losses are briefly discussed in the following sub-sections. 3.1 MOSFET Switching Losses Switching losses occur due to the positive product of current through the MOSFET and voltage across it during switching transition. The switching losses occur twice for every switching period during turn-on and turn-off. The figure below illustrates current and voltage during turn-on. Figure 3: MOSFET turn on waveforms 6 Small Signal OptiMOS™ 606 MOSFET in Low Power DC/DC converters Application Note AN 2012-12 V2.0 December 2012 Using linear approximations of the waveforms, the power loss components for the respective intervals can be estimated. The estimated power components in intervals T2 and T3 are given by: ID Fs 2 (1) VDS I D Fs 2 (2) P2 T2 VDS P3 T3 The total turn-on switching losses are the sum of the two components and are given by: 1 1 Pon P2 P3 T2 T3 VDS I D Fs TSW (on) VDS I D Fs 2 2 (3) Where TSW (on) is the turn-on time for the MOSFET it depends upon the gate drive voltage. The average gate drive currents during T2 and T3, IG, T2 and IG, T3 are given by: I G,T 2 Vcc 0.5 V pl Vth (4) Rtot I G,T 3 Vcc V pl (5) Rtot Vcc = gate driver power supply. Rtot = total gate resistance. Assuming that Ig, T2 charges the input capacitors of the MOSFET from Vth to Vpl and Ig,T3 is the discharge current of the gate to drain capacitor while the drain voltage changes from VDS to 0 then the approximate T2 and T3 are given by: T2 Ciss V pl Vth T3 Crss (6) I G ,T 2 (7) VDS I G ,T 3 The turn-on time, TSW (on) is the sum of T2 and T3 and is given by: TSW (on) Ciss V pl Vth Crss VDS T2 T3 I G ,T 2 I G ,T 3 (8) Similarly the turn-off switching losses can be estimated and are given by: (9) 1 Poff TSW ( off ) VDS I D Fs 2 Thus the total switching losses are the sum of turn-on losses and turn-off losses. 7 Small Signal OptiMOS™ 606 MOSFET in Low Power DC/DC converters Psw Pon Poff Application Note AN 2012-12 V2.0 December 2012 1 TSW ( on) TSW ( off ) VDS I D Fs 2 (10) Where, TSW (off) is the turn-off time for MOSFET. 3.2 MOSFET Gate Losses Energy is required in order to charge and discharge the gate capacitances of the MOSFET for each switching period. This energy is usually dissipated through gate series resistance in the gate driver circuit. Thus the gate losses are given by: Pg Qg Vcc Fs (11) QG = total gate charge, Vcc = gate driver power supply voltage and Fs = switching frequency of the converter. 3.3 MOSFET Output Losses The output losses are the energy losses when the MOSFET output drain to source capacitance is discharging during turn-on. Thus the output losses are given by: (12) 2 Pout 1 CDS VDS Fs 2 CDS = MOSFET output Drain to Source capacitance. 3.4 MOSFET Conduction Losses In the on-state, MOSFETs do not behave like an ideal switch with zero impedance, but instead they have a small resistance typically called RDS (on). Due to this resistance there will be power losses in the MOSFET and these are calculated by: Pcond RDS ( on) I D,RMS (13) 2 Ids, RMS = RMS value of the current through the MOSFET. The RMS values can be calculated by the current waveform in the MOSFET. When the converter is operating in continuous conduction mode the MOSFET current wave form will be as shown in the figure below. In the figure, IL, avg is the average current in the inductor, Ts is the switching period, D is the duty ratio and ΔIL is the ripple current in the inductor 8 Small Signal OptiMOS™ 606 MOSFET in Low Power DC/DC converters Application Note AN 2012-12 V2.0 December 2012 Figure 4: Current through MOSFET When the MOSFET is turned on, the current within it is equal to the current in the inductor. Thus the RMS value of the MOSFET current is given by: I D ,RMS (14) (I L ) 2 2 D. I L ,avg 12 So the total MOSFET losses will be the sum of all losses PMOSFET Psw Pg Pout Pcond 4 (15) Calculation of MOSFET Losses in the Application From the above section, BSL606SN losses can be estimated using the specified application values from Table 2. Assuming the MOSFET is driven by a 5 V gate drive power supply with 10 Ohms of total gate resistance and the input voltage is 8 V the following is the case. The switching losses of BSL606SN are: Psw Psw 1 TSW ( on) TSW ( off ) VDS I D Fs 2 1 4.5 10 9 s 0.15 10 9 s 25V 0.4 A 400 10 3 Hz 0.009W 2 9 (16) Small Signal OptiMOS™ 606 MOSFET in Low Power DC/DC converters Application Note AN 2012-12 V2.0 December 2012 TSW (on) and TSW (off) are calculated as discussed in section 3.1. The gate losses of BSL606SN are: Pg Qg Vcc Fs Pg 3.8 10 9 C 5V 400 103 Hz 0.008W (17) The output losses of BSL606SN are: 2 2 Pout 1 CDS VDS Fs 1 Coss Crss VDS Fs 2 2 1 1 2 2 Pout 180 10 12 1110 12 25V 400 10 3 Hz 169 10 12 F 25V 400 10 3 Hz 0.02W 2 2 The conduction losses of BSL606SN are: P R I cond DS (on) DS , RMS I DS , RMS 2 2 2 I L 0.24 A 2 2 D. I L ,avg 0.68 1.25 A 12 12 For the boost converter the duty ratio D VOUT VIN VOUT 1.03 A and the average current through the inductor is equal to the average input current for the converter and is given by I IN ,avg I L ,avg I OUT and the 1 D inductor ripple current ΔIL is given in table 2. 2 P 0.066 1.03 A 0.07W cond (19) 10 (18) Small Signal OptiMOS™ 606 MOSFET in Low Power DC/DC converters Application Note AN 2012-12 V2.0 December 2012 Thus the total losses of BSL606SN are summarized in the table below BSL606SN Conduction Losses Pcond 70 mW Switching Losses Gate Losses Output Losses Total MOSFET losses Psw Pg Pout 09 08 20 108 mW mW mW mW PMOSFET Table 3: BSL606SN losses in application 5 Comparison and Summary BSL606SN losses are compared with an automotive qualified competitor part of the same voltage class. The figure below illustrates the different losses in the respective device. Total Losses 180 160 Losses (mW) 140 120 Poss 100 Pdrv 80 60 Psw 40 Pcon 20 0 BSL606SN competitor part Figure 5: Loss comparison The chart shows that there are significantly less losses in the converter when BSL606SN is used in comparison to that of the competitor part. The benefits of using BSL606SN in low power DC/DC converters can be summarised as follows: The gate losses are comparitively very small in BSL606SN. This means that the gate requires little current even at logic level to turn it on. This makes it easy to interface to MCUs/ digital circuits. The switching losses are very small with BSL606SN. This gives the flexibility of increasing the switching frequency which in turn increases the transient performance. With the increase in switching frequency the inductance can be decreased which in turn decreases the total converter size and cost. 11

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