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FAIRCHILD Design Guide for Selection of Bootstrap Components

1. EMICONDUCTOR Application Note AN 9052 Design Guide for Selection of Bootstrap Components 1 Bootstrap Circuit 1 1 Bootstrap Floating Supply Using a N channel MOSFET as a high side switch requires a voltage supply referenced at the source of the MOSFET One of the most widely used method in supplying power to the high side circuitry is the use of the bootstrap floating supply due to its inherent simplicity and inexpensive fea tures This kind of floating supply is suitable for providing a gate drive circuitry to directly drive high side switches that operate up to rail voltages The basic circuit of the bootstrap supply shown in Figure 1 is formed by a diode Dbs and a capacitor Cbs But this type of floating supply has limita tions on refreshment of Cbs when duty cycle is very high or turn on time is very long In the case where the gate voltage is not enough to fully turn on the MOSFET Q1 the output of gate drive IC HO should be turned off to prevent the Q1 from operating in high dissipation mode The optional gate resistor Rg is used for the purpose of controlling the turn on turn off time of the Q1 and the bootstrap resistor Rbs is used to limit the current and prevent the bootstrap capacitor Cbs from overcharging figure 1 Bootstrap Circuit 1 2 Operation of Bootstrap Circuit The charged capacitor Cbs supplies the voltage to the tran sistors of the gate drive IC which is used to turn ON and OFF th
2. floating state of node X does not need to be accounted for when calculating the minimum bootstrap capacitor value Cbs Min The internal recharge switch of the FAN7085 pin VS provides a path to charge the bootstrap capacitor when S1 and S2 are turned off 2 2 Selection of Bootstrap Diode The maximum voltage rating should be higher than power rail VS and current rating can be multiplication of total charge and switching frequency A diode with a fast reverse recovery time is beneficial to minimize the leakage current 3 Examples of Bootstrap Capacitor Selection 3 1 Example 1 The basic operation conditions are Gate drive IC FAN7080 Switching device FDB8442 Bootstrap diode MMBD1405 Switching frequency 20KHz System operating voltage Vec 9 16V VS 38V 2008 Fairchild Semiconductor Corporation Rev 1 0 0 11 10 08 APPLICATION NOTE The known values from the datasheets are IQBS 150uA QLS 3nC assumed in fairchild 600V Gate drive IC ILK_Gs 100nA ILK_HS 50uA ILK_D 100nA VF 1 1V LK_C 0 VOP 5 5V is equal to VBSUV Qg 235nC 80A Vgs 10V RDS ON Max 5mohm Rbs and Rvs are not used The voltage drop is calculated as Vg SDrop 7 Voc Ve Vop Vx Vecmin7 LIV Vesuymax7 Rpsonmax our 9 1 5 5 0 4 2 0V The leakage current is calculated as lk Iuxesteusthxpoth c 100nA 50uA 100nA 0 50 2uA The total charged required i
3. Semiconductor Corporation Rev 1 0 0 11 10 08 APPLICATION NOTE 2 Selection of Bootstrap Components 2 1 Selection of Bootstrap Capacitor The maximum allowable voltage drop across the bootstrap capacitor to ensure enough gate source voltage is highly dependent to the internal undervoltage shutdown level of the gate drive IC and the voltage level at the source connection of Q1 node X VBSDrop Vcc Vr Vor Vx 2 Where Vcc gate drive IC supply voltage If static forward voltage drop of Dbs Vop minimum gate source voltage level required that will prevent Q1 to go into a high dissipation mode X MOSFET source connection If the minimum operating voltage VOP requires to be greater than UVLO voltage level then equation 2 can be denoted as follows V V V V OE SS Vx 3 BS Drop BSUVMax Where VBSUV Max is the maximum UVLO voltage level of gate drive IC The total charge Qbs required by the bootstrap capacitor can be calculated by summing the Q1 gate charge charge required for the level shifter in the gate drive IC and leakage charges resulting from leakage current Qgs Qyt Ux logs X Tont QLs 4 Where QBS total charge from Cbs Qg gate charge of Q1 ILK total leakage current JQBS operating current in gate drive IC TON Turning on interval of Q1 QLS level shift charge required per cycle The total leakage current is the summation of all of the indi vidual component s leakage cu
4. UA x 50uSec 3nC 39nC 20nC 3nC 62nC The minimum capacitor value needed to prevent UVLO con dition is calculated as Cre E Oss _ 62nC _ 200nF Beh Vasprop E The minimum capacitor value with a margin taken into ac count when allowable voltage drop is set to VUVBSH ex plained in sec 2 1 is calculated as DISCLAIMER Qes _ 62nC Chs y BSUVH The initial charging time when S1 and S2 are turned off is calculated as teu 2 5 x Rrecharge X Cgs 5 x 5000hm x 3 1uF 7 75mSec Where Rrecharge is the equivalent series resistance of the switch when the switch is turned on FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY FUNCTION OR DESIGN FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS NOR THE RIGHTS OF OTHERS LIFE SUPPORT POLICY FAIRCHILD S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION As used herein 1 Life support devices or systems are devices or systems which a are intended for surgical implant into the body or b support or sustain life or c whose failure to perform when properly used in accordance with instructions for use p
5. Vbs will decrease due to leakage current and consequently the gate drive IC will go into UVLO condition Controlling the low side switch properly the bootstrap capacitor can be recharged and maintain the voltage level needed by the gate drive IC The gate drive IC FAN7085 block diagram shown in Fig 2 has a built in recharge switch that will charge the bootstrap capacitor regardless of the application If Q1 is turned off and FAN7085 is used as a gate drive IC in the bootstrap circuit shown in Figure 1 the internal recharge switch of the FAN7085 will activate to provide the path to charge the bootstrap capacitor Cbs fully Let s look at a specific application of the FAN7085 which is shown in Fig ure 3 In the event that both of the switch S1 and S2 are turned off the internal recharge switch of the FAN7085 will provide the path to charge the bootstrap capacitor If the cur rent level passing through the recharge switch is higher than the leakage current bootstrap capacitor will charge through the recharge path The voltage level on the VS pin of the FAN7085 at a given current level when the recharge switch is turned on is defined on the datasheet VB Pulse Filter T Under l4 Voltage Reset j gt p pup EIOP o HO VB to VS eee E Voltage TA ig Reset VCC to GND Ci vs CO Lz iz Level Shifter LZ Logic ON EZ Filter Level Shifter OFF Fecharge Path 7 Figure 3 Application example of FAN7085 2008 Fairchild
6. e external high side switch Q1 The bootstrap capacitor Cbs gets charged from the voltage supply VCC through the bootstrap diode Dbs when the voltage at node X VX is pulled down to ground or even below ground level The bootstrap capacitor needs to be sized properly to 2008 Fairchild Semiconductor Corporation Rev 1 0 0 11 10 08 www fairchildsemi com account for the case when Vx is pulled down to ground which Vbs is at its lowest level and cause under voltage lockout UVLO malfunction Most gate drive ICs have und ervoltage detection circuit that prevents from driving an external switch when Vbs drops below a certain level speci fied in datasheets as VBSUV level The WsUvV level depends on the external switch that it is driving The under voltage level for IGBTs are in the 9V 10V range and for MOSFETs in the 4V 5V range In the case where the node X goes below the ground level Cbs will be overcharged by the level in which it goes negative There are negative tran sients at node X caused by the parasitic inductances and peak forward voltage drop Vfp of the body diode at the low side switch that needs to be considered also All of the overcharg ing affect mentioned above needs to be taken into account in determining the size of Cbs Adding resistors Rbs Rvs and using a diode with a low Vfp value are other possible solu tions to limit the overcharge effect on Cbs Let us now look at the case that causes the Cbs to d
7. ischarge Cbs discharges when Q1 turns on or node X is floating The associated dis charging factors are gate drive power leakage current in each component and current consumption in the gate drive IC From an application point of view specific conditions such as the duty cycle of PWM that causes ripple voltages on Cbs operation frequency and the type of modulation at which Q1 operates needs to be examined to make sure that Cbs can handle 1 3 Initial Charging and Refreshment of Boot strap Capacitor Another key parameter in selecting bootstrap components is initial start up time The initial charging time ch can be cal culated from the following equation Vig cc saul te 20 XR X2 X In ch bs F D Veg Vosmin ad V v Where RT Rbs Rvs with low side switch and no load RT Rbs Rvs RL with loads including equivalent impedance at node X D duty cycle In the case where PWM is not used the load not connected and the low side switch turned on the charging time at the www fairchildsemi com AN 9052 start up phase can be defined by the time constants Rbs Rvs and Cbs When the load is connected and forms the charge path in the bootstrap circuit the initial charging time is defined by Cbs and the relationship between Rbs Rvs and the load impedance RL Most designs the value of Cbs is picked with some margins which leads to longer star up time If node X is left floating for a long time
8. rovided in the labeling can be reason ably expected to result in significant injury to the user 2008 Fairchild Semiconductor Corporation Rev 1 0 0 11 10 08 2 A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness www fairchildsemi com
9. rrents lLk Inxes tIexust IL kpt hx c 5 Where ILK GS gate leakage current of the MOSFET ILK HS high side floating supply leakage current ILK D bootstrap diode leakage current www fairchildsemi com AN 905 ILK C capacitor leakage current which can be ignored if it is not an electrolytic capacitor The guiding criteria for calculating the minimum required bootstrap capacitance can be obtained through the following equation CbsMin 2 J 6 Equation 6 is a basic equation for calculating the minimum value for the bootstrap capacitor It uses the value of Vbs Drop which is the minimum operating voltage required and does not account for the any margins The margin that is needed is to compensate for the ripple voltage on Vbs by the PWM and the overcharging due to negative transients at node X It implies that Vbs Drop should be changed to a smaller value Vbs Min In regards to the ripple voltage com pensation the value of VBSUVH is assigned to Vbs Min in cal culating the bootstrap capacitor Cosmin 2 7 Where VBSUVH is hysteresis voltage of UVLO in gate drive IC The minimum bootstrap capacitance calculated in Equa tion 7 might need to account for additional margin depend ing on the specific application conditions Conditions at node X such as long floating state and negative transients with deep and long duration needs to be accounted for Using the FAN7085 unlike other gate drive ICs the long
10. s calculated as Qgs Qo t CLK Ips x Tont QLs 235nC 50 2uA 150uA x 50uSec 3nC 235nC 10nC 3nC 248nC The minimum capacitor value needed to prevent UVLO con dition is calculated as QBs _ 248nC CBsMin y soy 124nF BS Drop The minimum capacitor value with a margin taken into ac count explained in sec 2 1 is calculated as QBs _ 248nC BST Vesuvy 0 2V C 1 2uF Depending on the specific operating conditions additional margins needs to be accounted for in calculating the mini mum required capacitor value 3 2 Example 2 The basic operating conditions are Gate drive IC FAN7085 Switching device FDB42AN15A0 Bootstrap diode MMBD1405 Switching frequency 50KHz System operating voltage Vcc 7 16V VS 130V The known values from the datasheets are www fairchildsemi com AN 9052 APPLICATION NOTE IQBS 200uA QLS 3nC ILK_Gs 100nA ILK_HS 200uA ILK_D 100nA VF 1 1V LK_c 0 VOP 4 3V is equal to VBSUV Qg 39nC 12A Vgs 10V RDS ON Max 107mohm Rbs and Rvs are not used The voltage drop is calculated as Vasprop Voe Ye Yop Y V X 11V V R xI CCMin BSUV Max 7 1 1 4 3 1 3 0 3V DSONmax OUT The leakage current is calculated as lk Ixestiikusthx pthc 100nA 200uA 100nA 0 200 2uA The total charged required is calculated as Qgs Qa UK logs X Tont Qrs 39nC 200 2uA 200

FAIRCHILD Design Guide for Selection of Bootstrap Components

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