Application Note: AN-400 INTEGRATED CIRCUITS DIVISION IX2127 Design Considerations AN-400-R03 www.ixysic.com 1 AN-400 INTEGRATED CIRCUITS DIVISION 1 High-Side Gate Driver Using the IX2127 This application note provides general guidelines for designing a desaturation detection circuit, and selecting bootstrap components for use with IXYS Integrated Circuits Division’s IX2127. The IX2127 is a high-voltage, high-speed driver optimized to drive IXYS high voltage power MOSFETs and IGBTs. The IXYS high voltage SOI process and the IX2127 high voltage level shifters allow this driver to Figure 1 VCC operate at up to 600V. In addition, proprietary common mode techniques insure stable operation in high dV/dt noise environments. The IX2127 features an on-board comparator that detects an over-current condition in the external power switch, and then terminates the drive to that switch. An open drain output, FAULT, indicates to the system controller that an over-current shutdown has occurred. IX2127 Block Diagram Low Side VCC VB High Side Undervoltage Lockout HO Buffer Data Latch Transmitter IN Low-High Level Shift Receiver VS Enable Blanking Signal Delay FAULT Q Enable R Receiver COM S High-Low Level Shift Transmitter + Data Latch CS _ Comparator 2 www.ixysic.com R03 AN-400 INTEGRATED CIRCUITS DIVISION Figure 2 Application Circuit Diagram VCC IN FAULT 2 Rbs Dbs 1 VCC VB 8 2 IN HO 7 3 FAULT CS 6 4 COM VS 5 Cbs Simple Bootstrap Circuit Operation The bootstrap circuit is a simple and inexpensive way to provide power to the high side driver circuitry. It consists of resistor, diode and capacitor. The sequence of bootstrap charging is as follows. When Vs is pulled below VCC or is pulled down to ground by the load, the bootstrap capacitor begins to charge through the resistor and bootstrap diode from the VCC supply. This charge continues until VS is pulled up to a higher voltage than VCC by the external high side power MOSFET. VBS (the difference voltage between VB and VS) starts to float, and the bootstrap diode begins to reverse bias and block the high rail voltage. 3 Bootstrap Capacitor The bootstrap capacitor is selected based on the maximum allowable voltage drop when the high side switch is on. This voltage is determined by the power MOSFET’s minimum VGS voltage which is required to keep the high-side switch turned on, a parameter usually given in the MOSFET datasheet. The capacitor voltage drop can be expressed by: V boot V CC – V fbs + V GS min Where: • VCC = Supply voltage • Vfbs = Bootstrap Diode Forward Voltage Drop • VGS(min) = Minimum Gate to Source Voltage of Power MOSFET R03 The minimum value of the bootstrap capacitor is given by the formula: C bs = Q total V boot Where Qtotal is the total amount of charge supplied by the capacitor. The total charge can be expressed as follows: Q total = Q g + I lkcap + I lkgs + I qbs + I lk + I Cbs leak t on + Q ls Where: • Qg = Total gate charge of power MOSFET or IGBT • Ilkgs = MOSFET gate to source leakage current (given in MOSFET datasheet as IGSS) • Ilkcap = Bootstrap capacitor leakage current (can be ignored if a ceramic capacitor is used) • Iqbs = Bootstrap circuit quiescent current • Ilk = Bootstrap circuit leakage current • Qls = Charge of IX2127 internal level shifter (5nC) • ton = High-side switch-on time • ICbs(leak) = Bootstrap leakage current The bootstrap capacitor voltage is determined by the VCC power supply, and in this case a 25V to 50V capacitor should be adequate. Note that the leakage current in a ceramic capacitor is very low, and can be ignored in most calculations. The use of electrolytic capacitors is strongly discouraged due to their contribution of excessive leakage currents at higher temperatures. www.ixysic.com 3 AN-400 INTEGRATED CIRCUITS DIVISION 3.1 • • • • • • Example: Calculate the bootstrap capacitor value based on the following circuit components, an operating frequency of 20 kHz, and at a Duty Cycle of 50%: • • • • • IXTA5N60P - Power MOSFET MURS160-13-F- Bootstrap Diode VCC = 12V Qgate = 14.2nC maximum Ilkgs = 100nA maximum –9 Ilk cap = 0 Iqbs = 1000uA Ilk = 50uA Qls = 5nC ton = 25uS ICbs(leak) = 10nA If the maximum allowable voltage drop on the capacitor is 1V during the high-side switch-on state, then the minimum capacitor value can be calculated: –9 Q total = 14.2 10 + 100 10 –3 –6 + 1 10 + 50 10 = 31.25nC –9 –9 For instance, if Rbs = 3, Cbs = 0.1uF, and D = 10%, then the time constant can be evaluated: The value of the bootstrap capacitor is: Cbs = Qtotal / Vboot = 31.25nC / 1V = 31 nF = 3 0.1 0.1 = 3s Note that the voltage drop of the bootstrap diode contributes about 0.8V to the total voltage drop The value obtained from the above equation is the absolute minimum required. In some cases, though, a low value capacitor can cause overcharging and ripple on Vbs so the value obtained from the equation should be evaluated in an application circuit, and increased if required. 4 –6 + 10 10 25 10 + 5 10 Bootstrap Resistor The bootstrap resistor, Rbs , can be used to control charging current into the bootstrap capacitor. The value of this resistor has to be selected very carefully so that the bootstrap capacitor is able to charge fully under all operating conditions. This resistor will introduce a voltage drop that can be approximated with this formula: V drop = I chg – R bs t chg 5 Bootstrap Diode The bootstrap diode provides a voltage-charging path for the bootstrap capacitor, and also serves to block the full power rail voltage. This diode must have a very fast recovery time in order to minimize the amount of charge injected back from the bootstrap capacitor to VCC. Here are some parameters that should be looked at when selecting this diode: • Vrrm = Power Rail Voltage • trr = 100nS or less • If = Qtotal x Frequency Note that reverse recovery time (trr) is a function of the forward diode current. 6 Desaturation Detection Circuit The voltage across the MOSFET will increase during an over-current condition; hence, we can use a basic circuit consisting of R1, D2, R2 and R3 to detect this overload condition. Where, • Ichg = Bootstrap capacitor charging current • Rbs = Bootstrap resistor • tchg =Bootstrap capacitor charging time In practice Rbs will be a small value of up to 10. Let's look closer at the time constant that depends on Rbs , Cbs , and the duty cycle, D, of the switching power switch. R1 is selected to be 10k. This value minimizes Miller capacitance from D2, and ensures that there is no significant current being drawn from the high-side HO output pin. Due to the presence of the high voltage rail, D2 can be the same as DBS. = R bs C bs D 4 www.ixysic.com R03 AN-400 INTEGRATED CIRCUITS DIVISION 6.1 Example: Let R2=33k, and Solve for Vx: VX = VA R3 R2 + R3 Calculate component values in the event of an 8V overload condition (use the circuit in Figure 3): 0.26V = 9.2V R 3 33k + R 3 This calculation is based on a D2 maximum voltage drop of 1.2V. The voltage at VA can be expressed as: 0.26V 33k + R 3 = 9.2V R 3 VA = Vdiode + VDS 8580 + 0.25V R 3 = 9.2V R 3 VA = 1.2V + 8V 8580 = 8.94 R 3 R 3 = 960 VA = 9.2V The CS threshold is 260mV; hence, we need to divide VA so that when VA=9.2V, then VX=260mV. Figure 3 Saturation Detection Circuit R D bs bs V A IX2127 1 VCC V B R 2 8 C PWM 2 IN HO 7 µController 3 FAULT CS 6 4 COM V S 5 R HV BUS 2 1 bs V R g X R 7 D 3 To Load Gate Resistor Rg is the gate resistor, which is chosen to optimize switching speed and losses. Let's select a gate resistor that will provide100nS switching time at 12VCC . In this example case, the IXTA5N60P MOSFET is selected. From the IXTA5N60P and IX2127 datasheets we get the following parameters: • • • • • R total = V CC – V gs th I gate avg R total = 12V CC – 5.5V gs th 0.096A = 68 The driver-on resistance can be approximated: R driver on = V CC I OH+ VCC=12V Qgs=4.8nC Qgd=4.8nC Vgs(th)=5.5V tSW=100ns Typical IOH+ is listed as 250mA in the IX2127 data sheet. R driver on = 12V CC 250mA = 48 We define average gate current as: I gate avg = Q gs + Q gd t SW I gate avg = 4.8nC + 4.8nC 100ns = 0.096A R03 The total resistance can be expressed: The difference of Rtotal and Rdriver(on) will yield a 20 resistor that will provide 100ns switching time at 12VCC. www.ixysic.com 5 AN-400 INTEGRATED CIRCUITS DIVISION 8 Layout Considerations Proper layout techniques are important when designing high-speed gate drivers. The decoupling capacitors, the bootstrap resistor, and the gate resistor, Rg, should be close to the gate driver. For additional information please visit our website at: www.ixysic.com IXYS Integrated Circuits Division makes no representations or warranties with respect to the accuracy or completeness of the contents of this publication and reserves the right to make changes to specifications and product descriptions at any time without notice. Neither circuit patent licenses nor indemnity are expressed or implied. Except as set forth in IXYS Integrated Circuits Division’s Standard Terms and Conditions of Sale, IXYS Integrated Circuits Division assumes no liability whatsoever, and disclaims any express or implied warranty, relating to its products including, but not limited to, the implied warranty of merchantability, fitness for a particular purpose, or infringement of any intellectual property right. The products described in this document are not designed, intended, authorized or warranted for use as components in systems intended for surgical implant into the body, or in other applications intended to support or sustain life, or where malfunction of IXYS Integrated Circuits Division’s product may result in direct physical harm, injury, or death to a person or severe property or environmental damage. IXYS Integrated Circuits Division reserves the right to discontinue or make changes to its products at any time without notice. Specification: AN-400-R03 ©Copyright 2013, IXYS Integrated Circuits Division All rights reserved. Printed in USA. 6/24/2013 6 www.ixysic.com R03

- Similar pages
- AN-301: CPC9909 Design Considerations
- AN-300 MXHV9910.fm
- MIC4605 - Micrel
- MICREL MIC4102BM
- MICREL MIC4101BM
- FAIRCHILD FAN5193
- FAIRCHILD AN-6076
- dm00071167
- ZARLINK WL800/KG/TP1R
- MITEL WL800KGTP1R
- IX2127
- CLARE IX2127NTR
- CLARE IX2127N
- MAXIM MAX4981ETA
- AN 447: Interfacing Altera Devices with 3.3/3.0/2.5 V LVTTL
- ADDTEK AMC7140
- PHILIPS TEA1792ATS
- NEC UPA1458