www.mosaic-industries.com

1 1 MOTOROLAAN1542 Active Inrush Current Limiting Using MOSFETsPrepared by: C. S. MitterMotorola filter design has been an integral part of power supplydesigns. With the advent of input filters, the designer musttake into consideration how to control the high inrush currentdue to rapid rise of voltage during the initial application ofpower to the power supply. Depending on the input busvoltage level and the output power required by the load, thesupply designer must also design the inductor (if used) tosupport the DC current without saturating the core. Theinductor and capacitor is designed to meet EMI initial inrush current with inductor can become verylarge in size and weight, and in most cases size and weight isa crucial requirement to the this section, a review of various active and passivemethods of inrush limiting techniques are presented.

2 It isshown that a new and innovative method can be applied usinga single MOSFET and a minimal number of components inmany of the circuits requiring dv/dt control in order to limit thehigh current spikes. Its design methods and simple yeteffective equations are also presented. A variety ofapplications of this dv/dt control circuit into other areas areproposed. The simplicity and the advantage of this technique,as opposed to other techniques, is shown given itseffectiveness in different applications requiring dv/dt new inrush limiting is beneficial because dv/dt controlreduces the EMI due to current and voltage spikes, and thelifetime of capacitors and the semiconductor devicessurrounding the circuitry is increased.

3 This technique will alsoincrease the reliability of the devices and the capacitors. Andbecause of its minimal parts count, the design is very power supply designs, the input filter design is an integralpart of the design. In most designs input filter designsincorporate both inductor and capacitors. The inductor andcapacitors need to behave in such way as to provide EMIreduction and provide supply hold up requirement in case ofshort duration line dropout. This requirement along withderating of the capacitors for temperature variations results inhaving to use large filter innovations in technology, the process formanufacturing of capacitors allows for very low equivalentseries resistance (ESR), and thus they behave like nearlyperfect short circuits during initial power application to thepower supply [1].

4 The initial power applied to the power supplyposses a very high dv/dt. This high dv/dt interacting with thefilter capacitors will introduce short duration of high peakcurrent which can exceed far beyond the device ratings(semiconductor devices, fuses, circuit breakers), and canseriously damage or destroy the semiconductor devices, burnout the fuses or false trigger the circuit breakers. The high rateof rise of the voltage and fast rise of the current may activateother circuitry that are dv/dt and di/dt sensitive. This high dv/dtand di/dt introduces unwanted EMI is clear that a new methodology of controlling dv/dt withoutaffecting the inductor size and power supply efficiency neededto be INRUSH LIMITING TECHNIQUEST raditionally, most of the inrush current limiting is done byusing a large oversized inductor, or resistors in series with thecapacitors.

5 These techniques do not optimally utilize thesurface area, weight and power dissipation. In applicationswhere large DC current is required at the input of the powersupply, the inductor not only has to be designed for low EMI,but it needs to be designed to meet DC current capabilitywithout degrading the inductance value. Reduction ininductance will mean that the EMI noise attenuation capabilityis reduced. Therefore, the design of inductance becomes verylarge because with increase in operating current, the corebecomes larger. If a series resistor is used, unnecessarypower is lost because of I2xR.

6 This in turn degrades the powersupply efficiency. In order to overcome the power dissipationof the series resistor, many designers incorporate a parallelswitch with a resistor (semiconductor devices or relays).Depending on the operating current, the relay can becomeexcessively large and heavy. In addition, control circuit mustbe implemented in order to control the turn on and turn off ofthe relay. In the cases where semiconductors are used, suchas an SCR, the device can become very bulky and application also requires unique control circuitry in ordercontrol the SCR turn on and turn method of inrush current limiting is done using anNTC thermistor.

7 This device has a negative temperaturecoefficient and its resistance decreases as current is passedthrough the device (current flow increases the temperature ofthe device and decreases the resistance) [2]. The drawbackof this device is that it requires a cool off time after the poweris removed in order to reset to high resistive mode. Dependingon the power rating of these devices, its physical sizebecomes significant. The cool off time can be overcome byusing an active device along with the NTC device. But thisdefeats the purpose of using the NTC device in first place; thatis, the simplicity of application and minimal parts this documentby AN1542/DMOTOROLASEMICONDUCTOR APPLICATION NOTE Motorola, Inc.

8 1995AN1542 2 MOTOROLAVdcRTNLCDC DCPOWERCONVERTERF igure 1. InductiveAC INPUTD1D4D2D3RC+DC DCPOWERCONVERTERCONTROLCIRCUITSCRF igure 2. Passive and ActiveAC INPUTD1D4D2D3C+DC DCPOWERCONVERTERF igure 3. NTC ThermistorNTCNTCINTRODUCTION TO ACTIVE CURRENT LIMITERIn low to medium power levels which require few hundredvolts of blocking capability, MOSFETs are an ideal devicesbecause they posses following characteristics: 1) fastswitching time due to majority carrier devices, 3) lowerswitching loss due to fast rise and fall times, 2) simple gatedrive, 3) and low RDS(on) which helps to increase the efficiencyby decreasing the voltage drop across the device duringsteady state conduction.

9 Because the active current limiting isdone by using MOSFET devices, it is essential that onecomprehend the switching characteristics of this devices. Byunderstanding the switching characteristics, the designengineer will be better equipped to use the proposed circuitwithout any Switching CharacteristicsMOSFETs are charge controlled devices and can berepresented with the simplified equivalent circuit shown inFigure 4. The gate source capacitance (Cgs) is largelydependent on gate oxide and source metallizationcapacitance and can be measured and considered drain capacitance (Cgd) consists of the gate drainoverlap oxide capacitance and gate drain overlap depletioncapacitance.

10 This capacitance is nonlinear due to its voltagedependency. Drain source capacitance is the depletioncapacitance of the drain source junction. The followingexpressions can be obtained from the circuit shown inFigure 4. Equivalent Circuit for MOSFETI drainCiss = Cgs + Cgd; Cds shorted(1)Crss = Cgd,(2)Cgs CgdCgs + CgdCoss = Cds +; Cgs shorted(3) Cds + CgdThe expressions shown in equations 1 through 3 areparameters that are available from the MOSFET data sheetsand curves provided therein. Capacitance Ciss is equivalentinput capacitance, Crss is the reverse transfer capacitance,and Coss is the equivalent output capacitance.