Inrush Current Control

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Powering Communications and Technology

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Inrush Current ControlApplication Note

Table Of Contents1. INPUT FILTER 12. INRUSH CURRENT 13. INRUSH CURRENT CONTROL 2

3.1 INRUSH CURRENT CONTROL BY A RESISTOR 23.2 INRUSH CURRENT CONTROL BY A THERMISTOR 23.3 ACTIVE INRUSH CURRENT CONTROL 23.4 USING LT1640L FOR INRUSH CURRENT AND HOT SWAP CONTROL 4

1. Input FilterPower-One DC/DC converters have an input filter toreduce magnitude of the ripple current reflectedback to the input voltage source. The input filterusually contains both capacitors and inductor and isconfigured as a filter. See Figure 1 for theschematic of the typical input filter.

2. Inrush CurrentWhen a DC/DC converter is connected to the inputvoltage source, it causes a large peak inrush currentas a result of the application of the high dv/dt to thefilter capacitance. These filter capacitors (bothinternal and external) act like a short circuit,producing an immediate inrush surge current with afast rise time.

A similar phenomenon occurs during hot swapping -inserting and removing boards in systems alreadyconnected to the voltage source.

The peak inrush current can be significantly greaterthan the steady state current. If the inrush current isnot limited, it may burn out fuses, damage connectorpins, cause glitches in the input voltage, andgenerate high di/dt and dv/dt. Therefore, the peakcurrent and current ramp must be controlled.The picture in Figure 2 shows the inrush current of aPower-One's DC/DC converter model HLS30ZE,connected to the 48V source via 1' cable.

Fig. 1 Typical configuration of the input filter

Fig. 2 Inrush current of HLS30ZEThe table below contains parameters of the inputfilters for several series of Power-One DC/DCconverters.

Series C1, F L1, H C2, F HBS/HES 1 0.65 8

HAS 0.22 1 6 HBD 0.5 0.1 7

QBS/QES 0 1 3 TES 1 1 9 TQD 1 1 9 HLS 1 0.5 4 QLS 1 1 2 HLD 1 1 5 TLD 1 1 8

The magnitude of the inrush current depends onmany factors, such as the input voltage, the sourceand supply line impedance, the internal inputinductance and resistance, as well as capacitanceand ESR of the internal input filter of the DC/DCconverter. Some of those parameters depend onparticular system design and layout and are difficultto calculate. The most accurate way to determinethe inrush current is to measure it in the application.

Make sure that the current sensor used to measurethe inrush current does not limit its magnitude.Non-invasive sensors such as Hall sensors arerecommended for the inrush currentmeasurements.

Note that the parameters in the table are nominal and will varydepending on the line and load conditions.

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An addition of a 100F aluminum electrolyticcapacitor significantly increases the inrush currentas can be seen in Figure 3.

Fig. 3 Inrush current of HLS30ZE with an external 100Felectrolytic capacitorThe pictures demonstrate that the inrush currentand, especially, the I2t are determined mostly by theexternal capacitor connected across the input of theDC/DC converter.

3. Inrush Current ControlThere are several possible ways to achieve this.3.1 Inrush current control by a resistorThe schematic in Figure 4 shows the resistor R1which limits the inrush current during pluggingboards in the live system. The resistor is shortedout by the short pins when the board is fully inserted.C1 represents total capacitance of the externalcapacitor and the DC/DC converter's input filter.

Fig. 4 Inrush current limiting by a resistor

The problem with this method is that the insertiontime is not controlled. A very fast insertion may nothave the capacitor C1 fully

charged before the resistor is shorted out. There isalso no inrush current limit when power is applied toa system which has boards already inserted.3.2 Inrush current control by a thermistorThe resistor can be replaced with a NTC thermistorwhich does not need to be shorted out by the shortpins when the board is fully inserted. A thermistor isa thermally sensitive resistor with a resistance thatchanges significantly and predictably as a result oftemperature changes. The resistance of the currentlimiting NTC thermistor decreases as itstemperature increases due to the current flowingthrough the device. As the thermistor self-heats, itsresistance begins to drop and a relatively smallcurrent charges filter capacitors. After thecapacitors become charged, the self heatedthermistor introduces very low resistance in thecircuit.

Because current limiting thermistors heat after theysuppress inrush currents, these devices require acool-down time after power is removed. This cool-down or "recovery" time allows the resistance of thethermistor to increase sufficiently to provide therequired inrush current suppression the next time itis needed. A cool-down time varies according to theparticular device, its mounting method and theambient temperature. The typical cool-down time isroughly one minute. It may be unacceptable insystems requiring high availability.

NTC thermistors are available from a variety ofmanufacturers, namely Ametherm, RTI Electronics,Thermometrics, and others.

3.3 Active inrush current control3.3.1 Using MOSFETs for inrush current controlThe limitations of passive methods can beovercome by utilizing MOSFET devices for inrushcurrent control. MOSFETs are useful due to thesimple gate drive requirements and low Rds_on. Theactive inrush current control can be accomplishedwith a single MOSFET and a few external passivecomponents.

MOSFETs are charge controlled devices and can berepresented with the simplified equivalent circuitshown in Figure 5.

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Fig. 5 Equivalent circuit of a N-channel MOSFET with external capacitance C2

Capacitance Cgd, Cgs, and Cds can be determinedfrom the following equations:

)3(,)2(,

)1(,

rssossds

rssissgs

rssgd

CCCCCC

CC

=

=

=

where, Crss, Ciss, and Coss are parameters that areavailable from the MOSFET datasheets.

How fast the capacitance are charged anddischarged determines how fast the MOSFET willturn on or off. In order to ensure that the transitionfrom state to state is linear and predictable, anexternal capacitor C2 is added in parallel with Cgd.

If the external capacitance is much larger than theinternal Cgd, it will diminish the effect of the highlynonlinear capacitance Cgd on the transition process.

C2 acts as an integrator and is used to accuratelydetermine the switching characteristics of theMOSFET. The ability to control the constant linearslope of the drain voltage transition allows accuratecontrol of the inrush current.

3.3.2 Active inrush control circuit descriptionThe schematic in Figure 6 shows a self startingMOSFET based circuit that provides the activeinrush current control.

The MOSFET Q1 is placed in the return path of theDC/DC converter. Upon application of the inputvoltage or insertion of the board into the poweredbackplane, pin 1 of the DC/DC converter rises tothe input voltage level. The control circuit thenbrings it down to the ground potential at a fixedrate. The drain voltage decreases at a linear slopewhich is determined by C2R2 time constant. Thisslope determines the maximum magnitude of theinrush current.

C2 is selected from the following condition:

( ) )4(,2 gdgs CCC +>>R2 is determined by the desired inrush current:

)5(,2

2 maxinrush

load

ICVCR

=

where Vmax is the maximum input voltage, Cload is asum of C3 and the DC/DC converter's input filtercapacitance, and Iinrush is the magnitude of the inrushcurrent.

Fig. 6 Active inrush current control circuit

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R3 is a damping resistor and can be arbitrarilychosen, if the following condition is met:

)6(,23 RR

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From (4): C2>>1700pF. Select C2=0.01F;From (5): R2=252.5k. Select R2=240k;Select R3=270

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Fig. 8 Inrush control circuit based on LT1640L

DC/DC converter is a good starting point. Changesin the target inrush level, MOSFET type, and,possibly, changes in the MOSFET manufacturer willinfluence the values of C1 and C2. The users arestrongly advised to calculate these values ratherthan copy example values from the datasheets andapplications.

The function of R18 is to decrease turn off time ofQ1, while R3 negates the presence of C2 at theinitial application of the input voltage. Values from10K to 15K are suitable for R3.

3.4.1.2 POWER GOOD OUTPUT SIGNALWhile there are both high and low PWRGD outputversions of the controller (LT1640H and LT1640L),the L version is recommended for its better currentsinking capability. For the applications where opticalisolation is employed, the LED of the optocouplercan be driven directly by a LT1640L. Thoseapplications include mostly systems where an EMIfilter is connected between the inrush control circuitand the input of a DC/DC converter. Furthermore,the optocoupler can be used to interface withDC/DC converters requiring either active high oractive low ON/OFF control. Active low ON/OFFcontrol is usually indicated by suffix "N" in a Power-One converter part number.

Applications with active low ON/OFF control inputsand without intermediate EMI filters can use thedirect connections illustrated in Figure 8.

3.4.1.3 INSERTION/EXTRACTION SIGNALIt is strongly recommended that a board

insertion/extraction input signal be applied to theLT1640L. The most convenient method of applyingthis signal is via undervoltage lockout input (pin 3).Early warning of extraction assures that the Q1 isswitched off by the time the power pins breakconnection. This guarantees a "dry" or "cold"disconnect and alleviates the input voltage glitchesand disturbance during the extraction.

Two possible circuit approaches are shown inFigure 9.

Approach a) illustrates the simplest way to generatethe insertion/extraction signal and approach b)shows the scheme with isolated source to permithost control.

While short contact pins are shown in theschematics, there are alternative methods whereboard ejector mechanical switches are employed.This could eliminate the need for the short pins atthe expense of more complicated mechanics.

Fig. 9 LT1640 control options

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3.4.1.4 UV AND OV DIVIDER STRINGSThe schematic in Figure 8 shows separate dividerstrings for the UV and OV functions. Separatestrings simplify the resistors value calculations at theadded expense of one resistor when compared tothe approach shown in the LT application note. TheUV and OV input threshold voltage is 1.223V so thenearest 1% resistor value of 12.4KOhm is selectedfor a string current of 100A. The top resistors aredetermined based upon the required UV and OVthresholds. The values shown in Figure 8 set theUV threshold at 30V and OV threshold at 74V Notethat the two upper resistors have nearly the full inputvoltage across them.

The capacitors C5 and C6 are for noise protection.It is strongly recommended that they are used andplaced as close to the respective pins of theLT1604L as possible.

3.4.1.5 VIN FILTERR4 and C7 form a low pass filter on the input voltagerail of the LT1640L. For effective decoupling, C7needs to be placed as close to the IC as possible.

3.4.1.6 CIRCUIT BREAKERThe schematic in Figure 8 does not include theelectronic circuit breaker described in the LTapplication note. There are two reasons for this:

First, safety standards require the use of fuses forprotection from a catastrophic failure of the capacitorC3 or the input filter capacitor of the DC/DCconverter. The fuses make the electronic circuitbreaker redundant.

Second, the implementation of the electronic circuitbreaker requires very careful attention to the PCBlayout, noise protection and deglitching. Extracomponents need to be added to make the circuitwork properly under different conditions.

Because the function is redundant and difficult toimplement, users are advised to omit it.

3.4.1.7 OPTIONAL PROTECTIONAccording to Linear Technology, some systemshave demonstrated problems that have been solvedthrough the addition of a RC snubber and/or atransient clamp across the input voltage bus. Theinclusion of those components as "do not stuff"items is a cheap insurance against an unnecessaryboard spin in the event they end up being required.

It is recommended that some bulk capacitance beplaced on the backplane. As a rule of thumb, thesuggested amount of capacitance is two to fourtimes the value of capacitance that is located on theboard (C3 in Figure 8).

3.4.2 Design exampleGiven: Vmax=75V

Iinrush=3AQ1 is IRF540S

Select R18=270 and R3=12kFrom (11): C2=1380pF. Select C2=1500pF;From (12): C1=0.058F. Select C1=0.1F.

Figure 10 illustrates the operation of the circuitshown in Figure 8 at the input voltage of 48V andno load on the output of the DC/DC converter.

Fig. 10 Operation of the LT1640 inrush control circuit at 48Vand no load.

When the voltage is applied to the input of theDC/DC converter, the ON/OFF line is pulled up bythe converter's internal voltage source. As soon asthe capacitor C3 and the filter capacitance arecharged, the PWRGD signal pulls the ON/OFF linelow, enabling the DC/DC converter.

The picture in the Figure 11 illustrates the operationof the circuit at the input voltage of 48V and full loadon the output of the DC/DC converter.

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Fig. 11 Operation of the LT1640 inrush control circuit at 48Vand full load

The bottom trace represents the output voltage ofthe DC/DC converter. The voltage starts rising assoon as the PWRGD signal is applied to theON/OFF pin of the converter (see Figure 10). Sincethe converter is now loaded, the input currentincreases too, but only to its steady state leveldetermined by the input voltage and the output load.

The pictures in Figure 12 and Figure 13 illustrate theoperation of the circuit at the input voltage extremesand full load on the output of the DC/DC converter.



The inrush current reaches its maximum atVin=75V and is 2.4A, 20% less than the designgoal. The difference can be due to capacitorstolerances and variation of MOSFET parameters.

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Inrush Current Control