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SSeemmiiccoonndduuccttoorr CCuurrrreennttRReegguullaattoorrss PPrrootteecctt CCiirrccuuiittssSam Ochi, IXYS Corporation, Santa Clara, California

Current regulators (non-switchable and switch-able) are a new family ofuseful and versatile

devices targeted for use in commu-nications, networking, and powerconversion circuits. Current regu-lators (or current sources) tradi-tionally consist of resistors andother discrete active components,and are extensively used in thedesign of high performance analogand mixed signal circuits and sys-tems. These simple-to-use, two-terminal fixed current regulators,or switchable three-terminal cur-rent regulators, allow designers to simplify their circuits.Additionally, a family of non-switchable non-polarized accurrent regulators are also available for applications requir-ing current limiting in ac circuits.

Figure 1(a) shows the equivalent circuit of a dc currentregulator. M1 is a high voltage discreteMOSFET whose gate is driven by

amplifier A1, and whose invertinginput is the intersection of the M1source and the top of a currentsense resistor, R1. A1�s other inputis the output of a reference voltage,Vr. The negative terminal of Vr con-nects to the lower side of R1 andbecomes the cathode terminal ofthe current regulator.

In operation, when the M1drain (the anode of the current reg-ulator) is more positive than itssource, the current through it iscontrolled so that the voltageacross R1 is regulated in feedbackby A1 to be Vr. Vr is designed to be

approximately 3.0V so that if R1 is chosen as 300W, thenthe regulated current is 10mA. In the ac current regulatorequivalent circuit (Figure 1b), two current regulator circuitsare placed back-to-back to limit current flow in eitherdirection. Figure 1(c) shows the equivalent circuit of the

switchable current regulator in whichthe current sensing resistor can beselected by the user to provide regu-lated currents up to 100mA. A plot ofplateau current, ID(P), vs externalresistance, RK, is in Figure 2(a). Figure2(b) shows the circuit with an externalresistance.

Figure 1(a). Current Regulator. Equivalentcircuit. Figure 1(b). AC equivalent circuit.

Figure 1(c). Switchable current regulatorequivalent circuit.

This article originally appeared in PCIM Power Electronic Systems Magazine (http://www.pcim.com)and is copyrighted © 2000 by Adams/Intertec International Inc. Reprinted here by permission.

A family of currentregulators can be used to

create minimumcomponent, high

performance powersupplies, power supply and

network protectioncircuits, active noise

filters, and wide dynamicrange Class A amplifiers.

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Some current regulator applica-tions involve voltages >48V, and soprecautions should be taken. Also,many or all of the designs may requireheat sinking, overcurrent andovertemperature protection.

Power Supply ProtectionPower supply applications tend to

be cost sensitive and require high reli-ability. The main system power supplymay have built-in protection, such as

short-circuit, overcurrent, andovertemperature. Assume a systempower supply (Figure 3) drives m loadsand load #3 is shorted. Protection fea-tures built into this system powersupply will detect the short and grace-fully shut it down. Because of theshort, all m loads will lose their power.In fact, the system power will beunavailable until the shorted load isremoved.

Figure 4 shows the same power dis-tribution system with current regula-tors in series with all m loads. Assumeagain that load #3 is shorted, so thevoltage into load #3 will collapse tozero. The current regulator, IC3,allows load #3 to go to zero withoutpulling down or overloading the sys-tem power supply. The other m-1loads will continue to receive power.In fact, one of the loads may evenhave redundant circuitry built into itthat reports to the CPU that load #3went down, and that it will take overwhatever tasks #3 was doing. Othersimilar strategies may be used toimplement overall system robustnessand uninterrupted operation. Here,current regulators, IC1 through ICm, inseries with the m loads provide addedflexibility and system capability.

In some applications the load maybe located away from the systempower source. Therefore, it is essen-tial to account for the possibility ofcabling shorts not only to ground butto some other potential, such as the ac

mains, where potentials may be ofopposite polarity or even greater thanthe voltage provided by the systempower supply. Instead of a dc currentregulator, (IXCP10M45 throughIXCP100M45) a bidirectional ac cur-rent regulator such as theIXCP50MAC45 would be needed. Ifmore than 100mA is required, currentregulators may be paralleled.

Protecting SMPSSwitching power supplies generate

high frequency noise that is difficultto reduce or eliminate. The noise maybe due to: internal clocking tran-sients, high speed turn-on and off ofthe output switches, momentarycross-conduction of the upper andlower switches, the snappiness of theswitching free-wheeling diodes, strayinductances, and other factors.Instead of using bulky passive induc-tors and capacitors for filtering, cur-rent regulators can be used to emulatean infinite value inductor (analogousto a voltage source that can emulatean infinite value capacitor).

An LM317 linear regulator acts anoise filter in the SMPS of Figure 5(a).Figure 5(b) shows a current regulatorwith a zener diode as an SMPS noisefilter. Both circuits are designed tooutput ~ +29V @ 80mA, with a ~6Vinput-to-output drop. For noise rejec-tion, the key parameter is input powersupply ripple rejection; the better itis, the better the noise rejection. At

Current Regulators

Figure 2. Current regulator characteristics.(a). Plateau current vs. external resistance.

Figure 2(b). Resistor, RK, in series with (-)or �K� pin to achieve different values of ID(P)

Figure 3. Standard power distribution system Figure 4. Current regulator as load resistor.

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first, it may appear that linear regula-tor would work better, especiallybased on its typical data sheet specifi-cation of 80 dB rejection with a mini-mum of 66dB at 120Hz. Noise com-ponents from a switcher, especiallyone running at several hundreds ofkHz, has noise components beyond10MHz. A linear regulator�s 80dB rip-ple rejection at 120Hz is 17db at1MHz. Figure 6 compares theLM317�s ripple rejection with that ofthe current regulator. The currentregulator ripple rejection starts at58dB and is still 46dB at 1MHz, 19dBbetter power supply rejection at1MHz!

Simple IC solutions don�t exist fora 48V power supply active noise filter.Both circuits in Figure 7(a) and (b) pro-vide 58db of noise rejection at 1MHzwith an input-to-output drop of ~6V,as before. The circuit in Figure 7(a)uses a resistor and filter capacitor; theone in Figure 7(b) uses a current regu-lator to do an equivalent task. The fil-ter capacitor is a 1000mF, 63V, low ESRtype (0.05W ESR, Panasonic ECE-A1JFS102). The physical size of thiscapacitor is almost two orders of mag-nitude larger than that of the currentregulator.

Off-line Power SuppliesHigh voltage current regulators

simplify design of universal, off-linepower supplies operating from 90Vacto 260Vac by minimizing componentcount and cost. The off-line powersupply in Figure 8(a) provides a regu-lated +43Vdc @ 80mA. A simple, 5W,zener diode, Z1, sets the 43V output.Key points to note are the small sizeand value of the input and output fil-ter capacitors, C1=22mF and C2=39mF, respectively. As a comparison,similar ripple and output current per-formance for a universal off-line powersupply is in Figure 8(b). The conven-tional circuit of Figure 8(b) uses muchlarger input and output capacitors:C1=100mF and C2=220mF, respec-tively. Due to the wide variation incurrents sourced by the widely varyingac input, three 5W zener diodes, Z1,Z2, and Z3, are needed to meet thepower dissipation requirements. Also,the dropping resistor value is 1kW, at100W. In contrast, the current regula-tor, IC1, IXCP100M45 only needs todissipate 33W.

The multiple output, off-linepower supply in Figure 9 has two zenerdiodes stacked to produce 15V and+5V. By inspection, the +5V outputcurrent, Ioutb, is what is left over fromthe +15V output current, Iouta. Forexample, with the maximum of 20mAavailable from IC1, IXCP20M45, ifIouta consumes 5mA, then Ioutb cannotbe greater than 10mA. Figure 10 showsthat an off-line negative output sup-ply can be created just as easily.

Current regulators in universal, off-line power supplies reduce overall

cost, size and heat generation.Because no inductors are used, thereare inherently no radiated emissions,as is the case with off-line switchers.As an added benefit, the low valueinput capacitor, C1, minimizes powerfactor issues. Using a dropping resistorto reduce the rectified off-line volt-ages is no longer necessary and is anexpensive alternative. Current regula-tor-based off-line power supplies arean attractive alternative for outputrequirements less than 100mA.

Protecting CablesCurrent regulators in series with

network and telecommunicationscables will protect sensitive, missioncritical equipment from inadvertentnetwork cable shorts to ground or todestructive ac power. Network cablescan be snaked next to ac power cablesas well as grounded conduit. Theymay even run unprotected across fac-tory floors and can short at somepoint. An ac current regulator placedin series with networking equipment� one at the output of such equip-ment and the other at the input of anetwork card � can protect both sys-tems from cable shorts to ground or tothe other more dangerous (and lethal)ac mains power. It is not unusual thatnetwork cabling becomes snaked orhas its insulation scraped when cross-

Figure 5(a). SMPS applications. LM317 asSMPS noise filter.

Figure 5.(b). IXCP100M35 as SMPS noisefilter.

Figure 6. Ripple rejection comparisonbetween LM317 and IXCP100M35 solutions.

Figure 7. SMPS noise filter applications.(a). Resistor and capacitor noise filter.

Figure 7. (b). IXCP100M35 noise filter.

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ing an ac mains. It may even melt,which then results in a short to one ofthe network conductors and the acmains. The current regulators in Fig-ure 11 illustrate their placement withrespect to the twisted wire cabling,network hub and the remotes.

If you had a current regulator pro-tection system as in Figure 11, theworst that would occur will be thenetwork card somehow misbehavingdue to noise. (The current limited acmains into your network card will def-initely be seen by your network cardas unwanted noise.) You would callyour network administrator � com-plain that your net isn�t working prop-erly. The administrator will confirmthat there is something wrong withthe network cabling and runs anothercable to your office from the networkhub. As a word of caution, check withyour network administrator to be surethat the components added in serieswith the network shown in Figure 11don�t affect the reliability of the net-work connection.

Current regulators can also act asstart-up devices for universal off-lineSMPS. These power supplies mustcontend with input power voltages aslow as 90Vac and as high as 260Vac.This translates to full wave rectifiedvoltage range of 127Vdc to 368Vdc.Figure 12 shows a start-up circuit for auniversal off-line SMPS. Dependingon the application, the designerselects the current range required �from 2mA and up. Because the powerdissipation across a current regulatoronly goes directly with the voltageacross it, the circuit in Figure 12 pro-vides the most efficient solution with-out shutting off the start-up circuitduring normal operation.

Figure 13 shows a start-up circuitusing the switchable current regulatorcircuit, IXCP10M45S, which is shutdown whenever the SMPS provides aPower Good (PGD) signal. This cir-cuit improves the overall efficiency ofthe SMPS by turning off unneeded

circuits during normal operation. R1 isused to adjust the IXCP10M45Sdevice regulating current to 1mA, asshown in Figure 2(a).

Floating DC PowerCurrent regulators can be used to

replace isolated dc-dc convertersneeded to power most gate driversand current sensors that surroundtoday�s high power IGBTs and MOS-FETs. The circuit in Figure 14 imple-ments a floating dc voltage source topower the IC gate drivers used todrive and protect high power IGBTsand MOSFETs. This same circuit can

also be used to power other circuitry,such as current sensors, temperaturesensors and others.

The 10ma current regulators, IC3through IC5, provide the trickle cur-rent needed to keep the IC gate driv-er power supply storage capacitor fromdischarging during periods when theIGBT or MOSFET is in either the onor off state for long periods. Thesecurrent regulators are stacked in seriesto provide the full 1200Vdc break-down needed by the application. Thebootstrap circuit with D1and R1 sup-plies the greater power needed duringswitching. Zener diode, Z1, keeps the

Current Regulators

Figure 8. Universal off-line power supply. (a). Low-cost version.

Figure 8.(b) Conventional version without current regulator uses larger input and output capaci-tors.

Figure 9. Multiple output universal off-line power supply.

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floating VCC power below +15Vbetween VCC and the emitter orsource of the driven IGBT and MOS-FET. The anode return of the currentregulator is at a voltage greater thanVBUS by 15V to

17V. The power supply circuit for thislow current supply is the simplecharge pump circuit also in Figure 14.Here, the TSC427 is used to providebuffering to an oscillator output run-ning at 100kHz. Capacitors C1 andC2 act as the charge pump capacitorsto turn on D1 and D3 during positivegoing edges and D2 and D4 clamp thetop plate of C1 and C2 during thenegative going edges.

Advantages of the circuit providedin Figure 14 are:� Very low cost compared with other

floating power supply solutions �such as modular dc-dc converters

� No magnetics to take up spaceand introduce additional EMI

� Very low capacitance to the float-ing node � only the reversebiased junction capacitances ofthe bootstrap diode and the cur-rent regulator are all that are con-nected to the floating VCC node

� Low component count and spacerequirements

Figure 10. Negative output universal off-line power supply.

Figure 11. Network of telecommunications application.

Figure 12. Universal off-line SMPS start-upcircuit with non-switchable current regulator.

Figure 13. Universal off-line SMPS start-up circuit with switchable current regulator.

Figure 14. Floating dc-dc converter for voltage inverters with VBUS to 1200V.

Figure 15. Wide dynamic range Class A amplifier.

Figure 16. Class A buffer with ±180V input-to-output range.

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Amplifier Circuits Current regulators can also be employed in high voltage,

wide dynamic range analog circuits. Class A circuits are stillthe only way to obtain uncompromising amplifier perform-ance where absolute linearity coupled with excellent tran-sient response are needed. Figure 15 shows a Class A ampli-fier with -3db points at 10Hz and 2MHz. It provides a gainof 55, and a ±180V peak-to-peak output. The high peak-to-peak output can provide unclipped and uncompresseddynamic range of over 160dB � equivalent to a 28-bitA/D-D/A combination. The amplifier maintains its lineari-ty performance with source degeneration and keeping thechange in drain current during operation to no more than10% of its nominal range provided by IC1, the 100mAIXCP100M45 current regulator.

The source follower in Figure 16 uses IC1, the switch-able current regulator IXCP10M45S, as the input devicefollowed by a pull-down current source, the IXCP100M45.Zener diodes Z1 and Z2, 1N5245 are used to protect thegate of IC1. By placing a resistor in series with the upperdevice, the input to output offset can be set to zero. Thiscircuit will swing within ±10V of the full supply voltage of±200V.

Figure 17 uses the source followers in combination withthe Class A amplifier to create a feedback amplifier withbuffered input and output. The first source follower, IC1,buffers the input and drives the source of IC2. The second

Current Regulators

Figure 17. AVCL=100 Class A amplifier with ±170V output.

source follower, IC5, buffers the high impedance drain out-put of IC2 from the load, which is modeled as a 2KW in par-allel with 100pF. The Class A amplifier circuit in Figure 17is designed for a closed loop gain of 100 with its -3dB band-width corners fixed at 10Hz and 2MHz.

Current regulators are available in fixed current rangesfrom 1mA to 100mA. Also available are ac current regula-tors from 10mA to 50mA ranges. For additional design flex-ibility, switchable current regulators can be externally pro-grammed to currents from under 1mA to above 100mA.Presently available are current regulators rated at 350V and450V, and higher voltages are presently in development.

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