Designing Low Leakage Current Power Supplies 2

8/12/2019 Designing Low Leakage Current Power Supplies 2

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Figure 2: An example of conducted EMI for a 3W cellphone charger: (a) with, and (b) without a Y-capacitor.

By Peter Vaughan

Product Applications Manager

Power Integrations Inc.

Majority of AC/DC power sup-plies provide isolation from thehigh-voltage AC input to thelow-voltage DC outputs. Safetystandards, such as the UL1950,specify both the strength ofthe isolation barriere.g. a3,000VAC withstand voltageas well as the maximum leakagecurrent. This is the current thatflows across the primary to sec-ondary isolation barrier when

accessible parts are connectedto the protective earth groundthrough a specified impedance

value. Standards in leakage cur-rent ensure human safety, pre-

venting users from becomingpart of a path for substantial cur-rent to ground when touchingthe output or the enclosure of apower supply.

The maximum leakage cur-rent allowed to flow is based onthe specific classification of theapplication. In the past, lowleakage currents were requiredonly for special applicationssuch as medical equipmentwhere patient contact was eitherhighly likely or necessary. Those

applications had to meet limits,which were considerably morestringent than those required for

IT equipment (Table 1).However, there are other

reasons why a low leakage de-sign is desirable. For example,with many cellphones now en-

Designing low leakagecurrent power supplies

closed in metal housings, theirchargers often must meet

leakage-current specificationsset by the handset manufac-turer that are below thosespecified by the applicablesafety standard.

This is to prevent users fromfeeling the touch currentwhen they hold the phone whileit is charging, especially indamp environments like asteamy bathroom. Power sup-plies that support telephoneequipment (cordless phones,

answering machines and DSLmodems) must often have lowline-frequency leakage to pre-vent the coupli ng of audibl ehum onto the telephone lines.There may also be potentialcost savings from designing forlow leakage, such as reducingthe size and/or number of theEMI filter components re-quired in the power supply.

Leakage current sourceOne of the biggest contributors

to leakage current in AC/DC

Figure 1: (a) The Y-capacitor shunts much of the EMI current, keeping most of it within the supply;

(b) With no Y-capacitor, electromagnetic interference currents must flow external to the supply.

EMI current flow outside supply

(measured as conducted EMI)

EMI current flow inside supply

Csps : Stray capacitance primary

to secondary

CY : Y-capacitorCsse : Stray capacitance

secondary to earth ground

Csse

CSSE

Csps

Csps

Vds

Vds

CY

AC

AC

(a)

(b)

+

+

-

-

Condition Limit

All equipmentFrom protective earth ground

to accessible parts not

connected to protective ground

0.25mA

HandheldFrom protective earth ground

terminal (earthing conductor)

to protective ground

0.75mA

Portable

Information

technology

(IEC60950)

Stationary

From protective earth groundterminal (earthing conductor)

to protective ground3.5mA

Medical

(IEC60601-1)Patient leakage 100A

Equipmenttype

Table 1: Examples of leakage (touch) current specifications.

(a) (b)

(b)

(a)


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Figure 4: Circuit diagram of a TinySwitch-II based 3W (5.1V 600mA) charger power supply.

switching power supplies is theY-capa cit or a safety agenc yrated capacitor (typically orangeor blue in color) that can be usedto bridge the isolation barrier(Figure 1a). It is used to returndisplacement currents (gener-ated by the switching process)back to their source, preventingEMI. Any HF currents that flowout of the power supply will re-turn via the AC input lines, pro-ducing what is measured asconducted EMI. In Figure 1a,the Y-capacitor shunts much ofthe EMI current, keeping most

Figure 3: Overview of the paths where EMI currents flow as they are driven by the switching waveforms.

of it within the supply, while inFigure 1b, it all must flow exter-nal to the supply.

In general, the larger theval ue of the Y-capac itor, thelower the magnitude of the EMIthat the supply generates. Con-versely, the larger the value ofthe Y-capacitor, the higher willbe the le ak ag e cu rre nt th atflows across the isolation bar-rier. The expression:

CY(MAX) =

=0.25 10-3

265 2 50

ILEAK(MAX)

50pF =2.95nF

VAC(MAX) 2 fLINE

Estimates the maximum valueof a Y-capacitor that can beused, without exceeding thesafety limits. For a two-wire(without a protective earth con-nection), universal input powersupply with a floating output,

rounding down to the next stan-dard capacitor value gives amaximum Y capacitance of ap-proximately 2.2nF. For a 100/115VAC-only design, this wouldincrease to 3.3nF.

Simply removing or reduc-

ing the value of the Y-capacitoris usually not feasible, as doing

so typically increases EMI sig-nificantly (Figure 2b). More-over, adding a common modechoke or other filter compo-nents increases cost. Therefore,attention must be focused onreducing EMI currents thatmake use of a Y-capacitor nec-essary to begin with.

Reducing EMI currentsFigure 3 gives an overview ofthe paths where EMI currentsflow as they are driven by theprimary and secondary switch-ing waveforms. Table 2showssome of the ways of reducingcommon mode EMI currents.Techniques like using tape toincrease the spacing between

Techniques Pro Cons

Extra tape between

winding layers

Simple Does not significantly reduce common

mode EMI currents.

Adding a common mode

input choke

Simple Increases cost and board size, and may not

provide enough improvement.

Slow down rise/fall time

of switching waveforms:add snubbers or reduce

MOSFET gate drive

Simple Increases dissipation and lowers efficiency.

Increased cost and board size.Typically does not provide enough

improvement.

Modulation of switching

frequency (jittering)

Provides up to 6dB reduction in

quasi-peak and average EMI

Difficult to implement in discrete designs

Transformer shield

windings

Large reduction in common mode

EMI currents.

Allows significant reduction or

elimination of Y-capacitor.

Low cost

Increased transformer complexity.

Optimization of shields is time consuming.

Extremely consistent transformer

manufacturing is required to maintain

consistent EMI performance.

Table 2: Techniques for reducing common-mode EMI currents.


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Figure 6: An example of (a) conducted EMI performance with (b) an additional EMI filter choke on the input.

leakage current from to 183Ato 18A (Figure 5c). Close tothe same EMI performance isobtainable without the shieldwinding, but requires a commonmode choke L2in addition tothe existing differential modefilter inductor L1in the inputstage (Figure 6a and Figure6b). Depending on the require-ments of the particular appli-cation, a third shield could beadded to the transformer, whichcould reduce the value of theY-capacitor even further.

As shield windings react tothe primary side switching andthe secondary side commuta-tion, they reduce or cancel thegeneration of common-modedisplacement currents, makinga detailed analysis unnecessary.However, the requirements foreach design will vary due to fac-

tors such as component loca-tion on the PCB layout, theproximity of the board and mag-netic components to chassismetal, transformer size, volt-seconds rating, turns wiregauge and winding turns ratios.Therefore, trial-and-error willbe re quired to opti mize th eshield windings for each design.However, the basic principlesof shield winding placementtypically apply consistently.

Many of todays power supplyspecifications require lowerleakage current values due tohuman interaction with theproducts being powered. There-fore, power supply designers arebeing asked to eliminate or re-duce the size of the Y-capacitorthat they use to make the designpass EMI. By using transformershield windings, the value of theY-capacitor can be significantlyreduced or eliminated, to lowerthe leakage current while still

meeting conducted EMI limitswith adequate margin. Usableand affordable solutions for ac-complishing this objective arereadily available today.

Figure 5: Using shield windings in a switching transformer is the most effective way to reduce common-mode

electromagnetic interference currents, while having a minimal impact on the overall cost of the supply.

winding layers can also reduceinterwinding capacitance. Thismethod alone reduces little EMIcurrents. Shield windings havelong been used in line frequencytransformers to reduce noiseand coupling, and can be used inSMPS transformers as well. Asthe EMI plots in Figure 5show,the use of shield windings in aswitching transformer is the

most effective way to reducecommon mode EMI currents,while having a minimal impacton the overall cost of the supply.

Figure 4 is the circuit dia-

gram of a 5.1V, 600mA powersupply, based on a TinySwitch-II IC, and a simple two-windingtransformer. The IC is self-pow-ered, so an auxiliary trans-former winding is not required.The design is typical of the typeof adapter used as a charger forcellphones, PDAs or digitalcameras.

The TinySwitch-II IC modu-

lates its switching frequency(called jittering) to reduce EMI.However, without shield wind-ings in the transformer, thisdesign still requires a 2.2nF Y-

capacitor to meet conductedEMI (Figure 2a). Removingthe Y-capacitor produces theEMI, which is clearly unaccept-able (Figure 2b).

Adding a single turn foilshield between the primary andsecondary windings reduces themeasured EMI by about 10dB(Figure 5a). Supplementingthat with an additional shield

winding achieves a further 10dBreduction (Figure 5b). A goodscan with 10dB of margin can beobtained with a Y-capacitor ofonly 220pF, which reduces the

(a) (b)

(a)

(b)

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Designing Low Leakage Current Power Supplies 2