95-Wat Laptop PSU 12V in - 19V 5A Out1

95-wat Laptop PSU

elektor electronics - 1/200642

HANDS-ON POWER SUPPLIES

Michael Schön

12 V in, 19 V at 5 A out!

Anyone who regularly uses a laptop or notebook computer while they are away from the home oroffice knows that sooner or later they will need to plug into a mains outlet to top up the batteries.The car cigarette lighter socket in the car is also an electrical outlet but it can only supply 12 V. That’sno problem for the Laptop PSU Adaptor described here; it can deliver ample voltage and current fortoday’s portable computers from a car battery. It’s also simple to build and has excellent efficiency.Modellers will also be interested in this design to recharge their battery packs in the field.

charged’ and the resultant inducedvoltage is added to the supply voltageto produce an output voltage higherthan the supply. Unlike a standardtransformer inverter the basic circuitrequires just three components: a coil,a power switch (in this case a MOS-FET) and a diode. All the control func-tions are performed by IC1, this deviceproduces the pulse width modulated(PWM) switching signal for the powerswitch. The input and output voltagesare smoothed by a series of low-losscapacitors.The principle of the step-up converteroperation is shown in Figure 1. Whenthe power switch (MOSFET T) con-ducts (Figure 1a) current flowsthrough the coil and produces a mag-netic field. The anode of the diode (D)will be at ground potential (almost) soit will not be conducting and any

energy previously stored in the capac-itor C will be supplied to the load (RL).The coil core has a 1 mm air gap toensure that the core material is notdriven into saturation during thisphase of the cycle. When saturationoccurs it is not be possible to store anymore energy in the magnetic field andthe coil effectively becomes just a lowimpedance resistor which would short-circuit the input voltage.When T turns off (Figure 1b) currentthrough the coil no longer has a path toground through T, the magnetic fieldstarts to collapse inducing a voltageacross L of the opposite polarity to theswitch-on phase. The induced voltagerises above the supply voltage, for-ward biasing D and transferringenergy to C and RL. Energy stored in Cduring this phase is transferred to theload during the next switch-on phase.

Adaptor

1/2006 - elektor electronics 43

The laptop adapter described hereplugs into a car cigarette lighter socketand produces a 19 V nominal outputvoltage adjustable by ± 0.5 V. The input

voltage range is from 9.2 V to15 V and the

o u t p u tv o l t a g e

s h o w sgood regu-

lation evenwith large

fluctuationsof the input

voltage. Theoutput can sup-

ply 5 A continu-ously with brief

excursions up to10 A. The powersemiconductor

heatsinks aredimensionedfor 5 A contin-uous so

extended oper-ation up to 10 A

will increase dissipa-tion in the adapter and

in extreme cases willcause the input fuse to

complain. The high effi-ciency of this design (typi-

cally 95 %) reduces loading onthe car battery and also dissi-

pation within the adapter unit.At 60 mm _ 100 mm the PCB out-

line is not much bigger than themains adapter unit that was suppliedwith the laptop or notebook PC.

The step-up ConverterThe majority of laptops require a sup-ply voltage in excess of the 12 V avail-able at the cigarette lighter outputsocket, even with the engine runningthe supply will not rise much above13.8 V and this is still too low for mostlaptops which typically require 19 V.The 12 V supply can be boosted withthe help of a step-up converter design.The basic principle is simple: an induc-tor is repeatedly ‘charged up’ and ‘dis-

L D

CT RL

0500029 - 12a

VIN

I IN

tON

IOUT

VOUT

L D

CT RL

0500029 - 12b

VIN

I IN

tOFF

IOUT

VOUT

a

b

Figure 1. The step-up converter principle: Switch-on phase (a) and Switch-off phase (b).

Adapter specification • Output current: • Output voltage:

5 A (10 A momentarily) 19 V ± 0.5 V (adjustable)

• Input range: • Minimum input voltage: 10 V to 15 V 9.2 V

• Switching frequency: • Maximum continuous output power:42 kHz 95 W

• Efficiency: • PCB dimensions: 95 % (typical) 59 mm x 98 mm

Figure 2 shows the correspondencebetween the output voltage VOUT andthe transistor switching time (tON andtOFF). With short tOFF times it is impor-tant to use low loss devices for thediode and capacitors to ensure they donot exceed their SOA (Safe OperatingArea), otherwise the high energytransfer may lead to over heating andcomponent failure. From a design pointof view if you need an output morethan about three to four times theinput voltage it would be better to usea transformer in the design. This

would reduce the current ripple andlower the switching current peaks.The step-up regulator current and volt-age waveforms are shown in Figure 3.Waveform C shows the coil ripple cur-rent, increasing the coil size wouldreduce ripple but would also increasethe physical size of the unit. It isimportant to ensure that the coil is notmade so small that it cannot supplysufficient energy during the switch-offphase; this would produce an outputvoltage with very poor regulation. The56 µH coil used in this design produces

a ripple current of around 2 A which isapproximately 40% of the maximumoutput.

Just one ICThe input voltage in the range from 10to 15 V is connected to terminals K1and K2 in the circuit diagram (Figure4). A PCB mounted fuse is fitted in linewith the positive supply and the spec-ified fuse holder is equipped with twosolder tags for each of the two fusecontact clips, these help to reduce theresistance of the fuse assembly whichis particularly important because itpasses a high current. Four electrolyticcapacitors (C1 to C4) buffer the inputvoltage. The adapter circuit switchesthe supply current very quickly andrequires special low-loss capacitorssuitable for use in switched modepower supplies, the internal imped-ance of normal electrolytics is too high,they would overheat and may burst.C5 decouples any high frequency sig-nals on the supply voltage.The coil (L1) is made up from severallengths of enamel covered wire con-nected in parallel to reduce the skineffect which becomes a problem withhigh switching speed. The high powerswitching element is a HEXFET madeby International Rectifier. This devicehas source/drain resistance of just8 mΩ when it is conducting. The lowresistance ensures very low power dis-sipation in the device and allows it toswitch a hefty 104 A maximum whichshould ensure reliable operation in ourapplication. Both the HEXFET anddiode are fitted with finned heat sinks.Schottky diode D1 has a TO 220 pack-age outline. It has a 45 V maximumoperating voltage and a current of 16 Awhich gives a forward voltage drop ofaround 0.63 V, again for reliability thedevice is conservatively rated. LowESR type electrolytic capacitors arespecified again for output smoothingcapacitors C6 to C9. Capacitor C10 isused for high frequency decouplingand the regulated 19 V is output fromconnectors K3 and K4.A block diagram of the UC3843 (IC1) isshown in Figure 5. It contains a pulsewidth modulated (PWM) controller out-put signal and an internal voltage refer-ence. The adapter output voltage isdivided down by R1, R2, R3 and P1 andconnected to the voltage feedbackinput (pin 2) of the controller IC (theerror amplifier inverting input). C2improves the amplifier stability as doesR4, R5 and C12 connected between



VOUT[V]

tON [µs]

60

50

40

30

20

125 10 15 20

050029 - 13

VOUT = VIN •tON + tOFF

tOFF

VOUT = 12VtON + tOFF = 25 µs

t

a

t

050029 - 14

b

t

I IN

UT

ILID

∆ IL

VIN

VOUT

IOUTc

t

d

tON tOFF tON

Figure 2. Output voltage control using PWM.

Figure 3. Current and voltage waveforms with 50% on/off switching. (a) The PWM signal; (b) Voltage acrossthe MOSFET (VT); (c) Current through the coil (IL); (d) Current through the diode (ID).

the output (Pin1) and non invertinginput of the difference amplifier. The ICclock frequency is defined by the RCnetwork formed by R8 and C13. Thecomponent values shown give an oper-ating frequency of around 42 kHz. C14provides decoupling of the internal ref-erence voltage at pin 8. The supplydecoupling network formed by R12,C15 and C16 on the supply pin 7 isimportant for reliable operation of IC1.The current sense input (pin 3) is notused in this application.The power FET has a gate capacitanceof around 5,000 pF which must becharged and discharged 42,000 timesper second if the FET is to be switchedsuccessfully. It is important that thetransition between on and off occursas quickly as possible so that there isminimum dissipation in the FET. It istherefore necessary to use a lowimpedance driver (T2 and T3) betweenthe PWM output of IC1 and the gatepin of the FET. The BD139 and BD140complimentary pair can handle around1.5 A limited by resistors R9 and R10.R11 will turn off T1 if for any reasonboth T2 and T3 become non-conduct-ing or IC1 is defective.

Printed circuit boardThe complete circuit fits on the single-sided PCB (Figure 6) without the needfor any wire links. Before any of the

components are fitted it is best to con-struct the coil. The PCB layout alsoincludes pads for an off-the-shelf 56 µHinductor but it must be able to handlethe high switching currents. The pro-totype circuit used a coil with an ETD29 core and was constructed as fol-lows:The coil has a total of urns but tomake things a bit more interesting thewire consists of 10 strands of 0.5 mmdiameter enamelled copper wire(ECW) connected in parallel. The effec-tive cross sectional area of all the

strands is 1.96 mm2. This method ofconstruction produces a coil withmuch better properties at this switch-ing speed than would be achievedwith a single wire with a cross sec-tional area of 1.96 mm2 (the skin effecthas less influence).The strands are first carefully twistedtogether before they are soldered tothe coil termination posts, if the twist-ing is too tight the there will not beenough space to fit all the 21 turns.Each strand is 2 m long so overall 20 mof 0.5 mm enamel covered wire will be


R8

15k

R6

15k

R12

100

Ω

R1

5k6

R2

51k

R3

9k1

R11

10k

R9

6Ω

8

R10

6Ω

8

C13

2n2

C14

100n

C15

100n

C5

1µ

C10

1µ

C11

22n

IRL2505

T1

S

D

G

R7

27k

UC3843

ISENSE

RT/CT

IC1

COMP

VREF OUT

VFB

6

7

5

1

2

8

4

3

R4

1M

R5

4k7

C12

1µ

5k

P1

T2BD139

T3

BD140

D1

MBR1645

C16

10µ63V

C1 C2 C3 C4 C6 C7 C8 C9

L1

56µH

K3

K4

K1

K2

10A T

F1

10...15V 19V / 5A

050029 - 11

4x 3300µ / 16V 4x 2200µ / 25V

VrefUndervoltage

Lockout

LatchingPWM

5.0 VReference

Oscillator

ErrorAmplifier

VCCUndervoltage

Lockout8

R

R

Vref

4RT/CT

2Voltage

FeedbackInput

1OutputCompensation

5

GND

7 VCC

6 Output

5 PowerGround

3

050029 - 15

CurrentSenseInput

UC3843

Figure 4. The laptop PSU adaptor circuit diagram.

Figure 5. Block diagram of the PWM IC UC3843.

required. The start of the windings aresoldered to the first four pins on oneside of the coil former and after wind-ing the ends are soldered to the firstfour pins on the other side of the for-mer. It will be necessary to remove theenamel covering and tin the ends ofthe wire before they are soldered to thepins. Be careful here because the plas-tic coil former is not particularly tolerantto high temperatures and the pins maymove out of position if the plastic isallowed to get too hot. The seven pinscan be supported while soldering byfitting them into a spare strip of perfo-rated prototyping board. For the proto-type three layers were necessary forthe 21 turns, the first and second layerboth comprise of eight turns with fiveturns left for the last layer. Once thewindings are finished and before theend of the windings are soldered inplace try assembling the core pieces tomake sure there is enough room. When

there is insufficient space you canleave off the final turn; 20 turns insteadof 21 will not make a lot of difference tothe unit’s performance. Alternatively,you can remove one strand from thecoil wire so that there are nine ratherthan ten strands. The coil can also bewound with 16 strands of 0.4 mm ECW(effective cross sectional area of2.01 mm2) or RF braid provided thatthe cross sectional area is around2 mm2.When the winding is complete a layerof insulating tape is wound around thecoil. Standard insulating tape is suit-able here; the coil temperature neverexceeds hand-warm during operation.The parts list specifies two identicalcore halves with a 0.5 mm gap, oncethe coil is assembled the total air gapis 1 mm. If you use core halves withoutany air gap it will be necessary to sep-arate the two halves with some0.5 mm thick non-conducting material



(C) ELEKTOR

050029-1

C1C2

C3

C4C5

C6

C7

C8

C9

C10C11

C12

C13C14

C15

C16

D1

F1

IC1

L1

P1 R1

R2

R3

R4

R5

R6R7R8

R9R10

R11

R12

T1

T2

T3

+

19V

/5A

-

+

10 A

/T

-

10..1

5 V

050029-1

(C) E

LEK

TOR

05

00

29

-1

Figure 6. PCB Layout and component placement.

COMPONENTSLISTResistors:R1 = 5kΩ6R2 = 51kΩ (51kΩ1)R3 = 9kΩ1 (9kΩ09)R4 = 1MΩR5 = 4kΩ7R6,R8 = 15kΩR7 = 27kΩR9,R10 = 6Ω8R11 = 10kΩR12 = 100ΩP1 = 5kΩ preset

Capacitors:C1-C4 = 3300µF 16V, radial, low

ESR, diam. 12.5 mm, e.g.Panasonic EEUFC1C332 (Farnell)

C5,C10 = 1µF MKT, lead pitch 5mmor 7.5mm (larger version preferred)

C12 = 1µF MKT, lead pitch 5 mmC6-C9 = 2200µF 25V radial, low

ESR, diam. 12.5mm, e.g.,EEUFC1E222 (Farnell)

C11 = 22nF, lead pitch 5mmC13 = 2nF2, lead pitch 5mmC14,C15 = 100nF ceramic, lead

pitch 5mmC16 = 10µF 63V radial

Inductors:L1 = 56µH, 21 turns 10 x 0.5 mm

ECW, parallel1 x ETD 29 coil forner, vertical

mounting, Epcos B66359X1014T1(Schuricht # 331622)

2 x ETD 29 clamp, Epcos B66359-A2000 (Schuricht # 333862)

2 x ETD 29 core half, material #N67, air gap 0.5mm, EpcosB66358-G500-X167 (Schuricht #333840)

Semiconductors:D1 = MBR1645 (International

Rectifier) (e.g. Reichelt, Segor)T1 = IRL2505 (International Rectifier)

TO-220AB case, (e.g., RSComponents)

T2 = BD139T3 = BD140IC1 = UC3843N (Texas Instruments)

(e.g. Reichelt, Segor)

Miscellaneous:K1-K4 = 2-way spade terminal,

vertical, PCB mountF1 = fuse, 10A/T (slow) 6.3 x 32

mm (1⁄4 x 11⁄4 inch) + 2 fuse holdersfor 6.3 mm diameter and PCBmounting

2 x heatsink type SK104-STC (or STS)TO220 38.1mm, 11K/W (FischerElectronic)Isolating washers for T1and D1 (TO-220AB) + isolatingbushes

PCB, order code 050039-1 (seeElektor Shop pages or www.elektor-electronics.co.uk)

in order to achieve the 1 mm air gap.Larger outline capacitors were chosenfor C5 and C10 with a lead spacing of7.5 mm, these are better at handlinghigh current and have lower losses.Begin assembly by fitting the resistorsto the PCB then the fuse holder, nextcomes the spade connectors then the

preset followed by the electrolyticcapacitors (observe correct polarity)and the coil assembly.Before the FET and diode are solderedinto the PCB a bend should be intro-duced in the leads to help reduce themechanical stresses on the solderedjoints when the device heats up. Both

semiconductors must be insulated fromtheir heat sink using insulating wash-ers and an insulated bush for the M3mounting bolts and don’t forget toapply a little thermal compound.Ensure that these devices are well sol-dered in place because the leads willbe carrying several amps. The heatsink


Figure 7. All components including the wound coil fitted to the prototype board.

Test results and efficiencyInput voltage

(V)Input current

(A)Input power

(W)Output voltage

(V)Output current

(A)Output power

(W)Efficiency η

(%)10.8 4.25 45.9 18.93 2.32 43.9 95.710.8 5.59 60.4 18.92 3.05 57.7 95.610.8 8.49 91.7 18.90 4.59 86.8 94.610.8 11.23 121.3 18.88 5.99 113.1 93.212.0 3.84 46.1 18.93 2.34 44.3 96.112.0 5.06 60.7 18.92 3.08 58.3 96.012.0 7.63 91.6 18.90 4.62 87.3 95.412.0 10.00 120.0 18.89 6.00 113.3 94.513.2 3.48 45.9 18.94 2.33 44.1 96.113.2 4.56 60.2 18.93 3.06 57.9 96.213.2 6.91 91.2 18.91 4.61 87.2 95.613.2 9.03 119.2 18.89 5.99 113.2 94.914.4 3.18 45.8 18.95 2.33 44.2 96.414.4 4.16 59.9 18.94 3.05 57.8 96.414.4 6.29 90.6 18.92 4.61 87.2 96.314.4 8.24 118.7 18.90 6.00 114.4 95.6

can be taller, i.e., 50.8 or 63.5 mm with9 K/W or 8 K/W respectively. The proto-type is fitted with 38.1 mm high heatsink at 11 K/W. From the efficiencyviewpoint it would be better if thediode had a smaller heat sink becausethe diode’s forward conduction voltagedecreases with increasing temperature.However a smaller heat sink may leadto overheating if the output is over-loaded. A 20TQ045 diode from IRF canbe substituted for D1; this device has aslightly smaller forward voltage drop.There is no advantage in letting theFET run warmer, its channel resistance(RDSon) increases with increasing tem-perature. Finally fit transistors T2 andT3 along with IC1 and take the oppor-tunity to double check all the compo-nents and soldering before you moveon to the next phase and connectpower to the adapter.

Power onBefore the unit is powered up for thefirst time turn the preset P1 fully clock-wise to produce the minimum output

voltage. For testing use either a powersupply capable of supplying 12 V at10 A or a 12 V vehicle battery. Initiallyadjust the power supply current limitto 200 mA or, if you are using a battery,connect a 5 W resistor (47 or 56 Ω) inseries with the positive supply lead tothe adapter. Double check that the pos-itive lead of the power supply is con-nected to K1 and the negative lead toK2 (nearest to the PCB corner). Whenno obvious malfunction occurs with thepower connected (no excess currentdrawn from the supply and no smokesignals…) the current limit can beincreased (or the series resistorremoved). Connect a multimeter to theadapter output connections K3 and K4(the negative terminal is nearest theboard corner again) and check that theoutput voltage lies in the range of 18 to19 V. With no load connected to theadapter slowly adjust P1 to ensure thatit can achieve an output voltage of19.4 V. Now use either an electronicload or a combination of high powerresistors (4.8 Ω at 100 W) and adjust P1for 19 V output voltage at full load (5 A).

You may notice that when the adapter isoperating under full load conditions theoutput voltage will bounce up brieflyby 1 V whenever the load is discon-nected. The output quickly returns toits regulated level and in practice thisdesign ‘feature’ is not a problem for alaptop adapter application.The finished unit can be mounted inan enclosure. The high efficiencyachieved by this design means thatvery little heat is produced by the unitand there is no need for a fan. A fewholes in the case will be sufficient justto allow a little air circulation. Beforethe unit is connected to the cigarettelighter socket make sure that it (andthe plug) are capable of handling 10 A.Connect the adapter to the PC with asuitable lead and connector, doublecheck that the plus and minus are con-nected to the correct input pin of thelaptop or notebook PC — neverassume that it is equipped withreverse voltage protection!

(050029-1)

Michael Schö[email protected]



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95-Wat Laptop PSU 12V in - 19V 5A Out1