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W Run your laptop in your car W Charge SLA batteries W Run 24V equipment from a 12V battery Adjustable DC-DC converter for cars Need to run electronic equipment in your car but require more than 12V? Or do you want more voltage than your 12V battery can deliver? This versatile circuit will let you do it. Run your laptop, charge 12V SLA batteries or whatever. By JOHN CLARKE A T SILICON CHIP we regularly get requests from readers wanting to power electronic equipment in their car. Often they want to run a laptop computer in the car or perhaps charge 12V SLA batteries or whatever. In the past, our standard answer has been to advise them to modify the SLA battery charger circuit from the July 1996 issue. However, that was a 68  Silicon Chip bit of hurdle for many readers, so we have improved and updated the circuit to make it capable of delivering any voltage from 13.8V up to 24V DC. Typically, laptops require 15V DC or more in order to operate cor­rectly and this voltage is not available directly from the car battery. A car battery normally supplies only a nominal 12VDC when the engine is not running and • Main Features Steps up 12V to between 13.8V and 24V • Maximum current 2A • Charge 12V 6.5Ah or bigger SLA batteries • Efficient switchmode design • Fuse and reverse polarity protection • Power indication between 13.8V and 14.4V when being charged by the car’s alternator. Hence, if you want to run a laptop, you need this DC-DC converter. The unit is housed in a plastic zippy box measuring 130 x 68 x 43mm and www.siliconchip.com.au Fig.1: the basic operating principle of the DC-DC converter. When S1 is closed, current flows through L1, which then stores energy in the magnetic flux produced by the inductor. When S1 opens, the energy stored in the inductor is dumped via diode D1 to capacitor C1 and the load. Fig.2 (right): block diagram of the Motorola MC34063 DCDC converter IC. can be plugged into your car’s cigarette lighter socket. The output can be set to the desired level by adjusting a trimpot. By the way, for those people who want to run electronic equipment at less than 12V in a car, have a look at the “Power­Pack” published in the May 2001 issue of SILICON CHIP. This puts out a regulated supply at 3V, 6V, 9V and 12V. Performance Maximum output current ....................................... 1.1A <at> 24V, 2A <at> 15.7V Recommended continuous output ....................... 500mA <at> 24V, 1A <at> 16V Output ripple .......................................typically 50mVp-p when delivering 1A Load regulation ..............................better than 98% from no load to full load Performance The performance of the DC-DC Converter is shown in the graph of Fig.3. The output current ranges from a maximum of 2A at 15.7V, dropping to 1.1A at 24V, while still maintaining full regulation. Mind you, if you want to draw this level of current continuously, you would need to improve the heat dissipation of the circuit. We’ll come back to this point later. Output ripple and noise is quite low, nominally 50mV peak-to-peak when delivering 1A. Load regulation is better than 98% from no load to full load. How it works Fig.1 shows the basic operating principle of the DC-DC Converter. It incorporates an inductor, a diode, a switch and a capacitor. When switch S1 is closed, current (I1) flows through the inductor L1 and S1, which then stores energy in the magnetic flux produced by the inductor. When S1 opens, the energy stored in the inductor is dumped via diode D1 to capacitor C1 and the load. In practice, the switch is a transistor or Mosfet and the on/off times of the transistor’s conduction are varied to www.siliconchip.com.au Fig.3: the unit has a maximum output current of 2A at voltages up to 15.7V, dropping to 1.1A at 24V while still maintaining full regulation. main­ tain the desired load voltage. Our circuit uses a Motorola MC34063 DC-DC converter IC as the control device. Its internal circuit is shown in Fig.2. The MC34063 IC contains all the necessary circuitry to produce either step-up, step-down or an inverting DC converter. Its internal components comprise a 1.25V reference, a comparator, an oscillator, RS flipflop and output transistors T1 and T2. The switching frequency of the switching transistor (or Mosfet) is set by the capacitor connected to pin 3. We used 1nF to set it at about 30kHz. The oscillator is used to drive the flipflop which in turn drives the output tranJune 2003  69 sistors. Inductor current is sensed at pin 7 and when this reaches its peak the flipflop and the output transistors are switched off. The time when the output transistors are switched on is determined by the comparator which monitors the output voltage. When the pin 5 comparator input exceeds the 1.25V reference, which means the output voltage exceeds the required level, the comparator goes low to keep the flipflop from setting. This holds the transistors off. Conversely, if the output voltage is too low, the inverting input of the comparator will be below the 1.25V reference and so the output transistors can be toggled by the RS flipflop at the rate set by the oscillator. Circuit details Fig.4 shows the full circuit diagram of the DC-DC Convert­er. The internal transistors of IC1 are connected as a Darlington to drive the gate of Mosfet Q1 high via diode D2 to switch it on. Current then begins to flow in inductor L1. A 0.1Ω 5W resistor between pins 6 & 7 sets the peak current delivered to the inductor to 0.33V/0.1Ω or about 3.3A peak. The average cur­rent delivered to the load via diode D2 is limited to 2A. When pin 2 goes low to turn off Mosfet Q1, transistor Q2 discharges Q1’s gate capacitance for a rapid turn-off. This gives better efficiency than if the gate capacitance was dis­charged via a resistor (as it was in our 1996 design). Each time Q1 turns off, the voltage at its drain rises because of the energy stored in inductor Q1. Because the current can no longer flow in Q1 it is diverted by diode D1 and dumped in the two 470µF capacitors. Diode D1 is a Schottky type which has a fast response to cope with the high switching frequencies (ie, 30kHz). It also has a low forward voltage which reduces power dissipation and improves efficiency. The output capacitors are low ESR (effective series resistance) types suitable for high frequency switchmode operation. Voltage regulation Fig.4: the circuit uses IC1 to drive the gate of Mosfet Q1 via diode D2, while Q2 discharges Q1's gate capacitance each time pin 2 of IC1 goes low. Voltage regulation is provided by the feedback network connected between the output and pin 5 of IC1 (ie, the 22kΩ & 1.2kΩ resistors & trimpot VR1). 70  Silicon Chip Voltage regulation is provided by the feedback network from the output to pin 5. This comprises the 22kΩ resistor from the output and the 1.2kΩ resistor and series 1kΩ trimpot (VR1) connect­ing to ground. The output voltwww.siliconchip.com.au Fig.5: install the parts on the PC board as shown here, taking care to ensure that all polarised parts are correctly oriented. The text has the winding details for inductor L1. age is maintained when the voltage at pin 5 voltage is equal to the internal reference of 1.25V. So, for example if VR1, is set to 0Ω, the output will be 24V since when this is divided down by the resistors [ie, 1.2kΩ/(1.2kΩ + 22kΩ) or divided by 19.33], the voltage at pin 5 is 1.25V. Similarly, if VR1 is set to 1kΩ, the divider now will be (1.2kΩ + 1kΩ)/ (22kΩ + 1.2kΩ + 1kΩ) or divided by 11 and so the output will be 13.75V when pin 5 is at 1.25V. Power for the circuit comes in via a 3A fuse and diode D3, a Schottky power diode included for reverse polarity protection. Supply filtering is provided by two 1000µF 25V low ESR capacitors while further transient voltage protection is provided by the 16V zener diode, ZD1. There is a secondary reason to include diode D3 and this is to ensure that SLA batteries are not overcharged when the car battery voltage goes as high as 14.4V. Since this is a step-up voltage circuit, it cannot normally deliver less than the input voltage since the Mosfet is permanently off, if this situation is called for. When this happens, there is a direct current path via inductor L1 and diode D1 from the car battery to the SLA battery. Hence, the extra voltage drop via diode D3 helps ensure that SLA batteries are only charged to 13.8V. Construction Construction is easy, with the parts all mounted on a PC board coded 11106031 and measuring 120 x 60mm. Fig.5 shows the parts layout. This larger-than-life-size view shows the assembled PC board. The toroid is secured in place using cable ties. www.siliconchip.com.au June 2003  71 shown. Make sure that the wire ends are correctly stripped of insulation before soldering, by scraping it off with a sharp utility knife. L1 is secured in place with two cable ties which loop around it and through holes in the PC board. Spread the windings near Q1’s heatsink and the 100nF capacitor so that they are clear of these parts. The completed PC board is housed in a plastic case measuring 130 x 68 x 43mm. Fit the label to the front panel and drill out the holes for the LED and switch S1. You will also need to drill out the holes at each end of the case for the grommets. Clip the PC board into the case; it clips into the integral side clips within the case. Test the lid to check that the LED passes through the holes with correct alignment. You can adjust it for best fit and height by bending the leads. Wire up a cigarette lighter plug or alligator clip connec­tors to a length of twin automotive wire and pass the other end of the lead through the grommet. Terminate the wires to the input PC board terminals and wire switch S1 as shown. Similarly, connect a second length of automotive wire to the output terminals on the PC board and secure with a grommet. The completed PC board fits neatly into a standard plastic case. Note the rubber grommet between the heatsinks attached to Q1 & D1. You can begin construction by checking the PC board for shorted tracks or breaks in the copper pattern. Fix any defects you discover before going further. Then insert the PC stakes for S1 and inductor L1 and the wire links. Insert and solder in all the resistors using Table 1 to guide you in the colour codes. Insert the IC and zener diode taking care with correct orientation. The capacitors can be mounted next, along with trimpot VR1. The fuseholder clips must be inserted with the correct orientation. The easiest way to make sure the clips are oriented correctly is to fit the fuse into the clips, before inserting them into the PC board. The input and output terminals can now be mounted. D1, D3 and Q1 are mounted vertically on the PC board, each with a heatsink secured with a screw and nut. Note that diode D1 and Mosfet Q1 are held apart with a rubber grommet spacer between their heatsinks. This grommet is held between the heatsink mount­ing screws and prevents the two from making contact which would cause a short circuit. Next, mount Q2 and the LED. LED1 is mounted so that its top is 29mm above the PC board. Testing To test the unit, first apply power from a 12V battery or DC supply and check that the LED lights. If not, check that the LED is oriented correctly. Now measure the voltages on IC1 with a multimeter. There should be about 12V between pins 4 and 6. Now connect a multimeter across the output leads and adjust VR1. The Winding the inductor Inductor L1 is wound with 1mm enamelled copper wire. Draw half the length of wire through the centre of the core and neatly wind on 16 turns, side by side. Then with the other end of the wire, wind on another 16 turns so that the toroid has a total of 32 turns neatly wound around the core. The windings are terminat­ed onto the PC stakes as Table 2: Capacitor Codes Value IEC Code EIA Code 100nF (0.1µF)   100n   104 1nF (.001µF)   1n0   102 Table 1: Resistor Colour Codes o No. o  1 o  1 o  1 o  2 o  1 72  Silicon Chip Value 22kΩ 2.2kΩ 1.2kΩ 1kΩ 47Ω 4-Band Code (1%) red red orange brown red red red brown brown red red brown brown black red brown yellow violet black brown 5-Band Code (1%) red red black red brown red red black brown brown brown red black brown brown brown black black brown brown yellow violet black gold brown www.siliconchip.com.au Parts List Fig.7: here are the full-size artworks for the front panel and PC board pattern. voltage range should be from 13.8-24V. Note that the voltage will take several seconds to drop from a higher voltage to a lower setting since the only load is the voltage sensing resistors and these need to discharge the output capaci­tors. Set the voltage to that required for your application. If you want to charge SLA batteries, set the output to 13.8V. Now connect the unit to the appli- ance using a suitable connector. Be sure the output connector polarity is correct before running the appliance. Check that Mosfet Q1 and diodes D1 & D3 run warm rather than hot. Finally, if you need to continuously run the DC-DC convert­er at its full rated output of 2A, it would be wise to run it in a ventilated metal case and possibly use larger heatsinks for SC Q1, D1 & D3. This oscilloscope trace shows the gate drive to the Mosfet Q1. There is almost 11V drive with fast rise and fall times. The fast fall time is improv­ed using the Q2 gate discharge transistor which quickly discharges the gate capacit­ ance. www.siliconchip.com.au 1 PC board, code 11106031, 120 x 60mm 1 plastic case, 130 x 68 x 43mm 1 panel label, 126 x 64mm 1 powdered iron core (Neosid 17-742-22; Jaycar LO-1244; L1) 1 SPST rocker switch (S1) 2 2-way PC-mount screw terminals 8.25mm pin spacing (Altronics Cat. P-2101 3 mini heatsinks, 19 x 19 x 10mm 2 M205 PC-mount fuse clips 1 M205 3A fast-blow fuse (F1) 2 cordgrip grommets 1 14mm OD rubber grommet 1 plug for automotive cigarette lighter socket 1 1m length of red automotive wire 1 1m length of black automotive wire 1 1.2m length of 1mm enamelled copper wire 1 60mm length of 0.7mm tinned copper wire 2 100mm long cable ties 3 M3 x 10mm screws 3 M3 nuts 4 PC stakes 1 1kΩ horizontal trimpot (coded 102) (VR1) Semiconductors 1 MC34063 DC-DC converter (IC1) 1 MTP3055E N-channel Mosfet (Q1) 1 BC327 PNP transistor (Q2) 2 MBR735 7A 35V Schottky diodes (D1,D3) 1 5mm red LED (LED1) 1 1N914, 1N4148 diode (D2) 1 16V 1W zener diode (ZD1) Capacitors 2 1000µF 25V low ESR electrolytic (Altronics Cat. R-6184) 2 470µF 50V low ESR electrolytic (Atronics Cat. R-6167) 1 100nF MKT polyester 1 1nF MKT polyester Resistors (0.25W, 1%) 1 22kΩ 2 1kΩ 1 2.2kΩ 1 47Ω 1 1.2kΩ 1 0.1Ω 5W June 2003  73