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DC DC converter

Project main features

High power 24V DC DC converter



The DC/DC converter has the following main features:
  • Input voltage from 10V to 18V: a traditional car battery can be used as input source.
  • Output voltage from 20V to 28V adjustable with a simple trimmer for better cell pack charge optimization (up to 14 cells).
  • Output current up to 5A for 3300mAh battery packs fast charge.
  • Compact dimension (80x60mm), the same of the Advanced Charger. The DC/DC converter has the output connector in the same position of the Advanced Charger power input, in order to stack and couple the two boards with only 20mm dimensions increase.
  • Very cheap design: simple architecture, non critical and commonly available components, with a maximum total cost under 10 Euro.
  • Simple design but very high efficiency: from 90 to 95% (depending on components quality and type), giving the possibility to have no heat sink nor fan coolers even delivering up to 140W to the load.


DC DC converter schematic description

Description of the DC/DC converter project .

The schematics for the DC/DC converter is built around the UC3843 generic, low cost PWM controller. This very common PWM controller generate a duty-cycle modulated square wave ranging from 0 to 100%, at a user fixed frequency of 100KHz. The operating frequency is fixed via the C13 capacitor. In practice the UC3843 sense the output voltage via the resistive partitor P1/R5 that (thanks to the P1 trimmer) allow the user to change the DC/DC output voltage.

The input voltage can be set from 8V (lower limit for the UC3843) and 20V (upper limit for the mosftets gate voltage). The DC/DC output voltage can be changed between the input voltage (the output cannot be lower than the input for the circuit topology) and about the breakdown voltage of the output components. The main limitation is the total dissipated power: in fact we measured an efficiency of about 90-95% (depending on the quality of components): if the output power is about 120W (24V@5A) the dissipated power is 6-12W and without cooler the risk is to overheat the output capacitors, the inductor, the schottky diode and the mosfets. In practice with the output at 24V, 4A no particular components must be used, in case of higher power only "selected" components (see table) must be used. At the increase of the power the criticals components are the following:
  1. Input capacitors: choose low ESR capacitors (selected for DC/DC converters).
  2. Output capacitors: choose low ESR capacitors (selected for DC/DC converters).
  3. Schottky diode: choose high current and lower voltage drop diode.
  4. Inductor: choose wire with bigger diameter.
  5. Mosfets: choose mosfet with lower Rds(on).

The ESR value of the capacitors must be minimized at 100KHz frequency, the ESR resistance at 50-60Hz, commonly listed in the capacitors datasheets, is not useful for DC/DC converter selection.

Inductor
: the inductor toroid must be selected with high care. The right toroid must have a quite low "Al" parameter (below 100nH/n^2) and have sufficient room to fit all thewire turns. In the reference example we used an iron powder toroid with the Al=80nH/n^2, with 15 turns of 1.25mm diameter wire. The inductor value should be in the range 15..20uH, and we obtained L1=80*15*15=18uH. The room in the PCB for the inductor is about 23-24mm, for this select an appropriate size for the toroid and wire.


DC/DC (below 100W) critical components partlist

In the "Below 100W" DC/DC converter version, the components are non critical, for this can be replaced with some equivalents with similar characteristics.
  • Q1,Q2,Q3: IRF540N.
  • L1 wire: 0.8-1mm diameter.
  • D1: MBR1060.
  • C1,C2,C3,C4: 470uF, 63V not selected.

DC/DC (over 100W) critical components partlist

In the "Over 100W" DC/DC converter version, the components are critical, for this can be replaced with some equivalents with selected characteristics.
  • Q1,Q2,Q3: IRF1404.
  • L1 wire: 1.25-1.5mm diameter.
  • D1: MBR1545.
  • C1,C2,C3,C4: 470uF, 63V low ESR, selected for DC/DC converters.

DC/DC converter non-critical components partlist

The non critical components are the following.
  • U1: UC3843BN.
  • R1,R2,R11: 10 Ohm, 1/4W.
  • R5: 1KOhm, 1/4W.
  • R4: 4.7KOhm, 1/4W.
  • R3: 7.5KOhm, 1/4W.
  • R6: 27KOhm, 1/4W.
  • R7: 10KOhm, 1/4W.
  • P1: 20KOhm, multiturn trimpot.
  • C10: 100nF, 50V.
  • C11: 100nF, 50V.
  • C12: 100pF, 50V.
  • C13: 1nF, 50V.

DC DC converter PCB assembly

How to build the DC/DC converter project

Start the DC/DC assembly with the resistors and the capacitors. All the assembly should start with the lower height components. Mount a socket for the IC, then go to soldering the TO220 packages (mosfets and diode). These components are connected directly to a copper plane, for this the soldering iron temperature should be quite high (400-450 celsius degrees) in order to have a good junction. Mount at the end the inductor (remove the isolator with high care) and the input/output capacitors (470uF, 63V). To test the circuit, power supply the circuit with 12V and with a light load measure the output voltage: set via P1 the right voltage and check that there are no components with high temperature. Start to use the DC/DC converter, with attention to use "normal" components for power under 100W and "selected" components for power over 100W (see schematic description).





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Oloap,
Nov 5, 2013, 1:04 PM
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Oloap,
Nov 5, 2013, 1:04 PM
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Oloap,
Nov 5, 2013, 1:04 PM
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Oloap,
Nov 5, 2013, 1:04 PM
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