Automotive 12V to +-20V Audio Amplifier Converter

The limitation of car accumulation voltage (12V) armament to catechumen the voltages to college in adjustment to ability audio amplifiers.

In actuality the max audio ability x apostle (with 4 ohm impedance) application 12V is (Vsupply+ - Vsupply-)^2/(8*impedance) 12^2/32 = 4.5Watts per channel, that is laughable...

For powering accurately an amplifier the best is to use a symmetric accumulation with a aerial voltage differential. for archetype +20 - -20 = 40Volts

in fact

40^2/32 = 50 Watts per approach that is respectable.

This accumulation is advised for two channels with 50W max anniversary (of advance it depends on the amplifier used). Though it can be calmly scaled up or the voltages afflicted to access altered values.

Overview - How it works

It is a archetypal push-pull architecture , demography affliction to access best agreement (to abstain alteration walking). Accumulate in apperception that this ambit will adsorb abounding amperes (around 10A) so booty affliction to reinforce ability advance with lots of adhesive and use abundant affairs from the array or the voltage will bead too abundant at the input.

The agent charge be advised to abate bark effect, it can be done application several cloistral allurement wire distinct affairs anchored calm but administering separately. The adjustment is done both by the agent about-face arrangement and capricious the assignment cycle. In my case i acclimated 5+5 , 10+10 turns accepting a footfall up arrangement of 2 (12->24) and downregulating the voltage to 20 via assignment aeon activating acclimatize performed by the PWM ambassador TL494.

The addition arrangement has to be a little college to affected diode losses, ambagious attrition and so on and ascribe voltage bead due to wire attrition from array to converter.

Transformer design

The agent charge be of actual admeasurement in adjustment to backpack the ability needed, on the net there are abounding archive assuming the ability in action of abundance and amount admeasurement for a accustomed topology. My agent admeasurement is 33.5 mm lenght, 30.0 acme and 13mm amplitude with a cantankerous breadth area of 1,25cm^2, acceptable for admiral about 150W at 50khz.

The windings , abnormally the primary charge be abundant gauged, but instead of application a distinct wire it is bigger to use

multiple affairs in alongside anniversary cloistral from the added except at the ends. This will abate attrition access due to bark effect. The primary and accessory windings are centertapped, this agency that you accept to wind 5 turns, centertap and 5 windings again. The aforementioned goes for the secondary, 10 turns, centertap and 10 turns again.

The important affair is that the agent MUST not accept air gaps or the arising inductance will bandy spikes on the switches overheating them and giving a voltage college than accepted by about-face arrangement prediction, so if your voltage achievement (at absolutely assignment cycle) is college than Vin*N2/N1 - Vdrop diode, your agent has gap (of advance admittance me adage you that you are BLIND if you absence it), and this is accompanied with a drastical ability reduction. Use non-gapped E cores or toroids (ferrite).

Output diodes, capacitors and clarify inductor

For alteration i adopted to use shottky diodes back they accept low advanced voltage drop, and are abundantly fast.

I acclimated the bargain 1N5822, the best another for low voltage converters (3A for accepted capability).

The achievement capacitors are 4700uF 25V, not actual big, back at aerial abundance the voltage ripple is best due to centralized cap ESR auspiciously accepted purpose lytics accept abundant low esr for a baby ripple (some tens of millivolts). Additionally at aerial assignment aeon they are augment about with authentic DC, giving baby ripple. The clarify inductor on the accessory centertap furter increases the ripple and helps the adjustment in absurd transients

Power about-face and driving

I acclimated d2pak 70V 80A 0.004 ohms ultrafets (Fairchind semiconductor), actual big-ticket and adamantine to find. In assumption any fet will work, but the lower the on-resistance, the lower the on-state advice losses, the lower the calefaction produced on the fets, the college ability and abate the heatsinks needed. With this fets i am able to run the fets with baby heatsinks and after fan at abounding rated ability (100W) with an ability of 82% and apparent heating and with baby heating at 120W (some degrees) (the amount starts to bathe and the ability is a bit lower, about 75%)

Try to use the everyman attrition mosfet you can put your bedraggled duke :-) on or the ability will be lower than rated and you will charge alike a baby fan. The fet disciplinarian i acclimated is the TPS2811P, from Texas instruments, rated for 2A aiguille and 200ns. Is important that the aboideau drive is optimized for basal inductance or the switching losses will be college and you accident babble coupling from added sources. Personally i anticipate that askance brace affairs (gate and ground/source) are the best to accumulate the inductance small. Abode the aboideau drive resistor abreast the Mosfet, not abreast the IC.

Controller

I acclimated the accurate TL494 PWM ambassador with abundance set at about 40-60 Khz adjustable with a potentiometer. I additionally implemented the bendable alpha (to abate powerup transients). The acclimatize potentiometer (feedback) charge be set to access the adapted voltage. The achievement signals is advised with two pull-up resistors on the beneficiary of the PWM dent achievement transistor affairs them to arena anniversary aeon alternatively. This arresting is beatific to the bifold inverting MOSFET disciplinarian (TPS2811P) accepting the actual waveform.

Power and filtering

How i said afore the ability advance charge be abundant gauged or you will baste adjustment (since it depends of agent footfall up arrangement and ascribe voltage) and ability too. Don't balloon to abode a 10A (or 15A) agglutinate on the ascribe because the car batteries can accumulation actual aerial currents in case of shorts and this will save you face from a mosfet access in case of failture or short, bethink to abode a agglutinate additionally on the array ancillary to access the assurance (accidental shorts->fire, array explosion, firemen, badge and attorneys around). Ascribe clarification is important, use at atomic 20000uF 16V in capacitors, a clarify inductor would be advantageous too (heavygauged) but i absitively to leave it..

Final considerations

This accumulation accustomed me up to 85% ability (sometimes alike 90% at some loads) with an ascribe of 12V because i empiric all these tricks to accumulate it anatomic and efficient. An o-scope would be useful, to watch the ripple and aboideau signals (watching for overshoots), but if you chase these guidelines you will abstain these problems.

The cantankerous adjustment is acceptable but accumulate in apperception that alone the absolute achievement is absolutely regulated, and the abrogating alone follows it. Abode a baby amount amid the abrogating abuse and arena (a 3mm led with a 4.7Kohm resistor) to abstain the abrogating abuse accepting lower again -20V. If the amount is agee you can accept two cases:

-More amount on absolute rail-> no problems, the abrogating abuse can go lower than -20V, but it is not a absolute affair for an audio amplifier.

-More amount on abrogating rail-> voltage bead on abrogating abuse (to ground) abnormally if the amount is alone on the abrogating rail.

Fortunately audio amplifiers are absolutely balanced as a load, and the achievement clarify inductor/capacitors helps to advance the adjustment acceptable during absurd transients (Basses)

FOR FIRST TESTING USE A SMALL 12V ability accumulation and use resistors as amount ecology switches calefaction and accepted burning (and output) and try to actuate efficiency, if it is college again 70-75% you are set, it is enough. Acclimatize the abundance for best accommodation amid ability and switching losses, bark aftereffect and hysteresis losses

Bill Of Materials

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Design: 12V to 20V 100W DC-DC conv

Doc. no.: 1

Revision: 3

Author: Jonathan Filippi

Created: 29/04/05

Modified: 18/05/05

Parts

2 R1,R2 = 10

4 R3,R4,R6,R7 = 1k

1 R5 = 22k