Automotive 12V to +-20V converter for audio amplifier
The limitation of car supply voltage (12V) forces to convert the voltages to higher in order to power audio amplifiers.
In fact the max audio power x speaker (with 4 ohm impedance) using 12V is (Vsupply+ - Vsupply-)^2/(8*impedance) 12^2/32 = 4.5Watts per channel, that is laughable...
For powering correctly an amplifier the best is to use a symmetric supply with a high voltage differential. for example +20 - -20 = 40Volts
in fact
40^2/32 = 50 Watts per channel that is respectable.
This supply is intended for two channels with 50W max each (of course it depends on the amplifier used). Though it can be easily scaled up or the voltages changed to obtain different values.
Overview - How it works
It is a classic push-pull design , taking care to obtain best symmetry (to avoid flux walking). Keep in mind that this circuit will adsorb many amperes (around 10A) so take care to reinforce power tracks with lots of solder and use heavy wires from the battery or the voltage will drop too much at the input.
The transformer must be designed to reduce skin effect, it can be done using several insulated magnet wire single wires soldered together but conducting separately. The regulation is done both by the transformer turn ratio and varying the duty cycle. In my case i used 5+5 , 10+10 turns obtaining a step up ratio of 2 (12->24) and downregulating the voltage to 20 via duty cycle dynamic adjust performed by the PWM controller TL494.
The step-up ratio has to be a little higher to overcome diode losses, winding resistance and so on and input voltage drop due to wire resistance from battery to converter.
Transformer design
The transformer must be of correct size in order to carry the power needed, on the net there are many charts showing the power in function of frequency and core size for a given topology. My transformer size is 33.5 mm lenght, 30.0 height and 13mm width with a cross section area of 1,25cm^2, good for powers around 150W at 50khz.
The windings , especially the primary must be heavy gauged, but instead of using a single wire it is better to use
multiple wires in parallel each insulated from the other except at the ends. This will reduce resistance increase due to skin effect. The primary and secondary windings are centertapped, this means that you have to wind 5 turns, centertap and 5 windings again. The same goes for the secondary, 10 turns, centertap and 10 turns again.
The important thing is that the transformer MUST not have air gaps or the leakage inductance will throw spikes on the switches overheating them and giving a voltage higher than expected by turn ratio prediction, so if your voltage output (at fully duty cycle) is higher than Vin*N2/N1 - Vdrop diode, your transformer has gap (of course permit me saying you that you are BLIND if you miss it), and this is accompanied with a drastical efficiency reduction. Use non-gapped E cores or toroids (ferrite).
Output diodes, capacitors and filter inductor
For rectification i preferred to use shottky diodes since they have low forward voltage drop, and are incredibly fast.
I used the cheap 1N5822, the best alternative for low voltage converters (3A for current capability).
The output capacitors are 4700uF 25V, not very big, since at high frequency the voltage ripple is most due to internal cap ESR fortunately general purpose lytics have enough low esr for a small ripple (some tens of millivolts). Also at high duty cycle they are feed almost with pure DC, giving small ripple. The filter inductor on the secondary centertap furter increases the ripple and helps the regulation in asymmetrical transients
Power switch and driving
I used d2pak 70V 80A 0.004 ohms ultrafets (Fairchind semiconductor), very expensive and hard to find. In principle any fet will work, but the lower the on-resistance, the lower the on-state conduction losses, the lower the heat produced on the fets, the higher efficiency and smaller the heatsinks needed. With this fets i am able to run the fets with small heatsinks and without fan at full rated power (100W) with an efficiency of 82% and perceptible heating and with small heating at 120W (some degrees) (the core starts to saturate and the efficiency is a bit lower, around 75%)
Try to use the lowest resistance mosfet you can put your dirty hand :-) on or the efficiency will be lower than rated and you will need even a small fan. The fet driver i used is the TPS2811P, from Texas instruments, rated for 2A peak and 200ns. Is important that the gate drive is optimized for minimal inductance or the switching losses will be higher and you risk noise coupling from other sources. Personally i think that twisted pair wires (gate and ground/source) are the best to keep the inductance small. Place the gate drive resistor near the Mosfet, not near the IC.
Controller
I used the trusty TL494 PWM controller with frequency set at around 40-60 Khz adjustable with a potentiometer. I also implemented the soft start (to reduce powerup transients). The adjust potentiometer (feedback) must be set to obtain the desired voltage. The output signals is designed with two pull-up resistors on the collector of the PWM chip output transistor pulling them to ground each cycle alternatively. This signal is sent to the dual inverting MOSFET driver (TPS2811P) obtaining the correct waveform.
Power and filtering
How i said before the power tracks must be heavy gauged or you will scarify regulation (since it depends of transformer step up ratio and input voltage) and efficiency too. Don't forget to place a 10A (or 15A) fuse on the input because the car batteries can supply very high currents in case of shorts and this will save you face from a mosfet explosion in case of failture or short, remember to place a fuse also on the battery side to increase the safety (accidental shorts->fire, battery explosion, firemen, police and lawyers around). Input filtering is important, use at least 20000uF 16V in capacitors, a filter inductor would be useful too (heavygauged) but i decided to leave it..
Final considerations
This supply given me up to 85% efficiency (sometimes even 90% at some loads) with an input of 12V because i observed all these tricks to keep it functional and efficient. An o-scope would be useful, to watch the ripple and gate signals (watching for overshoots), but if you follow these guidelines you will avoid these problems.
The cross regulation is good but keep in mind that only the positive output is fully regulated, and the negative only follows it. Place a small load between the negative rail and ground (a 3mm led with a 4.7Kohm resistor) to avoid the negative rail getting lower then -20V. If the load is asymmetric you can have two cases:
-More load on positive rail-> no problems, the negative rail can go lower than -20V, but it is not a real issue for an audio amplifier.
-More load on negative rail-> voltage drop on negative rail (to ground) especially if the load is only on the negative rail.
Fortunately audio amplifiers are quite symmetrical as a load, and the output filter inductor/capacitors helps to maintain the regulation good during asymmetrical transients (Basses)
In fact the max audio power x speaker (with 4 ohm impedance) using 12V is (Vsupply+ - Vsupply-)^2/(8*impedance) 12^2/32 = 4.5Watts per channel, that is laughable...
For powering correctly an amplifier the best is to use a symmetric supply with a high voltage differential. for example +20 - -20 = 40Volts
in fact
40^2/32 = 50 Watts per channel that is respectable.
This supply is intended for two channels with 50W max each (of course it depends on the amplifier used). Though it can be easily scaled up or the voltages changed to obtain different values.
It is a classic push-pull design , taking care to obtain best symmetry (to avoid flux walking). Keep in mind that this circuit will adsorb many amperes (around 10A) so take care to reinforce power tracks with lots of solder and use heavy wires from the battery or the voltage will drop too much at the input.
The transformer must be designed to reduce skin effect, it can be done using several insulated magnet wire single wires soldered together but conducting separately. The regulation is done both by the transformer turn ratio and varying the duty cycle. In my case i used 5+5 , 10+10 turns obtaining a step up ratio of 2 (12->24) and downregulating the voltage to 20 via duty cycle dynamic adjust performed by the PWM controller TL494.
The step-up ratio has to be a little higher to overcome diode losses, winding resistance and so on and input voltage drop due to wire resistance from battery to converter.
Transformer design
The transformer must be of correct size in order to carry the power needed, on the net there are many charts showing the power in function of frequency and core size for a given topology. My transformer size is 33.5 mm lenght, 30.0 height and 13mm width with a cross section area of 1,25cm^2, good for powers around 150W at 50khz.
The windings , especially the primary must be heavy gauged, but instead of using a single wire it is better to use
multiple wires in parallel each insulated from the other except at the ends. This will reduce resistance increase due to skin effect. The primary and secondary windings are centertapped, this means that you have to wind 5 turns, centertap and 5 windings again. The same goes for the secondary, 10 turns, centertap and 10 turns again.
The important thing is that the transformer MUST not have air gaps or the leakage inductance will throw spikes on the switches overheating them and giving a voltage higher than expected by turn ratio prediction, so if your voltage output (at fully duty cycle) is higher than Vin*N2/N1 - Vdrop diode, your transformer has gap (of course permit me saying you that you are BLIND if you miss it), and this is accompanied with a drastical efficiency reduction. Use non-gapped E cores or toroids (ferrite).
Output diodes, capacitors and filter inductor
For rectification i preferred to use shottky diodes since they have low forward voltage drop, and are incredibly fast.
I used the cheap 1N5822, the best alternative for low voltage converters (3A for current capability).
The output capacitors are 4700uF 25V, not very big, since at high frequency the voltage ripple is most due to internal cap ESR fortunately general purpose lytics have enough low esr for a small ripple (some tens of millivolts). Also at high duty cycle they are feed almost with pure DC, giving small ripple. The filter inductor on the secondary centertap furter increases the ripple and helps the regulation in asymmetrical transients
Power switch and driving
I used d2pak 70V 80A 0.004 ohms ultrafets (Fairchind semiconductor), very expensive and hard to find. In principle any fet will work, but the lower the on-resistance, the lower the on-state conduction losses, the lower the heat produced on the fets, the higher efficiency and smaller the heatsinks needed. With this fets i am able to run the fets with small heatsinks and without fan at full rated power (100W) with an efficiency of 82% and perceptible heating and with small heating at 120W (some degrees) (the core starts to saturate and the efficiency is a bit lower, around 75%)
Try to use the lowest resistance mosfet you can put your dirty hand :-) on or the efficiency will be lower than rated and you will need even a small fan. The fet driver i used is the TPS2811P, from Texas instruments, rated for 2A peak and 200ns. Is important that the gate drive is optimized for minimal inductance or the switching losses will be higher and you risk noise coupling from other sources. Personally i think that twisted pair wires (gate and ground/source) are the best to keep the inductance small. Place the gate drive resistor near the Mosfet, not near the IC.
Controller
I used the trusty TL494 PWM controller with frequency set at around 40-60 Khz adjustable with a potentiometer. I also implemented the soft start (to reduce powerup transients). The adjust potentiometer (feedback) must be set to obtain the desired voltage. The output signals is designed with two pull-up resistors on the collector of the PWM chip output transistor pulling them to ground each cycle alternatively. This signal is sent to the dual inverting MOSFET driver (TPS2811P) obtaining the correct waveform.
Power and filtering
How i said before the power tracks must be heavy gauged or you will scarify regulation (since it depends of transformer step up ratio and input voltage) and efficiency too. Don't forget to place a 10A (or 15A) fuse on the input because the car batteries can supply very high currents in case of shorts and this will save you face from a mosfet explosion in case of failture or short, remember to place a fuse also on the battery side to increase the safety (accidental shorts->fire, battery explosion, firemen, police and lawyers around). Input filtering is important, use at least 20000uF 16V in capacitors, a filter inductor would be useful too (heavygauged) but i decided to leave it..
Final considerations
This supply given me up to 85% efficiency (sometimes even 90% at some loads) with an input of 12V because i observed all these tricks to keep it functional and efficient. An o-scope would be useful, to watch the ripple and gate signals (watching for overshoots), but if you follow these guidelines you will avoid these problems.
The cross regulation is good but keep in mind that only the positive output is fully regulated, and the negative only follows it. Place a small load between the negative rail and ground (a 3mm led with a 4.7Kohm resistor) to avoid the negative rail getting lower then -20V. If the load is asymmetric you can have two cases:
-More load on positive rail-> no problems, the negative rail can go lower than -20V, but it is not a real issue for an audio amplifier.
-More load on negative rail-> voltage drop on negative rail (to ground) especially if the load is only on the negative rail.
Fortunately audio amplifiers are quite symmetrical as a load, and the output filter inductor/capacitors helps to maintain the regulation good during asymmetrical transients (Basses)
MK1 Transforming Coffee Table
Sometimes tiny apartments are just a fact of life, making you do the best you can with a small amount of space. So, if your room is at a premium, you need the MK1 Transforming Coffee Table ($1,300). It's a coffee table when you want it, and a dining room table when you need it, easily transforming from one to the other in just two simple movements.
Made from either solid oak, ash, or walnut and birch plywood, this table is available in a number of attractive finishes to suit your space. All you need to do to transform is into a dining room table is fold out the eaves — and fold them back in when you're finished.
Made from either solid oak, ash, or walnut and birch plywood, this table is available in a number of attractive finishes to suit your space. All you need to do to transform is into a dining room table is fold out the eaves — and fold them back in when you're finished.
Mushkin Atom 3.0 USB flash drive introduced
I am quite sure that by now, many of us have already amassed quite a collection of USB flash drives over the months and years that have gone by. Hardly any conference or road show is devoid of a USB flash drive which more often than not contains the press release of a particular company, or a portfolio of new products. Having said that, it can be pretty difficult trying to keep track of just where exactly have you placed that particular file over in the myriad of USB flash drives that you have in your collection, and instead get a personal USB flash drive for yourself so that you can keep personal things private and secure.
Just in case you are in the market scouting for a new USB flash drive, have you considered checking out the Mushkin Atom 3.0? The Mushkin Atom 3.0 USB flash drive will place plenty of emphasis on convenience, where it comes in an extremely small form factor which is slightly less than the size of your average quarter. All of this without having to sacrifice on performance, since the Mushkin Atom 3.0 will still be able to boast of USB 3.0 extreme performance even when exercising low power consumption. Apart from that, you do not have to worry about legacy systems, since the Atom USB 3.0 flash drive will still play nice with USB 2.0 and 1.1 ports.
As for the 8GB model, it will feature 80MB/s read, 5.5MB/s write, and consumes 0.65W while idle, or 0.905W at load. There is also a 16GB flash drive model that boasts of 155MB/s read and 11.5MB/s write on 0.25W idle and 0.67W load. As for the 32GB model, it reads at 155MB/s with 21.5MB/s write, and low power consumption of 0.2475W when idle and 0.75W under load. Since it is so small, you can always opt to leave the Atom USB 3.0 flash drive in the USB port while working on-the-go without having to suffer from any obstruction. Notebook and Ultrabook owners, especially, would be able to appreciate its tiny design and performance. [Via]
Just in case you are in the market scouting for a new USB flash drive, have you considered checking out the Mushkin Atom 3.0? The Mushkin Atom 3.0 USB flash drive will place plenty of emphasis on convenience, where it comes in an extremely small form factor which is slightly less than the size of your average quarter. All of this without having to sacrifice on performance, since the Mushkin Atom 3.0 will still be able to boast of USB 3.0 extreme performance even when exercising low power consumption. Apart from that, you do not have to worry about legacy systems, since the Atom USB 3.0 flash drive will still play nice with USB 2.0 and 1.1 ports.
As for the 8GB model, it will feature 80MB/s read, 5.5MB/s write, and consumes 0.65W while idle, or 0.905W at load. There is also a 16GB flash drive model that boasts of 155MB/s read and 11.5MB/s write on 0.25W idle and 0.67W load. As for the 32GB model, it reads at 155MB/s with 21.5MB/s write, and low power consumption of 0.2475W when idle and 0.75W under load. Since it is so small, you can always opt to leave the Atom USB 3.0 flash drive in the USB port while working on-the-go without having to suffer from any obstruction. Notebook and Ultrabook owners, especially, would be able to appreciate its tiny design and performance. [Via]
Logitech Folio Style Protective iPad Mini Case
Logitech recently not only released its latest keyboard case for iPad mini, but also launched a new folio style protective case. The Folio is a well-designed folio styled protective case for iPad mini. The iPad mini case weights 196g, and as we can see from the images, the protective case features sleek, ultra-thin design, and is made from durable, water-repellent materials using cutting-edge manufacturing techniques in order to effectively protect your iPad mini from bumps and scratches.
Moreover, the iPad mini case doubles as an iPad stand to hold your device in several different positions. Apart from that, its SecureLock system keeps your iPad mini securely in place, and built-in magnets activates iPad mini’s auto wake/sleep functions. Logitech Folio iPad mini case is available in three colors, each one is priced at $49.99 USD. If you’re interested, jump to Logitech official site for more details.
Moreover, the iPad mini case doubles as an iPad stand to hold your device in several different positions. Apart from that, its SecureLock system keeps your iPad mini securely in place, and built-in magnets activates iPad mini’s auto wake/sleep functions. Logitech Folio iPad mini case is available in three colors, each one is priced at $49.99 USD. If you’re interested, jump to Logitech official site for more details.
PureGear Folio iPhone 5c Case
Need a suitable protective case to match the color of your iPhone 5c and hold your bank cards? Take a look at PureGear’s Folio iPhone 5c case, it may be able to meet your requirements. The Folio is a versatile wallet-style protective case designed for iPhone 5c.
As we can see from the images, the iPhone 5c case features elegant slim design along with bright color to meet your iPhone 5c’s design, and three internal slots allow to hold your ID and credit cards. Moreover, the iPhone 5c case is made of rubberized plastic and lined with soft microfiber in order to protect your iPhone 5c from impacts and scratches, and custom cutouts let you access all iPhone buttons and functions.
The Folio iPhone 5c case is available 4 gorgeous colors, each one costs $39.99 USD. If you’re interested, jump to PureGear official site for more details.
As we can see from the images, the iPhone 5c case features elegant slim design along with bright color to meet your iPhone 5c’s design, and three internal slots allow to hold your ID and credit cards. Moreover, the iPhone 5c case is made of rubberized plastic and lined with soft microfiber in order to protect your iPhone 5c from impacts and scratches, and custom cutouts let you access all iPhone buttons and functions.
HoneyDru USB Car Power Adapter
Having anything that is “cute” does seem to work these days, and considering how many of us carry a fair number of gadgets, it can be quite a task trying to make sure that all of them are fully juiced at the beginning of each day. Having said that, having a car charger in your ride is not too bad an idea after all, since you can never quite tell just when you might need to fall upon some backup power on the way to your meeting, with your smartphone running on empty. Why not get something cute while you are at it.
The $19.99 HoneyDru USB Car Power Adapter will certainly not disappoint, where this small, bee-shaped car power adapter will come with light-up eyes and wings for added “coolness” effect, especially during the evenings. Good thing nobody should be allergic to this bee-like USB car power adapter, as it does not sting at all, and the only thing it does well would be to power up those thirsty batteries in your well used gadgets.
The HoneyDru USB Car Power Adapter would be plugged into most vehicles’ power outlet cigarette lighter, and it is capable of 5V2A output, which should be more than some of the native chargers which accompany your devices. Apart from that, it will also feature a coiled cord which can be stretched to 3 feet and retracts to 8 inches for adaptability and flexibility.
The HoneyDru USB Car Power Adapter would be plugged into most vehicles’ power outlet cigarette lighter, and it is capable of 5V2A output, which should be more than some of the native chargers which accompany your devices. Apart from that, it will also feature a coiled cord which can be stretched to 3 feet and retracts to 8 inches for adaptability and flexibility.
Smart AC Automotive Power Strip
Women might say that they will never be able to have enough handbags and shoes (generally speaking), while men might never get enough of their power tools and hardware toys, and kids with their video games, but I am quite sure that something most of us will be able to agree on would be this – you can never have enough power outlets around, considering the rather large number of gadgets that we carry around with us these days. Smartphones, tablets, digital cameras, smart watches, the list goes on and on, and Thinkgeek intends to help you out with the $34.99 Smart AC Automotive Power Strip.
The Smart AC Automotive Power Strip will be able to juice up just about all things, where this inverter plugs into your cigarette lighter socket. It will come with a couple of USB power ports and two AC outlets, and fret not about it melting down or catching fire, since the safety features which have been thrown into the mix include over-voltage, overload, short-circuit, high-temperature protection, and low voltage shut-off. The whole family should be more than happy with this bad boy, since you can juice smartphones, tablets, e-readers, laptops, DVD players, game consoles, and digital cameras among others while you are on the go. [Via]
The Smart AC Automotive Power Strip will be able to juice up just about all things, where this inverter plugs into your cigarette lighter socket. It will come with a couple of USB power ports and two AC outlets, and fret not about it melting down or catching fire, since the safety features which have been thrown into the mix include over-voltage, overload, short-circuit, high-temperature protection, and low voltage shut-off. The whole family should be more than happy with this bad boy, since you can juice smartphones, tablets, e-readers, laptops, DVD players, game consoles, and digital cameras among others while you are on the go. [Via]
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