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Old 21st May 2003, 03:02 PM   #1
Paulr is offline Paulr  United Kingdom
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Default DC to DC converters

Does anyone have any experience with DC to DC converters? I'm looking at getting 15-0-15 (dc) at <100 ma from a car battery to power a few op-amps & this seems like the easiest way.

Does anyone know of any low power wide input ones. The wide input ones are usually on larger (5w up) & the ones I have seen have all been single output.

Cheers

Paul
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Old 21st May 2003, 03:35 PM   #2
PMiczek is offline PMiczek  United States
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Default DC-to +/-15V converter

Maxim MAX742, +/- 15V output with just a few parts. Ckt design examples are in the datasheet.

10V max input, but with such a low current output you can lower the input voltage easily, and build in some protection for transients and noise at the same time. In other words, I would pre-regulate the car battery supply to, say 9 volts first, with a filter and transorb for spikes, or maybe a couple inductors depending on the application, and put the DC-DC converter afterwards. Then filter the converter outputs just like the output of any +/- DC supply.

Maxim web site is http://www.maxim-ic.com/PowerSupplies.cfm

Maxim also has evaluation boards which can often be used for diy projects with a few extra parts and wires.

If you can lower the +/- voltage requirement, there are more choices.

PM
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Old 5th June 2003, 02:18 PM   #3
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Default dc to dc converter options

At only 100 mA output, I would suggest a non-isolated approach.

For the -15 volt output, a simple single-switch inverting buck-boost dc/dc converter should work well. Minus 15 volts at 0.1 amp is 1.5 watts, similar to a -4 volt, 0.425 amp design I'm currently working on. One wide range voltage mode controller I'm considering is the TI UC3572 (8 to 35 volts input), with pulse-by-pulse current limiting. Also, Natl Semi's series of simple switchers would be great here. Their app notes detail how to use a "buck", or step-down converter in the inverting mode. Another great choice is On Semi MC34063A. App note 920-D details inverting buck-boost operation. Input voltage range is a few volts up to 40 V. No external transistors are needed. At 0.1 amp, I'd consider operating in the discontinuous conduction mode (DCM). DCM is easier to stabilize and has faster transient response as opposed to CCM (continuous conduction mode). Input and output capacitors are bigger, however for DCM. If noise is an issue, CCM would be quieter. The MC34063A operates in the critical conduction mode (CRM), with variable frequency, a very effective approach. Also, do you really need -15 volts? Would -12 V be sufficient?

For the +15 V (or +12 V), a two-switch non-inverting buck-boost dc/dc converter works well. On Semi's MC34063A works well here without external transistors, since it has two npn parts built in. App note 920-D covers this.

Since both converters are non-isolated, no special transformer is needed, and a simple off the shelf inductor can be used. This will greatly simplify the design and speed up the project. The MC34063A requires no external frequency compensation. It is stable as long as the output filter cap(s) are electrolytic (tantalum, aluminum). Do not use film or ceramic output caps or the converter could become unstable. The ESR of the cap is relied upon for feedback loop compensation, and film and ceramic caps have near zero ESR. Also, the MC34063A has pulse by pulse current limiting. I hope this helps. Best regards.
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Old 5th June 2003, 04:48 PM   #4
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Default if you are using it to power opamps

then I assume you want "quiet" -- don't use a "simple switcher" here -- I would build a push pull switcher -- you can use the LM3524 or SG2524 as the switcher chip to drive the primary side of the transformer -- on the secondary side, wind for about 18 volts. Rectify and filter per usual, use a 100uH choke on each secondary side and run the output into an LM317LZ and LM337LZ positive and negative linear regulators set to produce + and - 15 volts. These burn a little energy but they will catch some of the switching transient from the LM3524. It may seem a little redundant, but it's an old fashioned way of making switching supplies a bit less noisy and has been referred to in a few manufacturer's data sheets. Not the most energy efficient way, but it works. The LM3524 or SG3524 is good for this application and will handle 200ma.

You can also use a special low noise switcher chip in push-pull configuration, but these cost $6 to $11 apiece in singles (LT1533) and I don't think its warranted in an automotive environment.
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Old 5th June 2003, 05:06 PM   #5
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Or you can spend $17 at mouser.com and
get a real DC-DC converter for +12vin and +15/-15 vout
and comes with all the protection circuits built in.

heheh

Takes the fun outta DIY.
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Old 5th June 2003, 07:30 PM   #6
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Default again, if it's just op amps

if you use something like the LT1026 voltage converter chip(www.linear.com) -- you can get +/- 18 volts out. I've used it with a opamps -- it's a trifle noisy -- compared to using a Lambda +/- 15 VDC supply, i.e. THD from an LT1013 was max 0.10%, min 0.03% when using the LT1026 converter to power the device compared with a threshold level (0.001%) with the Lambda regulated supply.

again you need to put something like a 100uH choke on the outputs and make sure that it is drawing at least 10ma.

I have a bunch of these if anyone cares.
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Old 16th June 2003, 01:35 PM   #7
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Default Try ON Semi's MC33025

For currents of +/- 100mA, try using ON Semi's MC33025. Except for a few differences, its pinout is very similar to the '3525. Like the '3525, it is a push-pull dual output chip. However, it can source/sink up to 1A peak. Its oscillator is good for about 1.5MHZ, yielding a top switching frequency of about 750kHz. And, like the '3525, it can use voltage-mode control, but has the option of using current-mode control, by the way you wire it.

You don't even need external switching MOSFETs- simply connect the transformer's primary across the A-output pin (11) and B-output pin (14), and you're ready to roll! Best of luck,

Steve
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