Sine wave drive dc-dc converter - Page 1
Forum section: electronics / beginners

author: newbie666
author_rank: Regular Contributor
timestamp: February 23, 2019, 04:11:53 pm
content: I've been reading this application note from Analog:https://www.analog.com/media/en/technical-documentation/application-notes/an29f.pdfFigure 4 from this app note really piqued my interest as it shows a 5 - 15V dc dc converter with output noise of 30uV (full load) and very low EM noise thanks to driving the transformer with a sine wave.To generate drive signal app note uses wein bridge oscillator made with discrete parts which to be honest I really don't feel like doing. So my question isis there an IC that would drive a transformer with a sine wave for me?What frequency should I aim at?What are limitations of such a circuit?I can imagine the efficiency won't be great, but what kind of max power output should I expect?edit: schematic of the converter

author: ogden
author_rank: Super Contributor
timestamp: February 23, 2019, 08:26:48 pm
content: Quote from: newbie666 on February 23, 2019, 04:11:53 pmFor primary side circuit I would use microcontroller with DAC as sine generator and AB-class audio amplifier IC as driver for transformer. Other option would be to uselightbulb+opamp sine generatorwith audio amplifier - in case you want purely analog solution.QuoteDid you read appnote carefully? Answer is there: "The sine wave’s 16kHz frequency is a compromise between the op amps available gain bandwidth, magnetics size, audible noise, and minimization of wideband harmonics."

author: David Hess
author_rank: Super Contributor
timestamp: February 23, 2019, 09:13:35 pm
content: The power amplifier could be replaced with a power operational amplifier or audio amplifier however this would further sacrifice efficiency because the design as shown has a rail-to-rail output stage minimizing dropout voltage. There are alternative boosted operational amplifier designs which could be used for similar performance however.Ogden is right that a simpler Wein bridge oscillator could be used however doing so would then require an adjustment for output level. I think the author was aiming to make a design which required no adjustments.Notice that another feature of this inverter is galvanic isolation between the input and output. This type of low noise isolated power supply is especially suitable for powering sensitive analog circuits like high resolution digital multimeter front ends.The text says efficiency is poor at 30%. I thought it would be much higher because the class-AB linear power amplifier stage can operate close to saturation.Output power is limited by the low efficiency producing heat and the current limit of the LT1010 based output regulators so considerably higher output power is possible if the output regulators are changed and the high power loss is acceptable. The text says as shown, the output current is 75 milliamps per output which is consistent with the LT1010.

author: ogden
author_rank: Super Contributor
timestamp: February 23, 2019, 09:26:25 pm
content: Quote from: David Hess on February 23, 2019, 09:13:35 pmAll this can be solved with modern low noise (0.8uV RMS) LDO'shttps://www.analog.com/en/products/lt3042.htmlThere are complementary negative regulator as well.

author: David Hess
author_rank: Super Contributor
timestamp: February 23, 2019, 10:46:22 pm
content: Quote from: ogden on February 23, 2019, 09:26:25 pmThe LT1010 has the advantage of being able to source and sink current from the output.I think the noise argument will be a wash because noise is controlled within the loop bandwidth of the LT1013 and noise from the buried zener reference is lower than an integrated regulator with a bandgap reference can achieve alone. You do not actually need a low noise output stage for low noise with the LT1013 and LT1021 determining the output noise.

author: ogden
author_rank: Super Contributor
timestamp: February 23, 2019, 11:18:22 pm
content: Quote from: David Hess on February 23, 2019, 10:46:22 pmSure, yet most of the (positive) linear regulators can only source current and everybody is fine with that. Could you name some common circuit(s) which requires push/pull power supply? - I'm curious. IMHO mentioned frontent of the multimeter do not require such.

author: David Hess
author_rank: Super Contributor
timestamp: February 24, 2019, 12:08:47 am
content: Quote from: ogden on February 23, 2019, 11:18:22 pmI think the advantage here is that the class-ab output stage of the LT1010 performs more consistently over a wide range of output currents because it never operates in cutoff. Some high performance references use a similar output circuit and they can source and sink current as well.The LT1010 could also be replaced with a 4 transistor complementary diamond buffer and there are some advanced low dropout complementary boosted output operational amplifier configurations which could be used also. (1) The LT1010 has the advantage of simplicity though and I suspect Linear Technology made it an understandable point in these older datasheets to show their own products like the LT1010 in application circuits even if higher performance alternatives existed.(1) See below for an example similar to what I would try however the common emitter output configuration may not have good enough power supply rejection and requires a single operational amplifier instead of a dual or quad.