When To Use Voltage Doubler, Boost Switching Controller, Switching Regulator - Page 1
Forum section: electronics / projects

author: Charles Creations
author_rank: Contributor
timestamp: November 28, 2014, 09:24:36 pm
content: Hi,To drive some ultrasonic transducers I need a >40V supply. At the moment, I am using a LC circuit, but the problem is it takes about 10 cycles at 40KHz to get the circuit to resonate with enough power. Ideally, I want to reduce the number of cycles (and time duration of the pulse) while maintaining peak output power.To do this, I have been looking into boosting my 5V supply up to higher voltages around 40V with a switching regulator but these require more parts then my LC circuit setup now. I am also looking at using a CWhttp://en.wikipedia.org/wiki/Cockcroft%E2%80%93Walton_generatorvoltage doubler. This is appealing because I have an atmega 328 can can output using a tone function up to around 60KHz to drive the doubler.My design constraints are mainly cost. I want the highest output power with my ultrasonic transducer with the lowest price for parts. What is my best option for generating my >40V supply? When should I use a purpose designed IC for a switching regulator vs using my microcontroller?Constraints:5V input supplySMDDriving Ultrasonic Transducer (low current)Desire a intermediate voltage tap for some opAmps (around 12-20V)

author: penfold
author_rank: Frequent Contributor
timestamp: November 28, 2014, 10:27:47 pm
content: Power is your biggest determining factor here. Practical CW voltage multipliers lose their attractiveness when you start drawing any 'significant' current, this could be in the region of a few mA before you start having to use large capacitance and you end up with a nasty lumpy current at the input and the whole thing becomes a noisy mess. And that's somewhere in the region of 4 stages of multiplication or so. I wouldn't really want to attack anything with more than 6 stages of multiplier, and less so anything with substantial current in which case Id keep to no more than 3 or 4 stages.If I were you and looking to get 40 V from 5 V, I would use a push-pull transformer to get to about 20 V, and then voltage double up to 40 V. This will put you in a very nice place as far as noise and ripple goes, as long as you design with appropriate components for the current you're dealing with then you can probably be happy up to a couple of amps or so.Whether or not to use a microcontroller, if its only a mediocre micro like an AVR (you're not trying to do any fancy control), then board space is probably your biggest concern, if you've got some logic level fets and power/efficiency/noise etc will let you switch directly from the micro, then pick the micro, but on the other hand, if you need a gate drive or two, then you're probably better looking for an integrated circuit with built in gate drives.Just as a hunch, it might be worth revisiting some of your LC network, I've had something similar with can get resonating in about 3 cycles. Depends on the Q factor of the transducer really, but I'm sure you could push it a bit faster than you're getting.

author: Charles Creations
author_rank: Contributor
timestamp: November 28, 2014, 10:39:46 pm
content: Thank you for the quick feedback. As far as current draw, my duty cycle for the transducer is around 1% so the output capacitor on the voltage doubler could be large and have a significant amount of time to recharge between pulse events of the ultrasonic transducer.I am looking at less then a 200us pulse at 40KHz with the pulses occurring at about 8 Hz so every 125ms. So basically there will be 200us of driving for every 125ms of charging and I could place several uF of output capacitance to sustain each pulse.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2014, 12:08:23 am
content: CW or a charge pump converter or anything like that, is okay if you don't need a lot of average power. If you're running a pulsed transducer, you might get away with enough bulk capacitance on the end with a multiplier or whatever driving it.If you need continuous power, don't piss around with charge pumps. Go for good old inductive conversion.You may also consider a transformer, since, that's all you're doing anyway, just transforming the inverter side. A simple 1:2 or 1:3 transformer (if you're lucky, enough flux and current capacity may be available off the shelf!) should get your Q down low enough to be useful. Of course, the actual loop Q still needs to be reduced, since increasing the inverter voltage just allows it to drive a lower Q circuit, it doesn't change the Q yet. So, the sucky part is you either need to modify the transducer (better coupling to air?) or add a resistor (which wastes power, and doesn't extend the transducer bandwidth itself).Note: I'm assuming the transducer has good coupling between electric and acoustic modes. I don't know how reasonable that is. If not, reducing the electrical Q (with a resistor) won't have as strong an effect on the actual acoustic risetime.Tim

author: langwadt
author_rank: Super Contributor
timestamp: November 29, 2014, 02:57:27 am
content: Quote from: Charles Creations on November 28, 2014, 09:24:36 pmThere are lots of single IC boost converters that'll generate you ~20V and you can stick a capacitive doubler/trippler on it to generate your +40Vhttp://www.maximintegrated.com/en/images/appnotes/660/660Fig03.gif(b)

author: Mad ID
author_rank: Regular Contributor
timestamp: November 29, 2014, 11:53:57 am
content: Quote from: Charles Creations on November 28, 2014, 09:24:36 pmI think you are are doing it the wrong way...use a small transformer to drive the transducer..that's how it usually done..You can get 100V+ without a problem for drive voltage.

author: Charles Creations
author_rank: Contributor
timestamp: November 29, 2014, 07:02:34 pm
content: I think you are are doing it the wrong way...use a small transformer to drive the transducer..that's how it usually done..You can get 100V+ without a problem for drive voltage.Thank you for the feedback. Could you point me in the direction of suppliers you would recommend for such a transformer? I have found some CFL transformers like this:http://www.digikey.com/product-detail/en/PM61300-2-RC/M8101CT-ND/774641But its bigger then I need. My average power requirements are so small that I really only need 1 to 2 VA.

author: Mad ID
author_rank: Regular Contributor
timestamp: November 29, 2014, 07:57:20 pm
content: Quote from: Charles Creations on November 29, 2014, 07:02:34 pmSure, take a look here.http://www.prowave.com.tw/english/item/sr.htmThis company has excellent app notes and a small development board. Have a look at the schematic.Also, Murata at their web site have nice schematics using transformers for ultrasound