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Electronics => Projects, Designs, and Technical Stuff => Topic started by: luky315 on April 25, 2014, 08:49:02 am

Title: DC/DC Module 100% ON
Post by: luky315 on April 25, 2014, 08:49:02 am

I am searching for a DC/DC converter module (=ready to use and EMI tested...) with 5V output (0.8A) and an input voltage range of 5 to 28V. This means that it is necessary to allow 100% ON-Mode to deliver (nearly) 5V from a 5V input. Are there any modules out there that I overlooked? A Buck/Boost Module with an input Range down to 3V would be even better, but is not absolutely necessary.
By the way: Are the MagI³C Modules from Würth and the LMZ Series from National/TI the same thing? Each of them claim that they are own developments but the specs are exactly the same, down to details like ON/OF timings.

Title: Re: DC/DC Module 100% ON
Post by: nickm on April 25, 2014, 01:49:14 pm

You will have a very hard time finding a integrated buck converter that can do 100% on time.  The reason is bucks require high side drive on the fet and the high voltage rail is a bootstrapped power supply.  To charge up this power supply the top switch needs to turn on and the bottom fet or diode needs to conduct for  a brief period of time to charge the bootstrap supply up again.  This is why on time is limited to about 97% on most bucks.  Now if you were just making your own you would add a second rail that wasn't bootstrapped and it would do 100% no problem.

A better choice would be a flyback but that will be physically larger to handle the same power.  An even better choice would be to ask yourself if you really need to support down to 5V.  A lot of times people get carried away overspeccing things and doing a lot of extra work when they really don't have to.

Title: Re: DC/DC Module 100% ON
Post by: qno on April 26, 2014, 07:37:42 am

Look for a SEPIC converter.

Title: Re: DC/DC Module 100% ON
Post by: Psi on April 26, 2014, 08:29:02 am

If you want 5v in 5v out it has to be sepic or an isolated transformer based converter

Title: Re: DC/DC Module 100% ON
Post by: DaveW on April 26, 2014, 11:03:31 am

The LTC3630 will go to 100% duty cycle, so the only losses will be the resistance of the internal FET and the resistance of the inductor. not a module I know, but at least shows they are out there...

Title: Re: DC/DC Module 100% ON
Post by: luky315 on April 28, 2014, 08:15:29 am

There are a few Buck Converter capable of 100% ON.
Are there any Buck-Boost Modules out there?

Title: Re: DC/DC Module 100% ON
Post by: Marco on April 28, 2014, 02:48:35 pm

Quote from: luky315 on April 28, 2014, 08:15:29 am

There are a few Buck Converter capable of 100% ON.
Are there any Buck-Boost Modules out there?

Having a special mode for when the voltage is EXACTLY correct doesn't make much sense.

There might be (non-inverting) buck boost controllers which go into a linear mode when Vin is slightly higher than the desired output voltage though.