Hi there,

I have to design a power switch to take power from a DC source and distribute it to two identical loads. The input voltage will vary from 24-28 V DC, and the load voltage is 20 V DC.

Frustratingly, it has been difficult to get the current rating of the loads from the supplier, despite frequent pushing. The only information I have to go off at this point in time is that the entire system consumes approximately 1500 W. I have therefore made the assumption that each load consumes 750 W and have calculated the currents accordingly. See the diagram below (at 28 V)



Assuming these assumptions are valid, I’m having a bit of difficulty with the internals of the switch. I want to use a DC-DC switching regulator, and finding an off the shelf component to regulate the 24-28 V input to 20 V is not a problem. However, most of these regulators are designed for output currents in the 1 – 5 A range. I know that Darlington transistors can be used to amplify current but they would not be able to amplify it to to the extent that I need.

The regulators with the highest currents that I could find are in the 10-15 A range with 15 V:

http://www.homemade-circuits.com/2013/04/15v-10-amp-adjustable-voltage-regulator.html

I've had a little bit of exposure to power circuits in the past, but I still have a lot to learn and need advice from people who are much more experienced. Do my assumptions look correct given the lack of information? Is there a regulator or any other circuit out there that you know of that could satisfy my requirements? Or am I missing something entirely?

Thanks for your help.

 

The linked regulator is a linear type, not a switcher. Have you considered a switcher such as this?
It will not be trivial to home-brew the supply you require.
An off-the-shelf DC/DC buck converter would probably be cheaper than a home-brew one.

 

Although the link above for the LTC3891 is probably the way to go. Especially as it is a "synchronous" design - has an active flyback diode. However, designing a switching power supply for 30A is not an easy task. You will need power MOSFETS, and special magnetics. The winding on the inductor will be using copper tape or multiple parallel (thick) wires. The core area must be large enough and the correct material. The input and output capacitors will be multiple parallel low ESR types.

You will have to take special care with the PCB layout: ground planes and minimising common impedance is going to be very important. The peak current could easily be twice the output current and the operating frequency will be 100kHz plus. We are talking sub micro-second timings here. This is a serious bit of electronics design and you won't get it right first time!

Best to see what you can buy ready made.

 

Thanks for the responses! I had come across the LTC3891. My concern is, as you mentioned Marley, the high current which will need quite a bit more design work on top of just using this chip. I may need to contact some suppliers to develop the circuit in this instance, and focus on the I/O interface instead.

Will definitely need to brush up on my circuit design skills for the future though!