Cost-effective power supply scenario for this project - Page 1
Forum section: electronics / projects

author: XaviPacheco
author_rank: Regular Contributor
timestamp: July 05, 2018, 07:05:11 pm
content: I'm working on a project that needs two separate voltages: 5V and 12V.12V for a gate driver and 5V for a microcontroller. Both have to be isolated, they must not share ground.I do not have the time now to design a switching power supply, so I'm looking for possible PCB mount switching power supplies that I can adapt to my PCB. Their input is 120VAC/60Hz from mains. So, it's a AC/DC type converter.About the load requirements, the 12V is just for the gate driver to provide the peak currents. So, I think that anything between 1 to 3A would work. The 5V will supply two microcontrollers and other ICs. I expect the load current for the 5V source to be 3.5A max (worst scenario).I physically have these two switching power supplies:EPS-65S-12EPS-65S-5But I find them kind of big in size for the project. I need something that I can save as much space as possible.What do you recommend?

author: D-Jack
author_rank: Contributor
timestamp: July 05, 2018, 07:24:01 pm
content: Have you checked here:https://www.digikey.com/products/en/power-supplies-board-mount/ac-dc-converters/923?

author: XaviPacheco
author_rank: Regular Contributor
timestamp: July 05, 2018, 07:45:56 pm
content: What do you think ofThis one? Kind of "expensive".

author: jbb
author_rank: Super Contributor
timestamp: July 05, 2018, 08:42:08 pm
content: Quote from: XaviPacheco on July 05, 2018, 07:05:11 pmThe nice thing about MOSFET / IGBT gate drivers is that they only need the peak currents for a very short period (<100ns). That means that the pulse can be provided by a suitable cap (e.g. 10uF + 100 nF chip ceramics) and you only need to supply theaveragecurrent. Unless you're switching something very big or very frequently, you'll only need mA per gate driver. Check your gate driver chip for power consumption.

author: XaviPacheco
author_rank: Regular Contributor
timestamp: July 05, 2018, 08:49:27 pm
content: Quote from: jbb on July 05, 2018, 08:42:08 pmWell, my gate driver is the 1EDI60N12AF. I will switch a Power MOSFET at PWM frequency between 1-10 kHz.

author: ajb
author_rank: Super Contributor
timestamp: July 05, 2018, 08:52:26 pm
content: Quote from: XaviPacheco on July 05, 2018, 07:45:56 pmIt's expensive because it's 30W! Like jbb said, it's not likely that you actually need anywhere near that much average power. It's entirely possible that a $2.50 1W converter will do the job, but you'll need to do some calculations to determine the amount of average power you actually need, based on the MOSFET/IGBT characteristics and switching frequency. Or, since it sounds like maybe you already have a working solution, you could just measure the current consumption at the 12V power supply.You might also be able to develop a gate drive supply from the supply to the load, but how practical that will be depends on the voltage and switching topology.EDIT:Quote from: XaviPacheco on July 05, 2018, 08:49:27 pmWhat's the MOSFET? A part number and some idea of the topology (is it really just a single transistor?) and the load voltage/current will be necessary to estimate the amount of energy required per switching cycle. But if you're only doing up to 10kHz, the average gate drive power is certainly going to be a lot lower than 30W for any reasonable transistor!

author: XaviPacheco
author_rank: Regular Contributor
timestamp: July 05, 2018, 08:57:50 pm
content: What I like about that power supply is that it gives me the two isolated voltages that I need.

author: XaviPacheco
author_rank: Regular Contributor
timestamp: July 05, 2018, 09:02:42 pm
content: Quote from: ajb on July 05, 2018, 08:52:26 pmThe MOSFET is the IRF250P224. It's a simple low side switching topology.

author: D-Jack
author_rank: Contributor
timestamp: July 06, 2018, 12:37:32 am
content: Quote from: XaviPacheco on July 05, 2018, 07:45:56 pmHoly moly!! That's waaaay too expensive! I bet you don't need that kind of power and spec. Check these two:https://www.digikey.com/product-detail/en/mean-well-usa-inc/IRM-45-12/1866-3053-ND/7704678https://www.digikey.com/product-detail/en/mean-well-usa-inc/IRM-10-5/1866-3032-ND/7704657And even that I think is overpowered. The logic side can be sized down I reckon. With some trimming I believe you can replace the 5V module with a lower power and much smaller module

author: XaviPacheco
author_rank: Regular Contributor
timestamp: July 06, 2018, 12:49:23 am
content: I think that what makes it expensive is having the two voltages in one module. Two modules will increase the size of the board by far and I want to save as much space as possible.

author: D-Jack
author_rank: Contributor
timestamp: July 06, 2018, 01:09:15 am
content: Quote from: XaviPacheco on July 06, 2018, 12:49:23 amYes that's why I would suggest you do a deep analysis of the power consumption of your board, especially the logic side because with some tweaking you could greatly reduce the size of one of the modules and thus reduce the overall size.And also, this may be silly question but why do you need both of them isolated from each other? Just to have a better picture of the project.

author: XaviPacheco
author_rank: Regular Contributor
timestamp: July 06, 2018, 01:28:42 am
content: Quote from: D-Jack on July 06, 2018, 01:09:15 amIt's a PMDC motor controller @90-130VDC. The mains will be rectified and filtered. This is the supply of the power mosfet. The gate driver, which converts the 3.3V to 12V, has isolation, so it isolates the PWM signal from the power mosfet. I simply do not want to share the power bridge rectifier ground with the microprocessor stage ground. I will also measure the current thorugh the motor, and the sensor that I will use also has galvanic isolation. In other words, I want to isolate the power stage from the low voltage stage.The 12V source will, of course, share the ground with the power rectifier. I attach below the gate driver that I will use.

author: D-Jack
author_rank: Contributor
timestamp: July 06, 2018, 02:06:31 am
content: Ohh I see...There is another solution. You can use a AC-DC for the 12V and a isolated DC-DC to step-down from 12 to 5V. The idea is to give you more flexibility when choosing the components and since it is DC-DC you can find equivalent power supplies in power for a cheaper price

author: ajb
author_rank: Super Contributor
timestamp: July 06, 2018, 06:18:15 am
content: Okay, I misread the page for that power supply you linked--I thought you were looking at a 5V->12V isolated DC-DC converter, which would be a quite sensible option. If you ACTUALLY need 3.5A @ 5V but are only driving a single MOSFET gate at 12V, then you could use a suitable 5V power supply (something like an EPS-25-5, maybe) and a 1W isolated DC-DC converter (example) to provide your 12V supply from that.(You could go the other way, with a 12V AC-DC supply and a 12V->5V DC-DC, but if you actually need >17W on the 5V rail isolated DC-DC converters at that power level tend to be pretty expensive. Whereas your gate drive certainly needs <1W and 1W DC-DC converters are cheap and widely available.)

author: XaviPacheco
author_rank: Regular Contributor
timestamp: July 06, 2018, 06:43:05 am
content: Quote from: ajb on July 06, 2018, 06:18:15 amI find this AC/DC converter good:https://www.digikey.com/product-detail/en/mean-well-usa-inc/IRM-15-5/1866-3037-ND/7704662It's 1 Output 5V/3A. Then, to convert those 5V to 12V isolated, I find that DC/DC converter good. But I'm still confused about something. That DC/DC converter is 84 mA max. But that's continous, right? It can supply high peaks current during a very short time, right?

author: ajb
author_rank: Super Contributor
timestamp: July 06, 2018, 01:52:08 pm
content: Quote from: XaviPacheco on July 06, 2018, 06:43:05 amThe converter doesn't have to supply those peaks--that's what capacitors are for. The datasheet for your gate driver will probably have some specific guidance about placing a certain amount of capacitance as close to the IC as possible.

author: XaviPacheco
author_rank: Regular Contributor
timestamp: July 07, 2018, 01:26:44 am
content: So, that 12V source will supply an average current of I = Qxfs, where Q is the total charge of the mosfet and fs my pwm frequency. According to my mosfet datasheet the total charge is 160nC, my pwm frequency is 10 kHz. So I = 160nC * 10000 = 1.6 mA... am I right?