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Help designing SMPS for my project

user5262794
Prodigy 50 points
Other Parts Discussed in Thread: UCC28704, UCC28740

Hi,

I'm new to Switched Mode Power Supply design, and need some help designing my first SMPS for my microcontroller project. The microcontroller is an Atmega 324p that will be running on 5VDC, and will control two solenoid valves running on 24VDC. I need to design the power supply for the valves and for the microcontroller. So I am thinking to first make an isolated flyback SMPS to go from mains voltage to 24VDC, and then a buck converter that steps 24VDC down to 5VDC.

The 5V circuit will draw a maximum of around 50mA, and minimum 1mA.

The 24V circuit will draw a maximum of around 0.8A. If the 5V is stepped down from the 24V supply, then the minimum current on the 24V rail will be approximately 0.2mA. If not, the minimum current on the 24V is 0A.

Project description: 
The SMPS will be connected to mains (mostly 230V, 50Hz, but if the SMPS can be made for universal voltages, that would be preferred) and supply a circuit that will most of the time (let's say 99% of the time) be in sleep mode. Let's assume sleep mode draws around 1mA @ 5V. Valves will be off in this state. The remaining 1% of the time the microcontroller will be awake and controlling the two solenoid valves (one 10W, 24VDC valve, and one 8W, 24VDC valve). First the 10W valve will be on for around 5 seconds. After that, the 8W valve will be run for around 5 seconds. Sometimes they can also be run simultaneously for a couple of seconds.

I tried WEBENCH, and found many simple circuits with high efficiency for stepping down from 24VDC to 5VDC, but when it comes to the 230VAC to 24VDC, I could not find any good solution that had high efficiency all the way from 0.8A down to 0,2mA. Or is this simply not possible? Do I have to add some more components to the circuit, like LEDs, to make the circuit draw more current in sleep mode?

If someone would kindly help me out, it will be very appreciated. Thank you.

  • 0
    TI__Expert 7080 points
    Hi,

    Thanks for your interest in TI's product.

    Can you share more detailed specifications of your power supply? for example:
    1. what the accuracy requirement of 24V and 5V, I mean the tolerance allowed for 24V and 5V ?
    2. If any power sequence requirement between 24V and 5V ? I can find you prefer firstly convert AC to 24V , then use buck converter for 5Vout.
    3. You mentioned 99% of the time the system is in sleep mode , if any standby power requirement?
    4. You also mentioned the maximum and minimum load , if the system only operating in the two modes? the load step is from min. to max.and vice versa. if so , how about he dynamic load requirement ?
    5. How about the efficiency requirement ? both AC to 24V and 24V to 5V.
    6. If any concerns on total size , cost , or some special requirements?

    After the spec. are clarified , we will proposal the solution to you.
    Thanks.
  • 0 in reply to Jaden Ning
    Prodigy 50 points

    Hi,

    Thank you for your response. I will try to answer you as good as I can.

    1: 24V +/- 5%. 5V +/- 0.2V.

    2: No requirement here. I just thought 230V-24V-5V would be the best solution with fewest components. But if it is better some other way, like an intermediate rail, no problem.

    3: No standby power requirement. "As low as possible".

    4: We have the following states:
    a) Microcontroller is in sleep mode and everything else is off.
    b) Microcontroller awake and 16x2 LCD is on.
    c) microcontroller awake and valve 1 on.
    d) microcontroller awake and Valve 1 and Valve 2 on
    e) microcontroller awake and Valve 2 on
    f) LCD can be on in any of the states mentioned above, except for state a).
    Did some research and the LCD will draw less than 20mA. To add a little margin, the 5V should be rated for around 100mA output current. 24V should be rated for around 0.9A output current or more.

    5: There is no specific requirement here, but I was hoping to have at least 80-90% efficiency on both over the whole load range, but if this is not possible, then we will just have to find the most optimal solution.

    6: No size requirement, but the smaller the better :). For example; two parallell isolated flyback transformers (mains to 24V and mains to 5V) will probably take up too much space. It will be inside a small container together with the Pcb for the microcontroller and the pcb with the LCD. The container is not designed yet until we know how much space everything is going to take. As for the cost, this is just a small part of the total system, so if the power supply is a bit more expensive than "normal" power supplies, cost should not be a problem.

    Sorry, I know the requirements are a little vague. If anything needs to be more specific, please come back to me.

    Thanks !

  • 0 in reply to user5262794
    TI__Mastermind 25570 points

    Hello

    It is possible to wind two secondaries on a Flyback transformer - you could get 230V to 24V/5V. The windings may be separate and isolated from each other or you could use the 5V winding to form part of the 24V winding - 5T for 5V, and 5T+19T for 24V (you will get better cross regulation with this arrangement).

    TI has lots of useful PSU design information at http://www.ti.com/ww/en/power-training/login.shtml  you could look at topics on Flyback design, Magnetics design and Control for exampe. We also have many reference designs at http://www.ti.com/reference-designs/index.html 

    The performance of a Flyback circuit is very sensitive to the detail of the transformer design - have a look at https://www.ti.com/seclit/ml/slup338/slup338.pdf    for lots of useful information.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Prodigy 50 points
    Hi,

    Do you know of a reference design that takes out both 24V and 5V, or some similar reference design that I can have a look at as an example?

    Could you please also point me in the right direction, for selecting a controller etc. that will work according to my spec? Thanks.
  • 0 in reply to user5262794
    TI__Expert 7080 points

    Hello,

    For Primary side regulation controller , I proposal UCC28704 which don't need 431 and opto-coupler, 5% voltage regulation can meet 24Vout requirement. you can use a DC/DC converter for 5V on secondary side for a accurate 5Vout , you can find some BUCK converters on TI.com

    For Secondary side regulation controller , I proposal UCC28740 to you. as colin proposed , you can wind two outputs on a transformer .

    Attached one reference design which have 1 output for your reference.

    Thanks

    PMP30251_RevB.pdf

  • 0 in reply to Jaden Ning
    TI__Mastermind 25570 points
    Hello

    There is some useful information about cross regulation effects in Flyback transformers in the article at www.ti.com/.../slup261.pdf - see Page 9 and Page 12 especially.

    www.ti.com/.../slup204.pdf may be of interest too.

    Regards
    Colin
  • 0 in reply to Jaden Ning
    Prodigy 50 points

    Hi.

    I opened the UCC28704 and UCC28740 in WEBENCH, and noticed the efficiency at 1mA is very poor, and duty cycle very low. Are you sure these controllers will work (and work efficiently) at such low currents?

    Thanks.

  • 0 in reply to user5262794
    TI__Expert 7080 points
    Hello,

    Usually , we don't calculate efficiency at such low output current (1mA). But call it standby power , that is why I asked what is your standby power requirement in previous reply . Just imaging , if the output current is 0A , there are still some power losses on the converter .
    How we to calculate the efficiency ? in light load condition , the controller will entry very low switching frequency and lower peak current mode to saving the power loss and for target output regulation .

    You don't need to worry about the bad efficiency performance at so lower load condition , since the total power loss also very light.
    Thanks.