Author Topic: Voltage too high  (Read 1594 times)

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Offline volyaire

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Voltage too high
« on: January 03, 2018, 11:02:24 pm »
I have built a voltage boost circuit using the LMR64010 chip. With 7.5v input it gives 18.5v output at no load which is correct, but when I add a 390 ohm load the voltage doubles to 36v. Why?
 

Offline jim_griff

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Re: Voltage too high
« Reply #1 on: January 04, 2018, 12:31:41 am »
Could you post a picture of the schematic? Might help diagnose it. :)
 

Offline volyaire

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Re: Voltage too high
« Reply #2 on: January 04, 2018, 05:33:46 pm »
Here is the circuit. It is built on a piece of veroboard 1.5" x 0.5"
 

Offline Cyberdragon

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Re: Voltage too high
« Reply #3 on: January 04, 2018, 07:29:26 pm »
What's your layout? This circuit is sensative being high frequency.
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Offline james_s

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Re: Voltage too high
« Reply #4 on: January 04, 2018, 08:15:59 pm »
Building high frequency circuits on perfboard is tricky. I would suggest trying deadbug or island construction on blank copper PCB for prototyping, it's likely to work much better. Study the datasheet for the IC carefully paying particular attention to layout guidelines.
 

Offline volyaire

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Re: Voltage too high
« Reply #5 on: January 04, 2018, 11:43:19 pm »
I've tried several different layouts and at worst it went up to 57v! It should be 18v. Can anyone suggest a similar chip to boost 7.5v to 20v but at a lower frequency like 100kHz?
 

Offline fable

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Re: Voltage too high
« Reply #6 on: January 04, 2018, 11:52:32 pm »
MC34063
 

Offline james_s

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Re: Voltage too high
« Reply #7 on: January 05, 2018, 12:37:34 am »
The chip you have will work fine if you get the layout right. The fact that you've tried several different layouts doesn't mean you've got it right. You can also get ready made modules both buck and boost types very cheaply from Chinese sellers. Most of the ones I've tried work fine.
 

Offline volyaire

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Re: Voltage too high
« Reply #8 on: January 05, 2018, 12:58:50 am »
OK.   If I increased the 120p capacitor would the frequency drop?
 

Offline eblc1388

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Re: Voltage too high
« Reply #9 on: January 05, 2018, 01:00:45 am »
I've tried several different layouts and at worst it went up to 57v!

I would eliminate the possibility that the meter is not given me the correct reading, by using another meter.
 

Offline RoGeorge

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Re: Voltage too high
« Reply #10 on: January 05, 2018, 01:08:37 am »
Can you put a closeup picture of it (with load included)?

Offline volyaire

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Re: Voltage too high
« Reply #11 on: January 05, 2018, 02:13:25 am »
You can also get ready made modules both buck and boost types very cheaply from Chinese sellers. Most of the ones I've tried work fine.

Can you give me a link? Thanks.
« Last Edit: January 05, 2018, 02:17:38 am by volyaire »
 


Offline volyaire

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Re: Voltage too high
« Reply #13 on: January 05, 2018, 04:12:47 pm »
Thanks for those links. I'll probably have one more go at re-arranging it before I give up. My meter is OK. Here is the picture of the layout. Is the inductor too big? It is 10uH.
 

Offline james_s

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Re: Voltage too high
« Reply #14 on: January 05, 2018, 04:44:52 pm »
You'd have to look at the datasheet regarding the inductor, but those long spindly leads going to the IC are likely a big part of the problem. You need to have short relatively thick connections for the switching side. Stick the IC onto the board and run the wires across to it, might be easiest to stick it to the bottom.
 

Offline volyaire

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Re: Voltage too high
« Reply #15 on: January 05, 2018, 04:48:58 pm »
Thanks
 

Offline RoGeorge

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Re: Voltage too high
« Reply #16 on: January 05, 2018, 05:24:52 pm »
Sometimes I cut a perfboard pad in half in order to accommodate cases with pitch smaller than 0.1 inch, something like this: https://hackaday.io/project/7574-the-devil-is-in-the-details/log/26074-diy-breadboard-friendly-soic-breakout-boards


Offline james_s

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Re: Voltage too high
« Reply #17 on: January 05, 2018, 05:49:59 pm »
I usually cut strips of polyamide tape to insulate under pins I want to isolate, there are a variety of different methods though. For prototyping don't be afraid to bend a pin up or down to clear other things.
 

Offline Zero999

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Re: Voltage too high
« Reply #18 on: January 05, 2018, 08:34:17 pm »
Sometimes I cut a perfboard pad in half in order to accommodate cases with pitch smaller than 0.1 inch, something like this: https://hackaday.io/project/7574-the-devil-is-in-the-details/log/26074-diy-breadboard-friendly-soic-breakout-boards
I don the same thing with stripboard, except I often don't bother with a breakout board. I cut the strips in half, all around the IC. It works quite well.
 

Offline jim_griff

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Re: Voltage too high
« Reply #19 on: January 06, 2018, 05:06:43 pm »
Thanks for those links. I'll probably have one more go at re-arranging it before I give up. My meter is OK. Here is the picture of the layout. Is the inductor too big? It is 10uH.

Definitely parasitic inductance judging by the relatively large inductor and the long leads soldered to the chip.

I've been lucky enough in the past to have built RF circuits upwards of 400MHz on breadboard/perfboard, but usually anything above a few hundred kHz can get wonky quite easily. Some circuit configurations may work perfectly for a while then suddenly stop working. With HF in the circuit, it's all about getting the layout right.

Short lead lengths and star ground configurations (if you don't have a solid ground plane) can help when prototyping. Manhattan style is quick and useful for prototyping if you're using leaded components (although mostly used for RF circuits).
 

Offline volyaire

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Re: Voltage too high
« Reply #20 on: January 11, 2018, 07:38:47 pm »
I redesigned the layout roughly copying the data sheet and using a much smaller 10uH inductor (see picture) and now it works well
 

Offline james_s

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Re: Voltage too high
« Reply #21 on: January 11, 2018, 10:05:39 pm »
Good to hear. This is an excellent example of the importance of layout when dealing with high frequency circuits. Seemingly tiny amounts of inductance and capacitance that would be irrelevant at low frequencies can have huge effects.
 


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