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I'm kaffufled
By bigkahoonaa, Section Diaries Posted on Sat May 10th, 2008 at 04:37:45 AM MST
Power electronics

I went to one of my favourite electronic shop. They had a huge closing sale, and I scored a big box of components for $20. Examples of a few power supplies:

Even had some logic switches:

Most of the stuff works. Not bad for less than $0.50 each. My goal is to learn more about power electronics, but that's not my conundrum. This is:

The schematic:

I get current flowing through, even when there's no gate voltage and I disconnect the power caps. I'm kaffufled.

I'm kaffufled | 7 comments (7 topical, 0 editorial)

Re: I'm kaffufled (3.00 / 0) (#1)
by ve6aym on Fri May 9th, 2008 at 11:15:35 PM MST
(User Info)

One of your irf730 is shorted from source to drain or you have a short from b+ to ground in your circuit.. fets usually short..

Re: I'm kaffufled (3.00 / 0) (#2)
by wooferhound (tim((NoSpamAt))wooferhound.com) on Sat May 10th, 2008 at 05:22:56 AM MST
(User Info) http://wooferhound.com

My favorite electronics store is going out of business too
in fact, today is their last day
I have $60 spendable and I'll be going over there and see what I can get on the Last Day Clearance

I guess the internet is killing the Brick & Mortar stores
and I guess that I am part of the problem cause I get piles of electronics from eBay and other internet locations.
W o o f -={(

Re: I'm kaffufled (3.00 / 0) (#3)
by Flux on Sat May 10th, 2008 at 10:16:22 AM MST
(User Info)

Don't understand your circuit. Capacitors in strange places, no freewheel diode. No indication of where the gate drive is referenced to and no mosfet driver.

If the gate drive is referenced to the wrong place then it may be permanently on. Looks like a fet killer circuit so if one is dead I wouldn't be surprised.

Flux

[ Parent ]

Re: I'm kaffufled (3.00 / 0) (#4)
by bigkahoonaa on Sat May 10th, 2008 at 03:20:46 PM MST
(User Info)

Thanks guys. I looked at the circuit again, and I finally realised that I was sending the "pot" the wrong way. Good thing no one can see my red face on the internet.

The driver signal comes from a Picaxe PWM pin. I tried putting a freewheel diode but it never seemed to make any difference.

Re: I'm kaffufled (3.00 / 0) (#5)
by oztules (oztules__at__bigpond.com) on Sat May 10th, 2008 at 06:18:01 PM MST
(User Info) http://www.anotherpower.com/gallery/Oztules-toys

Its a mystery to me how this actually works at all.
I must be reading this wrong, as I feel the cap is in the wrong place and reversed. (the neg terminal is at a more positive potential than the pos terminal).

As to my way of thinking, the drain is at a lower potential than the source in your diagram. Usually, a conducting diode closes the circuit on the inductor when the fet turns off. In this case (to my thinking anyway), then the body diodes would perform this function. This may explain why adding conducting diodes made no obvious difference.

In it's current config, it may struggle... but work. The back emf generated by the conductor may current spike the fets body diode though, so this just doesn't work for me.

By changing the pot reference point, you would make the gate more negative with respect to the source, and this should turn it off I guess, but it just seems all wrong... the gs voltage may go excessive when the fet is open..... ie the source will go to gen+ almost, while the gate is tied to the drain by the pot.

Note, I have not built a buck, but if I did, it would not be in this guise, if it works well like this then .... but the fets seem vulnerable.... and the cap is still wrong to me.

I may be completely wrong with this, but it just do't look right.

.........oztules
Flinders Island Australia

Re: I'm kaffufled (3.00 / 0) (#6)
by oztules (oztules__at__bigpond.com) on Sun May 11th, 2008 at 03:22:17 AM MST
(User Info) http://www.anotherpower.com/gallery/Oztules-toys

After looking at your circuit again, I am completely wrong.
I neglected to check the frame of reference for the genny. It has the most negative point on its genny ground, I was looking at battery - being the most negative point. But it is not.

So the cap is around the right way (still think it's the wrong place) and the drain is less negative than the source. So that makes it a bit clearer to me now, but there is still a goodly amount of mud in my vision on this one.

Where is the pwm referenced to??.... gate and ?

The body diode is still being used as the freewheel diode, but through the impedance of the genny.... is this correct? I think a diode from drain to battery+ should do better.

........oztules
Flinders Island Australia
[ Parent ]

Re: I'm kaffufled (3.00 / 0) (#7)
by bigkahoonaa on Tue May 13th, 2008 at 07:13:05 PM MST
(User Info)

Hi Oztules

The most negative point is genny ground, so the caps are the right way around. It would have clearer had I posted all the circuit. It's here. I've tried adding diodes but the LTSpice model never shows anything but a few microamps going thru the diode. The real circuit may be different. I like your idea of adding a diode going back to battery +. It should limit voltage at the fets to whatever the genny produces plus the drop across the diodes.

I got it working again. I trashed the idea of the pot and added 10K resistors at the base of the fets between gate and source. I was hoping to see if there was any changes with resistance. Anyway, I'm going to try to see what happens when I change PWM frequency, power caps, and inductors.

[ Parent ]

I'm kaffufled | 7 comments (7 topical, 0 editorial)

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