[SOLVED]Half bridge low current

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fedaye

Member level 2
Hey guys i have a half bridge like below, its all voltage waveforms are okay but the flowing current is extremely small. my resistor is a short circuited copper cable (despite the skin effect its resistance value becomes very small i have calculated it, roughly 0.09 Ω)
my operating frequency is 200 kHz.

this is the waveform of the "a" point in the half bridge picture:

by the way if i decrease the frequency from 200 kHz to 150 kHz then the current is increasing. (from 0.46 A to 0.72 A)

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edit:
my capacitors are electrolytic and i have read somethings about this. someones say electrolytic capacitors can act as an inductor at high frequencies. they are not convenient for high frequency and i must use film capacitors instead of them.
i think this is sensible.

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barry

First of all, you obviously have some inductance in that circuit, as evidenced by the ringing on your edges, and, yes, aluminum electrolytics can have significant inductance. But you don't tell us any thing about your switching devices, your power supply or anything else. What are you trying to do here, besides having a short-circuit? I think you need some resistance for your load.

Orson Cart

above about 100kHz, electrolytics are inductors (or worse resonant), hence film caps used in power electronics, this why current goes up as F goes down...

Anna Conda

Anna Conda

Points: 2

fedaye

Member level 2
hey guys i have changed the capacitors (they are 0.68uF film capacitors) and i have a rheostat as a load but there is another problem for now;
at low currents the waveforms are okay (except effects of inductance etc)
for 0.02 Amp

but at high currents waveforms are not normal
for 0.44 Amp:

and for 3.8 Amp:

barry

Maybe you're killing your power supply. Look at the voltage across the bridge. Maybe add a large cap right across there.

fedaye

fedaye

Points: 2

Orson Cart

Yep, your power supply is running out of gas, or you have a big inductor in series?

Points: 2

Anna Conda

Points: 2

fedaye

Member level 2
my circuit is now like the picture below but the low current problem is still valid. All of the voltage waveforms are okay, very very interesting situation, i don't have any inductor, maybe there are inductive effects of the cables

current must be 1-2 A here not 0.02 A so interesting

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Orson Cart

looks normal, you are hard switching, but otherwise all OK...

Anna Conda

Anna Conda

Points: 2

fedaye

Member level 2
looks normal, you are hard switching, but otherwise all OK...
is there any solution to current?
26V and 0.02A; impedance must be 1300 ohm for this, not 10 ohm

Orson Cart

how inductive is your resistor? You need to show a photo of your entire circuit and close ups, you are doing something wrong...

Anna Conda

Anna Conda

Points: 2

fedaye

Member level 2
how inductive is your resistor? You need to show a photo of your entire circuit and close ups, you are doing something wrong...

it is a rheostat but not important same situation is valid with short circuited load. i will try to upload a picture

fedaye

Member level 2
it must carry high current therefore i have used many cables

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Orson Cart

Ha, what is the resonant freq of your load with your 10uF caps (20uF effective)? if you strike the early part of a resonant half cycle then the circulating current can be in the order of a few amps but the power supply will be supplying only the losses in the ckt, i.e. fet res losses and I^2R losses elsewhere..... this looks to be what is happening....

p.s. a high power wire-wound rheostat will be inductive......

Points: 2

Anna Conda

Points: 2

fedaye

Member level 2
Ha, what is the resonant freq of your load with your 10uF caps (20uF effective)? if you strike the early part of a resonant half cycle then the circulating current can be in the order of a few amps but the power supply will be supplying only the losses in the ckt, i.e. fet res losses and I^2R losses elsewhere..... this looks to be what is happening....

p.s. a high power wire-wound rheostat will be inductive......

did you mean there is a parallel resonant therefore the current is very small?
i think i must try lower capacitances than 10uF like 0.68uF then if there is a resonant i can get rid of it
true?
there are 5 parallel cables between Drains and sources of the mosfets maybe they have some inductances that can cause the resonant with 10 uF capacitors.

fedaye

Member level 2
and also i will try this to reduce inductances of the parallel wires

fedaye

fedaye

Points: 2

fedaye

Member level 2
a rheostat roughly like this but same result is occuring without any load (with short circuit)

Orson Cart

So - highly inductive then, if you have the same result with a short as a load, then there must be something wrong somewhere with the connections (open circuit some where?) as the two cases are markedly different - re-check your build carefully....!

A true short for a load should destroy one or both mosfets pretty quickly unless your rail splitting caps are very small....

fedaye

fedaye

Points: 2

fedaye

Member level 2
So - highly inductive then, if you have the same result with a short as a load, then there must be something wrong somewhere with the connections (open circuit some where?) as the two cases are markedly different - re-check your build carefully....!

A true short for a load should destroy one or both mosfets pretty quickly unless your rail splitting caps are very small....

i really wonder the reason, if there is an open circuit how can voltage waveforms become true or how can 3A current flow with short circuit?
i can not imagine anything this is an unusual problem
i checked many place by a multimeter against open circuit (by measuring resistance)
they looked normal
maybe i will surrender

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if i decrease the frequency:

if i increase the current by decreasing load resistance:

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Orson Cart

you said the result was the same for a short, but now you admit it is not the same: 3A versus 0.02A with your "load" (unspecified).

Changing the operating frequency will give you some insights...

how about showing us the voltage across the load (not simply the fet mid point to gnd)

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