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More Efficient Controlled Bridge Rectifier

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Tom Moore

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I am looking to improve the efficiency of a battery charger design that we currently produce. It is of the type which uses phase angle controlled SCRs to regulate the DC voltage which is passed to the battery. The component that is being used currently is a half controlled bridge module made by International Rectifier, part No. P101. I am looking at making my own rectifier out of Discrete PCB mounted components as these modules are fairly expensive and we would like to move all of the components onto a PCB.

I have found lots of information on google on using FETs as ideal diodes to improve the efficiency of bridge rectifiers but I have not found anything to do with improving the efficiency of controlled bridges.

I was thinking that I could replace the diodes with Schottky types to improve the efficiency there but I would also like to replace the SCRs with something more efficient like a combination of MOSFETs and Schottky diodes. My current idea is to make a standard bridge rectifier out of Schottky diodes and control the output of the rectifier with a low Rds P-Channel FET. I have attached a schematic which illustrates this idea and the schematic of the bridge that is currently being used.

Would this even work at all? Does anyone have any experience with this sort of circuit? Any ideas or input is welcome
 

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I am looking to improve the efficiency of a battery charger design that we currently produce. It is of the type which uses phase angle controlled SCRs to regulate the DC voltage which is passed to the battery. The component that is being used currently is a half controlled bridge module made by International Rectifier, part No. P101. I am looking at making my own rectifier out of Discrete PCB mounted components as these modules are fairly expensive and we would like to move all of the components onto a PCB.

I have found lots of information on google on using FETs as ideal diodes to improve the efficiency of bridge rectifiers but I have not found anything to do with improving the efficiency of controlled bridges.

I was thinking that I could replace the diodes with Schottky types to improve the efficiency there but I would also like to replace the SCRs with something more efficient like a combination of MOSFETs and Schottky diodes. My current idea is to make a standard bridge rectifier out of Schottky diodes and control the output of the rectifier with a low Rds P-Channel FET. I have attached a schematic which illustrates this idea and the schematic of the bridge that is currently being used.

Would this even work at all? Does anyone have any experience with this sort of circuit? Any ideas or input is welcome


Look into "synchronous" rectifiers or similar.. those designs use MOSFETS for exactly the reasons you are after.
 
**broken link removed**

seems like this is the sort of thing you were seeing. just hand a control FET on the output of this bridge. trying to control the individual FETs is a nightmare if this works for you.

You neglected to mention how much current you need. I get dual FETs that will handle 2A for $0.20 each, but they would cost more in low quantities. We build our own CCFL backlight inverter with them.

Dan
 
I don't think you can use MOSFET active diodes because their blocking voltage is limited by their gate voltage and I've never seen anything higher than 20V. You might be stuck with schottkies. Ubergeeks' link is realy cool though in that there is gate-limiting zeners/resistors to overcome that. Too bad you have to use PFETs and lose some efficiency because of that though. It'd be nice if you could use all NMOS but the gate voltage is too low.

I find it strange they were using thyristors and not MOSFETs to regulate the output, especially since it's DC after the rectifier- do you have any idea why? Where they regularing the AC input rather than the DC output?

You might be able to use an N-channel which would make even more efficient. High-side would require some gate driver circuitry but you probably need that anyways since it sounds like you are using PWM. Low-side would might cause noise associated with the neutral shifting .
 
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I find it strange they were using thyristors and not MOSFETs to regulate the output, especially since it's DC after the rectifier- do you have any idea why? Where they regularing the AC input rather than the DC output?
Regulating the AC with phase-controlled switches is efficient. To get similar efficiency controlling the DC, you would need a switching regulator.
 
Isn't that what he's doing?
EDIT: Oh wait, does doing it in AC, rectifying it, and then smoothing it with a capacitor allow you to not need an inductor because the AC is, almost by definition "sloped" on it's own
 
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Full bridge rectifiers using MosFet power transistors require two P and two N-channel MosFets controlled by a highly accurate OpAmp.

If timing is not accurate, depending on many factors like resistor tolerance, gate capacitance etc. the rectifier will produce a dead short.

Secondly P-channel MosFets generally have a higher RDSon than N-channel types which degrades the effectiveness of the bridge rectifier.

To avoid those risks (and saving components, hence expenses) I have made an "active rectifier" as two way rectifier using N-channel MosFets and a dual OpAmp for optimum gate control.

That kind of rectifier just requires a transformer with a center tap on the secondary winding.

Boncuk
 
Isn't that what he's doing?
EDIT: Oh wait, does doing it in AC, rectifying it, and then smoothing it with a capacitor allow you to not need an inductor because the AC is, almost by definition "sloped" on it's own
If you are charging a battery, you can use unfiltered rectified AC, you don't need any smoothing capacitors. The battery rather acts as a large capacitor.
 
Thanks for all your replies.

Thanks for the link to the EDN article, it is one that i have seen before and is maybe what i had in mind. If i put the P-Channel FET after this circuit as i have in my diagram i think i should be able to make a controlled rectifier.

The charger that i am designing is to have a power output of at least 20A hence why i am trying to make the bridge rectifier more efficient. The SCR controlled bridges get very hot and waste a lot of power, which means an even larger transformer is required just to cope with the power loss.

Boncuk, I previously looked at using a center tapped transformer so that only 2 'diodes' would be required but this makes less efficient use of the transformer, hence a larger transformer would be required. I would be interested to see how you do the gate drive using the op-amps though.

crutschow, you are right in that there is no smoothing on the output of the rectifier. The battery itself acts as a large smoothing capacitor. This is a very common design of battery charger.
 
This circuit makes advantage of the antiparallel diodes contained in the MosFets by generating initial supply voltage for the OpAmp.

The rest should be self explanatory.

Transformers normally have two secondary windings to connect them either parallel or in series. Just use it as a transformer with center tap. The power output is not higher than normal.

Boncuk
 

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Tom,
I agree that a Schottky-diode bridge with a FET controlling the output will work and may be your shortest and least problematic path.
Please also note that Boncuk's circuit is an active rectifier that will always rectify the entire cycle, even without the FET being ON, via the FET's body-diode; so that FET configuration can't be switched on/off in the way you are looking for.
Mike
 
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