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Old 24th August 2005, 06:06 AM   #1
Pierre is offline Pierre  France
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Default Assymmetric outputs in half-bridge

Hello all.
I am building and testing an half-bridge SMPS for +/-55V output. It works very nice so far, although it has been "only" tested up to 125W, but the mosfets and diodes remaing absolutely cool.
It is running unregulated by the moment, and I only have a problem: one of the outputs puts +50V, but the other puts a bit more, around -57V, with the same load.
I can't figure out is causing that assymmetry, as 4 diodes are the same (MUR1560), the inductors (4.7uH) and capacitance (6600uF+100nF) are identical for both rails.

Another thing I would like to do is to measure the switching node voltage with respect to GND. I tried with 4 47k resistors, taking the output from one (1/4 voltage divider), but the RC constant with the capacitance of my 60MHz probe is enough to filter all the details and obtain a rather roundy waveform that doesn't tell me nothing useful. Could I connect the 10x probe directly or is that a bit too much voltage for a typical oscilloscope?

Thanks!!!
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Old 24th August 2005, 07:05 AM   #2
Eva is offline Eva  Spain
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Are your output inductors coupled? Using two independent output inductor produces poor cross-regulation.

Concerning switching waveform, you can connect your 10x probe to the switching node if it's rated at 400V or more, but be aware that very fast switching transients of high amplitude may cause a bit of partial breakdown in the attenuator that will show as overshoot. I had to live with that problem until I found an affordable 1200V 100:1 probe (there are a lot of good high voltage probes in the market but they are expensive).

You may also build your own high voltage attenuator but you would need to place a high voltaje adjustable capacitor in paralell with the resistors in order to compensate for probe capacitances (that capacitor is the one you can adjust through the calibration screw that most attenuating probes have).
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Old 24th August 2005, 09:25 AM   #3
Pierre is offline Pierre  France
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Thanks, Eva.
No, my inductors are independent by the moment.
However, with equal loads and 1 ampere of current flowing, I cannot find the cause that makes the difference in voltages. Can it be an assymmetry in both PWM signals? I don't think so, but... who knows?

Thanks for the advice on the probes also.

Best regards,
Pierre
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Old 24th August 2005, 12:35 PM   #4
Eva is offline Eva  Spain
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Note that 4.7uH is a very small inductor value, so current ripple will be very high (same applies for voltage ripple).

Also, with such a small inductor value, any slight imbalance in duty cycle between each side of the half bridge will cause a severe output voltage imbalance.

You should try a coupled inductor. If you are using iron-powder toroid cores you can unwind your current inductors, stack the cores, and wind them back together in a bifilar fashion. You would get a dual inductor of approx. 4.7uH (actually 2x 9.4uH when exciting only one winding) by keeping the same turn counts. Additional magnet wire would be required, though.

Theoretically, an ideal dual coupled inductor forces energy transfer between both sides until the voltage across them is the same.
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Old 24th August 2005, 06:24 PM   #5
Pierre is offline Pierre  France
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Sorry. The inductors are 47uH, not 4.7uH, They are wound on a drum core, so i will have to substitute them.

Unfortunately, this afternoon I was testing the thing a with a bit more duty-cycle, giving around 140W. Everything was quite cool (the diodes were warm but they have no heatsink), when one of the mosfet exploded.

Prior to this, I had measured the switching waveform by directly connecting my 10X probe. There is no visible overshoot, and the tops are flat. But there is ringing (about 10-20% of the total amplitude) at the dead time. I suppost that's quite normal.

The first thing that has come to my mind is that the same problem that causes the voltage imbalance has caused the failure. The core is a ETD44, with 26 primary turns, the secondaries being 13+13 turns.
Half primary is wound, then the secondaries, then the rest of the primary, to minimize inductance. It was not wound with great care, so perhaps the secondary is not absolutely symmetrical.

Can you point to the possible cause of the failure? Thanks Eva, your help is greatly appreciated!

Pierre
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Old 24th August 2005, 06:35 PM   #6
Eva is offline Eva  Spain
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Are you adjusting the duty cycle directly with a potentiometer?

Is your prototype mounted in breadboards or something that could lose contact?

Are you sure that your probe didn't slip and made a short?

Have you implemented some kind of current limiting in the primary side? This avoids a lot of catastrophic failures (one of my prototypes that implemented that survived for some time even with one of the switching devices shorted).

Concerning inductors, you may still turn yours into coupled if you wind half of the desired dual coupled inductor on each core in a bifilar fashion and connect them in series. Be careful with polarities.
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Old 24th August 2005, 10:06 PM   #7
Pierre is offline Pierre  France
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Yes, by the moment duty cycle is manually adjusted by means of a potentiometer. Maximum setting is about 44%, with 80KHz sw. frequency, 700ns dead-time.
The prototype is built on a double-sided board, with carefull layout, and I think no false contact or short was produced in that moment.

Some details that may give some clue: the mosfets are driven by a IR2110, with 10 ohm gate resistors and antiparallel schottky diode, mosfet are (were) IRFB11N50. The driver has also died.

There is a current sense transformer, whose output voltage is rectified and goes to the shutdown pin of the SG3535, but I don't think that has worked. There is also a 5A fuse before the bridge, but it hasn't blown (one of the mosfet exploded so no shortcircuit was finally produced )

My impression is that, as the transformer is coupled by means of a 1uF/250V capacitor, it shouldn't walk into saturation, and the number of turns limits the max flux to an acceptable level, if I have run the numbers correctly.

Thanks for your help!
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Old 25th August 2005, 06:11 AM   #8
mzzj is offline mzzj  Finland
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Any possibility that you have made 12R and 13R secondaries?
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Old 25th August 2005, 06:58 AM   #9
Pierre is offline Pierre  France
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Yes, it is possible.
But with an standard connection of the 4 bridge diodes, would that produce an assymmetry in the voltage of both rails or only an error in the output voltage of both of them with respect with the expected output?

Anyway, I have replaced the driver and mosfets, let's see if it doesn't fail anymore and I can start putting heavier loads. I have discovered that one of the primary ends was not correctly soldered at the transformer pins, perhaps it has suddenly disconnected ¿¿¿???

Best regards,
Pierre
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Old 25th August 2005, 08:15 AM   #10
Pierre is offline Pierre  France
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Please verify that the formula I am using for calculation of the flux density vs primary turns is correct:

Np=(V*10^8)/(4*B*f*A)

where...
V=320/2=160V
B is expressed in Gauss
f is expressed in Hz
A is ¿EFFECTIVE AREA? in cm^2 (172mm^2=1.72cm^2 for ETD44) ???

That gives 1100 gauss for 26 primary turns, at 80KHz that's about ok for a 3C90 core, isn't it?

Thanks for the clarifications!
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