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Old 10th November 2006, 01:46 PM   #1
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Join Date: May 2005
Location: Avesnes/Helpe
Default half bridge SMPS secondary voltage drop problem ?

hi everyone,

Let me explain my project:

230Vac----->+/- 40 VDC
350-400W

230V AC mains---->320VDC rectified
2*660uF/200V capacitors
Half bridge SMPS topology
SG3525 oscillator and modulator (fosc=200kHz;fsw=100kHz)
Rdeadtime is 50 ohms.
Rt=6,8k
Ct=1nF
12V auxiliary power supply
No feedback
ETD39 core
Bmax=100mT
N1=32 turns
N2=16 turns center tap
capacitor 1uF in serie with primary connected to +160V (middle point of capacitors)
2*IRF840 driven by gate transformer (3C90 core 13.5mmOD, 12:12:12)
gate transformer driven by transistors mounted in push pull topology (2N3906 and 2N4401)

My problem is:

With no load, voltage accross secondary windings (8 turns) is about 42V peak peak so no problem...

With a 10 ohms load between + and - (rectified), voltage drops to +/- 30V
i think it's a bit much than could be expected ?

and rectified voltage accross primary capacitors drops from 320VDC to 300VDC...

here are some pics...
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Old 10th November 2006, 01:47 PM   #2
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voltage accross one secondary winding (8 turns) unloaded...
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Old 10th November 2006, 01:49 PM   #3
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voltage accross the same winding but with a 10 ohms load, 6A delivered...
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Old 10th November 2006, 01:50 PM   #4
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with these informations, is someone able to tell me more about this voltage drop, is it normal, are the capacitors on the primary side enough for the ripple...?
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Old 10th November 2006, 01:53 PM   #5
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pic of the proto board
note that gate driver board is on the left side...
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Old 10th November 2006, 02:12 PM   #6
Elvee is offline Elvee  Belgium
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Hi,

Looks pretty normal to me, especially if your no-load condition is truly and completely no-load, without even a bleeding resistor: the switching spikes can raise the output voltage several volts.
And in no-load condition, you have a nice, full square wave due to the magnetizing inductance, but with the load, the magnetizing current is insufficient to drive the load and dead time becomes apparent; maybe your diodes are a bit slow too, but it's difficult to tell.
I'd add a turn or two to the secondary, plus a small bleeding resistor to keep the no-load voltage at reasonnable values.
You could also measure the leakage inductance of your transformer (seen from the primary) and adjust the 1 cap to make it resonate at the switching frequency; this would somewhat improve the "stiffness" of the supply.
LV
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Old 10th November 2006, 03:19 PM   #7
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sorry ;-)
here is the right pic:
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