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alexclaire 10th November 2006 01:46 PM

half bridge SMPS secondary voltage drop problem ?
hi everyone,

Let me explain my project:

230Vac----->+/- 40 VDC

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.
12V auxiliary power supply
No feedback
ETD39 core
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...

alexclaire 10th November 2006 01:47 PM

2 Attachment(s)
voltage accross one secondary winding (8 turns) unloaded...

alexclaire 10th November 2006 01:49 PM

2 Attachment(s)
voltage accross the same winding but with a 10 ohms load, 6A delivered...

alexclaire 10th November 2006 01:50 PM

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...?

alexclaire 10th November 2006 01:53 PM

2 Attachment(s)
pic of the proto board
note that gate driver board is on the left side...

Elvee 10th November 2006 02:12 PM


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.

alexclaire 10th November 2006 03:19 PM

2 Attachment(s)
sorry ;-)
here is the right pic:

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