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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I have a first attempt at a buck converter running at 37khz. The output has spikes at 37khz and appears fuzzy when I increase and decrease the voltage as seen on the video.
Here is an image of the waveform when I load it with my induction heater. The spikes occur at the switching frequency.
My question is how do I get a smooth output? I can vary the switching frequency, I can increase the inductance. Right now, the smoothing capacitor is 900v @ 750uf. The inductor is a standard E-core power transformer. Does it matter if I use an E-core transformer for the inductance, or should I wind a ferrite toroid?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
A little bit of spiking like this is to be expected around any switching regulator. They spray RF and noise all over the place. You may even be able to pick it up with your scope probe tip shorted to the ground clip.
If the circuit performs adequately and nothing burns up, there's no need to worry about it.
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837
A power transformer with an open secondary can have a resonant frequency in the same range as your switching frequency (lots of stray capacitance). That doesn't seem conducive to a well functioning buck converter.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
wrote ... The inductor is a standard E-core power transformer.
I never noticed that. If it's a laminated iron cored one the performance will be terrible. You really should use ferrite with an air gap, or an iron powder toroid.
Also. a layer wound or single layer winding helps to minimize stray capacitance.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Yes, I was going to change this as I was questioning in my opening question. Mind you, this is just a test version to iron out some of the kinks and get familiar with this topic.
I plan on changing the gate resistor to 3.3R and add some EMI suppression caps. I have some ferrite cores ready for winding.
On the output use a smaller capacitor, followed by a common mode choke then the output filter capacitor, that should be a large electrolytic in parallel with a smaller non polarized cap.
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help with buck converter output
