The switching frequency is 400kHz, not 400Hz.
XL6009 based DC/DC Converter for sensitive audio voltages like e.g. phantom power?
- Thread starter rock soderstrom
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thor.zmt
Well-known member
This PCB, you are quite sure? And you are sure the bodged up 34063 works nice and stable without subharmonic oscillation?
It's not what I see on the 'scope shot in the back of the pic, BWTFDIK.
Thor
rock soderstrom
Tour de France
Thanks, I have ordered a couple of suitable chokes. Yesterday, a test with my new tube microphone preamp using an ordinary RC filter yielded positive results. There is certainly still room for improvement, but overall the whole thing was promising.
Among other things, I actually "measured" something around 400Hz, which irritated me. As I said, my approach could be improved and it could also be due to the upstream power supply, possibly interference?
I am trying to understand the function of the small board in detail with this test, I am currently researching on the web. I'm beginning to understand better how it works.
The journey continues.
thor.zmt
Well-known member
The circuit was designed to power nixies. Absolute stability and noise were not a relevant consideration.
In typical China fashion, it was then copied badly and applied badly.
Hard to say, these days SMPS typically use either 67kHz, 132kHz fixed, much higher frequencies or variable (resonant and quasi resonant) switching frequencies, QRP/RP style valley/zero switching supplies are normally targeted at 23kHz minimum frequency under maximum load and ~80khz at idle.
What we typically see is this:
Normally a linear regulator for heaters & this for the HT. Switching frequency will be below 20kHz
An alternate schematic in common use on china boards (and a bit better in theory) is this:
This should switch at ~ 30khz.
By comparison, using a suitable MOSFET as cascode on a modern Switcher IC is a much better choice:
This circuit uses a SPOT-23-6 6Pin SMT IC with 400mA maximum current and compact MOSFET with minimal additional parts and switches at 1MHz. It can be very stable and low noise (around like an adjustable 3-pin regukator).
Of course, this actually needs DESIGN WORK, not just copy/pasta, so you will not find it on china aliexpress and e-bay boards.
I have used this with the Richtek RT9297 with ~ 3.8A switch current and 1.2MHz switching frequency to generate high voltages with many Watts of power. Using a USB-C Brick supplying 20V/5A (100W) we can easily get 4A Heaters at any voltage > 18V you like and around 25W HT, using two IC's, one mosfet, one USB-C trigger chip and a few small size passive parts.
Thor
rock soderstrom
Tour de France
Thanks, that helps a lot.
thor.zmt
Well-known member
Here two boards I got to make a HT and heater for a 5670 based circuit.
HT uses SG3525 which is a proper SMPS controller, works up to 400kHz (no idea what is actually used here, have to check) with a push-pull switching stage and bridge rectifier.
Rest is left to the user - no filtering whatsoever integrated.
LT is (fake) LM2596HV with generic elcaps that need replacing with Solid electrolyte types.
So these are a cut above the usual AliExpress junk.
Thor
