Isolated Power Supply world-wide AC input

Part 8

Changing Equipment

Aufbau
Figure 169
It is necessary to develop a small circuit running with the PFC. Unfortunately I can`t show the purpose of this circuit, please don`t expect an explanation for the purpose of the circuit.

working on the isolated 28V low voltage secondary side, no risk to use now a more comfortable oscilloscope with lower maximum input voltage range.

Note the labtop, measure and simulate at the same time, a good method to develop a circuit.


soldered circuit
Figure 170
Device under Test


Lötplatz
Figure 171
These are the things you should have to solder SMD PCB`s and prototypes.

From left to right:
  • Soldering workstation with hot-air, adjustable in temperature and air stream
  • Desoldering iron with vacuum pump for removing liquid soldering wire
  • Vacuum tools for removing soldered IC safely from the PCB, different metal cans heating exactly on the wanted PCB area.
  • Micro tweezers cartridge tool, desoldering 2-pin SMD parts, resistors, capacitors.......
  • Two other handpiece soldering iron, with smaller and higher power
  • Approx. 20 different types of cartridges, different sizes and shapes (not shown on photo)
  • Handpiece soldering iron with Nitrogen Air for highest soldering quality (not shown on photo)
  • Cold light source with mechanical adjustable double halogen light bulb, medical light source for Chirurgie
  • (in future I build an professional adapter for the light-cable suitable to the microscope)
  • Stereo Zoom Microscope with comfortable adjustable viewing angle.
If you solder often, use quality tools.

With the hot-air tool it is easy to solder the small copper-PCB-pieces on the ground plane, it is like placing a SMD part. Whitout an hot-air tool use e.g. fast 2K-Epoxy-Glue - but be aware glued copper-pieces remove accidentially sometimes, possible fix them with a large soldering iron (not easy, little tricky). Easiest, solder them with the hot-air tool.

Soldering on a real wooden table and chair (hard-oil furnished or natural, no plastic paint) - no further need for ESD protection. Wooden floor (hard-oiled or natural surface),  100% wooden chair and use 100% cotton clothes - avoid all plastic and synthetic in the working area. Natural cotton and wood under high relative humidity (no drying air-conditioning) is in general a safe ESD condition for almost all parts.

Of course very sensitive parts like laserdiodes and high impedance HF electronic require special ESD environmental and handling procedures better than the protection shown on the photo.

It is possible to handle such also very sensitive devices under hobby conditions - but think very well what you are doing, each movement of your hand and your body has to be considered. Best read a physic book about electrostatic, you must understand first in detail why things are charging before understanding ESD protection.


In my opinion, soldering a circuit direct on copper is an efficient method developing a circuit:
  • probing excellent
  • low parasitic inductance
  • low capacitance coupling
  • high insulation resistance
  • excellent ground potential
  • each node accessable with voltage probes
  • each wire accessable with a current probe (require only a small additional wire)
  • if required, connect ten or more voltage probes together with four current probes at the same time
  • all probes connected very tight to the soldering junctions
  • change parts as often you want, no damaged PCB footprint copper, due often soldering
  • very flexible changing the schematic
  • always enough space for increasing the circuit
  • SMD and THT parts can be used at the same time
  • fast build-up
  • lowest cost for copper pcb

In my opinion disadvantages of the following methods developing a circuit:

Breadboard with springs and flexible wires:
  • probing difficult
  • springs on a bad quality boards causing sometimes contact problems
  • higher parasitic inductance
  • probing on the wires, more difficult
  • less ground potential
  • not recommended for higher currents
  • breadboards with excellent mechanical quality, price

Prototyping PCB`s with stripes and holes:
  • probing difficult
  • take time for thinking where to place the parts
  • thinking where to place the parts before start evaluating the circuit
  • unflexible for changing circuit
  • somtimes long ways
  • bad ground potential
  • good for known circuits

Selfmade protoyping PCB`s with etching or milling machines:
  • probing difficult
  • require a time demanding layout
  • require time for milling or etching.....
  • environmental - etching chemicals
  • environmental - milling, very small particles (requires excellent vaccum cleaner during milling)
  • schematic changes difficult
  • selfmade PCB`s are excellent for already working circuits, not recommended for a design start.
  • good for known circuit

Manufactorer Evaluation Boards: 
  • probing difficult
  • changing parts and schematic difficult
  • changing many times a part - destroys foot prints
  • after measuring all signal and currents, board requires repairing
  • some boards are good, same are terrible - using 0402 (bad) and 0603 (better) parts for low-frequency applications? - why? These board designers are not interested in customers requirements using an Eval Board.
  • Eval Boards, fast starters, useful for QFN, DFN and BGA packages.
  • Eval Boards useful for Layout hints. 
Note: the Eval Board of this PFC project was good for developing the circuit, big enough and parts easy accessable for most measurements. My tip: start with the Eval Board and change to another method when necessary.

Developing the circuit on the first Proto-PCB delivered to a customer: 
  • probing difficult
  • changing parts difficult
  • unflexible changing circuit
  • changing many times a part - destroy foot prints
  • haywires
  • designer don`t want to solder in an ugly way on a customer PCB => not tested
  • increased time pressure to deliver PCB
My opinion: PCB should be the last step and not the first step for a circuit development.

The worst item for all these methods:

"difficult probing" - often results in NOT testing an node.


Ein kleiner Gruss an die deutsch sprachigen Leser:

zweifelt an Methoden, wie: "zur Evaluierung einer Schaltung müssen alle Drähte und Bauteile gerade verlegt sein oder im 90° Winkel". Bei diesen Methoden wird dann suggeriert, dass das sogenannte "ordentliche Arbeiten" sehr wichtig ist, die eigentliche Aufgabe eine gute Schaltung zu entwickeln tritt dabei vollkommen unbeachtet in den Hintergrund.

Die "sogenannte Sauberkeit am Arbeitsplatz" ist für die Entwicklung einer Schaltung einer der unwichtigsten Punkte überhaupt. Ist es bei mir zu Hause dreckig? - ich bin mir sicher - nein.

Viel wichtiger hingegen ist die "Sauberkeit auf der PCB", insbesonders eng aufgebaute PCB`s leiden unter diesem Punkt, der Luftaufbau hat an diesem Punkt keine Schwierigkeiten. Eine total mit Flussmitteln versiffte Test PCB, ist in der Regel ein Beweis dafür was die Schaltung taugt: - wenig - außer viel Siff darauf.

Wurde die manuelle SMD Prototypen Schaltung komplett unter dem Mikroskop gelötet mit gutem Licht, Flussmittel und Lötausstattung - wenn ja prima - oder erfolgte die Lötung unter Plastik Leselupen, Kopflupen oder sonstigen optisch dürftigen Hilfsmitteln? Die Preise selbst für ein gebrauchtes Stereo Mikroskop sind erträglich. Aussagen wie: "ich habe gute Augen" - zeugen von nicht verstandener Notwendigkeit. Selbst die schärfsten Augen sind einem Mikroskop um Längen unterlegen.

Verheerend ist die Fehleinschätzung der Ansicht zu sein, stets alles theoretisch vorab entwickeln zu wollen - ohne Test direkt in die Serie hinein.


Woran erkennt man sofort auf einen einzigen Blick innerhalb von wenigen Sekunden eine schlecht dimensionierte, meist auch nicht gut funktionierende Schaltung?

An der Art und Weise wie der Schaltplan gezeichnet ist.

Die Mühe und die Verständlichkeit, die sich jemand gibt einen Schaltplan zu zeichnen ist das Spiegelbild zur schaltungstechnischen Qualität. Es spielt keine Rolle ob Handzeichnung oder CAD Tool, von Interesse ist das Erkennen des Willens die Schaltung einem anderen so zu präsentieren, dass dieser mit dem geringstmöglichen Aufwand an Zeit eine Chance hat die Schaltung zu verstehen.

Wenn man Gelegenheit hatte den Schaltplan einzusehen, weiß man in den meisten Fällen innerhalb von Sekunden alles weitere über Schaltung und Dokumentation.

Some last Screenshots without Comments

(demonstrates many measurements are necessary for a fine running circuit)

Figure 172



 Figure 173



Figure 174



Figure 175



Figure 176


Figure 177 - SAFE


Figure 178



Figure 179



Figure 180



Figure 181



Figure 182


Board nach Pre Development
Figure 183

Board after finishing this pre-development level.

- Ready to go for the first PCB layout -

Eval Board looks now quite different to it´s original condition. A full development of a circuit require special methods, in my opinion, build up the circuit on a self made evalboard, after knowing all voltages and currents start with the first PCB.

This application note for the pre-development stops here with Part 8. Operates excellent now, final layout will be easy, without any unwanted surprising or time delay.


END

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