Tests on Switching power supply


This is integrated into the USB_ICD2 programmer that I described in another page of my site. It supplies a DC 13V voltage which is the usual programming VPP voltage for PIC micro-controllers. The DC 5V voltage to power the various components of the circuit is taken directly from the USB port, but the current absorbed by the assembly is minor!

The oscillogram is based on the 0V level. You will notice that one distinguishes perfectly the phases of damping of the tension. The pulse at the 0V level corresponds to the off state of the switching transistor internal to the integrated circuit LM3578. The signal that is displayed corresponds to the output ( pin 8 ) of the LM3578 before filtering ( after filtering, you have a perfectly continuous and stable +13V voltage free of all interference generated by the high switching frequency ).

When switching from the low state (0V) to the high state ( ≈ +12.5V peak ) thanks to the conduction of the transistor integrated in the LM3578, there is an accumulation of energy for ≈ 1.5µs ( duty cycle ≈ 7.5% ) then cut off this voltage by the regulation internal to the LM3578. The direction of the current in the inductor is then reversed, and it restores the energy accumulated to the load. The cycle begins again according to a predetermined duration ( RC circuit represented here by the assembly R23/R17/P1 and C7/C9, the potentiometer P1 makes it possible to adjust the output voltage VPP ... )

Here are more photos of the signal that I was able to measure for you. We see in particular the voltage levels, as well as the blocking and saturation times of the transistor integrated into the LM3578:









The diagram corresponding to this power supply is as follows:

diagram of the Alim USB ICD2

oscillogramYou can see some similarities with the oscillogram opposite which corresponds to the test on the filtering transformer of the previous article ...

For more details on the theoretical operation and from a "mathematical" point of view, I strongly urge you to consult the LM3578 datasheet ( in English ) pdf3

Important note:

I carried out all these operations either with a function generator delivering signals of 200 mV (-40 dB) of maximum amplitude, or on my ICD2 programmer which is only supplied with DC 5V by the USB socket.

It is obviously possible to make the same kind of measurements on a switching power supply supplied by the sector ( voltage between 100-240V ~ 50/60 Hz ), however I would like to warn you about the dangerousness of these manipulations ( RISK ELECTRIC SHOCK ) if you do not take the basic usual precautions, namely to use an isolation transformer .
Also pay attention to the very high voltages circulating in the primary circuit, of the order of DC 300V !

My responsibility can in no way be incurred for all damage, both technical and physical, which may be caused to you, both by lack of precaution and ignorance.
You have been warned!

There are ready-made switching power supplies, in a protective box such as this model shown opposite:XPPOWER ECES-10

It has the advantage of being totally isolated, and displays very interesting characteristics to constitute a small powerful power supply and without danger for the user.

Here are its main characteristics:

  • Power output type: Fixed
  • VAC input voltage: 85 V to 264 V
  • Number of output channels: 1
  • Nominal Output Voltage: 12 V
  • Input power consumed no-load: <0.3 W
  • Switching frequency: 130 KHz
  • Max Output Current: 410 mA
  • Power, max.output : 5 W
  • Depth: 25.4 mm
  • Width: 25.4 mm
  • Height: 15.24 mm

Complete datasheet available for download herepdf3

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