transformer and diode question

Discussion in 'General Electronics Chat' started by jaygatsby, Dec 8, 2011.

  1. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
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    I first started looking in to switched mode power supplies when I tore apart a cell phone charger, saw that it had only a tiny transformer in it, realized that all the 120VAC step-down transformers online were physically huge compared to it, and asked about it here. So I think that the small transformer I saw in the cell phone charger was not connected to the mains, but somewhere else in that circuit?

    Q1: I want to learn how to build a switched mode transformer, but physically isolating the supply from the mains, at the mains, is (I'm told) the only non-dangerous solution; the cell phone had a small transformer, too small for it to be directly connected to the mains (3/4 inch cubed maybe). What am I missing?

    Q2: Is the forward voltage rating of a diode the max voltage it can handle? The data sheet for a large 'power diode' that I have says its forward voltage is 1V. It seems like it should be more, considering its huge size. Again, what do I not understand about diodes?

    Thanks,
    J
     
    Last edited: Dec 8, 2011
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The trick is the frequency that the current through the transformer is being switched. The higher the switching frequency, the smaller the transformer can be physically - within certain practical limits, of course. The cell phone charger was probably switching at a couple hundred kHz so that the physical size could be small; reducing the amount of materials that had to be used to build it; reducing it's direct cost.

    The mains power was rectified, and probably stored in (a) capacitor(s); then an oscillator or switching circuit excited the primary of the small transformer, inducing a voltage on the secondary. There was some type of feedback to the primary side switcher to cause its' output to decrease when the voltage on the output was up to specifications.

    If you're interested in pursuing switching supplies, you might have a look at Ronald Dekkers' "Flyback Converters for Dummies" page; it's a great resource:
    http://www.dos4ever.com/flyback/flyback.html
    Cautions: in figures 3 and 16, replace R4 with a 47k or smaller resistor to keep the output voltage at a safe level. Also, in figure 16, change the turns ratio to 1:3.

    For standard silicon diodes, it usually works out that the rated current is just about when the diode Vf is 1v. This is not always precisely the case, but it's not a bad "rule of thumb".

    Schottky diodes have a lower Vf than standard silicon diodes. They are also MUCH faster than silicon diodes; even supposedly "ultrafast" silicon diodes.

    Standard silicon rectifier diodes work just fine with low frequencies; up to around 500Hz sine waves. After that, they start causing power losses because they turn off very slowly. Schottky diodes turn off in the 15nS-25nS range. Silicon diodes are in the microsecond range.

    That reminds me, I found this handy table of diode technologies in a Rohm application note:

    [​IMG]

    That little table is a real "keeper".
    SiC (Silicon Carbide) diodes are quite new, and are not affordable for the hobbyist at this point - Schottky diodes are lower voltage, but just about as fast.

    When you're dealing with low frequency stuff, the diode you use is frequently not very important. When you get into switching supplies, you need bleeding-edge fast diodes, or you will lose a considerable amount of power because they won't turn off quickly enough.
     
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  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    @Jaygatsby: As you may or may not have picked up from SgtWookie's notes, it is the VBR, VRRM, (or sometimes a rating called Peak Inverse Voltage, PIV) that sets the maximum limit of operating voltage for a rectifier. These ratings describe how much reverse voltage the diode can block without breaking down. In some cases like a simple half-wave rectifier feeding a capacitor, the PIV needs to be at least double the peak input voltage. Can you think why?

    The forward voltage tells us how much voltage appears across the diode during normal conduction (at a given current). It is actually desirable for the this voltage to be as low as possible, as it represents a loss, leading to reduced output voltage and heating. This is one reason (apart from their switching speed) why Schottky diodes are preferred for some applications.
     
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