| Electronics > Beginners |
| Weird switching topologies you don't see anymore |
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| T3sl4co1l:
I used an analogous drive method in this buck converter, Here the drive transformer couples load current into the base, forcing a ~constant hFE(sat), and also the inductive energy provides rapid turn-off (shunting of B-E charge). Once triggered, it remains on (positive feedback), until the transformer core saturates, yanking it off. The values shown give a pulse width of about 5us. The control is then a simple variable frequency multivibrator to regulate output current. A similar circuit was used in ATX power supplies for decades; there, a symmetrical (half bridge) arrangement is used instead, with two base windings are provided, and the feedback winding is in series with the common output. Drive was provided from a shorting-mode circuit, so that base voltage is held at zero when the transistors should be off, and one or the other side is driven on when desired. Typically a TL494 and pair of transistors (2SC1815 or such) performed this. Tim |
| lordvader88:
--- Quote from: T3sl4co1l on July 24, 2019, 04:03:58 pm ---Nah, they're driving it with a triple darlington, it's slow enough not to care. In the modern day of MOSFETs throwing off harmonics beyond 200MHz, we'd have to observe that the transformer can be a 1:1 transmission line, and so can be made of twisted pairs or quads, that the impedance should be near the switching impedance and the leakage inductance is proportional simply to winding length. --- End quote --- What do u mean by that part " the impedance should be near the switching impedance " ? What's the switching impedance ? Is it just what the overall mosfet impedance is at operational frequencies, and then wire pairs should have about the same valve? |
| chris_leyson:
@David Hess. The original Royer was designed as a linear voltage to frequency converter and was not really meant to be a power converter. "A Switching Transistor D-C to A-C Converter Having an Output Frequency Proportional to the D-C Input Voltage" July 1955. For the Royer converter, the self oscillating frequency is proportional to the input voltage, whereas in the Jensen, the time taken to reach saturation is determined by the base drive transformer and the required base drive voltage which is more or less constant compared to a potentialy varying input voltage. So, for the Jensen converter the operating frequency remains reasonably constant. Also, you don't get the large collector current spikes that you do with the Royer. There are also proportional base drive versions of it as well. |
| David Hess:
--- Quote from: chris_leyson on July 24, 2019, 10:49:10 pm ---So, for the Jensen converter the operating frequency remains reasonably constant. Also, you don't get the large collector current spikes that you do with the Royer. There are also proportional base drive versions of it as well. --- End quote --- I suspect the low current spikes were why Tektronix used it. They required no post regulation although they did include pi filters on the low voltage outputs. Proportional base drive allowed primary side regulation although they were selecting the inverter output transistors for something. |
| T3sl4co1l:
--- Quote from: lordvader88 on July 24, 2019, 10:37:15 pm ---What do u mean by that part " the impedance should be near the switching impedance " ? What's the switching impedance ? Is it just what the overall mosfet impedance is at operational frequencies, and then wire pairs should have about the same valve? --- End quote --- Yeah, an informal impedance: peak switch voltage divided by peak switch current, or something along those lines. The loop stray inductance and switch (transistor/diode) capacitance also form a ratio sqrt(L/C) with the same units (ohms), related for the same reason. In a transformer coupled topology (flyback, forward, SEPIC, Ćuk..), the leakage inductance is part of the loop stray inductance. The traditional rule of thumb "minimize inductance" is actually trying to do this: set 1/sqrt(Lstray*Csw) (the loop resonant frequency) higher than the switching speed, and set the impedance low enough, that you do not experience significant peaking or ringing. (This is a rather outdated rule, as modern devices are faster than the strays that can be achieved with leaded components.) But if you can't avoid that, then you can at least manage it, and that's where knowing the loop impedance comes into play. What the impedance means is, for a step change in inverter state, and accompanying step change in voltage or current, what the respective peak current or voltage will be. So the transistor carrying say 10A, turning off suddenly, will generate 10A * Zloop peak voltage. Zloop also tells you what value resistor to use, in an RC damper. Tim |
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