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Switching Power Supplies

quantumtangles

Dec 19, 2012
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As I understand it, the essential requirements for a switching power supply are:

Gated oscillator (anywhere north of 100kHz) x 1
Operational amplifier x 1
Voltage reference
Resistors x 2 (voltage divider)
NPN (darlington pair) series pass transistors x 2
Filtering Caps x 2 for input and output
Catch diode x 1
Inductor x 1

Has anyone here actually built a switching power supply circuit using gated oscillators etc and if so, what is the trade off between using a super high frequency gated oscillator (to move packets around really quickly) and the downside of high frequency packet transference.

Is it simply the case that high frequency gated oscillators in the 'on' state result in fully saturated series pass transistor pairs where the respective bases are almost continually supplied with the required voltage to allow collector/emitter action (and therefore it consumes more electrical energy because it is in the 'on' state most of the time), or is it more subtle than this?

In other words, why not use a 1Ghz gated oscillator to move the packets around ever faster. Is there a downside to ever increasing packet speed, and if so, what is it?

Thanks in advance for any responses:D
 
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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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As I understand it, the essential requirements for a switching power supply are:

Gated oscillator (anywhere north of 100kHz) x 1

You can have a lower frequency.

Operational amplifier x 1

Well, you need something with gain, not necessarily an op-amp

Voltage reference

This is only required if you want a regulated output. Some switchmode power supplies do not require this.

Resistors x 2 (voltage divider)

There is no need for a voltage divider. Even though one is often used.

NPN (darlington pair) series pass transistors x 2

Well... you only need one pass element and it doesn't necessarily need to be a BJT.

Filtering Caps x 2 for input and output

Only the output one is required.

Catch diode x 1
Inductor x 1

Yep.

In essence all you need is a switch, an inductor, a diode and a capacitor.

For an example of a SMPS lacking several of the things you indicate above, see here: https://www.electronicspoint.com/smps-boost-regulator-using-schmitt-trigger-oscillator-t251353.html

It has no input capacitor, voltage reference, the frequency is under 100kHz.

The first design has a voltage divider, but the final design does not.

Has anyone here actually built a switching power supply circuit using gated oscillators etc and if so, what is the trade off between using a super high frequency gated oscillator (to move packets around really quickly) and the downside of high frequency packet transference.
The final design in the thread above uses a gated oscillator.

The downside of very high frequencies is capacitance.

you'll have a harder time getting fast rise and fall times, and eventually the switching element won't switch. Any capacitance will cause a low pass filter effect, and the regulator (in some configurations) will end up operating more like a linear than a switching regulator.
 

quantumtangles

Dec 19, 2012
153
Joined
Dec 19, 2012
Messages
153
Thanks Steve. Awesome answer. The thread on SMPS is fantastic. Beginning to find switching power supplies fascinating :D
 
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