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Switching transistor (Simetrix)

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Newbie level 2
Aug 21, 2015
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Im trying to simulate a double frequency buck converter and i observed that the transistor switch is acting weird. It fluctuates after the transient response. im not sure why. The base of the BJT transistor is connected to a PWM with 15V peak to peak 7.5V offset and the supply of the circuit is a 12V voltage supply. Here is the schematic diagram


thanks in advance for your help

Your Q3 transistor appears upside-down. I suppose that is because it conducts upwards (synchronous operation)? Can you substitute a diode instead? That will make it easier to interpret what the circuit does.

Also it is worth a try to put PNP type for Q1 & Q2.

It's unclear in which regard the design is acting as "double frequency buck converter". You should show the timing of all control waveforms.

The circuit is at best a simplified model of a real converter circuit. Transistor maximum ratings (Ic, Vbe) are considerably exceeded, this might explain unexpected waveforms. Why don't you use ideal switches in a first attempt?

Thanks for the replies. We dont have any software that can simulate the circuit with ideal switches. also we are required to create a prototype for this circuit so we are looking for transistors which can handle the high current and high frequency of the design (3A and 50kHz). Here is the circuit diagram with ideal switches


The switches S1 and S2 are complementary with high frequency
The switches S3 and S4 are complementary with low frequency

The Q3 in the first schem has a PWM connected to its base with the same frequency as Q2 but has a 180 degree phase shift so it can act as a complementary switch with Q2.

Looking at your schematic...

It is possible for gating signals to close S2 & S4 simultaneously. This would cause undesired current flow in L1. To solve this, I believe you ought to detach the righthand end of L1, and connect it to the upper end of the capacitor. Your circuit will then become a twin interleaved buck converter.

[edited to describe another scenario which is more likely than closing S1 & S2 simulataneously]
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We dont have any software that can simulate the circuit with ideal switches.
??? Any SPICE based software can, of course Simetrix, too. Review the refernce manual, chapter 4 analog devide reference, voltage controlled switch.

For the real circuit, you need transistors with sufficient current handling, preferably MOSFETS, and control signals according to voltage swing present at the respective switch terminals.

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