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I made a project with the objective of changing the supply voltage of a system (VOUT in the circuit) through a PWM signal.

The initial objective was to vary the supply voltage of a secondary circuit (bridge H) from 0V to 110V continuous.

I thought of doing this using a system with an op amp. I put an RC circuit on the amplifier input to convert the PWM signal into an analog signal. My intention was as follows: 50% duty = 55V at the output; 100% duty = 110V (approximate considering drops) at the output.

But what I have in VOUT is a pulsing signal with amplitude of 120V and RMS voltage varying (that wasn't my intention).

can anybody help me?

schematic

simulate this circuit – Schematic created using CircuitLab

EDIT 1:

The VOUT load will be an H-bridge. Similar to the circuit below:

schematic

simulate this circuit

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  • \$\begingroup\$ What is the frequency of your PWM signal? What does the voltage across C1 look like? \$\endgroup\$
    – Elliot Alderson
    Commented Jun 10, 2021 at 19:47
  • \$\begingroup\$ Is V1 110VAC or 110VDC? It is shown on the schematic as DC. \$\endgroup\$
    – Voltage Spike ♦
    Commented Jun 10, 2021 at 19:57
  • \$\begingroup\$ @ElliotAlderson The frequency is easily changed by uc if necessary, but the tests I did were with 100kHz. The voltage at C1 seems to continue. 1.6V with 50% duty. \$\endgroup\$
    – Eduardo Cardoso
    Commented Jun 10, 2021 at 20:04
  • \$\begingroup\$ @VoltageSpike It's 110VDC continuous. \$\endgroup\$
    – Eduardo Cardoso
    Commented Jun 10, 2021 at 20:05
  • \$\begingroup\$ What does the load look like? This circuit is an oscillator. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Jun 10, 2021 at 20:27

1 Answer 1

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The loop is not compensated correctly, you will need to run a spice AC analysis to ensure that there the overall loop has no resonance points and has a good roll-off (it should look like a low pass filter).

Try making it more of a low pass filer by making a pole in the system on the feedback loop by putting a cap in parallel with R7.

I'd run the analisys but I'm away from my machine

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  • \$\begingroup\$ Even if the loop is changed will the circuit still behave like an oscillator on VOUT? \$\endgroup\$
    – Eduardo Cardoso
    Commented Jun 11, 2021 at 11:12
  • \$\begingroup\$ Don't know, run the analysis in spice \$\endgroup\$
    – Voltage Spike ♦
    Commented Jun 11, 2021 at 14:01

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