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# LT spice -SPWM- transformer output not filtering

#### thannara123

I am learning to design circuits in LTSpice. I have a schematic where an SPWM signal is generated using an H-bridge, which is connected to the primary of a transformer. The secondary is connected to a filter capacitor and a load resistor.
LC filter is required to obtain a 50 Hz sine wave, which is my fundamental frequency. However, I am not achieving a sine wave output.

I tried placing an inductor across the load in series with a capacitor and took the load across the capacitor, aiming to achieve a pure sinusoidal wave. ANd i got it .
But whenever use the transformer am in trouble .

Despite trying various random LC values, I have not succeeded.
How can I achieve a smooth 50 Hz sine wave in this setup?
switching freqency is taken as 20khz

Last edited:

Hi,

a single capacitor is no filter! A filter consists of at least two parts, a series one and a parallel one.
If the capacitor is parallel to the load ... you additionally need a series impedance.

The transformer and it´s given inductance acts not as a sereis impedance. Only it´s (tiny) leakage inductance (not given in your schematic) acts as series impedance.

Klaus

thannara123

### thannara123

Points: 2
Thanks I added Leakge inductunce .

Hi,

I would not rely on leakage inductance. It may vary a lot, from part to part, from manufacturer to manufacturer, from charge to charge, from version to version.

Unless: you order custom made transformers with precisely designed in leakage current... according your specification and tolerance.

Klaus

The filter must attenuate the switching frequency while allowing the 50 Hz signal to pass through.
The cutoff frequency of the LC filter should be set between the fundamental frequency and the switching frequency, typically much lower than the switching frequency to effectively filter out high-frequency components.
You have to calculate the values of LC using the cut-off frequency formula: https://www.omnicalculator.com/physics/lc-filter

It would take a 10 mH coil to create a resonance at 50 Hz with 1 mF using as a current source or common emitter and not a low RdsOn to a parallel LC tank.

This won't work.
--- Updated ---

The H bridge is an RdsOn switched Voltage Source.

The LC tank is a high Z resonance does not resonate when shunted by a voltage source, rather, only a high Z current source.