# General question about the workings of a high frequency inverter?

#### tesla000

Joined Jun 12, 2021
12
Hello,

i am designing a pure sinewave inverter, for which i will first use a sg3525 to step up the voltage to 380v and then the voltage will be converted to sine wave using spwm and lc filter. I am currently testing the first half of the circuit, that is the sg3525 booster part. I got most of the information of the inverter design from electronoobs video (https://electronoobs.com/eng_circuitos_tut83.php). I only slightly modified the circuit that i want it to be a universal voltage input from 12-24v, so i made a voltage feedback on sg3525 and it works, the only thing that's bothering me is that it's not isolated since it's sharing the same ground for voltage measurement and also i don't understand why the voltage has to be stepped up to 380V?, isn't the max voltage of 230v outlet around 322V, because 230V*sqrt2=322.

I would really appreciate some tips here.

#### schmitt trigger

Joined Jul 12, 2010
765
You are correct, it doesn’t.
380 volts DC is a common output voltage on PFC circuits, precisely because it is above the crest voltage of the sinewave.

#### schmitt trigger

Joined Jul 12, 2010
765
If your SG3525 timing values are indeed as shown, it will be oscillating at 2500 Hz, per the SG3525 datasheet.
The poor GBU406 is a standard recovery bridge rectifier. At that switching frequency, it will overheat and probably fail. As it is the main rectifier bridge, it may take down your Mosfets with it.
Replace it with fast recovery diodes.

#### tsan

Joined Sep 6, 2014
138
i don't understand why the voltage has to be stepped up to 380V?, isn't the max voltage of 230v outlet around 322V, because 230V*sqrt2=322.
It's stepped higher than 322V because there are voltage drops caused by for example resistance, maximum duty cycle limitation and output filter. It's possible that DC voltage is actually less than 380V even transformer rating is 12/380V.

#### Ian0

Joined Aug 7, 2020
8,940
You are correct, it doesn’t.
380 volts DC is a common output voltage on PFC circuits, precisely because it is above the crest voltage of the sinewave.
As it is drawn there is no feedback to regulate the 380V bus, so 380V at 12V in makes sure that there will be enough output voltage when the battery drops to 10.8V

#### Ian0

Joined Aug 7, 2020
8,940
If your SG3525 timing values are indeed as shown, it will be oscillating at 2500 Hz, per the SG3525 datasheet.
The poor GBU406 is a standard recovery bridge rectifier. At that switching frequency, it will overheat and probably fail. As it is the main rectifier bridge, it may take down your Mosfets with it.
Replace it with fast recovery diodes.
10nF/33k gives 200us charge time, 2500Hz operating frequency as you say. It seems like an order of magnitude too low to me for the size of transformer. I would expect a transformer of that size to work at 25kHz-50kHz.
I bet it whistles annoyingly!