How inverters work when the supply voltage is reduced.

Hi everyone,

Can anyone here explain the working of inverters with limitations involved when the supply voltage is reduced while keeping threshold voltage of transistors is constant?

Thank you.

Re: How inverters working when the supply voltage is reduced.

There needs to be a conversion method in the inverter, to increase the voltage step-up ratio, if the inverter is going to maintain the same output volt level.

If it is designed to do the above, then it must also draw more current from the supply. It must be designed to turn on more internal devices, or reduce the impedance of existing devices, or increase the duty cycle, etc.

Re: How inverters working when the supply voltage is reduced.

imperza said:

Hi everyone,

Can anyone here explain the working of inverters with limitations involved when the supply voltage is reduced while keeping threshold voltage of transistors is constant?

Thank you.

Click to expand...

You can see commercial UPS devices, and their transformers. Thise transformers have many taps to adjust correct voltage, electronics is charge for correct tap switching. Also look how transformer based voltage stabilizators works.

Re: How inverters working when the supply voltage is reduced.

hi tpetar
i hve few questions to ask from u
1) i have completed my first stage of inverter that is dc to dc converter ,,but there is aproblem ,my output goes upto 560volts when there is no load,,if i apply a 100w bulb on this dc bus,my output goes to 330volts ,,and if i apply 200watt bulb my output goes to 270 volts...so where is the problem? i am using sg3525 for driving the mosfet,,with 50khz frequency.also tell me how feedback works in sg3525???..i want to implement feedback ..if i reduce the no. of output turns on secondary side of the transformer ,output goes to 315 volt but when i put a 100watt bulb ,,output decreases drastically...so please help me ..
2) how can i calculate the value of resistance that will be used between gate to source teminal ,,,,and also the value of resistance that will be used between sg3525 pin 14 to gate of the mosfet ?
3) how can i protect the mosfet from burning? my mosfet hve burnt 3 times ,, i hve used 3 + 3 mosfet bank...

please help me tpetar....

Re: How inverters working when the supply voltage is reduced.

Make new thread with all details and circuit, describe problems, and what happen.

Re: How inverters working when the supply voltage is reduced.

mr bawa you have post this issue several times, you transformer is ok and the voltage is also ok what you need is your feed back loop to your SG3525 or 3524 ,we use pin 1 of the oscilator with pot to vary the potential at the pin depending on your desinge.

Re: How inverters working when the supply voltage is reduced.

--BawA-- said:

hi tpetar
i hve few questions to ask from u
1) i have completed my first stage of inverter that is dc to dc converter ,,but there is aproblem ,my output goes upto 560volts when there is no load,,if i apply a 100w bulb on this dc bus,my output goes to 330volts ,,and if i apply 200watt bulb my output goes to 270 volts...so where is the problem? i am using sg3525 for driving the mosfet,,with 50khz frequency.also tell me how feedback works in sg3525???..i want to implement feedback ..if i reduce the no. of output turns on secondary side of the transformer ,output goes to 315 volt but when i put a 100watt bulb ,,output decreases drastically...so please help me ..
2) how can i calculate the value of resistance that will be used between gate to source teminal ,,,,and also the value of resistance that will be used between sg3525 pin 14 to gate of the mosfet ?
3) how can i protect the mosfet from burning? my mosfet hve burnt 3 times ,, i hve used 3 + 3 mosfet bank...

Click to expand...

1) You face this problem due to lack of feedback and "minimum load". Your output is rectified with a bridge rectifier configuration and filtered with an LC filter that requires a minimum load to provide clean correct output DC. Without lack of minimum load, output may go too high and the feedback loop may go unstable. Increased inductance in the LC filter requires lower minimum load and vice versa.

2) Typically, you should use a resistance between 1k and 4.7k between the gate and source of the combined parallel MOSFET configuration. At 50kHz, the series gate resistor could be between 4.7R and 10R.

3) This depends on the circuit. The MOSFET could burn for a lot of reasons. Ensure you have snubbers/clamps in place. Ensure gate drive is proper.

Hope this helps.
Tahmid.

Re: How inverters working when the supply voltage is reduced.

thanx tahmid
plzz tell me what value ol L and C should i use?
which and how much minimum load should i connect to the dc bus?

Re: How inverters working when the supply voltage is reduced.

What is the minimum load you plan to use? The larger the minimum load, the smaller the required inductance and vice versa. I would say that you can start with something between 3mH and 5mH.

Re: How inverters working when the supply voltage is reduced.

Tahmid .I dont hve any idea of minimum load that should be connected permanently...should I connect a led as a load or somethig else ? Please specify...

Re: How inverters working when the supply voltage is reduced.

You can have a small resistor that would draw some power. However, this is an inefficient method.

You can select a large inductance so that minimum load is low and then use a power resistor to provide the minimum load. If you want you can use a relay or other switching device to disconnect the resistance once a larger load is connected.

Hope this helps.
Tahmid.

Re: How inverters working when the supply voltage is reduced.

tpetar said:

You can see commercial UPS devices, and their transformers. Thise transformers have many taps to adjust correct voltage, electronics is charge for correct tap switching...

Click to expand...

Another option is to work on inverter PWM duty-cicle based on lookup table, instead to perform a post regulation.
It is commonly used to compensate decayment of battery charge.

+++

Re: How inverters working when the supply voltage is reduced.

When you go to low input voltage, to obtain the output
voltage requires more duty cycle (perhaps, more than
designed or capable) and to obtain more current will
run more primary current. Both of these can threaten
the electronics. Going lower you may not have the gate
voltage drive needed to keep the power switch FETs from
becoming lossy and cook themselves. So you will see
undervoltage lockout as a design feature (just like, and
same reason, as you see it on the old Unitrode PWMs).

Now I have had an automotive inverter "flicker" in and
out of UVL when I used it on a truck with a weak charging
system, and the output bounced so badly that it fried my
serial port (ground loop). This feature is meant to stop
things from working, not meant to motorboat indefinitely.

So my advice to you is, don't use an inverter in a marginal
power quality environment, because its output behavior
is somewhat unknown but likely unhelpful.