Sine wave power inverter

[sougata]

Hi,

That was a great explanation walkura and thanks for the "human" explanation. Now I would try to dig into the software aspects a little. I am a complete self taught and know my lackings, so please correct me if I am conceptually wrong.

• While changing cycles within the software the loop latency may be variable if you are not using your own asm interrupts to deal with that. Since this is all about PBP my recommendations are for time critical routines use asm. As already proved asm+PBP=muscle. Considering a round-robin multitasking with no blocking codes would ensure specific time slots alloted to different routine on priority basis.
• Your load is imbalanced. I have seen a few cheap soldering irons which has a single diode inside them. Causing to draw current in only cylce.
• For ease of manufacturing I have seen MOSFETs on a common heatsink. This reduces a per channel MOSFET Rds-On difference and thus should be avoided for a stable design and utilize the MOSFETs property of negative temperature co-efficient for balancing.
• Per cycle flux reset may be applied during the dead-time with its own pros-cons (may not be applicable for ZVT implementation)
• Since the current drawn per cycle is also a function of the impedance it may be accomodated in software as well as almost all hardware designs support current sensing.
• Sometimes ADC based current sensing can take longer to come into action and kill your MOSFETs. So my advice is use a comparator based sensing to quickly turn-off your MOSFETs under fault conditions and use the current sense for your other tasks or PID.
• If you are into serious intelligent smps design then consider the dsPIC (No PBP support however). Your tasks become real easy for only smps implementation.

[mincing]

Hi all,
NOw I have completed quite a bit.yet there are two issues that are still dogging me.
1.Output stabitity,
2.Heat at no load( the power dissipation)

The output sine wave is quite stable as long as the system does not enter the closed loop.
Once it enters there is a continuous up/down swing of out put voltage and sine wave.

We could control it to quite a bit by putting a PI controller before the feed back.
still there is a fluctuation of about5 to 6 volts.


2.heat is toomuch.the no load currnent of the unit is 2A at 24V and when added with more during loading,i am afraid I can run unit like this.
any suggestions are welcome!

[sougata]

Hi Mincing,

Sorry for the late reply. I really do not get time (or get into weired problems) to log into this forum. Without specific details about your circuit like schematic, code it is impossible to suggest anything.

[mincing]

I had posted shchematics in the past and we had even discussed it in the forum.

[sougata]

Dear Mincing,

You reported heating. Without actual values of components and detailed circuit diagram it is very difficult to guess whats going on inside. Also if you are using de-saturation protection schemes as used in IGBTs your MOSFETs may heat up if not designed carefully. And without code it would be not possible to find out if it has got something to do with. Anyway I would be out of station for somedays and would check back later.

[mincing]

can you help me how to calculate snubbers for H-bridge inverter mosfet switches.

suggestions from any are welcome!

[walkura]

Good evening Mincing .

I seen your question about snubbers and can copy-paste you a relative simple answer .
But i have to add that this is a dissapative snubber (i'm not always happy with that ,but it solves the ringing)
The information that follows is from the website of ridley engineering (they give a clear calculation method)

If you have any ringing waveforms in your power circuit, these waveforms must be damped or they can lead to device failure, excessive EMI, or instability. In many cases, you can damp a ringing waveform with a series RC network across the offending device.

Note: if the ringing frequency is not almost two orders of magnitude higher than the switching frequency, you may be in trouble. It will be hard to damp the ringing without excessive dissipation, and alternate circuit solutions must be found.

First, you need to measure the natural frequency of the ringing waveform.
To design the snubber for the power FET of a flyback circuit, for example, first run the circuit at low power with an oscilloscope probe on the drain waveform with no snubber.

Make sure you are using a low capacitance scope probe, otherwise the waveforms will be modified by the connection of the probe. If you suspect that the probe capacitance is too high, just set the scope to a higher sensitivity, and without making electrical contact to the FET drain, just bring probe close to the device. You will see the high-frequency ringing waveform due to the radiated noise.

Observe the ringing waveform at turn-off on the drain. Use a reasonably high input voltage (without destroying the FET, or course) since the resonant frequency of the ringing will be voltage dependent. Record the resonant frequency.

The ringing is caused by an equivalent RLC network. For a low-loss circuit, it will be quite undamped, and the oscillations will continue for many cycles. Step 1 is to add a damping R across the device. First , you must know one of the resonant elements, L or C. On the primary switch, the leakage inductance is the dominant L, and should be well known. For a secondary ringing, the diode capacitance will be a known quantity.

Calculate the characteristic impedance, of the resonant circuit.

If you know L, Z = 2 x 3.14 x f x L

If you know C, Z = 1/(2 x 3.14 x f x C)

Try an initial value of snubber resistor of R = Z. This usually suffices to control the ringing.

Using just a resistor across the power device will control the ringing, but the dissipation would be very large. A series capacitor is used to reduce the power dissipation in the damping resistor.

Calculate the C needed in series with R according to: C = 1/(3.14 x f x R)

Increasing C beyond this value will increase dissipation, but will not improve the damping. In some cases you will be able to decrease it by 30% or so, but any less than this and the snubber will be less effective.

Size the resistor according to the dissipation it will see: P = C(VxV)Fs

where V is the voltage on the device when it is off. Depending on the circuit operation, the actual dissipation may be closer to half this value in some cases, and the design will be conservative. Use thermal data from your circuit to determine if the resistor size can be reduced.

Build the snubber (keep leads short) and test the circuit. You should be close to the final solution on this first attempt.

If the problem remains ,i would advice you the book switchmode powersupply's by Pressman .
They have a chapter on all sort of snubbers (active passive etc. etc.)

Good luck Walkura