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EMI issue in high power DC/AC inverter FCC PART 15B 3M

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adnan012

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Hi,

I am evaluating the performance of a DC/AC inverter. The inverter specs are as follows

Input:
40-48VDC Lithium ion cell power bank.

Output:
110VAC 60Hz @ 1600 Watts

Invert topology:

Push Pull DC/DC stage. There are two MOSFETS at each leg on primary side. The voltage is boosted to 230VDC. After Battery +iver terminal there is an electrolytic capacitor. After this there is large inductor (on a EE ferrite core ). After the inductor there are 4 ceramic type capacitors with a total capacitance of 20uF and this point is connected to power push-pull transformer. Push pull transformer is designed by using PQ40/40 type core.

Full Wave rectifier at output stage.
H-Bridge for converting DC voltage to pure sine wave.
There is a fly-back power supply which provide 3 isolated power sources.
The primary and secondary side sides re isolated.
The PWM controller is TL494.
Switching frequency is 50KHz.
There is no voltage or current feed-back to the TL494. The duty cycle is fixed and is about 40% -45% for each primary side MOSFETS.

On secondary side, MCU sense the output current via shunt resistor and it performs over load protection. Sine wave sampling frequency is 20 KHz.

There are 3 separate heat sinks. One for primary side MOSFETS, one is for full-wave diode rectifier and the 3rd one is for h-bridge.

I have tested the inverter in a proper EMC lab. There are frequency peaks from 30 to 70 MHz range. The standard is FCC PART 15B 3M. The peaks have more than 6 to 10dBuV/m above the limit.

I have to fix the emi issues. I want to know what type of equipment is required like spectrum analyzer and some special probes for debugging and fixing the issues.

There was a 7MHz ringing at the drain of MOSFETS during turn-off period. Its peak was almost 30Vpp. I have fixed it. There is no other issue regarding stability of the inverter.

I need help to solve this issue.

Regards
 

the rectifying diodes and the diodes in the inverter H bridge will be the next largest source of VHF RFI ...
 
Thanks for reply.

So i need to add snubbers across rectifier and bridge inverter diodes. Can heat sinks cause emi?

Regards
 

Yes, if a semi is heatsunk, and the tab has high dv/dt, a thin insulator will couple RFI to the heatsink - which can then travel every where ....
 
Hi,

Can heat sinks cause emi?
No, they can't cause EMI.
But they can act as (big) antennas spreading EMI. Thus in a way they can "amplify" EMI...not like an active amplifer, but they may "help" to spread the HF into the environment ... the EMI then is higger than without the heatsink.

Klaus
 
Hi,

I am evaluating the performance of a DC/AC inverter. The inverter specs are as follows

Input:
40-48VDC Lithium ion cell power bank.

Output:
110VAC 60Hz @ 1600 Watts

Invert topology:

Push Pull DC/DC stage. There are two MOSFETS at each leg on primary side. The voltage is boosted to 230VDC. After Battery +iver terminal there is an electrolytic capacitor. After this there is large inductor (on a EE ferrite core ). After the inductor there are 4 ceramic type capacitors with a total capacitance of 20uF and this point is connected to power push-pull transformer. Push pull transformer is designed by using PQ40/40 type core.

Full Wave rectifier at output stage.
H-Bridge for converting DC voltage to pure sine wave.
There is a fly-back power supply which provide 3 isolated power sources.
The primary and secondary side sides re isolated.
The PWM controller is TL494.
Switching frequency is 50KHz.
There is no voltage or current feed-back to the TL494. The duty cycle is fixed and is about 40% -45% for each primary side MOSFETS.

On secondary side, MCU sense the output current via shunt resistor and it performs over load protection. Sine wave sampling frequency is 20 KHz.

There are 3 separate heat sinks. One for primary side MOSFETS, one is for full-wave diode rectifier and the 3rd one is for h-bridge.

I have tested the inverter in a proper EMC lab. There are frequency peaks from 30 to 70 MHz range. The standard is FCC PART 15B 3M. The peaks have more than 6 to 10dBuV/m above the limit.

I have to fix the emi issues. I want to know what type of equipment is required like spectrum analyzer and some special probes for debugging and fixing the issues.

There was a 7MHz ringing at the drain of MOSFETS during turn-off period. Its peak was almost 30Vpp. I have fixed it. There is no other issue regarding stability of the inverter.

I need help to solve this issue.

Regards

You can check the ringing frequency across the diode & Mosfet. BTW what kind of diode,you have used???
Try using a snubber across diode..
 
Thanks for reply.

I have attached the datasheet of the diode.

What is the proper way of measuring ringing across the diode. If i place a wire loop (a short wire connected between Gnd and signal terminal of probe) near the inverter pcb, i can see many frequencies on scope in fft mode. I get bell shaped peaks at 40MHz and at 68MHz. And these are the frequencies which are present in EMI test results. By placing prob at the diodes i can see some ringing of few hundreds mV but these are difficult to observe by using scope.

I am using simple 200MHZ/1GS/s scope.

Regards
 

Attachments

  • SSP20300CG.pdf
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1)You can check with an oscilloscope across diode

2)What is the switching frequency of DC-DC stage...50khZ??

3)I guess you have followed the all the good layout practices in the Power PCB.
 

a small 5 turn coil connected to the scope tip ( no probe ) on x1 is a good sniffer probe - make sure it is well insulated ....
 

Thanks for reply.

During the testing I observed some high frequency at the cathodes of the high side diodes ( the diodes connected to the +I've of output DC capacitor ) with respect to the secondary dc ground. When I placed a capacitor between ground and cathode of the high side diode , I observed frequency shifting ( towards low frequency ) and a decrease in amplitude ( exactly at the diode terminal). When I did same to the other high side diode the amplitude of the previously shifted frequency decreased.

If I place capacitor at the out dc capacitor this frequency shifting and attenuation is not detectable.



Regards
 

A schematic would help a lot to follow your description... also a photo of the PCB/heatsink assembly.
 

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