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Topic Title: Circuit Help
Topic Summary: help with a circuit i am working on
Created On: 23 February 2010 10:59 PM
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 23 February 2010 10:59 PM
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zhassan11

Posts: 10
Joined: 23 February 2010

Hi everyone

I am an electronic/electrical engineer and i am currently working on a circuit and i am facing some difficulties with it.

If someone can help me i can email them the circuit design and an explanation of the problem in more detail.

the circuit is basically a 12V DC to 700VAC convertor, the problem i have with the H bridge is the two top IGBTs keep blowing up, i control the H bridge by using a 555 timer and NOT gates.

Three prototype's seem to work ok, However i could not get the fourth or the fifth prototype to work.

The problem seem to be two top IGBT gate signal's appear to be a saw tooth instead of HV square wave signal , i replaced the IGBT's with NPN's and the H bride start to work and generate 700 square wave signal but the two top NPN's start to get hot.

i need to understand am i deriving the H bride in a wrong way ? why the only first few prototypes work ok?

I would appreciate any help thanks,

if anyone can help can you please get back to me on this email: zhassan11@hotmail.com or you can PM me.

Thanks
Zahra

Edited: 24 February 2010 at 12:05 AM by zhassan11
 24 February 2010 11:03 AM
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amillar

Posts: 1918
Joined: 28 May 2002

How are you making sure that the top and bottom (high-side and low-side) transistors are not turning on at the same time? Because of the internal capacitances in the devices they will not turn off immdiately, so unless you put in a delay between turning one off and the next on there will be some overlap.

The other big problem can be getting enough drive current into the gates/bases to ensure that they switch cleanly.

Usually the best solution I know of is to use a high-side/low-side driver driver IC that is designed for the type of devices you are using. Many of these will allow you to set the 'dead time', the delay betwwen switching the devices, typically by changing a resistor value. Have a look at Farnell or RS for the range of devices they offer.

Good luck! And stand a long way away from that 700V!

-------------------------
Andy Millar CEng MIET CMgr MCMI

http://www.linkedin.com/in/millarandy

"The aim of argument, or of discussion, should not be victory, but progress." Joseph Joubert
 24 February 2010 08:01 PM
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westonpa

Posts: 1771
Joined: 10 October 2007

Have you tried simulating the circuit with OrCAD or similar software?

Regards.
 25 February 2010 10:07 AM
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ANFierman

Posts: 138
Joined: 25 July 2008

I can recommend LTspice: a far better spice simulator then most of the paid for offerings.

Free and unlimited, if a little clunky.

Runs on Windows and Linux (through WINE).

http://www.linear.com/designtools/software/ltspice.jsp

IGBT Spice models from various vendors such as ST and Fairchild.

-------------------------
Andy Fierman

---------------------------
http://signality.co.uk
---------------------------
 25 February 2010 11:25 PM
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zhassan11

Posts: 10
Joined: 23 February 2010

Just wanted to thank everyone for their response

I will do some experiments and see how it works out.

Thanks again.
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