AVR-Chat

At 08:11 PM 12/20/04 -0800, you wrote:
>i build a full hbridge cct with 4 mosfet and 2 IR2110 mosfet driver and 
>test it yesteday.
>i supply 5V to Vcc, 12V to Vcc and give signal with 5V to Hin or Lin 
>continuously to switching the 4 mosfet continuously at 40Hz like usual.

5V to Vdd I assume?


>i connect Vb to Vs with cap 1uF, a diode FR103 from Vcc to Vb, SD and Vss 
>to GND, cap 22uF from Vdd to Vss,cap 22uF from Vcc to com. HO to gate of 
>high side mosfet through 22ohm and its Source to Vs. Lo to gate of mosfet 
>low side through 22ohm and its source to com.
>
>after that i supply 5V to Vdd and 12 V to Vcc and 0V to my load. i found 
>that Vbs=11.0V, and Vgate of high and low mosfet or Vho and Vlo are around 
>4.0V.

11V on Vbs is about right.  4V on VHO is way too low.  You should get 
pretty close to the Vbs voltage.  How are you measuring these voltages?

>then i increase the load voltage (middle of hbridge like usual) 
>slowly.everthing looks find and the output is ok. but when i keep increase 
>the voltage, i found that the Vho or gate voltage decrease and finally 
>both high side mosfet turn off when the load voltage exceed 3.0V but both 
>low side mosfet still on.

First things first.  You need to figure out why your gate voltages are so 
low.  What do the gate drive signals look like on the oscilloscope?  Also 
how do you know the hi sides turn off?

40Hz is awfully slow to be running this thing at.

>
>then i increase the Vcc to 15V, the load output voltage can reach only 
>6.0V. after that both the high side mosfet off again. i check Vbs, it show 
>more than 10V so i think it is ok.
>
>(1) so what problem??    why Vgate of high side mosfet or Vho keep 
>decrease when i increase the load voltage? i need to apply high voltage 
>later, may be up to 200V and 10A to my motor(load) so what can i do to my cct.

More details please.  Specifically what does the gate drive waveform look 
like.  If the gate is really at 4V then it should be draining Vbs pretty 
quickly.  And on the lo side I would expect it to pull down your 12V power 
supply so I suspect that what you are reporting is glossing over what is 
actually happening.

Robert

" 'Freedom' has no meaning of itself.  There are always restrictions,
be they legal, genetic, or physical.  If you don't believe me, try to
chew a radio signal. "

                         Kelvin Throop, III