IR2153

That's an interesting idea, adding a pair of opamp input resistors. They shouldn't be necessary in a simple circuit, but we don't know how eehinjor changes his resistor values, what kind of wiring and panel switch, etc., he's using. These need to be isolated from the opamp's high-frequency feedback capabilities to prevent oscillation.

It works fine at 1Hz, or at DC, if a separate floating power supply is created for the HI-side driver section. A few years ago I posted the schematic of a high-speed 250V 50-ohm pulse generator using this approach with an IR2113, a chip with similar MOSFET drivers.

According to the diagrams on page 6, both outputs are source/sink. Something else is wrong.

Good luck.

John

Kell, A couple of things to check: ~ Are you matching the recommended values of Rt, Ct to your switching frequency? Also the Voltage on the cap for Vb must be higher than Vs, by an amount equal to or greater than gate threshold voltage of the FET you are using. Regards, Jon

Thanks for the replies. I'll go into a little more detail. At first I tried to run it with a .01 uF cap and a 10 k resistor for the timing, to drive a ferrite transformer, using rectified mains for power. I used a MUR160 for the bootstrap diode. I had a 10 uF cap for the Vb/Vs cap. I used IRF740 mosfets. I don't have a scope so when the circuit wouldn't work I decided to check it out by bringing the timing down to something I could check by eye with test lamps. I disconnected from the high voltage and used a 12 volt battery for the test. I connected little lamps from pin 6 to Vcc and ground and set the timing to about 1 hz with an electrolytic cap and a 10 k resistor. The lower mosfet worked, but not the upper. So I tested pin 7, which drives the upper mosfet. It was sinking but not sourcing. It is my understanding that the chip is supposed to provide its own power for the high side because the Vb cap charges up every time the lower mosfet turns on, and this provides power for the upper driver. My test illustrated that this was happening. During the test I used a

IR have an app note about providing a seperate hi side supply specifically for low frequency operation. Uses a 555 IIRC.

Graham

This note from IR addresses the subject. I still haven't figured out the problem.

Oops, forgot to post the link