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Help with using UC3823n for a boost converter

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darkfeffy

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Hi all,

Please I need some help using the UC3823N PWM controller for a boost converter (with synchronous rectifier). I want to obtain 189Vdc at the output of the converter from a 48Vdc battery (for now used a dc supply). I have attached a schematic of the controller set-up and another for the boost.
My problem is the boost output voltage is lower than expected (55Vdc). The controller output duty is locked at 20% in this condition instead of approx 75%. I am using (Rt = 3.6kOhm and Ct = 4.7nF) and a +15Vdc supply. These give a 94kHz approx. frequency (as expected). I am using voltage mode control. The feedback (PIN 1) is a simple voltage divider that should convert 189Vdc to 2.55Vdc (to compare with the 2.55V reference). I have not used the E/A out pin (PIN 3) nor the CLK/LEB Pin (Pin 4). These pins are left floating. Vc (pin 13) is +15Vdc (I had tried connecting the Vc pin to a +5V supply but the controller wouldn't work, so I connected it to Vcc (pin 15) then it worked - I don't understand why). I inverted the output through a NAND before sending it to an IR21844 gate driver (which re-inverts the signals). Pin 1 reads 1.2V (this is less than the 2.55V reference). When the boost is NOT supplied, the controller gives an 85% duty (max) as expected. I used by-pass and filter capacitors. Any one help??

P.S: I wish to add that I used this very controller and this very set-up (but without the NAND inverter and with Vc = 5V) for a buck converter (96V to 52V) and it worked alright.

Thanks. Edwin
 

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Suspected I might be having an issue with E/A pin unconnected, but I don't have much idea about doing compensation, control, etc...
Anyone help?
 
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