Current sense signal lags current on PWM controller
Current sense signal lags current on PWM controller
(OP)
Hi,
I am using a UC2842 PWM chip to control a boost converter, boosting to 120Vdc with a 70Vdc input. The issue is that the Isense input to the PWM chip (no slope compensation fitted) seems to lag the current through the Inductor and sense resistor (0.01R fitted in -ve line)- the two opamp inputs are connected to either side of this resistor.
Could anyone give me any pointers on how to align the Isense signal with the current signal?
I have attached a scope trace, colours are: Blue=UC2842 oscillator (52kHz), yellow = gate drive output, green = inductor current, purple=Isense signal into PWM.
Regards
Dave
I am using a UC2842 PWM chip to control a boost converter, boosting to 120Vdc with a 70Vdc input. The issue is that the Isense input to the PWM chip (no slope compensation fitted) seems to lag the current through the Inductor and sense resistor (0.01R fitted in -ve line)- the two opamp inputs are connected to either side of this resistor.
Could anyone give me any pointers on how to align the Isense signal with the current signal?
I have attached a scope trace, colours are: Blue=UC2842 oscillator (52kHz), yellow = gate drive output, green = inductor current, purple=Isense signal into PWM.
Regards
Dave





RE: Current sense signal lags current on PWM controller
Regards
Dave
RE: Current sense signal lags current on PWM controller
What else is connected to U1-3? You didn't show the entire net. Do you have the rest of the filter circuit (resistor to ground) shown on p. 5 of the UC2842 data sheet?
A 0.1uF or 0.01uF cap across C1 might help too; C1 has series inductance that can make it work poorly at higher frequencies. Make sure the new cap is mounted as close to U2-8 and U2-4 as possible, with as short as leads as possible.
You might also try reconfiguring the RJ/CB filter for less capacitance. The LM2904 data sheet states it can drive 50 pF; you might be getting close to that with the unspecified load of the UC2842D and other parasitic capacitance on the board.
Back to your original lag question, have you calculated the lag through your filter circuit?
Let us know how this all works out.
John D
RE: Current sense signal lags current on PWM controller
--dave
RE: Current sense signal lags current on PWM controller
John D