I have designed a flyback regulator circuit and am having troubles regulating the output voltage. I have used TI's flyback IC UCC28704 for the design.
My input voltage is 225VDC.
Oscilloscope image:
CH1 (yellow): Voltage at CS pin (ref to GND)
CH4 (green): Voltage at MOSFET drain (ref to GND)
CH7 (red): Snubber diode voltage (ref to GND)
CH8 (orange): Secondary voltage (ref to GND)
There is a significant ringing at the CS input during the turn-on of the switch. I have not found any documents that address the cause or how to fix this ringing. Otherwise, the ringing during turn-off of the switch and turn-off of the diode seems to okay.
The VS pin input is a fraction of the secondary voltage. It is used both to regulate the output voltage and ensure the switching is performed during the low points of the diode ringing waveforms to reduce power loss. As shown on the oscilloscope image, the VS pin is switching at the correct times. However, the output voltage is not regulated correctly.
I have a load attached to the output of the circuit. If the load is 100 ohm I get an output voltage of 16V. If the load is 70 ohm I get an output voltage of 9.5V. The voltage should always be 12V. The output isn't regulating in either direction. Between the two cases, the switching frequency changes but the ON time of the switch does not. The IC is therefore operating in the frequency modulation region of the control law (region 4 in the datasheet).
Any ideas on why the IC is unable to control the output voltage?? I have switched out all of the components to see if any are malfunctioning except for the flyback itself, which I don't want to re-wind.
Things that have been changed from the schematic to the actual IC during debugging:
C9 - removed - this does not have much effect on the CS input.
R4 = 53600
R6 = 26100
R5 = 3300
R7 = 7.2
C8 = 6.6u
R2 = 68000
C1 = 1.5n
My input voltage is 225VDC.
Oscilloscope image:
CH1 (yellow): Voltage at CS pin (ref to GND)
CH4 (green): Voltage at MOSFET drain (ref to GND)
CH7 (red): Snubber diode voltage (ref to GND)
CH8 (orange): Secondary voltage (ref to GND)
There is a significant ringing at the CS input during the turn-on of the switch. I have not found any documents that address the cause or how to fix this ringing. Otherwise, the ringing during turn-off of the switch and turn-off of the diode seems to okay.
The VS pin input is a fraction of the secondary voltage. It is used both to regulate the output voltage and ensure the switching is performed during the low points of the diode ringing waveforms to reduce power loss. As shown on the oscilloscope image, the VS pin is switching at the correct times. However, the output voltage is not regulated correctly.
I have a load attached to the output of the circuit. If the load is 100 ohm I get an output voltage of 16V. If the load is 70 ohm I get an output voltage of 9.5V. The voltage should always be 12V. The output isn't regulating in either direction. Between the two cases, the switching frequency changes but the ON time of the switch does not. The IC is therefore operating in the frequency modulation region of the control law (region 4 in the datasheet).
Any ideas on why the IC is unable to control the output voltage?? I have switched out all of the components to see if any are malfunctioning except for the flyback itself, which I don't want to re-wind.
Things that have been changed from the schematic to the actual IC during debugging:
C9 - removed - this does not have much effect on the CS input.
R4 = 53600
R6 = 26100
R5 = 3300
R7 = 7.2
C8 = 6.6u
R2 = 68000
C1 = 1.5n
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