M
Mook Johnson
- Jan 1, 1970
- 0
I have a UCC1806 that skips pulses at less than 20% of max load. Typically
they go down to 10% of max load. This one has an unusually large output
inductor which should extend the min current lower. When it skips a pulse
pair the output inductor goes discontinuous and causes spikes and high
ripple on the 5 V output.
Here are the parameters of the suppply.
1) push pull topology typical hookup with no current slope compensation.
2) 44(ct):24(ct) transformer
3) Input 30V : output 5V @ 4A max
4) Output inductor: 100uH, low esr electrolytics at 400uF on the output
5) current sense transformer 1:75 with 37 ohm burden resistor and a 499 ohm
with 100pF spike filter before the isense pin.
6) Ilimit pin is set to 5.6V though some 200K range voltage divider from a
10V reference (forget the exact values) and is decoupled with 2uF.
7) SD pin is tied low.
8) Sync and from a crystal osciliator and the Rt is tied to vref and Ct is
ground (external clock operation).
9) switching Freq is 100KHz
Is there any indication from the above that could cause a problem. @800mA
output current the current ramp is about .25V and clean. Then all of a
suddend evey other pulse pair stops occurng reliably. raise the output
curent to 1 amp and above and the supply acts normally.
any ideas on where to look?.
they go down to 10% of max load. This one has an unusually large output
inductor which should extend the min current lower. When it skips a pulse
pair the output inductor goes discontinuous and causes spikes and high
ripple on the 5 V output.
Here are the parameters of the suppply.
1) push pull topology typical hookup with no current slope compensation.
2) 44(ct):24(ct) transformer
3) Input 30V : output 5V @ 4A max
4) Output inductor: 100uH, low esr electrolytics at 400uF on the output
5) current sense transformer 1:75 with 37 ohm burden resistor and a 499 ohm
with 100pF spike filter before the isense pin.
6) Ilimit pin is set to 5.6V though some 200K range voltage divider from a
10V reference (forget the exact values) and is decoupled with 2uF.
7) SD pin is tied low.
8) Sync and from a crystal osciliator and the Rt is tied to vref and Ct is
ground (external clock operation).
9) switching Freq is 100KHz
Is there any indication from the above that could cause a problem. @800mA
output current the current ramp is about .25V and clean. Then all of a
suddend evey other pulse pair stops occurng reliably. raise the output
curent to 1 amp and above and the supply acts normally.
any ideas on where to look?.