Re: UCC1806 skips pulses at light load
- From: Terry Given <my_name@xxxxxxxx>
- Date: Thu, 11 Jan 2007 12:51:04 +1300
Mook Johnson wrote:
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?.
check out the current sense pin. At low load, the threshold of the current sense comparator is low (by definition) so a spike on the leading edge can (and often does) trip the comparator, terminating the pulse as soon as it begins. Some controllers include 100 - 150ns of Leading Edge Blanking to avoid this issue.
Its also why there is usually an RC filter. you have 1/2k and 100pF, so about 50ns or so, which is probably too low. you will need to probe this node carefully (coax probing technique - see Linear Technology AN47). As FAJ mentioned, layout is pretty important. The cap needs to be right at the controller. ditto for timing cap.
Also check the oscillator cap waveform, a spike can get in here and seriously annoy the x806. You ought to have a nice solid 0V plane under the x806 too.
BTW the current sense spike mostly comes from the gatedrive (some too from the xfmr primary & reflected secondary capacitance). When using a current sense resistor, you can eliminate it with an npn/pnp complementary emitter follower; tie the pnp collecter to to the switch return (Source), feed the npn collector from the smps IC supply via a small resistor (eg 10R), and place a 100nF cap between the npn and pnp collectors. draw the circuit, and trace the current paths - you will see that the spike now wont flow thru the current sense resistor (but the 100nF cap current does).
As you have a CT, this is even simpler (assuming the CT is tied to the Sources of both primary switches) - connect the x806 0V to the switch side of the CT rather than the 0V side; now, as above, the gate drive current wont flow thru the CT.
HTH
Cheers
Terry
.
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