ON Semiconductor AND8084D Technical data

AND8084/D

NCP1650 Benchtop
Assistance

Prepared by: Alan Ball

ON Semiconductor
Applications Engineer

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APPLICATION NOTE

The NCP1650 is a high–performance, Power Factor
Correction IC. It is capable of producing a high power factor
input current waveform under continuous and discontinuous
modes of operation. It is also a highly integrated device,
and as such, requires fine tuning for optimum performance.
The purpose of this application note is to assist in
troubleshooting and fine tuning this circuit.

Troubleshooting

When troubleshooting this circuit, always use an
oscilloscope. DVM readings will not show oscillations,
spikes or other waveforms that may be helpful in
determining the cause of the problem.

Be aware that this is a non–isolated power converter that
is connected to a high–voltage, AC line. The ground of this
circuit will be at an AC potential and could pose a shock
hazard. Use an approved isolation transformer before
connecting oscilloscopes or other test equipment to this
circuit.

Output Does Not Regulate

1. High Output Voltage If the output voltage is greater
than 8% of the level of the designed output voltage,
check the voltage divider from the output to pin 6.
Make sure that the resistor values are correct, and
that the resistors are connected properly.

2. High Output Voltage
approximately 8% above the designed output level,
the overvoltage comparator is controlling the loop.
The switching will be erratic as the overvoltage
comparator inhibits the operation of the loop.

The input to the error amplifier (pin 6) should be 4.3 volts
under this condition. T he o utput o f t he e rror a mplifier ( pin 7)
should be high (approximately 6.0 volts). If it is not high,
check connections to this node.

The voltage/power OR’ing network inverts this signal,
which should cause the output of the reference multiplier
(pin 4) to be approximately zero volts.

The averaged current signal on pin 10 of the current sense
amplifier should be less than the output of the reference
multiplier on pin 4.

If the output voltage is

3. Low Output Voltage
than the designed output level, check the values in
the output voltage divider that connects to pin 6 of
the IC.

If the voltage divider values are correct, check the output
of the power error amplifier at pin 8. If this voltage level is
higher than the output of the voltage error amplifier (pin 7),
the power circuit is limiting the output. Check to make sure
that the load is within the rated range, and that the values of
R10, R9, and the current shunt are correct.

Unit Does Not Start

1. Typically, the inability of this unit to start–up is due
to inadequate Vcc. The NCP1650 requires a
minimum of 10.5 volts to turn on, and 9.5 to maintain
operation. If the Vcc voltage drops below 9.5 volts,
the chip will shut down.

When the chip begins operation, the bias current will
increase from a level of 0.5 mA to about 5.0 mA. Depending
on the start–up circuit used, there may not be enough energy
available to get the unit started before the Vcc drops below

9.5 volts.

In this case, a higher value Vcc cap may solve the
problem, and/or a higher current start–up circuit.

If the start–up circuit is operating properly, check the
voltage on pin 6. This pin has a shutdown feature that
requires a voltage of greater than 0.75 volts for the chip to
come out of its shutdown mode and commence operation.

Failure of Power Switch or High Voltage Diode

Overheating is the main cause of failures of these devices.
The rectifier diode will experience significant heating due to
the reverse recovery spike (unless a special circuit is used to
reduce this effect). Measure the temperature of the package
of both of these devices with a thermocouple and assure that
they do not exceed the manufacturers ratings. Additional
heatsinking and/or alternative parts may be required to keep
the temperature in a safe range.

The power switch has several protection circuits within
the NCP1650 controller. The main one being the
instantaneous current limit. If peak current is a concern,
check the values per the Excel spreadsheet or review the
design equations in the data sheet.

If the output voltage is less

 Semiconductor Components Industries, LLC, 2002

May, 2002 – Rev. 0

1 Publication Order Number:

AND8084/D

AND8084/D

The voltage on the power switch will exceed the output
voltage by a diode drop plus any spikes that may occur. A
good layout will keep these spikes to a minimum. Observe
the drain pin of the power switch with a wide bandwidth
oscilloscope to look for spikes. Spikes can be reduced by
adding snubbers or modifying the layout to reduce path
lengths between the inductor, drain and rectifier anode.

Noise Problems

Noise issues can be identified by abrupt changes in the
current waveform. Instabilities will cause smooth
oscillations, but noise will cause sharp edges as the current
steps from one level to another.

Figure 1. Example of Input Current Waveform

Distortion Due to Noise Issues

Possible causes are:

1. Poor grounding. In general, one of two grounding
schemes should be used.

• Single Point Ground – This is sometimes referred

to as a “star ground”. All major power traces should
be routed as closes as possible to a single point, and
routed directly to that point. This includes the shunt
resistor, FET source, output capacitor, input bypass
capacitor, and one trace going to all signal circuitry.
The chip ground should be as close as possible to the
ground side of the shunt resistor.

• Ground Plane – One layer of the printed circuit

board is left as a solid copper plane and all grounds
are connected to this plane. Even with a ground
plane, it is recommended to keep the high power
grounds (as described in the above paragraph) close
to each other, as well as keeping the chip ground
close to the current shunt resistor ground.

2. Reduce rise and fall times of the power device.
Increasing the resistance in the gate lead of the power
FET will reduce the speed of its transitions. This will
result in increased switching losses in the power
switch. Snubber circuits can be added across the FET
and/or diode to reduce noise levels. There are several
types of snubbers including RC and RCD
configurations.

3. Noise can also be radiated from various sources. The
node of the FET drain, output rectifier, and boost
inductor is a very noisy source, with both high
voltages and high dv/dt’s. Sensitive components,
which include most bias components of the
NCP1650, should be kept away from this node.
Traces between these components should be kept as
short as possible to reduce these emissions.

Performance

Figure 2. AC Ref with Phase Delay Figure 3. AC Ref with Minimal Phase Delay

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