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Texas Instruments UCC28880, UCC28881 Audible Noise Reduction Techniques Application notes
Application Report
SLUA821 – March 2017
UCC28880, UCC28881 Audible Noise Reduction
Techniques
Mike O'Loughlin
1
Introduction
The economical UCC28880 and UCC28881 high voltage switcher/controllers were designed for offline
buck converters for low power applications to meet size and cost constraints. These devices use an
On/Off control scheme, like many others available today, that could potentially allow the effective switching
frequency to drop into the audible band and generate noise. By following the recommended design
techniques in this application note for UCC28880 and UCC28881 the problem can be avoided by staying
quite over the entire operating range.
2
Functional Schematic
RF
L2
D2
HVIN
VDD
UCC28880/1
CVDD
C1
+
DRAIN
RFB1
FB
VIN
C2
-
GND
CFB
RFB2
D4
L1
D1
D3
CL
RL
+
VOUT
-
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Figure 1. Offline High-Side Buck Converter with On and Off Control
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Circuit Operation
3
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Circuit Operation
The UCC28880 and UCC28881 uses what is known as On/Off control. This control scheme samples the
output during the freewheeling period through the RFB1 and RFB1 resistor dividers and when the voltage at
the feedback pin is less than 1.02 V it delivers 62-kHz of pulse width modulated (PWM) packets to
maintain the output voltage. The PWM packets are controlled by peak current mode control.
FB
1.02 V
On
Off
62 kHz
On
62 kHz
MOSFET
Gate
MOSFET
Current
Figure 2. On/Off Control Diagram
The output inductor (L1) is sized based on internal peak current limit (ILIMIT), output current (IOUT) and PWM
switching frequency of typically 62 kHz. The inductor selected should as close to the calculated value as
possible.
Table 1. UCC28880 ILIMIT Specification
PARAMETER
TEST CONDITIONS
MIN
TYP
Static, –40°C
ILIMIT
Current limit
Static, 25°C
170
Static, 125°C
140
210
MAX
UNIT
300
mA
260
mA
mA
Table 2. UCC28881 ILIMIT Specification
PARAMETER
TEST CONDITIONS
MIN
TYP
Static, 25°C
330
440
Static, 125°C
315
Static, –40°C
ILIMIT
Current limit
MAX
UNIT
630
mA
570
mA
mA
NOTE: VHVIN = 30 V, TA = TJ = –40 °C to 125°C in Table 1 and Table 2.
IOUT
L1
2
0.9 u ILIMIT
(1)
§
VOUT ·
VOUT u ¨ 1
¸
VIN ¹
©
2 u ILIMIT IOUT u 62kHz
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Circuit Operation
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Resistors RFB1 and RFB1 are also used to adjust the output voltage and capacitor CFB is used to set PWM
packet timing (tFB). The initial tFB is set to 1/10th the product of the output capacitance (CL) and load
impedance (RL) and then will be fine-tuned based on actual circuit performance.
tFB (RFB1 RFB2 ) u CFB CL uRL u 0.1
(3)
Control Methodology will operate in the Audible Range (<20 kHz).
Due to variations in peak current (ILIMIT) at maximum load the effective switching frequency will vary from
62 kHz down to 31 kHz. As the load decreases below 80% the converter can and will enter the audible
range (<20 kHz) The output ripple voltage and inductor ripple current will look similar to Figure 3 as the
load decreases.
Figure 3. Output Ripple Voltage (VOUT = CH1) and Inductor Ripple Current (IL1 = CH4)
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Sources of Audible Noise
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Sources of Audible Noise
Any components in the design that have energy packets being delivered through them at 20 kHz or less
can vibrate. This can include capacitors, diodes, inductor’s wires, connectors.
5
Determining the Source of Audible Noise
CAUTION
Follow high-voltage safety practices.
1. A sheet of paper can be rolled up into a tube and used like a hearing aid. Put one side of the paper
tube to your ear and point the other end of the tube toward the power supply circuit board in operation.
This can help you find the audible component.
2. A non-conductive stick can be used to determine which device is vibrating and creating the audible
noise. A number 2 wooden pencil with an eraser can be used for this. Touch the suspected component
that is contributing to the audible noise. If the sound dulls this may be the source of the audible noise.
6
Things the Designer Can Do to Reduce Audible Noise
1. Select the output inductor (L1) as close to the recommended value as possible. This will ensure at full
load the effective switching frequency is as close to the maximum as possible. This calculation was
covered earlier in this application note.
2. Set the tFB time constant as small as possible to get the converter to operate at the maximum
frequency possible. This may affect load regulation of the power converter and may require pre-load
resistor (RL) as shown in Figure 1. A Zener clamp or a series pass regulator on the output may be
required to regulate the output voltage at light loads. Please refer to Figure 4 for the circuit
configuration.
+
+
VOUT
-
VOUT
-
Figure 4. Zener Shunt or Series Pass Regulator to Improve Load Regulation
4
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Things the Designer Can Do to Reduce Audible Noise
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3. Select a buck inductor (L1) that is varnished and/or encased in magnetic potting material. TDK and
Wurth have a family of inductors that are encased in potting material. Avoid inductors that are not
varnished and/or encased in magnetic potting material. Examples of these types of inductors can be
found in Figure 5.
Figure 5. Potted and Varnished Inductor, Inductor without Varnishing and Potting.
4. Use electrolytic input and output capacitors because they are less likely to create audible noise over
ceramic capacitors that have piezoelectric characteristics. Please note that some electrolytic capacitors
can vibrate and create audible noise as well. They may have to be attached to the board in rubber
silicon or encased in potting material to reduce vibrations and make the design less audible.
5. Glue down any possible components that could vibrate with epoxy, rubber silicon or other like
adhesive.
6. In rare cases it may be necessary to varnish/encase the entire power supply in potting material to
reduce audible noise. There are many companies that make these electronics insulation resins and
polymers for these applications such as Elantas Electrical Insulation, Epic Resins and ITW Engineered
Polymers.
Figure 6. Potted Power Supply
7. After the initial design is complete and the audible noise is removed. It is recommended to do
temperature, production and verification testing. This process should involve multiple boards and
devices to verify the design.
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Summary
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Summary
The UCC28880 and UCC28881 economic offline buck converters use an On/Off control technique that
can make the design susceptible to audible noise (<20 kHz) As this application note discussed the audible
noise can be reduced by properly selecting input and output capacitors; as well as, inductors that are
vanished and encased in magnetic potting materials. In some cases it may require varnishing and/or
encasing the design in potting material.
8
Recap of Recommendations
1. Select the output inductor (L1) as close to the recommended value as possible.
2. Set the tFB time constant as small as possible to get the converter to operate at the maximum
frequency possible.
(a) Preloading, Zener and/or series pass regulator may be needed to improve load regulation.
3. Select a buck inductor (L1) that is varnished and or encased in magnetic potting material.
4. Avoid using ceramic capacitor for input and output capacitors, electrolytic capacitors are a better
choice.
5. Glue down components that can vibrate.
6. Encapsulate the power supply in potting material.
7. Conduct temperature, production and verification testing to account for variations in component and
device tolerances.
9
Reference Material
1. UCC28880 Data Sheet, High Voltage Switch for Non-isolated AC/DC Conversion
2. UCC28881 Data Sheet, 700-V, 225-mA Low Quiescent Current Off-Line Switcher
6
UCC28880, UCC28881 Audible Noise Reduction Techniques
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