
AN-39
B
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ISP × ESR. ISP is the secondary peak current, which is calculated
in the Transformer Secondary Design Parameters section of
the spreadsheet.
Tips for Clampless designs
The mechanical construction of the transformer will play a
crucial role in Clampless designs. Care should be taken to
reduce the leakage inductance and increase the intra-winding
capacitance of the primary winding. Intra-winding capacitance
is defined as the capacitance measured from one end of a
winding to the other end while all other windings are open.
This is best achieved by using a 2-layer primary winding. It
is common to use a layer of tape between 2 primary layers.
This should be avoided for Clampless designs, as this tends to
reduce intra-winding capacitance. For designs that do not use
a bias winding for damping the leakage ringing, there is no
restriction on strictly using a 2-layer primary winding. However,
for Clampless designs that do not use a bias winding, a 2-layer
primary winding must be used.
Even with the increased winding capacitance, no-load power
of < 150 mW is easily possible with LinkSwitch-LP.
For typical Clampless designs, the leakage inductance is below
90 µH and the intra-winding capacitance is at least 30 pF.
Minimizing Audible Noise
The cycle skipping mode of operation used in LinkSwitch-LP
can generate audio frequency components in the transformer.
To limit this audible noise generation, the transformer should
be designed such that the peak core flux density BM is below
1500 Gauss (150 mT). Following this guideline and using the
standard transformer production technique of dip varnishing
practically eliminates audible noise. Higher flux densities
are possible, however careful evaluation of the audible noise
performance should be made using production transformer
samples before approving the design.
Ceramic capacitors that use dielectrics such as Z5U, when used
in clamp circuits, may also generate audio noise. If this is the
case, try replacing them with a capacitor having a different
dielectric, for example a polyester film type.
Standard Transformer Designs
The LinkSwitch-LP family members have the same primary
current limit but different switching frequencies, which result
in different, output power capabilities. This allows additional
flexibility in design by allowing the same transformer design to
be used for different output powers and output voltages.
To illustrate this, Appendix A provides two reference
designs that in many cases may eliminate the need to design
a transformer. These two reference designs include Power
Integrationsʼ E-Shield windings to minimize EMI.
Table 9 lists a series of output voltages and current, which can
be used to select the correct LinkSwitch-LP device, reference
transformer design and feedback resistor values (assuming bias
winding feedback).
The table also lists, for information, the effective VOR. As
the output voltage is reduced from the nominal design the
VOR reduces and conversely increases as the output voltage
is increased. It is this that limits the effective output voltage
range that one transformer can cover without either excessive
peak drain voltage or the design entering continuous conduction
mode (KP < 1) with itʼs associated increase in EMI.
Note: The standard transformer designs assume that a bias
winding is used. Therefore to implement a Clampless design
the bias winding must be used with slow diode (D5) as shown
in Figure 10.
Example Designs Using Standard
Transformers
Figure 1 shows an example design for a cell phone charger power
supply. It is a universal input power supply with 6 V output at
a constant maximum current of 330 mA. The circuit uses no
Y capacitor, no primary side clamp and relies on a slow diode
used in the bias winding for damping the leakage spike. The
transformer uses a standard EE16 core and uses E-Shields to
meet the CISPR-22 EMI limits. Detailed transformer drawings
are shown in Appendix A and these can be used as a building
block for others. For slightly different output voltages (see
Table 9), the resistor divider in the bias winding may be adjusted.
For power below 2 W, either a smaller LinkSwitch-LP part may
be used or the primary inductance may be adjusted by changing
the length of the air gap.
Figure 10 shows another example design for a cell phone
charger power supply which is also a universal input voltage
range supply with an output voltage of 9 V at a maximum
constant current of 220 mA. This is also a Clampless design,
which relies on the bias diode to damp out the leakage spike
during turn off. Use of E-Shields allows the design to pass the
CISPR-22 EMI limits with 10 dB of margin, without the use
of a Y capacitor. Detailed drawings for this transformer are
shown in Appendix A.