Datasheet

CCR PS
CS XFMR
OCC
V N
R
2I
´
= ´ h
R
MAX MAX MAGCC
t
D 1 f D
2
æ ö
= - ´ -
ç ÷
è ø
UCC28710, UCC28711
UCC28712, UCC28713
UCC28714, UCC28715
www.ti.com SLUSB86 –NOVEMBER 2012
Transformer Turns Ratio, Inductance, Primary-Peak Current
The maximum primary-to-secondary turns ratio can be determined by the target maximum switching frequency at
full load, the minimum input capacitor bulk voltage, and the estimated DCM quasi-resonant time.
Initially determine the maximum available total duty cycle of the on time and secondary conduction time based on
target switching frequency and DCM resonant time. For DCM resonant time, assume 500 kHz if you do not have
an estimate from previous designs. For the transition mode operation limit, the period required from the end of
secondary current conduction to the first valley of the V
DS
voltage is ½ of the DCM resonant period, or 1 µs
assuming 500-kHz resonant frequency. D
MAX
can be determined using the equation below.
(12)
Once D
MAX
is known, the maximum turns ratio of the primary to secondary can be determined with the equation
below. D
MAGCC
is defined as the secondary diode conduction duty cycle during constant-current, CC, operation. It
is set internally by the UCC28710 family at 0.425. The total voltage on the secondary winding needs to be
determined; which is the sum of V
OCV
, the secondary rectifier V
F
, and the cable compensation voltage (V
OCBC
).
For the 5-V USB charger applications, a turns ratio range of 13 to 15 is typically used.
(13)
Once an optimum turns ratio is determined from a detailed transformer design, use this ratio for the following
parameters.
The UCC28710 family constant-current regulation is achieved by maintaining a maximum D
MAG
duty cycle of
0.425 at the maximum primary current setting. The transformer turns ratio and constant-current regulating
voltage determine the current sense resistor for a target constant current.
Since not all of the energy stored in the transformer is transferred to the secondary, a transformer efficiency term
is included. This efficiency number includes the core and winding losses, leakage inductance ratio, and bias
power ratio to rated output power. For a 5-V, 1-A charger example, bias power of 1.5% is a good estimate. An
overall transformer efficiency of 0.9 is a good estimate to include 3.5% leakage inductance, 5% core and winding
loss, and 1.5% bias power.
(14)
The primary transformer inductance can be calculated using the standard energy storage equation for flyback
transformers. Primary current, maximum switching frequency and output and transformer power losses are
included in the equation below. Initially determine transformer primary current.
Primary current is simply the maximum current sense threshold divided by the current sense resistance.
(15)
(16)
The secondary winding to auxiliary winding transformer turns ratio (N
AS
) is determined by the lowest target
operating output voltage in constant-current regulation and the VDD UVLO of the UCC28710 family. There is
additional energy supplied to VDD from the transformer leakage inductance energy which allows a lower turns
ratio to be used in many designs.
(17)
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