UCC28700, UCC28701, UCC28702, UCC28703

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SLUSB41A – JULY 2012 – REVISED DECEMBER 2014

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.

(23)

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.

(24)

(25)

The secondary winding to auxiliary winding transformer turns ratio (NAS) is determined by the lowest target

operating output voltage in constant-current regulation and the VDD UVLO of the UCC28700. 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.

(26)

9.2.2.8

Standby Power Estimate

Assuming no-load standby power is a critical design parameter, determine estimated no-load power based on

target converter maximum switching frequency and output power rating.

The following equation estimates the stand-by power of the converter.

(27)

For a typical USB charger application, the bias power during no-load is approximately 2.5 mW. This is based on

25-V VDD and 100-µA bias current. The output preload resistor can be estimated by VOCV and the difference in

the converter stand-by power and the bias power. The equation for output preload resistance accounts for bias

power estimated at 2.5 mW.

(28)

Typical start-up resistance values for RSTR range from 13 MΩ to 20 MΩ to achieve 1-s start-up time. The

capacitor bulk voltage for the loss estimation is the highest voltage for the stand-by power measurement,

typically 325 VDC.

(29)

For the total stand-by power estimation add an estimated 2.5 mW for snubber loss to the start-up resistance and

converter stand-by power loss.

(30)

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