Transformer Zero Energy Sense and Restart Timer Circuits

Enable Circuit

Zero Energy Detect Circuit

Current Reference Generator Circuit

Overvoltage Protection (OVP) Circuit

UCC28810

UCC28811

When all of the energy stored in the transformer has been delivered to the load, the voltage across the primary

winding falls to zero. This voltage can be sensed by monitoring the bias winding of the transformer. The internal

active clamp circuitry prevents the voltage from going to a negative or a high positive value. The clamp has the

capability to sink and source 10 mA. The resistor value in series with the bias winding should be chosen to limit

the pin current to less than 10 mA under all operating conditions. The rising edge threshold of the TZE sense

comparator can be as high as 2.0 V. If the bias winding is not used to power the controller then it should be

chosen such that the positive voltage (when the power MOSFET is off) at the TZE pin is greater than 2.0 V,

limited to less than 10 mA.

The restart timer attempts to set the gate drive high when the gate drive remains off for more than 400

µs

nominally. The minimum time period of the timer is 200

µs. This translates to a minimum switching frequency of 5

kHz. The primary inductance value is chosen for switching frequencies greater than 5 kHz.

The gate drive signal is held low if the voltage at the VSENSE pin is less than the ENABLE threshold. This

feature can be used to disable the converter by pulling VSENSE low. If the output feedback path is broken,

VSENSE is pulled to ground and the output is disabled to protect the power stage.

the EAOUT pin to approximately 2.4 V. This ensures that the zero power comparator is not activated during

transient behavior (when the slew rate enhancement circuitry is activated).

The current reference generator creates a current sense threshold signal that is approximately equal to 0.67 ×

zero-crossing distortion and hence the THD performance of the controller in the application. The dynamic range

of the inputs can be found in the electrical characteristics table.

The OVP feature in this device is not activated under most operating conditions because of the presence of the

slew rate enhancement circuitry present in the error amplifier. As soon as the output voltage reaches 5% to 7%

above the nominal value, as detected by VSENSE, the slew rate enhancement circuit is activated, and the error

amplifier output voltage is pulled below the dynamic range of the current reference generator. This prevents

further rise in the output voltage.

If the EAOUT pin is not pulled low fast enough, and the output voltage rises further, the OVP circuit acts as a

second line of protection. When the voltage at the VSENSE pin is more than 7.5% of the nominal value

the regulated value, and protects the other components of the system.

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