HT2802D

5/8

High performance primary sensing regulator (PSR)

www.hotchip.com.cn

Ver1.0

FUNCTION DESCRIPTION

HT2802D is specially designed for the charger/adapter of small power digital products. To use

PSR control technology to provide accurate (CV/CC) characteristics. The controller works in PFM

mode and the switching frequency can be automaticially adjusted as per the load. Optimized

driving circuit greatly promotes withstand voltage of the power BJT and minimizes the switching

loss, so as to make the circuit have excellent converting efficiency

CONSTANT VOLTAGE(CV) OPERATION

When the circuit is operates in CV area, FB will detect the output voltage when the power

BJT turn-off through the sensing voltage by auxiliary winding of the transformer, to make the

output voltage stablized in the fixed value. In the turn-on period of the output rectifier diode D,

it's given the output winding voltage and auxiliary winding voltage as follows

Vd)

(Vo

Ns

N

V

AUX

AUX







--------(5)

Vaux : auxiliary winding voltage； Nb：subsidiary winding turns；NS: output winding

turns； Vd：forward voltage drop of output rectifier diode

FB will test the auxiliary winding voltage while the power BJT off-time to regulate the output

voltage. In order to avoid the peak influence happens in power BJT turn-off resulting from the

leakage inductance of the transformer, and considering the accuracy of sampling voltage, FB

captures the auxiliary winding detection sampling points as below:

Portion of Tons

Tons

Valex

ov

See equation 5

CONSTANT CURRENT(CC ) OPETATION

HT2802D CC control circuit is requested the converter operating in DCM . In CC operation

mode , the CC control circuit will fix the proportion between the Tons and Toffs produced by

diode rectifier D. When Tons time, the primary energy of the transformer will be converted to the

secondary winding, via the rectifier diode then charge the output capacitor and supply load

simultaneously.

When Toffs, the primary coil stores the energy, the output filter capacitor makes load discharge.

In CC operation area, HT2802D switch frequency will follow the output load direct ratio contro

detected by FB Voltage

IS

OA

Tons

Toffs

Iout