A piece of power management IC-TNY277, a member of TinySwitch-III family produced by Power Integrations Inc., is used in the drive circuit. TNY277 incorporates a high-voltage power MOSFET with a power supply controller in one device, using an “ON/OFF” control scheme and offers a design flexible solution with a low system cost and extended power capability. The pin configuretion of TNY277 is shown in Figure 1, where D pin is the power MOSFET drain connection, providing internal operating current for both start-up and steady-state operation and S pin is internally connected to the output MOSFET source for high voltage power return and control circuit common. EN/UV pin has dual functions: enable input and line under-voltage sense. During normal operation, switching of the power MOSFET is controlled by this pin [2].

When the power MOSFET is turned off, there is a high voltage spike on the drain due to the transformer leakage inductance. This excessive voltage on the MOSFET may lead to an avalanche breakdown and eventually failure of the Tiny switch. Therefore, it is necessary to use an additional network to clamp the voltage. The RCD snubber circuit and MOSFET drain voltage waveform are shown in Figures 2 and 3, respectively. The RCD snubber network absorbs the current in the leakage inductance by turning on the snubber diode D_sn once the MOSFET drain voltage exceeds the voltage of node X as depicted in Figure 2. In the analysis of snubber network, it is assumed that the snubber capacitor is large enough that its voltage does not change significantly during one switching cycle. The snubber capacitor used should be ceramic or a material that offers low ESR. Electrolytic or tantalum capacitors are unacceptable due to these reasons [3-5].

The first step in designing the snubber circuit is to determine the snubber capacitor voltage at the minimum input voltage and full load condition (V_sn). Once V_sn is determined, the power dissipated in the snubber network at the minimum input voltage and full load condition is obtained as

where I_{ds peak} is the maximum peak current through the TNY277 at the minimum input voltage condition, f_s is the TNY277 switching frequency, L_lK is the leakage in-ductance, V_sn is the snubber capacitor voltage at the minimum input voltage and full load condition, V_R is the reflected output voltage and R_sn is the snubber resistor. V_sn Should be larger than V_R and it is typical to set V_sn to be 2.5 times V_R. Too small a V_sn results in a severe loss in the snubber network as shown in Equation (1). The leakage inductance is measured at the switching frequency on the primary winding with all other windings shorted. Then, the snubber resistor with proper rated wattage should be chosen based on the power loss. The maximum ripple of the snubber capacitor voltage is obtained as

where f_s is the TNY277 switching frequency. The snubber capacitor voltage (V_sn) of Equation (7) is for the minimum input voltage and full load condition, When the converter is designed to operate in CCM under this condition, the peak drain current together with the snubber capacitor voltage decrease as the input voltage increases as shown in Figure 3. The peak drain current at the maximum input voltage and full load condition (I_{ds Peak}) is obtained as

where the maximum input power P_in, is given by

where P_o is the output power and the η is the circuit efficiency.

And the transformer primary side inductance L_P is given by

where k is the is the ripple factor in full load and minimum input voltage condition and V_{DC min} is the minimum DC link voltage and the maximum duty cycle ratio D_max is given by

the snubber capacitor voltage under maximum input voltage and full load condition is obtained as

where f_s is the TNY277 switch frequency, L_lK is the primary side leakage inductance, V_R is the reflected output voltage and R_sn is the snubber resistor.

From Equation (7), the maximum voltage stress on the internal MOSFET is given by

where is maximum DC link voltage V_{DC max} given by

in this section the snubber cell elements selected successfully to improve the efficiency and performance of the proposed charger.

In general, a battery charger employs constant current (CC)/constant voltage (CV) control circuit for an optimal charge of a battery. Figure 4 shown a constant current (CC)/constant voltage (CV) circuit based on the flyback converter is proposed in this paper, a simple, low cost circuit using a comparator (U3) and shunt regulator (U4), being validated with a parametric detailed analysis provided in this section.

DC input voltage V_DC is generated by rectifying AC input voltage V_AC using a bridge rectifier, and its maximum V_{DC max} is specified in Equation (9).

Assume that the switching frequency and the “ON” period for the power MOSFET are f_s and t_on, respectively. Then duty ratio D is defined according to the following equation:

The input power P_i for the drive circuit is calculated by Equation (4) where P_o is the output power and the η is the circuit efficiency. Thus the average primary current I_i is represented as follows:

Assume that the initial current in primary winding is zero ampere.

During the MOSFET “ON” period, there is a fixed voltage across primary winding and current in it ramps up linearly. At the end of the “ON” period, the primary current has ramped up to I_p, as shown in Figure 5(a). The relationship between I_i and I_p can be represented as follows:

The power output of the flyback circuit is determined by the primary inductance L_P and the primary current I_p, where L_P can be calculated as

When MOSFET turns “OFF”, the current in the magnetizing inductance forces a reversal of polarities on primary winding. Since the current in an inductor cannot change instantaneously, at the instant of turn “OFF”, the primary current transfers to the secondary at an amplitude

where N_p, N_s, are the primary and secondary winding turns. During the MOSFET “OFF” period, the secondary current ramps down linearly, shown in Figure 5(b), with an average value calculated as

Finally, from (10)-(14) and by using Equation (4), the circuit output voltage V_o can be represented as the following equation:

where N is the primary/secondary turns ratio. Assume that the reference voltage in Figure 4 is V_ref, the voltage detected by R_d is V_d and the voltage across the zener diode U4 is V_z, then the V_r and V_d is given by follwing equations:

when V_d > V_r the photoelectrical coupler (U2) and the MOSFET turns “OFF” to reduce the output current. On the contrary, if V_d < V_r, the photoelectrical coupler leaves off work and the MOSFET turns “ON”, helping increasing the output current.

Constant voltage (CV) control: The voltage divider network of R₄ and R₅ should be designed to provide V_z to the reference pin of the U4. The relationship between R₄ and R₅ is given by

where V_o is the output voltage. By Assuming R₄ it is easily to obtained R₅.

The feedback capacitor (C_F) introduces an integrator for CV control. To guarantee stable operation, C_F of 470 nF is chosen.

R₁ should be designed to provide proper operating current for the shunt regulator (U4) and to guarantee the full swing of the feedback voltage for the TINY 277 Switch.

Finally a snubber cell and constant output current and constant output voltage elements are obtained thus the final scheme of the proposed fly back converter is shown in Figure 6.