november 2014 docid10240 rev 2 1/17 17 AN1897 application note vipower?: low-cost universal input dvd supply with the viper22a-e introduction in the past few years, many consumer products have been provided to the end user, such as dvd or vcd players. generally, their power supply requires multiple outputs to supply a variety of control circuits: mcu, motor, amplifier, vfd. offline switch mode power supply regulators from st?s viper ? family combine high voltage, avalanche rugged vertical power mosfet with current mode control pwm circuitry. the result is the innovative ac-dc converter, simpler, quicker, with reduced component count and cheap. the viper family complies with the ?blue angel? and ?energy star? norms, with very low total power consumption in standby mode, thanks to the burst operation. this document presents the application on dvd player power supply with the viper22a-e meeting the specifications in ta ble 1 . figure 1. viper22a-e evaluation board table 1. output specifications input output 1 output 2 output 3 output 4 output 5 output 6 universal line 5 v+/- 5% (1) 1. the accuracy of +/-5% is reached for a range of load combination only. see section 3.2 for cross- regulation results. +12 v+/- 5% (1) -12 v+/- 5% (1) -26 v+/- 5% (1) 3.3 v+/- 5% (1) 5 v stb +/- 5% (1) min. 85 v ac max. 265 v ac imin. 20 ma imax.1.5 a imax. 30 ma imax. 30 ma imax.50 ma imax. 150 ma imax.100 ma www.st.com
contents AN1897 2/17 docid10240 rev 2 contents 1 application description and design . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1.1 start-up phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1.2 auxiliary supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1.3 burst mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1.4 feedback loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1.5 primary driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 transformer consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 layout recommendation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3 experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1 efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.2 regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 4 transformer specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5 pcb layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 6 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
docid10240 rev 2 3/17 AN1897 application description and design 17 1 application description and design 1.1 schematics the overall schematic is shown in figure 3 . 1.1.1 start-up phase the viper22a-e has an integrated high voltage current source linked to the drain pin. at the start-up converter, it charges the v dd capacitor until it reaches the start-up level (14.5 v), and the viper22a-e starts switching. 1.1.2 auxiliary supply the viper22a-e has a wide operating voltage range from 8 v to 42 v, respectively minimum and maximum values for undervoltage and overvoltage protections. this function is very useful to achieve low standby total power consumption. the feedback loop is connected to 5 v output by d12 to regulate 5 v output. +5 v stb output is blocked by q3, so +5 v stb regulation is neglected. when the standby signal is present, the q3 v ce cannot provide enough voltage to maintain d12 conducted, so the 5 v output is blocked, and the +5 v stb output is connected to the feedback loop. in this condition the +5 v stb is regulated. thanks to the transformer structure, all the other secondary outputs and the auxiliary voltages are pulled down to a very low level, also pulling down the total power consumption. these features are below-indicated. ? in normal full load, the vdd voltage must be lower than the overvoltage protection. ? in short-circuit, the vdd voltage must be lower than the shutdown voltage. actually, this condition leads to the well-known hiccup mode. ? in no-load condition, the vdd voltage must be higher than the shutdown voltage. 1.1.3 burst mode the viper22a-e integrates a current mode pwm with a power mosfet and includes the leading edge blanking function. the burst mode allows the viper22a-e to skip some switching cycles when the energy drained by the output load goes below e = (t b *v in )2 * f sw /2l p (t b = blanking time, v in = dc input voltage, f sw = switching frequency, lp = primary inductance). the consequence is the reduction of the switching losses in case of low load condition by reducing the switching frequency. 1.1.4 feedback loop the 5 v output voltage is regulated by a tl-431 (u3) via an optocoupler (u2) to the feedback pin. if the output voltage is high, the tl-431 draws more current through its cathode to the anode and the current increases in the optocoupler diode. the current in optocoupler npn increases accordingly and the current into the viper22a-e fb pin increases. when the fb current increases, the viper22a-e skips some cycles to decrease turn-on time and lower the output voltage to the proper level (see figure 1 ). the 5 v output voltage is regulated thanks to the tl-431 reference voltage and the r8 and r9 resistive dividers.
application description and design AN1897 4/17 docid10240 rev 2 figure 2. viper22a-e fb pin internal structure 1.1.5 primary driver in a flyback power supply, the transformer is used as an energy tank during the on-time of the mosfet. when the mosfet turns off, its drain voltage rises from a low value to the input voltage while the secondary diode conducts, transferring to the secondary side the magnetic energy stored in the transformer. since primary and secondary windings are not magnetically coupled, there is a serial leakage inductance that behaves like an open inductor charged at ipk, causing the voltage spikes on the mosfet drain. these voltage spikes must be clamped to keep the viper22a-e drain voltage below the bv dss (730 v) rating. if the peak voltage is higher than this value, the device is destroyed. the rcd clamp (see figure 4 ) is a very simple and cheap solution, but it impacts on the efficiency and on the power dissipation in standby condition. besides, the clamping voltage varies according to the load current. rcd clamp circuits may allow the drain voltage to exceed the breakdown rating of the viper22a-e during the overload operation or during turn-on with high line ac input voltage. a zener clamp is recommended (see figure 5 ). however this solution gives higher power dissipation at full load, even if the clamp voltage is exactly defined. 1.2 transformer consideration on the electrical specifications of a multiple output transformer (cross-regulation, leakage inductance), the main efforts focused on the proper coupling between the windings. a lower leakage inductance transformer allows a lower power clamp to reduce the input power. it l leads to lower power dissipation on the primary side. auxiliary and secondary windings are swapped in order to decrease the coupling to the primary one. the secondary windings act as a shielding layer to reduce the capacitive coupling. fewer spikes are generated on the auxiliary windings, the primary and secondary windings have better coupling. n + ] 2 6 & |