Dual output forward 150W precision switching regulated power supply

The +5V, +12V dual output forward 150W precision switching regulated power supply circuit is shown in the figure. The power supply uses one piece each of TOP227Y (ICl), linear optocoupler CNYl7--2 (IC2, product of German Telefunken Company), and adjustable precision parallel voltage regulator TL431 (Ic3). The AC input voltage is 110V/220V. At low voltage input, the power efficiency can reach 77% (typical value). S is the power selection switch. When S is closed, the AC voltage llOV doubler rectifier circuit is selected. Its working principle is shown in the figure below. Assume that point a has a positive potential and point b has a negative potential in the positive half cycle of the alternating current. At this time, the diodes VDb and VDc in the rectifier bridge are turned on, and VDa and VDd are cut off. See Figure (a). The 11OV alternating current charges C1 along the following path: u→a→C1→VDc→b, charging Uc1 to about ×llOV=155V, with the polarity of the upper end positive and the lower end negative. During the negative half cycle, point b has a positive potential, and point a becomes a negative potential. The equivalent circuit is shown in Figure (b). At this time, VDa and VDd are on, VDb and VDc are off, and the current charges Cl0 along the path u→b→VDa→C10→a, so that Uc10≈115V, the polarity is still positive at the upper end and negative at the lower end. Obviously, the actual output voltage of the rectifier filter = Ucl + Ucl0 ≈ 2 × 155V = 310V, thus achieving voltage doubling rectification, and the rated DC high voltage can be obtained even at low voltage input. Since the conduction voltage drops of VDa and VDd are very small and the dynamic conduction resistance of TOP227Y is negligible, the load of the rectifier filter, that is, the primary impedance of the high-frequency transformer is ZT. R10 and R11 are balancing resistors that can balance the voltages on Cl0 and C1 to prevent a certain capacitor from being broken down due to excessive voltage drop. In addition, after the power is turned off, these two resistors also provide a bleed circuit for the capacitor. When S is disconnected, 220V AC is selected. At this time, Cl0 and C1 are connected in series, and the total capacitance becomes 235μF. The electromagnetic interference filter (EMI filter) connected at the AC incoming line is composed of common mode choke L2 and C6~C8, C3, and R12. The midpoint of C7 and C8 should be connected to ground. L2, C7, and C8 are used to suppress common mode interference, and C6 and C3 are specifically used to filter out series mode interference. Since C6 has a large capacity (0.47μF) and resistor R12 is connected in parallel to it, after power off, C6 is discharged through R12, which can prevent the power incoming terminals L and N from being charged. In view of the fact that the leakage inductance of the high-frequency transformer will produce a peak high voltage when TOP227Y is turned off, a clamping circuit composed of VD1, R13, and Cll is now used, and then the drain-source voltage UDS is clamped by the voltage regulator tubes VDzl~VDz3. The above measures can effectively protect the power MOSFET from damage. The two coils on the secondary side of the high-frequency transformer are N2s and N2b. The two common mode windings of filter inductor L1 are L1a and Llb in sequence. Utilizing the mutual inductance of L1a and Llb can increase the equivalent inductance of L1, which has a stronger inhibitory effect on common mode interference. The voltage on N2a is rectified and filtered by VD2, L1a and Cl5, and a regulated output of +12V and 6.2A can be obtained. VD2 uses MURl610A ultra-fast recovery diode, average rectification current Id=16A, reverse withstand voltage URM=100V, reverse recovery time trr=35ns, and adopts TO~220 package. VD6 and VD5 are freewheeling diodes. The voltage of N2b passes through VD3, L1b and C2. After rectification and filtering, a regulated output of +5V and 15A can be obtained. In order to improve the efficiency when rectifying high-frequency and large currents, VD3 uses MBR3045 Schottky common cathode pair tubes, and the two internal tubes are used in parallel. Its main parameters are as follows: Id=30A, URM=45V, trr<10ns, TO-220 package. C14 and R8 suppress high-frequency damped oscillations (also known as "ringing") on VD5. R2 is the minimum load of +12V output, which is used to improve the voltage regulation rate at light load. The external error amplifier consists of TLA31. For example, when the +5V output voltage changes, the sampling voltage obtained after dividing the voltage by R4 and R5 is compared with the 2.5V bandgap reference voltage in TL431, and an error voltage is formed at the K point and the LED operating current is IF produces corresponding changes, and then changes the size of the control terminal current Ic through the optocoupler IC2 (CNY17-2), adjusts the output duty cycle of TOP227Y, keeps Uo2 unchanged, and finally achieves the purpose of voltage stabilization. The +5V voltage regulation value Uo2 is set by TL431. R1 is the current limiting resistor of the LED. The frequency response of the error amplifier is determined by C9, R6, and R4. C12 can appropriately reduce the high-frequency gain of the error amplifier. R7 and C13 can improve the transient response of the error amplifier. C5 has three functions, namely filtering the peak voltage on the control terminal, determining the automatic restart frequency, and compensating the control loop together with C5 and R3.